PhD Judy M Bradley
email: jm.bradley@ulster.ac.uk

Lisa Kent

PhD Brenda O'neill

PhD Alan Nevill

Lesley Boyle

MD J Stuart Elborn

Cardiorespiratory measurements during field tests in CF: Use of an ambulatory monitoring system

Keywords: respiratory inductive plethysmography, LifeShirt, cystic fibrosis, 6 Minute Walk Test, Modified Shuttle Test respiratory inductive plethysmography, LifeShirt, cystic fibrosis, 6 Minute Walk Test, Modified Shuttle Test

Respiratory inductive plethysmography (e.g. LifeShirt) may offer in-depth study of the cardiorespiratory responses during field exercise tests. The aims of this study were to assess the reliability, discriminate validity and responsiveness of cardiorespiratory measurements recorded by the LifeShirt during field exercise tests in adults with CF. To assess reliability and discriminate validity, participants with CF and stable lung disease and healthy participants performed the 6 Minute Walk Test (6MWT) and Modified Shuttle Test (MST) on 2 occasions. To assess responsiveness, participants with CF experiencing an exacerbation performed the 6MWT at the start and end of an admission for intravenous antibiotics. The LifeShirt was worn during all exercise tests. Reliability and discriminate validity were assessed in 18 participants with CF (mean (SD) age: 26 (10)years; FEV 1 %predicted: 69.2 (23)%) and 18 healthy participants (age: 24 (5)years, FEV 1 %predicted: 92 (8)%). There was no difference in 6MWT and MST performance between days and reliability of cardiorespiratory measures was acceptable (bias p>0.05; CV<10%). Participants with CF demonstrated a significantly greater response to exercise (e.g. ventilation, respiratory rate) compared to healthy participants indicating discriminate validity. Responsiveness was assessed in 12 participants with CF: clinical measurements and 6MWT performance improved (61(81)m; p<0.05) however cardiorespiratory measurements recorded by the LifeShirt remained the same (bias: p>0.05; CV<10%). This study provides evidence that cardiorespiratory responses can be measured non-invasively during field exercise tests in adults with CF. Reliability and discriminate validity of key cardiorespiratory measurements recorded by the LifeShirt were demonstrated. Some information on responsiveness is reported.

INTRODUCTION

In cystic fibrosis (CF) exercise tests enable assessment of functional capacity and limitations, determination of level of fitness, safe and effective exercise recommendations, and evaluation of interventions. 1,2 Field exercise tests are often used in CF centres when clinical laboratories are unavailable. While field exercise tests provide information on exercise performance (i.e. how far a patient can walk or run) they do not provide in-depth information on the cardiorespiratory responses to exercise or what factors contribute to exercise limitation. Measuring cardiorespiratory responses during field exercise tests using ambulatory monitoring has been limited by the technology available. Some available ambulatory monitoring systems record a narrow range of measurements and others are too cumbersome. Respiratory inductive plethysmography (e.g. the LifeShirt) may offer the opportunity for more in-depth study of the cardiorespiratory responses during field exercise tests and potentially provide additional valuable information. 3 The LifeShirt ® (Vivometrics, Inc., Ventura, CA, U.S.A.) is a multi-function ambulatory system capable of detecting several cardiorespiratory measures of interest to clinicians and researchers working in CF, for example timing and volume components of breathing and heart rate. The LifeShirt consists of a CoolMax ® garment, data recorder, and computer-based analysis software (VivoLogic). The core technology is respiratory inductive plethysmography which provides non-invasive assessment of respiratory pattern. Unlike direct methods which must be coupled to the individual (e.g. pneumotach via a face-mask or mouthpiece and nose-clips), respiratory inductive plethysmography is unobtrusive and so can be used for extended periods in an ambulatory setting. The LifeShirt system incorporates a Lycra vest into which the respiratory inductive plethysmography bands are embedded in order to limit displacement. Before systems such as the LifeShirt can be used in clinical or research practice information is required on clinimetric properties (i.e. reliability, validity and responsiveness).

Assessment of clinimetric properties of the LifeShirt has mainly been conducted in healthy participants in a laboratory environment. [3][4][5][6] We have previously demonstrated that the LifeShirt can provide valid and reliable cardiorespiratory data across multiple assessments in healthy participants tested in a laboratory environment. 3 There has been one previous study that has used the LifeShirt during exercise in clinical populations and demonstrated the ability of cardiorespiratory measurements recorded by the LifeShirt (ventilation, respiratory rate and heart rate) to discriminate between patients with congestive heart failure, chronic obstructive pulmonary disease and healthy participants. 4 There is no available information on the clinimetric properties of cardiorespiratory measurements recorded by respiratory inductive plethysmography (i.e. the LifeShirt) during exercise tests in patients with CF.

The aims of this study were:

•

MATERIALS AND METHODS

Ethics

The protocol was approved by the Queen's University Belfast and Belfast Trust City

Hospital ethics and research governance procedures and written informed consent obtained. Recruitment commenced in November 2003 and ended in October 2007.

LifeShirt

LifeShirt data relating to various cardiorespiratory measurements were recorded during field exercise tests. These included inspiratory tidal volume (ViVol, mL); ventilation (V E , L•min -1 ); respiratory rate (Br/M, breaths•min -1 ); ratio of inspiratory time to total respiratory time (Ti/Tt); and heart rate (HR, beats•min -1 ). The method of calibrating the volume components of breathing was recommended by the manufacturers and involved patients inflating and deflating a fixed volume (400mL/800mL) bag seven times whilst wearing nose clips. This was performed in standing.

Spirometry and exercise tests

Spirometry was performed according to ATS/ ERS guidelines for standardisation of spirometry 7 and expressed as percentage of predicted normal values according to the equations developed by Quanjer et al. (1993) 8 . Lung disease was classified by FEV 1 as normal (>90% predicted), mild (70-89% predicted), moderate (40-69% predicted) or severe (<40% predicted). 9 For the 6MWT participants were asked to walk as fast as they could around a 30 metre course for a period of six minutes according to the ATS

Statistical analysis

The significance of differences in clinical measurements and exercise performance between Time 1 and Time 2 were analysed using paired t-tests. Breath-by-breath data recorded by the LifeShirt were summarised into minute means to facilitate analysis.

Exploratory assessment of the 6MWT (using repeated measures ANOVA) revealed that a steady state work rate was achieved by the fourth minute therefore minute means of the final two minutes were analysed. It was not possible to perform repeated measures ANOVA on the submaximal stages of the MST due to an unbalanced design (i.e. there was a large range of performances resulting in differences in the number of completed stages), therefore the minute means were calculated for the last two stages that participants reached on both days (i.e. the two highest comparable stages). When the two days were compared (Part 1 and Part 2), differences (residual errors) were found to be associated with the size of the measurements (heteroscedasticity) and as such, all data were log transformed. [START_REF] Atkinson | Statistical methods for assessing measurement error (reliability) invariables relevant to sports medicine[END_REF] Reliability and responsiveness were then assessed using repeated-measures ANOVA performed in Minitab (Minitab Ltd, Coventry, UK). Results are reported as significance of bias (systematic differences between measurements in a particular direction) and within-subject mean-square errors reported as coefficients of variation (CV) (unexplained error differences between measurements). 17,18 3 and4). Analysis for SpO 2 was not performed due to the large amount of missing values. 21 This is due to the increased residual volume to total lung capacity ratio as a consequence of hyperinflation and increased deadspace. 21 Ti/Tt also demonstrated some significant differences during stages 7 to 10 however the difference is small and unlikely to be of clinical relevance. From stage 8 of the MST the difference between participants with CF and healthy participants in HR response becomes significant with participants with CF having an exaggerated response.

F

There may be several reasons that this does not appear to be significant until later stages in the exercise test. Firstly there may be a compensatory increase in HR in response to inefficient ventilatory response. Secondly it may be due to a lower anaerobic threshold in participants with CF (i.e. there is increased demand on cardiorespiratory systems when individuals exercise above their anaerobic threshold). Thirdly, hyperinflation additionally contributes to the increased work of breathing by forcing the inspiratory muscles to work at a mechanical disadvantage and as oxygen is diverted away from exercising muscles to the inspiratory muscles there is increased demand on the cardiovascular system. Data on the later stages of the MST is not available for all patients and therefore the findings need to be confirmed in further studies.

It should be highlighted that the standard deviation appears to decrease for Ti/Tt and HR.

The cardiorespiratory data were log transformed as is appropriate for data demonstrating heteroscedasticity (errors associated with size of measurements). For ViVol, VE, and The reliability study was performed in a small group of participants with CF, in particular there were few patients with moderate (n=4) or severe (n=3) lung disease. Reliability may not be equivalent across different severities of disease as patients with more severe disease experience more frequent fluctuations in health status making it difficult to assess reliability of measurements. Future research should include larger numbers of patients with moderate and severe lung disease in order to assess reliability in subgroups of patients.

In this study, the distance walked during the 6MWT improved in response to an admission for intravenous antibiotic treatment for an exacerbation of lung disease in participants with CF. Cardiorespiratory measurements recorded by the LifeShirt during the 6MWT were unable to capture the mechanism by which exercise capacity improved.

The 6MWT is a self-paced functional test which is frequently used both in the CF clinical setting and as an endpoint in clinical trials. 22 A possible mechanism of improved performance on the test could be improved cardiorespiratory efficiency. During an exacerbation patients often present with many characteristics which would impair cardiorespiratory efficiency, such as impaired airflow and reduced gas exchange. 23 Therefore it would be expected that ventilatory response would be increased at the start Assessment of clinimetric properties of the LifeShirt has mainly been conducted in healthy participants in a laboratory environment. [3][4][5][6] We have previously demonstrated that the LifeShirt can provide valid and reliable cardiorespiratory data across multiple assessments in healthy participants tested in a laboratory environment. 3 There has been one previous study that has used the LifeShirt during exercise in clinical populations and demonstrated the ability of cardiorespiratory measurements recorded by the LifeShirt (ventilation, respiratory rate and heart rate) to discriminate between patients with congestive heart failure, chronic obstructive pulmonary disease and healthy participants. 4 There is no available information on the clinimetric properties of cardiorespiratory measurements recorded by respiratory inductive plethysmography (i.e. the LifeShirt) during exercise tests in patients with CF.

The aims of this study were:

•

LifeShirt

LifeShirt data relating to various cardiorespiratory measurements were recorded during field exercise tests. These included inspiratory tidal volume (ViVol, mL); ventilation (V E , L•min -1 ); respiratory rate (Br/M, breaths•min -1 ); ratio of inspiratory time to total respiratory time (Ti/Tt); and heart rate (HR, beats•min -1 ). The method of calibrating the volume components of breathing was recommended by the manufacturers and involved patients inflating and deflating a fixed volume (400mL/800mL) bag seven times whilst wearing nose clips. This was performed in standing.

Spirometry and exercise tests

Spirometry was performed according to ATS/ ERS guidelines for standardisation of spirometry 7 and expressed as percentage of predicted normal values according to the equations developed by Quanjer et al. (1993) 8 . Lung disease was classified by FEV 1 as normal (>90% predicted), mild (70-89% predicted), moderate (40-69% predicted) or severe (<40% predicted). 9 For the 6MWT participants were asked to walk as fast as they could around a 30 metre course for a period of six minutes according to the ATS

Statistical analysis

The significance of differences in clinical measurements and exercise performance between Time 1 and Time 2 were analysed using paired t-tests. Breath-by-breath data recorded by the LifeShirt were summarised into minute means to facilitate analysis.

Exploratory assessment of the 6MWT (using repeated measures ANOVA) revealed that a steady state work rate was achieved by the fourth minute therefore minute means of the final two minutes were analysed. It was not possible to perform repeated measures ANOVA on the submaximal stages of the MST due to an unbalanced design (i.e. there was a large range of performances resulting in differences in the number of completed stages), therefore the minute means were calculated for the last two stages that participants reached on both days (i.e. the two highest comparable stages). When the two days were compared (Part 1 and Part 2), differences (residual errors) were found to be associated with the size of the measurements (heteroscedasticity) and as such, all data were log transformed. [START_REF] Atkinson | Statistical methods for assessing measurement error (reliability) invariables relevant to sports medicine[END_REF] Reliability and responsiveness were then assessed using repeated-measures ANOVA performed in Minitab (Minitab Ltd, Coventry, UK). Results are reported as significance of bias (systematic differences between measurements in a particular direction) and within-subject mean-square errors reported as coefficients of variation (CV) (unexplained error differences between measurements). 17,18 There was no significant difference in FEV 1 %predicted over the two occasions in either group indicating that health status of participants was stable.

In order to facilitate analysis of the reliability of cardiorespiratory measurements recorded by the LifeShirt system it was essential that performance on the exercise tests was reliable between Time 1 and Time 2. There was no difference in exercise For both the 6MWT and the MST reliability of cardiorespiratory measures recorded by the LifeShirt was acceptable with no significant bias (i.e. p>0.05) and CV below 10%

(Table 1). However, exercise intensity at each stage of the MST was externally controlled (i.e.

patients with CF and healthy participants exercised at the same intensity) and therefore discriminate validity in the cardiorespiratory measurements could be assessed for each stage fully completed.

Cardiorespiratory measurements recorded by the LifeShirt generally demonstrated an exaggerated response to exercise. Significant differences (p<0.05) between participants with CF and healthy controls were observed from stages 1 to 8 for ViVol, stages 1 to 10

for V E , stages 1 to 12 for Br/M, stages 7 to 10 for Ti/Tt, and stages 8 to 12 for HR (Figures 1 to 5). Abnormalities in cardiorespiratory responses were still apparent when only participants with CF and normal lung function or mild CF-related lung disease (FEV 1 >70 %predicted, n=11) were compared to healthy participants: significant differences were observed from stages 1 to 8 for ViVol, stages 1 to 10 for V E , and stages 1 to 10 for Br/M. Ti/Tt and HR showed a trend for being higher in later stages however there were fewer participants completing these stages. 3 and4). Analysis for SpO 2 was not performed due to the large amount of missing values. resting ventilation to ventilation experienced during exercise). 1,2 This study confirms previous findings showing that performance (distance completed) is reliable for both the 6MWT and the MST. [START_REF] Bradley | Reliability repeatability and sensitivity of the modified shuttle test in adult CF[END_REF]19,20 All cardiorespiratory measurements recorded by the LifeShirt during the 6MWT and MST showed no significant bias (p>0.05) and acceptable CVs (<10%) over two occasions during a period of stability, indicating acceptable reliability. 21 This is due to the increased residual volume to total lung capacity ratio as a consequence of hyperinflation and increased deadspace. 21 Ti/Tt also demonstrated some significant differences during stages 7 to 10 however the difference is small and unlikely to be of clinical relevance. From stage 8 of the MST the difference between participants with CF and healthy participants in HR response becomes significant with participants with CF having an exaggerated response.

There may be several reasons that this does not appear to be significant until later stages in the exercise test. Firstly there may be a compensatory increase in HR in response to inefficient ventilatory response. Secondly it may be due to a lower anaerobic threshold in participants with CF (i.e. there is increased demand on cardiorespiratory systems when individuals exercise above their anaerobic threshold). Thirdly, hyperinflation additionally contributes to the increased work of breathing by forcing the inspiratory muscles to work at a mechanical disadvantage and as oxygen is diverted away from exercising muscles to the inspiratory muscles there is increased demand on the cardiovascular system. Data on the later stages of the MST is not available for all patients and therefore the findings need to be confirmed in further studies.

It should be highlighted that the standard deviation appears to decrease for Ti/Tt and HR. In this study, the distance walked during the 6MWT improved in response to an admission for intravenous antibiotic treatment for an exacerbation of lung disease in participants with CF. Cardiorespiratory measurements recorded by the LifeShirt during the 6MWT were unable to capture the mechanism by which exercise capacity improved.

The 6MWT is a self-paced functional test which is frequently used both in the CF clinical setting and as an endpoint in clinical trials. 22 A possible mechanism of improved performance on the test could be improved cardiorespiratory efficiency. During an exacerbation patients often present with many characteristics which would impair cardiorespiratory efficiency, such as impaired airflow and reduced gas exchange. 23 Therefore it would be expected that ventilatory response would be increased at the start Pediatric Pulmonology

8 guidelines. 10 14 Part 1 :

 810141 For the MST participants were asked to walk or run around a 10 metre course at increasing speeds dictated by an external bleep from a tape recorder.11-Reliability and discriminate validityPatients with CF and stable lung disease were recruited from the Northern Ireland Adult CF Centre in Belfast City Hospital. Age matched controls were recruited from staff and students at Belfast City Hospital and the University of Ulster. All participants wore the LifeShirt system during a 6MWT and MST performed on two occasions at least one week apart (Time 1 and Time 2). Exercise tests were carried out according to standardised protocols and in a standardised order. Sufficient rest periods were used between exercise tests for Borg breathlessness score and heart rate to return to resting values. All participants performed spirometry on each occasion.Part 2: ResponsivenessParticipants with CF undergoing intravenous antibiotic therapy for an acute exacerbation of respiratory disease were recruited from Northern Ireland Adult CF Centre in Belfast City Hospital. A course of intravenous antibiotics was chosen as a test platform to assess responsiveness of cardiorespiratory measurements recorded by the LifeShirt. This has been used in previous studies that have investigated the ability of an outcome measure to detect clinically important changes.[START_REF] Horsley | Lung clearance index is a sentive, repeatable and practical measure of airways disease in adults with cystic fibrosis[END_REF] All patients also performed spirometry and had Creactive protein (CRP) levels determined on two occasions (Time 1= beginning of admission for intravenous antibiotic therapy and Time 2= end of admission for ). All participants also wore the LifeShirt system during a 6MWT on each occasion.

  the heteroscedasticity was obvious and thus log transforming the data produced uniform standard deviations across the stages of the MST. For Ti/Tt and HR the data demonstrated heteroscedasticity, albeit less pronounced, and thus the log transformed standard deviations appear to decrease across the stages of the MST.

Figure 1 :

 1 Figure 1: Mean ln[ViVol] at each stage of MST, data for Time 1 and Time 2 combined.

Figure 2 :

 2 Figure 2: Mean ln[V E ] at each stage of MST, data for Time 1 and Time 2 combined.

Figure 3 :

 3 Figure 3: Mean ln[Br/M] at each stage of MST, data for Time 1 and Time 2 combined.

Figure 4 :

 4 Figure 4: Mean ln[Ti/Tt] at each stage of MST, data for Time 1 and Time 2 combined.

Figure 5 :

 5 Figure 5: Mean ln[HR] at each stage of MST, data for Time 1 and Time 2 combined.

Part 2 :

 2 ResponsivenessParticipants with CF undergoing intravenous antibiotic therapy for an acute exacerbation of respiratory disease were recruited from Northern Ireland Adult CF Centre in Belfast City Hospital. A course of intravenous antibiotics was chosen as a test platform to assess responsiveness of cardiorespiratory measurements recorded by the LifeShirt. This has been used in previous studies that have investigated the ability of an outcome measure to detect clinically important changes.[START_REF] Horsley | Lung clearance index is a sentive, repeatable and practical measure of airways disease in adults with cystic fibrosis[END_REF] All patients also performed spirometry and had Creactive protein (CRP) levels determined on two occasions (Time 1= beginning of admission for intravenous antibiotic therapy and Time 2= end of admission for ). All participants also wore the LifeShirt system during a 6MWT on each occasion.

performance between Time 1 and

 1 Time 2 in any of the tests in either the patients with CF (mean difference [95%CI] Time 1 vs. Time 2: 6MWT: -10 [-33 to 13]m; MST: -29 [-65 to 7]m) or the controls (mean difference [95%CI] Time 1 vs. Time 2: 6MWT: 10 [-8 to 28]m; MST: 38 [-59 to 136]m). As there were no differences between Time 1 and Time 2 for either group, this enabled us to combine the groups to determine the reliability of the cardiorespiratory measurements recorded by LifeShirt over a wide range of exercise capacities.

  Twelve participants with CF were recruited (6M:6F). Mean (SD) FEV 1 %predicted at the start of admission for antibiotics was 45(19)% and mean (SD) length of stay was 13 (4) days. FEV 1 %predicted (Mean(SD): Time 1: 45(19) vs. Time 2: 52(16), p=0.024) and distance walked during the 6MWT (Mean(SD): Time 1: 544(80), vs. Time 2: 605(98), p=0.025) improved significantly indicating a treatment benefit. A trend towards improvement, which approached significance was observed for CRP (Mean (SD): Time 1: 12 (15) vs. Time 2: 7 (11), p=0.061). There was no significant difference from Time 1 to Time 2 for ViVol, V E , Br/M, Ti/Tt and HR recorded by the LifeShirt during the 6MWT (Tables

DISCUSSION

  This is the first study to use the LifeShirt to examine the cardiorespiratory responses to the 6MWT and MST in participants with CF and in healthy participants. The study showed that the LifeShirt had good reliability and discriminate validity, and could assess the cardiorespiratory responses to an exercise test at the beginning and end of an admission for intravenous antibiotics. The 6MWT (endurance test) and the MST (peak test) are frequently used, both in the CF clinical setting and as an endpoint in clinical trials, and offer the opportunity to examine the full range of physiological responses (i.e.

Figure 1 :

 1 Figure 1: Mean ln[ViVol] at each stage of MST, data for Time 1 and Time 2 combined.

Figure 2 :

 2 Figure 2: Mean ln[V E ] at each stage of MST, data for Time 1 and Time 2 combined.

Figure 3 :

 3 Figure 3: Mean ln[Br/M] at each stage of MST, data for Time 1 and Time 2 combined.

Figure 4 :

 4 Figure 4: Mean ln[Ti/Tt] at each stage of MST, data for Time 1 and Time 2 combined.

Figure 5 :

 5 Figure 5: Mean ln[HR] at each stage of MST, data for Time 1 and Time 2 combined.

Figure 1 :

 1 Figure 1: Mean ln[ViVol] at each stage of MST, data for Time 1 and Time 2 combined. Adults with CF (diamond), healthy adults (square). Error bars represent standard deviation, *p<0.05, **p<0.01 256x174mm (96 x 96 DPI)

Figure 2 :

 2 Figure 2: Mean ln[VE] at each stage of MST, data for Time 1 and Time 2 combined. Adults with CF (diamond), healthy adults (square). Error bars represent standard deviation, **p<0.01, ***p<0.001 256x174mm (96 x 96 DPI)

Figure 3 :

 3 Figure 3: Mean ln[Br/M] at each stage of MST, data for Time 1 and Time 2 combined. Adults with CF (diamond), healthy adults (square). Error bars represent standard deviation, **p<0.01, ***p<0.001 256x174mm (96 x 96 DPI)

Figure 4 :

 4 Figure 4: Mean ln[Ti/Tt] at each stage of MST, data for Time 1 and Time 2 combined. Adults with CF (diamond), healthy adults (square). Error bars represent standard deviation, *p<0.05, **p<0.01 256x174mm (96 x 96 DPI)

Figure 5 :

 5 Figure 5: Mean ln[HR] at each stage of MST, data for Time 1 and Time 2 combined. Adults with CF (diamond), healthy adults (square). Error bars represent standard deviation, *p<0.05, **p<0.01 256x174mm (96 x 96 DPI)

  Time 1 and Time 2 combined. The significance of the difference in cardiorespiratory measurements between participants with CF and healthy participants was assessed by 2-sample t-tests (significant at p<0.05 level) on a stage by stage basis.

	F o r Part 1: Reliability and discriminate validity RESULTS Eighteen patients and eighteen age matched healthy participants were recruited [CF: 9M:9F, age 26 (10) years, FEV 1 %predicted: 69.2 (23)%, controls: 8M:10F, age: 24 (5) years, FEV 1 %predicted: 92 (8)%]. Within the group of participants with CF there was a range of severity in lung disease (normal: n=2; mild: n=11; moderate: n=4; severe: n=3). There was no significant difference in FEV 1 %predicted over the two occasions in either cardiorespiratory measurements were small and not clinically important (Table 2). Analysis for SpO 2 was not performed due to the large amount of missing values. Patients had a significantly lower mean (SD) exercise capacity as measured by the 6MWT (CF: 546 (90)m vs. controls: 705 (61)m (p<0.01)). Therefore since the two groups were exercising at different intensities the cardiorespiratory responses were not assessed for discriminate validity. Participants with CF also had a lower exercise Part 2: Responsiveness Twelve participants with CF were recruited (6M:6F). Mean (SD) FEV 1 %predicted at the start of admission for antibiotics was 45 (19)% and mean (SD) length of stay was 13 (4) days. FEV 1 %predicted (Mean(SD): Time 1: 45(19) vs. Time 2: 52(16), p=0.024) and distance walked during the 6MWT (Mean(SD): Time 1: 544(80), vs. Time 2: 605(98), p=0.025) improved significantly indicating a treatment benefit. A trend towards completed for F o r group indicating that health status of participants was stable. F o r capacity as measured by the MST (CF: 757 (266)m vs. controls: 1201 (232)m (p<0.01)). However, exercise intensity at each stage of the MST was externally controlled (i.e. F improvement, which approached significance was observed for CRP (Mean (SD): Time o r 1: 12 (15) vs. Time 2: 7 (11), p=0.061). There was no significant difference from Time 1
	P P In order to facilitate analysis of the reliability of cardiorespiratory measurements P patients with CF and healthy participants exercised at the same intensity) and therefore P to Time 2 for ViVol, V E , Br/M, Ti/Tt and HR recorded by the LifeShirt during the
	e e r R e v i e w e e r R e v i e w recorded by the LifeShirt system it was essential that performance on the exercise tests was reliable between Time 1 and Time 2. There was no difference in exercise (mean difference [95%CI] Time 1 vs. Time 2: 6MWT: -10 [-33 to 13]m; MST: -29 [-65 to 7]m) or the controls (mean difference [95%CI] Time 1 vs. Time 2: 6MWT: 10 [-8 to 28]m; MST: 38 [-59 to 136]m). As there were no differences between Time 1 and Time 2 for either group, this enabled us to combine the groups to determine the reliability of the cardiorespiratory measurements recorded by LifeShirt over a wide range of exercise R e v i e w Cardiorespiratory measurements recorded by the LifeShirt generally demonstrated an exaggerated response to exercise. Significant differences (p<0.05) between participants with CF and healthy controls were observed from stages 1 to 8 for ViVol, stages 1 to 10 for V E , stages 1 to 12 for Br/M, stages 7 to 10 for Ti/Tt, and stages 8 to 12 for HR (Figures 1 to 5). Abnormalities in cardiorespiratory responses were still apparent when w e i v e R performance between Time 1 and Time 2 in any of the tests in either the patients with CF e e r discriminate validity in the cardiorespiratory measurements could be assessed for each stage fully completed. r e e 6MWT (Tables
	capacities. only participants with CF and normal lung function or mild CF-related lung disease
	(FEV 1 >70 %predicted, n=11) were compared to healthy participants: significant
	For both the 6MWT and the MST reliability of cardiorespiratory measures recorded by differences were observed from stages 1 to 8 for ViVol, stages 1 to 10 for V E , and stages
	the LifeShirt was acceptable with no significant bias (i.e. p>0.05) and CV below 10% 1 to 10 for Br/M. Ti/Tt and HR showed a trend for being higher in later stages however
	(Table 1). The mean absolute differences between Time 1 and Time 2 for all there were fewer participants completing these stages.
	10 11 13

Bias was considered to be significant at p<0.05 level and a CV below 10% was considered acceptable. For discriminate validity means were calculated for each stage of the MST

  consumables from VivoMetrics. AN and LB do not have any conflict of interest to declare. All data analysis was performed independently by AN (VivoMetrics were not involved in the design or conduct of the study, analysis or interpretation of results, or the decision to publish).

	Pediatric Pulmonology
	F o F o ergometry enables the upper limb to be stabilized and therefore may provide a more suitable platform for assessing the clinimetric properties of the LifeShirt pulse oximeter. This study focused on inspiratory tidal volume, ventilation, respiratory rate, fractional inspiratory time, and heart rate. Other measurements may be relevant in CF (for example, the relative contributions of the ribcage and abdomen to breathing). Further study is needed to assess the clinimetric properties of the range of cardiorespiratory measurements available from the LifeShirt. F o 11. Singh SJ, Morgan MDL, Scott S, Walters D, Hardman AE. Development of a shuttle walking test of disability in patients with chronic airways obstruction. Thorax 1992; 47: 1019-1024. 12. Bradley JM, Howard JL, Wallace ES, Elborn JS. The validity of a modified shuttle test in adult CF. Thorax 1999; 49: 437-439. received equipment and F o F o F o F o F o
	r r r r r r r r
	P P P P P P P P
	e e r e e r e e r e e r e e r e e r e e r e e r
	R e v i e w R e v i e w R e v i e w R e v i e w R e v i e w R e v i e w R e v i e w R e v i e w
	16. Atkinson G, Nevill AM. Statistical methods for assessing measurement error
	In this study, participants also wore a pulse oximeter linked to the LifeShirt. However,
	(reliability) invariables relevant to sports medicine. Sports Med 1998; 26(4):217-
	due to large amounts of missing data analysis was not performed on SpO 2 . The reason
	238.
	for this may have been that movement of the upper limb during walking or running
	caused large amounts of movement artifact and hence a large noise-to-signal ratio. Cycle
	18 19 22

Page 16 of 61 John Wiley & Sons, Inc.

17 of an admission for intravenous antibiotic therapy. In other words more air would have to be moved in and out of the lungs (reflected by increased V E , ViVol, Br/M) to maintain gaseous concentrations at the alveolar-capillary membrane. It could be hypothesised that, with intravenous antibiotic treatment, airway inflammation, sputum load and gas trapping would decrease leading to improvements in airflow and gas exchange, and hence improved cardiorespiratory efficiency during exercise. Patients in this study improved their exercise capacity (i.e. walked further at Time 2). Cardiorespiratory measurements recorded by the LifeShirt did not change significantly from Time 1 to Time 2 therefore this may indicate that the increase in exercise performance masked any treatment induced improvements in cardiorespiratory efficiency. These improvements would have been more clearly demonstrated using an externally controlled constant load test (e.g. treadmill walking at constant speed Time 1 and Time 2). A number of findings would suggest that patients in this study were experiencing a "mild" exacerbation: the modest level of CRP at Time 1, and the modest changes in all the clinical outcome measures from Time 1 to Time 2. Improvement may also have been more evident in participants experiencing a severe exacerbation. Further evaluation of responsiveness is needed in participants with CF taking into consideration the exercise test chosen and the severity of exacerbation.

Conclusion

In conclusion this study adds to the growing body of evidence that cardiorespiratory response can be measured non-invasively during field tests such as the MST. This study demonstrated that key cardiorespiratory measurements recorded by the LifeShirt are reliable during field exercise tests and can discriminate between adults with CF and young healthy adults. This study provides some limited information on responsiveness of cardiorespiratory measurements recorded by the LifeShirt during the 6MWT. 13. Bradley JM, Howard JL, Wallace ES, Elborn JS. Reliability repeatability and sensitivity of the modified shuttle test in adult CF. Chest 2000; 117: 1666-1671. 14. Bradley JM, McAlister O, Elborn JS. Pulmonary function, inflammation, exercise capacity and quality of life in CF. Eur Respir J 2001; 17: 1-4. 15. Horsley AR, Gustaffson PM, Macleod KA, Saunders C, Greening AP, Porteous DJ, Davies JC, Cunningham S, Alton EWFW, Innes JA, 2008. Lung clearance index is a sentive, repeatable and practical measure of airways disease in adults with cystic fibrosis. Thorax 2008; 63: 135-140.

  Respiratory inductive plethysmography (e.g. LifeShirt) may offer in-depth study of the cardiorespiratory responses during field exercise tests. The aims of this study were to assess the reliability, discriminate validity and responsiveness of cardiorespiratory measurements recorded by the LifeShirt during field exercise tests in adults with CF. To assess reliability and discriminate validity, participants with CF and stable lung disease and healthy participants performed the 6 Minute Walk Test (6MWT) and Modified Shuttle Test (MST) on 2 occasions. To assess responsiveness, participants with CF experiencing an exacerbation performed the 6MWT at the start and end of an admission for intravenous antibiotics. The LifeShirt was worn during all exercise tests. In cystic fibrosis (CF) exercise tests enable assessment of functional capacity and limitations, determination of level of fitness, safe and effective exercise recommendations, and evaluation of interventions. 1,2 Field exercise tests are often used in CF centres when clinical laboratories are unavailable. While field exercise tests provide information on exercise performance (i.e. how far a patient can walk or run) they do not provide in-depth information on the cardiorespiratory responses to exercise or what factors contribute to exercise limitation. Measuring cardiorespiratory responses during field exercise tests using ambulatory monitoring has been limited by the

	Page 27 of 61	Pediatric Pulmonology
		F o r P e e r R e v i e w Cardiorespiratory measurements during field tests in CF: Use of an ambulatory monitoring system Judy M Bradley PhD 1,2 , Lisa Kent BSc (Hons) 1 , Brenda O'Neill PhD 1 , Alan Nevill PhD 3 , Lesley Boyle BSc (Hons) 2 , J Stuart Elborn MD FRCP 2,4 1 Health and Rehabilitation Sciences Research Institute, University of Ulster, Jordanstown, UK; 2 Respiratory Medicine, Belfast Health and Social Care Trust, Belfast City Hospital, UK; 3 School of Sport, Performing Arts and Leisure, University of Wolverhampton, UK; 4 Respiratory Medicine Group, Centre for Infection and Immunology, Queens University Belfast, UK Primary Institution: Respiratory Medicine, Belfast Health and Social Care Trust, Title: Authors: Affiliations: Belfast City Hospital, UK F o r P e e r R e v i e w cardiorespiratory measurements recorded by the LifeShirt remained the same (bias: F o r P e e r R e v i e w Keywords respiratory inductive plethysmography; LifeShirt; cystic fibrosis; Modified Shuttle Test F o r P e e r R e v i e w INTRODUCTION and researchers working in CF, for example timing and volume components of breathing F o r P e e r R e v i e w unobtrusive and so can be used for extended periods in an ambulatory setting. The LifeShirt system incorporates a Lycra vest into which the respiratory inductive plethysmography bands are embedded in order to limit displacement. Before systems such as the LifeShirt can be used in clinical or research practice information is required Summary: clinical measurements and 6MWT performance improved (61(81)m; p<0.05) however 6 Minute Walk Test; on clinimetric properties (i.e. reliability, validity and responsiveness).
		Adults with CF (diamond), healthy adults (square). Error bars represent standard p>0.05; CV<10%). This study provides evidence that cardiorespiratory responses can be and heart rate. The LifeShirt consists of a CoolMax ® garment, data recorder, and
		deviation, *p<0.05, **p<0.01. measured non-invasively during field exercise tests in adults with CF. Reliability and computer-based analysis software (VivoLogic). The core technology is respiratory
		discriminate validity of key cardiorespiratory measurements recorded by the LifeShirt inductive plethysmography which provides non-invasive assessment of respiratory
		were demonstrated. Some information on responsiveness is reported. pattern. Unlike direct methods which must be coupled to the individual (e.g. pneumotach
		via a face-mask or mouthpiece and nose-clips), respiratory inductive plethysmography is
		1 2 3 5
		John Wiley & Sons, Inc.

Funding:

Funding was received to support this project from the Northern Ireland R&D Office (£15,000). This study formed part of a PhD programme of research which was co-funded through a Department for Employment and Learning Co-operative Award in Science and Technology (DEL CAST) with VivoMetrics. DEL CAST award entails a studentship paid to LK (VivoMetrics contribution = £14760 from 2006 to 2009). LK, JB, SE, BO'N received equipment and consumables from VivoMetrics. AN and LB do not have any conflict of interest to declare. All data analysis was performed independently by AN (VivoMetrics were not involved in the design or conduct of the study, analysis or interpretation of results, or the decision to publish). Reliability and discriminate validity were assessed in 18 participants with CF (mean (SD) age: 26 (10)years; FEV 1 %predicted: 69.2 (23)%) and 18 healthy participants (age: 24 (5)years, FEV 1 %predicted: 92 (8)%). There was no difference in 6MWT and MST performance between days and reliability of cardiorespiratory measures was acceptable (bias p>0.05; CV<10%). Participants with CF demonstrated a significantly greater response to exercise (e.g. ventilation, respiratory rate) compared to healthy participants indicating discriminate validity. Responsiveness was assessed in 12 participants with CF: technology available. Some available ambulatory monitoring systems record a narrow range of measurements and others are too cumbersome. Respiratory inductive plethysmography (e.g. the LifeShirt) may offer the opportunity for more in-depth study of the cardiorespiratory responses during field exercise tests and potentially provide additional valuable information.

3

The LifeShirt ® (Vivometrics, Inc., Ventura, CA, U.S.A.) is a multi-function ambulatory system capable of detecting several cardiorespiratory measures of interest to clinicians

  For the MST participants were asked to walk or run around a 10 metre course at increasing speeds dictated by an external bleep from a tape recorder.[START_REF] Singh | Development of a shuttle walking test of disability in patients with chronic airways obstruction[END_REF][START_REF] Bradley | The validity of a modified shuttle test in adult CF[END_REF][START_REF] Bradley | Reliability repeatability and sensitivity of the modified shuttle test in adult CF[END_REF][START_REF] Bradley | Pulmonary function, inflammation, exercise capacity and quality of life in CF[END_REF] Patients with CF and stable lung disease were recruited from the Northern Ireland Adult CF Centre in Belfast City Hospital. Age matched controls were recruited from staff and students at Belfast City Hospital and the University of Ulster. All participants wore the LifeShirt system during a 6MWT and MST performed on two occasions at least one

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	guidelines. 10 Part 1: Reliability and discriminate validity
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week apart (Time 1 and Time 2). Exercise tests were carried out according to standardised protocols and in a standardised order. Sufficient rest periods were used between exercise tests for Borg breathlessness score and heart rate to return to resting values. All participants performed spirometry on each occasion.

  Time 1 and Time 2 combined. The significance of the difference in cardiorespiratory measurements between participants with CF and healthy participants was assessed by 2-sample t-tests (significant at p<0.05 level) on a stage by stage basis.

	F o o Part 1: Reliability and discriminate validity RESULTS Eighteen patients and eighteen age matched healthy participants were recruited [CF: 9M:9F, age 26 (10) years, FEV 1 %predicted: 69.2 (23)%, controls: 8M:10F, age: 24 (5) years, FEV 1 %predicted: 92 (8)%]. Within the group of participants with CF there was a completed for F range of severity in lung disease (normal: n=2; mild: n=11; moderate: n=4; severe: n=3).
	r r
	P P
	e e r e e r
	R e v i e w R e v i e w
	Bias was
	considered to be significant at p<0.05 level and a CV below 10% was considered
	acceptable. For discriminate validity means were calculated for each stage of the MST
	10 11

  The mean absolute differences between Time 1 and Time 2 for all small and not clinically important (Table2). Analysis for SpO 2 was not performed due to the large amount of missing values.

	Page 35 of 61	Pediatric Pulmonology
	cardiorespiratory measurements were Patients had a significantly lower mean (SD) exercise capacity as measured by the
	6MWT (CF: 546 (90)m vs. controls: 705 (61)m (p<0.01)). Therefore since the two
	groups were exercising at different intensities the cardiorespiratory responses were not
	assessed for discriminate validity. Participants with CF also had a lower exercise
	F o capacity as measured by the MST (CF: 757 (266)m vs. controls: 1201 (232)m (p<0.01)).
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  The results also indicate that cardiorespiratory measurements recorded by the LifeShirt can discriminate between groups of participants with CF and healthy participants at comparable exercise intensities during the MST. The patterns of cardiorespiratory response demonstrated in this study are in line with previously reported abnormalities in cardiorespiratory response to exercise in CF.21 Ventilatory response is exaggerated in CF compared to healthy participants. The data in this study suggest that initially this is due to both an increase in ViVol and Br/M. After stage 8 of the MST ViVol recorded in participants with CF appears to converge with that of healthy participants (i.e. no longer significantly E and Br/M continue to be significantly different to that of healthy participants until stage 10. This is in agreement with previous reports that as exercise intensity increases limitation of ViVol occurs and greater increases in Br/M must compensate in order to meet ventilatory demands.

	different), however V
	F o
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	15

  the heteroscedasticity was obvious and thus log transforming the data produced uniform standard deviations across the stages of the MST. For Ti/Tt and HR the data demonstrated heteroscedasticity, albeit less pronounced, and thus the log transformed standard deviations appear to decrease across the stages of the MST.

	Br/M The reliability study was performed in a small group of participants with CF, in particular
	there were few patients with moderate (n=4) or severe (n=3) lung disease. Reliability
	F may not be equivalent across different severities of disease as patients with more severe o r disease experience more frequent fluctuations in health status making it difficult to assess
	P reliability of measurements. Future research should include larger numbers of patients
	with moderate and severe lung disease in order to assess reliability in subgroups of e r e patients.
	R e v i e w
	The cardiorespiratory data were log transformed as is appropriate for data demonstrating
	heteroscedasticity (errors associated with size of measurements). For ViVol, VE, and
	16

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  for intravenous antibiotic therapy. In other words more air would have to be moved in and out of the lungs (reflected by increased V E , ViVol, Br/M) to maintain gaseous concentrations at the alveolar-capillary membrane. It could be hypothesised that, received equipment and consumables from VivoMetrics. AN and LB do not have any conflict of interest to declare. All data analysis was performed independently by AN (VivoMetrics were not involved in the design or conduct of the study, analysis or interpretation of results, or the decision to publish).

	Pediatric Pulmonology
	ergometry enables the upper limb to be stabilized and therefore may provide a more suitable platform for assessing the clinimetric properties of the LifeShirt pulse oximeter. of an admission with intravenous antibiotic treatment, airway inflammation, sputum load and gas trapping This study focused on inspiratory tidal volume, ventilation, respiratory rate, fractional
	would decrease leading to improvements in airflow and gas exchange, and hence inspiratory time, and heart rate. Other measurements may be relevant in CF (for
	improved cardiorespiratory efficiency during exercise. Patients in this study improved example, the relative contributions of the ribcage and abdomen to breathing). Further
	their exercise capacity (i.e. walked further at Time 2). Cardiorespiratory measurements study is needed to assess the clinimetric properties of the range of cardiorespiratory
	F o r recorded by the LifeShirt did not change significantly from Time 1 to Time 2 therefore F o measurements available from the LifeShirt. F o F o F o F o F o F o this may indicate that the increase in exercise performance masked any treatment induced r r r r r r r
	P improvements in cardiorespiratory efficiency. These improvements would have been P P P P P P P
	e e r R e v i e w more clearly demonstrated using an externally controlled constant load test (e.g. treadmill walking at constant speed Time 1 and Time 2). A number of findings would suggest that e e r e e r e e r e e r e e r e e r e e r patients in this study were experiencing a "mild" exacerbation: the modest level of CRP at Time 1, and the modest changes in all the clinical outcome measures from Time 1 to Time 2. Improvement may also have been more evident in participants experiencing a severe exacerbation. Further evaluation of responsiveness is needed in participants with CF taking into consideration the exercise test chosen and the severity of exacerbation. R e v i e w R e v i e w R e v i e w R e v i e w R e v i e w R e v i e w R e v i e w
	In this study, participants also wore a pulse oximeter linked to the LifeShirt. However,
	due to large amounts of missing data analysis was not performed on SpO 2 . The reason
	for this may have been that movement of the upper limb during walking or running
	caused large amounts of movement artifact and hence a large noise-to-signal ratio. Cycle
	17 18

Conclusion

In conclusion this study adds to the growing body of evidence that cardiorespiratory response can be measured non-invasively during field tests such as the MST. This study demonstrated that key cardiorespiratory measurements recorded by the LifeShirt are reliable during field exercise tests and can discriminate between adults with CF and young healthy adults. This study provides some limited information on responsiveness of cardiorespiratory measurements recorded by the LifeShirt during the 6MWT.

Table 1 Page 49 of 61 John Wiley & Sons, Inc.

 1

	Adults with CF (diamond), healthy adults (square). Error bars represent standard
	deviation, *p<0.05, **p<0.01.

Table 1 :

 1 Difference between Time 1 and Time 2 of measurements recorded by the LifeShirt during the final two minutes of 6MWT and final two comparable stages of the

	MST in participants with CF (n=18) during a stable period, and healthy participants
	(n=18) (repeated measures ANOVA)
		6MWT		MST
		Bias (p)	CV (%)	Bias (p)	CV (%)
	lnViVol	0.603	4.8	0.403	3.7
	lnV E	0.649	3.9	0.688	6.5
	lnBr/M	0.799	5.0	0.056	6.2
	lnTi/Tt	0.514	2.7	0.155	2.3
	lnHR	0.879	6.2	0.080	8.0
	Key: lnViVol: log of inspired tidal volume; lnV

E : log of ventilation; lnBr/M: log of respiratory rate; lnTi/Tt: log of ratio of inspiratory time to total respiratory time; lnHR: log of heart rate.

Table 2 Page 51 of 61 John Wiley & Sons, Inc.

 2

	Pediatric Pulmonology

Table 2 :

 2 Cardiorespiratory measurements recorded by the LifeShirt during the final two minutes of 6MWT and final two comparable stages of the MST at Time 1 and Time 2 in participants with CF (n=18) during a stable period, and healthy participants (n=18).

		6MWT			MST
		Time 1	Time 2	Difference (SD)	Time 1	Time 2	Mean difference
		Mean	Mean	[95% CI]	Mean	Mean	(SD)
		(SD)	(SD)		(SD)	(SD)	[95% CI]
	ViVol	1190	1171	18.7 (529)	1808	1717	91 (675)
	(mL)	(470)	(536)	[-106 to 143]	(803)	(752)	[-67 to 250]
	V E	41.1	41.1	-0.06 (19.9)	79.3	77.4	1.9 (31.0)
	(L•min -1 )	(18.2)	(22.5)	[-4.7 to 4.6]	(34.9)	(36.5)	[-5.4 to 9.2]
	Br/M	35.4	35.6	-0.2 (3.8)	45.0	45.7	-0.8 (4.4)
	(br•min -1 )	(6.2)	(6.5)	[-1.1 to 0.7]	(6.3)	(7.0)	[-1.8 to 0.2]
	Ti/Tt	0.457	0.459	-0.002 (0.022)	0.488	0.490	-0.002 (0.015)
		(0.029)	(0.027)	[-0.007 to 0.003]	(0.019)	(0.019)	[-0.006	to
							0.001]
	HR	126.6	126.9	-0.4 (20.3)	167.0	161.2	5.8 (19.7)
	(b•min -1 )	(19.7)	(19.2)	[-5.14 to 4.38]	(26.0)	(27.5)	[1.16 to 10.43]

Key: ViVol: inspired tidal volume; V E : ventilation; Br/M: respiratory rate; Ti/Tt: ratio of inspiratory time to total respiratory time; HR: heart rate.

Table 3 Page 53 of 61 John Wiley & Sons, Inc.

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Table 3 :

 3 Difference between Time 1 (start of admission for intravenous antibiotics) and Time 2 (end of admission for intravenous antibiotics) in measurements recorded by the LifeShirt during the final two minutes of 6MWT in patients with CF (n=12) (repeated Key: lnViVol: log of inspired tidal volume; lnV E : log of ventilation; lnBr/M: log of respiratory rate; lnTi/Tt: log of ratio of inspiratory time to total respiratory time; lnHR: log of heart rate.

	measures ANOVA)
		% difference	Bias between Time	CV between Time 1
		between Time 1	1 and Time 2	and Time 2
		and Time 2	(p)	(%)
	lnViVol	4.2	0.789	4.2
	lnV E	1.8	0.907	3.3
	lnBr/M	-2.4	0.414	5.6
	lnTi/Tt	-1.1	0.207	2.3
	lnHR	-0.4	0.126	3.1

Table 4 Page 55 of 61 John Wiley & Sons, Inc.

 4

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Table 4 :

 4 Cardiorespiratory measurements recorded by the LifeShirt during the final two minutes of 6MWT at Time 1 (start of admission for intravenous antibiotics) and Time 2 (end of admission for intravenous antibiotics).

		Time 1	Time 2	Mean difference
		Mean (SD)	Mean (SD)	(SD)
				[95% CI]
	ViVol	914.5 (392.2)	830.7 (246.2)	83.8 (434.7)
	(mL)			[-99.8 to 267.4]
	V E	33.0 (12.2)	31.2 (7.8)	1.8 (15.4)
	(L•min -1 )			[-4.7 to 8.3]
	RR	39.3 (8.2)	40.2 (8.4)	-0.9 (4.3)
	(br•min -1 )			[-2.7 to 0.9]
	Ti/Tt	0.456 (0.032)	0.461 (0.029)	-0.005 (0.016)
				[-0.012 to 0.002]
	HR	128.3 (19.5)	134.5 (15.9)	-6.2 (14.5)
	(b•min -1 )			[-12.3 to -0.02]

Key: ViVol: inspired tidal volume; V E : ventilation; Br/M: respiratory rate; Ti/Tt: ratio of inspiratory time to total respiratory time; HR: heart rate.

John Wiley & Sons, Inc.Pediatric Pulmonology

Acknowledgments

Funding was received to support this project from the Northern Ireland R&D Office (£15,000). This study formed part of a PhD programme of research which was co-funded through a Department for Employment and Learning Co-operative Award in Science and Technology (DEL CAST) with VivoMetrics. DEL CAST award entails a studentship paid to LK (VivoMetrics contribution = £14760 from 2006 to 2009). LK, JB, SE, BO'N Acknowledgments Funding was received to support this project from the Northern Ireland R&D Office (£15,000). This study formed part of a PhD programme of research which was co-funded through a Department for Employment and Learning Co-operative Award in Science and Technology (DEL CAST) with VivoMetrics. DEL CAST award entails a studentship paid to LK (VivoMetrics contribution = £14760 from 2006 to 2009). LK, JB, SE, BO'N

Funding was received to support this project from the Northern Ireland R&D Office (£15,000). This study formed part of a PhD programme of research which was co-funded through a Department for Employment and Learning Co-operative Award in Science and Technology (DEL CAST) with VivoMetrics. DEL CAST award entails a studentship paid to LK (VivoMetrics contribution = £14760 from 2006 to 2009). LK, JB, SE, BO'N received equipment and consumables from VivoMetrics. AN and LB do not have any conflict of interest to declare. All data analysis was performed independently by AN (VivoMetrics were not involved in the design or conduct of the study, analysis or interpretation of results, or the decision to publish).


Tolulope ; Saiki

MRCPCH Simon Hannam

PhD Gerrard F Rafferty

MD Anthony D Milner

MD Anne Greenough
email: anne.greenough@kcl.ac.uk

King'

Tolulope Saiki Bsc

Ventilatory response to added dead space and position in preterm infants at high risk age for SIDS

Keywords: prematurity, functional residual capacity, respiratory control, respiratory muscle strength prematurity, functional residual capacity, respiratory control, respiratory muscle strength

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Prone sleeping remains a significant risk factor for sudden infant death syndrome (SIDS) (1).

Premature birth is also a risk factor for SIDS (2) and prematurely born infants may be particularly at risk of SIDS if they are slept prone (3). A possible explanation for the vulnerability of prematurely born infants to SIDS in the prone position might be that they are less able to respond to a stress, such as hypercarbia, in that position. The response to added deadspace (tube breathing) is a simple and reproducible method of assessing the ventilatory response to hypercarbia. Healthy infants born at term can compensate fully for an imposed dead space by increasing their minute ventilation (3). Intrauterine exposure to smoking, however, another known risk factor for SIDS (4), is associated with a dampened response to tube breathing in the perinatal period in term born babies (5). In addition, at 36 weeks postmenstrual age (PMA), we found that prematurely born infants had a dampened response to tube breathing in the prone compared to the supine position (6). The aim, therefore, of this study was to test the hypothesis that prematurely born infants' ability to respond to added dead space in the prone position would be impaired at their high risk age for SIDS, that is 45-48 weeks postmenstrual age (2). In addition, we wished to determine whether any differences between positions in the response to added dead space were related to differences in respiratory muscle strength and/or respiratory drive and whether the response to added dead space was influenced by postmenstrual age.

MATERIALS AND METHODS

Infants born prior to 33 weeks of gestation were eligible for this study once they were older than 40 weeks PMA (ie post term). If their parents gave informed written consent, infants were studied supine and prone, after each position had been maintained up to three hours.

The order in which the positions were studied was randomised between babies. In each position, first the infant's response to added dead space was examined and then their respiratory muscle strength, respiratory drive and lung volume were assessed. Infants were studied while in quiet sleep, with their eyes closed and breathing quietly.

To assess the response to added dead space, a face mask was placed over the infant's nose and mouth. Tubing (7.0 mm internal diameter) connected the face mask to a three way tap via which a bias flow of air (2 L/min) was delivered to the breathing circuit (Figure 1). A second tube (7.0 mm internal diameter) led from the face mask to a pneumotachograph (PK Morgan, Rainham Kent, UK) via a Y connector. The length of the tube between the face mask and Y connector was adjusted such that its volume was 4.4 mls/kg bodyweight, twice the generally accepted anatomical deadspace (7). A third tube connected the three way tap to the remaining part of the Y connector. The pneumotachograph was attached to a differential pressure transducer (MP45, Validyne Corporation, Northridge CA, USA) the signal from which was amplified (DC280; Validyne Corporation) and displayed in real time on a computer running Spectra software (version XX; Grove Medical, London UK) with 200 Hz analog to digital sampling (DAQ 16XE-50; National Instruments, Austin, TX, USA). Tidal volume was obtained by digital integration of the flow signal using the Spectra software and minute volume (MV), inspiratory (Ti), expiratory (Te) and total respiratory cycle (Ttot) times and respiratory rate were calculated breath by breath. Baseline recordings of respiratory flow were made with the three way tap in the neutral position, so that the bias flow of 2 L/min of air to the face mask eliminated any dead space. When the tap was rotated the bias flow was fed via the third tube directly to the Y connector and the pneumotachograph, so that the the series of occlusions (8). Respiratory drive was assessed by measurement of the inspiratory pressure generated during the first 100 milliseconds of the first inspiratory effort during each occlusion (P 0.1 ). The reported P 0.1 was the mean of the series of occlusions.

Lung volume can affect respiratory muscle strength, hence at the end of each three hour period, lung volume was assessed by measurement of functional residual capacity (FRC) using a helium gas dilution technique (9). The FRC system (Equilibrated Biosystems Inc, Series 7700, Melville, NY) contained a rebreathing bag as the system reservoir and a specially designed infant circuit (total volume 95 mL). A facemask (Rendell Baker, Laerdal, Norway) was held snugly over the infant's nose and mouth. Silicone putty was used around the mask to achieve an airtight seal. The facemask was connected to the re-breathing bag via a three-way valve, actuation of which switched the infant from breathing ambient room air to breathing the test gas mixture (10% He, balance O 2 ) in the rebreathing bag. During the measurement, if there was no change in the helium concentration over a 15-second period, equilibration was deemed to have occurred. The initial and equilibration helium concentrations were used in the calculation of FRC, which was corrected for oxygen consumption (assumed to be 7 ml/kg/minute) (10)) and for body temperature, pressure and water vapor-saturated conditions. FRC was measured twice in each position and the results of the paired measurements were averaged and related to body weight. The co-efficient of repeatability of FRC in non ventilated infants is 3.9 ml/kg (11).

Statistical Analysis

Differences in the results in the two positions were assessed for statistical significance using the paired Wilcoxon Rank Sum Test. Spearman's correlation coefficients were calculated to determine the strengths of any relationships between the change in the minute volume or the time constant of the response to the added dead space and MIOP or P 0.1 . To assess whether the response to added dead space post term was poorer post term than at 36 weeks PMA, we compared the results from this study to those we have previously published from 25 infants, median gestational age 30 (range 26-32 weeks) studied at 36 weeks PMA (6).

Differences between results at 36 weeks PMA and post term were assessed for statistical significance using the Mann Whitney U-Test. Twelve infants were included in this and our previous study (6), differences in the time constant results in the two positions at 36 weeks PMA and post term were assessed for statistical significance using the paired Wilcoxon Rank Sum test. Using all the results from the two studies, Spearman's correlation co-efficients were calculated to determine the strength of any relationships between the change in minute volume or the time constant and postmenstrual age. Analysis was performed using SPSS version 12.0, SPSS Inc, Chicago, Illinois 60606, USA.

Sample Size

In a previous study (5), we demonstrated that the coefficient of repeatability of TC was 11 seconds. Recruitment of 20 infants allowed, with at least 80% power at the 5% level, detection of a difference in the Tc between positions of at least 11 seconds.

Patients

Twenty infants (10 males), median gestational age 30 (range 24-32) weeks and birthweight 1190 (SD 326) (range 543-1730) gm (mean (+ SD) z score 0 + 1) were recruited. They were studied at a postmenstrual age of 45 (41-47) weeks and weight of 3.72 (SD 0.95) (range 1.8-5.3) kgs (mean (+ SD) z score 0 + 1). Five infants were oxygen dependent beyond 36 weeks PMA, but none were oxygen dependent at the time of study. None of the infants were receiving any medication when studied. Three mothers had smoked during pregnancy. The twelve infants included in our previous study (6)

RESULTS

The addition of the dead space resulted in an increase in minute volume in both the prone and supine positions (Table 1). The difference between positions in the percentage change in minute volume was not statistically significant. The time constant of the response was significantly longer in the prone compared to the supine position (p=0.002) (Table 1) (Figure 2). There were no significant differences in the respiratory timings or minute ventilation at baseline between the two positions (Table 1). The median T i was significantly longer in the prone compared to the supine position after the dead space was added (Table 1).

The median MIOP tended to be lower in the prone compared to the supine position (p=0.07), but there was no significant correlations between the response to tube breathing and MIOP in either position. The median P 0.1 was significantly lower in the prone compared to the supine positions (p=0.003), but there were no significant correlations between the response to tube breathing and P 0.1 in either position. The median FRC was significantly higher in the prone compared to the supine position (p=0.031) (Table 1).

Comparisons of the results of the infants studied at 36 weeks PMA and those studied post term revealed no significant differences in the median changes in minute volume or the time constant, but that the median MIOP was significantly greater in both positions post term (p<0.001) (Table 2). The change in minute ventilation was not significantly correlated with postmenstrual age in either the supine (p=0.697) or the prone (p=0.25) position. The mean difference in the time constants between the prone and supine positions was 8.3 seconds at 36 weeks PMA and 21.3 seconds post term respectively. The mean difference between the difference at the two time periods was 13.4 seconds (95% confidence intervals of the differences was -2.5, 28.6) (p = 0.098). In the twelve infants who had been assessed at 36 weeks PMA and post term, the median time constant at 36 weeks PMA in the supine position was 25 (range 10-51) seconds and in the prone position was 34 (range 15-96) seconds (p=0.019) and post term in the supine position was 26 (range 2-40) seconds and in the prone

DISCUSSION

We have demonstrated that at a high risk age for SIDS (2), the response to added dead space in prematurely born infants was poorer in the prone compared to the supine position, as the time constant was significantly longer in the prone position. Hypercarbia is the most important stimulus to ventilation during added dead space (tube breathing), as measurements in air and 30% oxygen yielded similar results (12) and no effect of resistance was found in experiments involving prematurely born infants (12). Our results then suggest that prematurely born infants at the high risk age for SIDS have a dampened response to hypercarbia in the prone position. The time constant during tube breathing is shorter in active compared with quiet sleep (3). To avoid such a confounding effect, infants were always tested when they were in quiet sleep. Maternal smoking during pregnancy is associated with an impaired response to added dead space (5). Three infants studied had mothers who were smokers, but our results were not biased by their inclusion as we compared the response of all the infants in two positions.

The dampened response to added dead space in the prone position may indicate reduced respiratory drive in that position. Indeed we did find that the P 0.1 was significantly lower in the prone compared to the supine position. P 0.1 is influenced by respiratory muscle strength, but there was no significant difference in the median MIOP between positions. As we had demonstrated previously (13), the median FRC was higher in the prone compared to the supine position. Although the difference was significant, however, the magnitude of the difference was relatively small and unlikely to influence MIOP (14). It is possible, however, that the difference in FRC may have affected the strength of the Hering-Breuer inflation reflex. The Hering-Breuer inflation reflex which terminates inspiration and prolongs expiration in response to lung inflation (15) is stronger in the prone compared to the supine position (16,17). Differences in the strength of the reflex between positions in prematurely born infants have been shown to be significantly correlated with differences in lung volume (17). In addition, in healthy adults the strength of the Hering-Breuer reflex was directly proportional to increasing lung volume (18). A possible explanation for our results, then, is that the significantly increased FRC in the prone position was associated with increased feedback from pulmonary stretch receptors which reduced ventilatory drive as evidenced by a lower P 0.1 and a longer time constant in response to added deadspace.

Ideally, to have assessed the effect of postmenstrual age on the response to dead space, we would have undertaken serial measurements on the same group of babies, but were unable to do this only in a subset of our study population. The same equipment and methodology, however, was used in both studies. Indeed, one of the researchers was involved in making certain of the measurements in both studies and the same team analysed the results of the two studies using the same techniques. The only difference in the two studies was in the first one (6), the infants were studied while awake and in the one currently reported the infants were studied while in quiet sleep. In quiet sleep there is less variability in minute volume than when infants are in active sleep or awake, but this would not affect the time constant of the response. We, thus, feel it is appropriate to pool the results of the two studies.

Despite the median MIOP being significantly greater post term than at 36 weeks PMA, the response to added dead space was not significantly better, suggesting respiratory muscle strength (above a crucial level) may not be a major determinant of the response to added dead space. There were no statistically significant differences in the time constants in either position post term compared to 36 weeks PMA. The paired analysis in the study population overall and in the subgroup, however, demonstrated the levels of significance were greater post term than at 36 weeks PMA, indicating that a greater proportion of the infants had longer time constants in the prone compared to the supine position post term than at 36 weeks PMA. We also found a significant correlation between the time constant of the response to added dead space and postmenstrual age in the prone position, that is, the response to added dead space was slower with increasing postmenstrual age. A possible explanation for the reduced response rate to added dead space with increasing postnatal age might be maturational changes in the Hering-Breuer inflation reflex. There are few 12 longitudinal studies of maturational changes in the Hering-Breuer reflex, but in term born infants, the strength of the reflex appears to diminish between two and twelve months of age (19). In prematurely born infants an initial increase in the strength of the reflex from birth to 40 weeks PMA has been described, but the reflex strength was lower when the infants were examined four months after their expected date of delivery (20). Others (21) have demonstrated a reduction in the strength of the reflex in prematurely born infants from birth, but the study was performed over thirty years ago. Thus, it (21) would have included infants not exposed to either antenatal steroids or postnatal surfactant and with different lung function from the present population of very prematurely born infants. In addition, it is likely the infants were studied supine (21). We speculate that the strength of the Hering-Breuer reflex in the prone position may be maximal in prematurely born infants at the critical age for SIDS and could contribute to their reduced response to added deadspace.

In conclusion, we have demonstrated that prematurely born infants at the high risk age for SIDS had a poorer response to an added dead space in the prone compared to the supine position. In addition, the rate of response to the added dead space in the prone position was significantly damped with increasing postmenstrual age up to the critical age for SIDS.

These findings lend support to the hypothesis that a poorer response to a stress may contribute to the increased vulnerability of prematurely born infants to SIDS in the prone position. Apparatus used to measure the response to added dead space.

(A) The arrows show the direction of gas flow before the dead space is added.

(B) When the three way tap is turned to be in the vertical position, this diverts the bias flow from the face mask and the infant has to breathe from the added dead space tube. The vulnerability of prematurely born infants to Sudden Infant Death Syndrome (SIDS) in the prone position might be explained by a reduced ability to respond to a stress, such as hypercarbia, in that position; our objective, therefore, was to further explore the influence of position on the response to a stress.

Working hypothesis:

The ability of prematurely born infants to respond to added dead space in the prone compared to the supine position would be impaired at the high risk age for SIDS.

Patients: Twenty infants, median gestational age of 30 (range 24-32) weeks were studied at a median postmenstrual age (PMA) of 45 weeks. In addition, comparisons were made to the results of 25 infants studied at 36 weeks PMA.

Methodology: Infants were studied supine and prone. Breath by breath minute volume was measured at baseline and after a dead space was incorporated into the breathing circuit; the time constant of the response was calculated. The pressure generated in the first 100 msec of an occlusion (P 0.1 ), the maximum inspiratory pressure during an airway occlusion and functional residual capacity (FRC) were also measured in both positions.

Results:

The median time constant was longer (38 (range 15-85) versus 26 (range 2-40) seconds (p=0.002)). P 0.1 lower (p=0.003) and FRC higher (p=0.031) in the prone compared to the supine position. In the prone position, the time constant correlated with PMA (p=0.047), ie. the rate of response to added dead space was significantly damped with increasing postnatal age up to the critical age for SIDS.

Conclusions:

The dampened rate of response to added dead space in the prone compared to the supine position lends support to the hypothesis that a poorer response to a stress may contribute to prematurely born infants increased risk of SIDS in the prone position. Premature birth is also a risk factor for SIDS (2) and prematurely born infants may be particularly at risk of SIDS if they are slept prone (3). A possible explanation for the vulnerability of prematurely born infants to SIDS in the prone position might be that they are less able to respond to a stress, such as hypercarbia, in that position. The response to added deadspace (tube breathing) is a simple and reproducible method of assessing the ventilatory response to hypercarbia. Healthy infants born at term can compensate fully for an imposed dead space by increasing their minute ventilation (3). Intrauterine exposure to smoking, however, another known risk factor for SIDS (4), is associated with a dampened response to tube breathing in the perinatal period in term born babies (5). In addition, at 36 weeks postmenstrual age (PMA), we found that prematurely born infants had a dampened response to tube breathing in the prone compared to the supine position (6). The aim, therefore, of this study was to test the hypothesis that prematurely born infants' ability to respond to added dead space in the prone position would be impaired at their high risk age for SIDS, that is 45-48 weeks postmenstrual age (2). In addition, we wished to determine whether any differences between positions in the response to added dead space were related to differences in respiratory muscle strength and/or respiratory drive and whether the response to added dead space was influenced by postmenstrual age.

MATERIALS AND METHODS

Infants born prior to 33 weeks of gestation were eligible for this study once they were older than 40 weeks PMA (ie post term). If their parents gave informed written consent, infants were studied supine and prone, after each position had been maintained up to three hours.

The order in which the positions were studied was randomised between babies. In each position, first the infant's response to added dead space was examined and then their respiratory muscle strength, respiratory drive and lung volume were assessed. Infants were studied while in quiet sleep, with their eyes closed and breathing quietly.

To assess the response to added dead space, a face mask was placed over the infant's nose and mouth. Tubing the series of occlusions (8). Respiratory drive was assessed by measurement of the inspiratory pressure generated during the first 100 milliseconds of the first inspiratory effort during each occlusion (P 0.1 ). The reported P 0.1 was the mean of the series of occlusions.

Lung volume can affect respiratory muscle strength, hence at the end of each three hour period, lung volume was assessed by measurement of functional residual capacity (FRC) using a helium gas dilution technique (9). The FRC system (Equilibrated Biosystems Inc, Series 7700, Melville, NY) contained a rebreathing bag as the system reservoir and a specially designed infant circuit (total volume 95 mL). A facemask (Rendell Baker, Laerdal, Norway) was held snugly over the infant's nose and mouth. Silicone putty was used around the mask to achieve an airtight seal. The facemask was connected to the re-breathing bag via a three-way valve, actuation of which switched the infant from breathing ambient room air to breathing the test gas mixture (10% He, balance O 2 ) in the rebreathing bag. During the measurement, if there was no change in the helium concentration over a 15-second period, equilibration was deemed to have occurred. The initial and equilibration helium concentrations were used in the calculation of FRC, which was corrected for oxygen consumption (assumed to be 7 ml/kg/minute) (10)) and for body temperature, pressure and water vapor-saturated conditions. FRC was measured twice in each position and the results of the paired measurements were averaged and related to body weight. The co-efficient of repeatability of FRC in non ventilated infants is 3.9 ml/kg (11).

Statistical Analysis

Differences in the results in the two positions were assessed for statistical significance using the paired Wilcoxon Rank Sum Test. Spearman's correlation coefficients were calculated to determine the strengths of any relationships between the change in the minute volume or the time constant of the response to the added dead space and MIOP or P 0.1 . To assess whether the response to added dead space post term was poorer post term than at 36 weeks PMA, infants, median gestational age 30 (range 26-32 weeks) studied at 36 weeks PMA (6).

Differences between results at 36 weeks PMA and post term were assessed for statistical significance using the Mann Whitney U-Test. Twelve infants were included in this and our previous study (6), differences in the time constant results in the two positions at 36 weeks PMA and post term were assessed for statistical significance using the paired Wilcoxon Rank Sum test. Using all the results from the two studies, Spearman's correlation co-efficients were calculated to determine the strength of any relationships between the change in minute volume or the time constant and postmenstrual age. Analysis was performed using SPSS version 12.0, SPSS Inc, Chicago, Illinois 60606, USA.

Sample Size

In a previous study (5), we demonstrated that the coefficient of repeatability of TC was 11 seconds. Recruitment of 20 infants allowed, with at least 80% power at the 5% level, detection of a difference in the Tc between positions of at least 11 seconds.

Patients

Twenty infants (

RESULTS

The addition of the dead space resulted in an increase in minute volume in both the prone and supine positions (Table 1). The difference between positions in the percentage change in minute volume was not statistically significant. The time constant of the response was significantly longer in the prone compared to the supine position (p=0.002) (Table 1) (Figure 2). There were no significant differences in the respiratory timings or minute ventilation at baseline between the two positions (Table 1). The median T i was significantly longer in the prone compared to the supine position after the dead space was added (Table 1).

The median MIOP tended to be lower in the prone compared to the supine position (p=0.07), but there was no significant correlations between the response to tube breathing and MIOP in either position. The median P 0.1 was significantly lower in the prone compared to the supine positions (p=0.003), but there were no significant correlations between the response to tube breathing and P 0.1 in either position. The median FRC was significantly higher in the prone compared to the supine position (p=0.031) (Table 1).

Comparisons of the results of the infants studied at 36 weeks PMA and those studied post term revealed no significant differences in the median changes in minute volume or the time constant, but that the median MIOP was significantly greater in both positions post term (p<0.001) (Table 2). The change in minute ventilation was not significantly correlated with postmenstrual age in either the supine (p=0.697) or the prone (p=0.25) position. The mean difference in the time constants between the prone and supine positions was 8. We have demonstrated that at a high risk age for SIDS (2), the response to added dead space in prematurely born infants was poorer in the prone compared to the supine position, as the time constant was significantly longer in the prone position. Hypercarbia is the most important stimulus to ventilation during added dead space (tube breathing), as measurements in air and 30% oxygen yielded similar results (12) and no effect of resistance was found in experiments involving prematurely born infants (12). Our results then suggest that prematurely born infants at the high risk age for SIDS have a dampened response to hypercarbia in the prone position. The time constant during tube breathing is shorter in active compared with quiet sleep (3). To avoid such a confounding effect, infants were always tested when they were in quiet sleep. Maternal smoking during pregnancy is associated with an impaired response to added dead space (5). Three infants studied had mothers who were smokers, but our results were not biased by their inclusion as we compared the response of all the infants in two positions.

The dampened response to added dead space in the prone position may indicate reduced respiratory drive in that position. Indeed we did find that the P 0.1 was significantly lower in the prone compared to the supine position. P 0.1 is influenced by respiratory muscle strength, but there was no significant difference in the median MIOP between positions. As we had demonstrated previously (13), the median FRC was higher in the prone compared to the supine position. Although the difference was significant, however, the magnitude of the difference was relatively small and unlikely to influence MIOP (14). It is possible, however, that the difference in FRC may have affected the strength of the Hering-Breuer inflation reflex. The Hering-Breuer inflation reflex which terminates inspiration and prolongs expiration in response to lung inflation (15) is stronger in the prone compared to the supine position (16,17). Differences in the strength of the reflex between positions in prematurely born infants have been shown to be significantly correlated with differences in lung volume (17). In addition, in healthy adults the strength of the Hering-Breuer reflex was directly proportional to increasing lung volume (18). A possible explanation for our results, then, is that the significantly increased FRC in the prone position was associated with increased feedback from pulmonary stretch receptors which reduced ventilatory drive as evidenced by a lower P 0.1 and a longer time constant in response to added deadspace.

Ideally, to have assessed the effect of postmenstrual age on the response to dead space, we would have undertaken serial measurements on the same group of babies, but were unable to do this only in a subset of our study population. The same equipment and methodology, however, was used in both studies. Indeed, one of the researchers was involved in making certain of the measurements in both studies and the same team analysed the results of the two studies using the same techniques. The only difference in the two studies was in the first one ( 6), the infants were studied while awake and in the one currently reported the infants were studied while in quiet sleep. In quiet sleep there is less variability in minute volume than when infants are in active sleep or awake, but this would not affect the time constant of the response. We, thus, feel it is appropriate to pool the results of the two studies.

Despite the median MIOP being significantly greater post term than at 36 weeks PMA, the response to added dead space was not significantly better, suggesting respiratory muscle strength (above a crucial level) may not be a major determinant of the response to added dead space. There were no statistically significant differences in the time constants in either position post term compared to 36 weeks PMA. The paired analysis in the study population overall and in the subgroup, however, demonstrated the levels of significance were greater post term than at 36 weeks PMA, indicating that a greater proportion of the infants had longer time constants in the prone compared to the supine position post term than at 36 weeks PMA. We also found a significant correlation between the time constant of the response to added dead space and postmenstrual age in the prone position, that is, the response to added dead space was slower with increasing postmenstrual age. A possible explanation for the reduced response rate to added dead space with increasing postnatal age might be maturational changes in the Hering-Breuer inflation reflex. There are few infants, the strength of the reflex appears to diminish between two and twelve months of age (19). In prematurely born infants an initial increase in the strength of the reflex from birth to 40 weeks PMA has been described, but the reflex strength was lower when the infants were examined four months after their expected date of delivery (20). Others (21) have demonstrated a reduction in the strength of the reflex in prematurely born infants from birth, but the study was performed over thirty years ago. Thus, it (21) would have included infants not exposed to either antenatal steroids or postnatal surfactant and with different lung function from the present population of very prematurely born infants. In addition, it is likely the infants were studied supine (21). We speculate that the strength of the Hering-Breuer reflex in the prone position may be maximal in prematurely born infants at the critical age for SIDS and could contribute to their reduced response to added deadspace.

In conclusion, we have demonstrated that prematurely born infants at the high risk age for SIDS had a poorer response to an added dead space in the prone compared to the supine position. In addition, the rate of response to the added dead space in the prone position was significantly damped with increasing postmenstrual age up to the critical age for SIDS.

These findings lend support to the hypothesis that a poorer response to a stress may contribute to the increased vulnerability of prematurely born infants to SIDS in the prone position. (A) The arrows show the direction of gas flow before the dead space is added.

(B) When the three way tap is turned to be in the vertical position, this diverts the bias flow from the face mask and the infant has to breathe from the added dead space tube.

  the facemask. This resulted in the dead space of the second tube being added to the infant's respiratory system. The infant then breathed through the additional dead space of 4.4 mL/kg until the maximum minute ventilation (MMV) was deemed to have been reached. The breath by breath MV was plotted against time and a polynomial regression line (using Excel software (Microsoft Corp, Seattle, USA) was drawn through the data points collected from the point at which the additional dead space was added to the circuit. The response to added dead space was determined by calculating the time constant (Tc), defined as the time taken to achieve 63% of the increase in minute ventilation, and the change in MV expressed as a percentage of the baseline MV. The coefficient of repeatability of Tc had been previously shown to be 11 seconds (5) and at 36 weeks PMA was 7 seconds. Respiratory muscle strength was assessed by measurement of the maximum pressure generated during crying against a sustained end expiratory airway occlusion (MIOP). During the procedure neck position was carefully controlled to avoid flexion. A pneumotachograph (GM Engineering, Kilwinning, UK), attached to a differential pressure transducer (Validyne MP45, range +/-2 cmH 2 O, Validyne Corporation, Northridge CA, USA), was inserted in the distal end of a face mask which was held snugly, to avoid leaks, over the infant's nose and mouth. Airway pressure was measured from a side port on the pneumotachograph using a differential pressure transducer (Validyne MP 45, range +/-100 cmH 2 O, Validyne Corporation, Northridge CA, USA). The signals from both transducers were amplified (Validyne CD280, Validyne Corporation, Northridge CA, USA) and displayed in real time on a computer running Labview software (version 4.0, National Instruments, Austin TX, USA) with 100 Hz analog to digital sampling (DAQ 16XE-50, National Instruments, Austin TX USA).Airway occlusions were performed using a T-piece containing a one way non-rebreathing valve attached to the distal end of the pneumotachograph. Airway occlusion was initiated by occluding the inspiratory limb of the T-piece during the preceding expiratory phase, the valve in the expiratory limb allowed expiration, but not inspiration. Each occlusion was maintained until the infant had made at least five respiratory efforts. At least five sets of occlusions were performed. The reported MIOP was the largest inspiratory pressure generated during

  (range 15-85) seconds (p= 0.008). There was no significant difference in the difference in the time constant results in the supine and prone positions between the two time periods. Overall, the time constant of the response to added dead space had a significant positive correlation with postmenstrual age in the prone (r=0.305, p=0.047), but not the supine (r= 0.031, p= 0.844) position.

  FIGURE LEGEND:

Figure 2 :

 2 Figure 2:The time constants of the response related to position at 36 weeks PMA and post term

INTRODUCTION

  Key words: prematurity; functional residual capacity; respiratory control; respiratory muscle strength Prone sleeping remains a significant risk factor for sudden infant death syndrome (SIDS)(1).

5 bias

 5 (7.0 mm internal diameter) connected the face mask to a three way tap via which a bias flow of air (2 L/min) was delivered to the breathing circuit (Figure1). A second tube (7.0 mm internal diameter) led from the face mask to a pneumotachograph (PK Morgan, Rainham Kent, UK) via a Y connector. The length of the tube between the face mask and Y connector was adjusted such that its volume was 4.4 mls/kg bodyweight, twice the generally accepted anatomical deadspace(7). A third tube connected the three way tap to the remaining part of the Y connector. The pneumotachograph was attached to a differential pressure transducer (MP45, Validyne Corporation, Northridge CA, USA) the signal from which was amplified (DC280; Validyne Corporation) and displayed in real time on a computer running Spectra software (version XX; Grove Medical, London UK) with 200 Hz analog to digital sampling (DAQ 16XE-50; National Instruments, Austin, TX, USA). Tidal volume was obtained by digital integration of the flow signal using the Spectra software and minute volume (MV), inspiratory (Ti), expiratory (Te) and total respiratory cycle (Ttot) times and respiratory rate were calculated breath by breath. Baseline recordings of respiratory flow were made with the three way tap in the neutral position, so that the bias flow of 2 L/min of air to the face mask eliminated any dead space. When the tap was rotated the bias flow was fed via the third tube directly to the Y connector and the pneumotachograph, so that the flow bypassed the facemask. This resulted in the dead space of the second tube being added to the infant's respiratory system. The infant then breathed through the additional dead space of 4.4 mL/kg until the maximum minute ventilation (MMV) was deemed to have been reached. The breath by breath MV was plotted against time and a polynomial regression line (using Excel software (Microsoft Corp, Seattle, USA) was drawn through the data points collected from the point at which the additional dead space was added to the circuit. The response to added dead space was determined by calculating the time constant (Tc), defined as the time taken to achieve 63% of the increase in minute ventilation, and the change in MV expressed as a percentage of the baseline MV. The coefficient of repeatability of Tc had been previously shown to be 11 seconds (5) and at 36 weeks PMA was 7 seconds. Respiratory muscle strength was assessed by measurement of the maximum pressure generated during crying against a sustained end expiratory airway occlusion (MIOP). During the procedure neck position was carefully controlled to avoid flexion. A pneumotachograph (GM Engineering, Kilwinning, UK), attached to a differential pressure transducer (Validyne MP45, range +/-2 cmH 2 O, Validyne Corporation, Northridge CA, USA), was inserted in the distal end of a face mask which was held snugly, to avoid leaks, over the infant's nose and mouth. Airway pressure was measured from a side port on the pneumotachograph using a differential pressure transducer (Validyne MP 45, range +/-100 cmH 2 O, Validyne Corporation, Northridge CA, USA). The signals from both transducers were amplified (Validyne CD280, Validyne Corporation, Northridge CA, USA) and displayed in real time on a computer running Labview software (version 4.0, National Instruments, Austin TX, USA) with 100 Hz analog to digital sampling (DAQ 16XE-50, National Instruments, Austin TX USA).Airway occlusions were performed using a T-piece containing a one way non-rebreathing valve attached to the distal end of the pneumotachograph. Airway occlusion was initiated by occluding the inspiratory limb of the T-piece during the preceding expiratory phase, the valve in the expiratory limb allowed expiration, but not inspiration. Each occlusion was maintained until the infant had made at least five respiratory efforts. At least five sets of occlusions were performed. The reported MIOP was the largest inspiratory pressure generated during

7 we

 7 compared the results from this study to those we have previously published from 25

  3 seconds at 36 weeks PMA and 21.3 seconds post term respectively. The mean difference between the difference at the two time periods was 13.4 seconds (95% confidence intervals of the differences was -2.5, 28.6) (p = 0.098). In the twelve infants who had been assessed at 36 weeks PMA and post term, the median time constant at 36 weeks PMA in the supine position was 25 (range 10-51) seconds and in the prone position was 34 (range 15-96) seconds (p=0.019) and post term in the supine position was 26 (range 2-40) seconds and in the prone

9 position

 9 was 38 (range 15-85) seconds (p= 0.008). There was no significant difference in the difference in the time constant results in the supine and prone positions between the two time periods. Overall, the time constant of the response to added dead space had a significant positive correlation with postmenstrual age in the prone (r=0.305, p=0.047), but not the supine (r= 0.031, p= 0.844) position.

  maturational changes in the Hering-Breuer reflex, but in term born

Figure 1 :

 1 FIGURE LEGEND:

Figure 2 :

 2 Figure 2:The time constants of the response related to position at 36 weeks PMA and post term

Table 1 : The response to added deadspace in the prone and supine position

 1 Data are demonstrated as median (range)

		Prone	Supine	p
	Minute ventilation (ml/min/kg)
	Baseline	235 (71-446)	229 (105-537)	0.575
	Maximum post dead space	402 (104-771)	387 (138-692)	0.629
	Following addition of dead space:
	Change in minute ventilation (%)	66 (17-311)	72 (3-298)	0.794
	Change in tidal volume (%)	32 (-14-136)	41 (-5-222)	0.654
	Change in respiratory rate (%)	3 (-8-71)	6 (-7-31)	0.822
	Time constant (secs)	38 (15-85)	25.5 (2-39.7)	0.002
	Respiratory rate (bpm)
	Baseline	54 (35-91)	55 (37-87)	0.167
	Added dead space	57 (39-98)	59 (39-94)	0.211
	Tidal volume (ml/kg)
	Baseline	4.3 (1.5-9.5)	3.6 (2.1-8.5)	0.191
	Added dead space	6.2 (1.8-10.8)	5.9 (2.1-10)	0.287
	Ti (sec)
	Baseline	0.56 (0.26-0.88)	0.54 (0.30-0.91)	0.108
	Added dead space	0.56 (0.29-0.92)	0.52 (0.33-0.71)	0.030
	Te (sec)
	Baseline	0.52 (0.40-1.02)	0.56 (0.33-0.87)	0.313
	Added dead space	0.51 (0.36-0.88)	0.49 (0.33-0.71)	0.331
	Ttot (sec)
	Baseline	1.12 ((0.66-1.71)	1.11 (0.63-1.58)	0.156
	Added dead space	1.08 ((0.67-1.71)	1.04 (0.62-1.56)	0.073
	MIOP (cm H 2 O)	63.8 (47.9-104.6)	66.6 (50.6-112.9)	0.070
	P 0.1 (cm H 2 O)	3.82 (2.18-6.21)	4.72 (2.65-9.99)	0.003
	P 0.1 /MIOP ratio	0.06 (0.03-0.11)	0.07 (0.02-0.14)	0.033
	FRC (ml/kg)	30.2 (21.4-41.9)	26.9 (18.7-40.7)	0.031

Table 2 : Comparison of data at 36 weeks PMA and post term.

 2

	Data are demonstrated as median (range)
		36 weeks PMA	Post term	p
	% change in MV
	Supine	85.4 (6.5-242)	71.6 (2.6-298)	0.315
	Prone	60.3 (14.8-174)	66.1 (16.9-311)	0.553
	Time constant (secs)
	Supine	22 (6-92)	26 (2-40)	0.819
	Prone	26 (8-106)	38 (15-85)	0.201
	MIOP (cm H 2 O)
	Supine	36.9 (13.8-63.8)	66.6 (50.6-112.9)	<0.001
	Prone	25.9 (16.8-53)	63.8 (47.9-104.6)	<0.001
	P 0.1 (cm H 2 O)
	Supine	3.66 (1.87-8.77)	4.72 (2.65-9.99)	0.118
	Prone	2.64 (1.39-13.20)	3.82 (2.1-6.21)	0.011
	P 0.1 /MIOP ratio
	Supine	0.121 (0.05-0.26)	0.071 (0.02-0.14)	0.001
	Prone	0.105 (0.03-0.33)	0.060 (0.03-0.11)	<0.001

John Wiley & Sons, Inc.Pediatric Pulmonology

John Wiley & Sons, Inc.

Dr Saiki was supported by the Foundation for the Study of Infant Deaths. Dr Saiki was supported by the Foundation for the Study of Infant Deaths.