Benoit Borel
email: benoit.borel-2@univ-lille2.fr

Erwan Leclair

Delphine Thevenet

; Laurent Beghin

EA Claudine Fabre

Comparison of mechanical ventilatory constraints between continuous and intermittent exercises in healthy prepubescent children

Keywords: children, exercise modality, ventilatory response, flow/volume loop Interrelations nutrition, inflammation, et infection children, exercise modality, ventilatory response, flow/volume loop 23 children, exercise modality, ventilatory response, flow/volume loop 23

Background: The aim of this study was to evaluate the occurrence and severity of mechanical 2 ventilatory constraints in healthy prepubescent children during continuous and intermittent exercise. 3 Methods: Twelve prepubescent children (7 -11 years old) performed 7 exercises on a treadmill: one 4 graded test for the determination of maximal aerobic speed (MAS), three continuous exercises (CE) 5 at 60, 70 and 80% of MAS and three intermittent exercises (IE), alternating 15s of exercise with 15s of 6 passive recovery, at 90, 100 and 110% of MAS. During each CE and IE, tidal flow/volume loops were 7 plotted within a maximal flow/volume loop (MFVL) measured at rest before each exercise. Expiratory 8 flow limitation (expFL expressed in %Vt) was defined as the part of exercise tidal volume (Vt) meeting 9 the boundary of MFVL. Breathing strategy was estimated by measuring inspiratory capacity relative 10 to forced vital capacity and tidal volume relative to inspiratory capacity. Other breathing pattern 11 parameters (ventilation VE, Vt, respiratory frequency f) were continuously recorded during exercise. 12

Results: An "intensity" effect was found for E

V & during CE (p<0.001) but not during IE (p=0.08). The 13 increase in E V & was predominantly assumed by an increase in f for both exercise modalities. During 14 each exercise, several children heterogeneously experienced expFL ranging between 10 -90%Vt. For 15 all exercises, Vt was predominantly regulated by an increase in Vt/IC with no change in IC/FVC from 16 rest to exercise. Finally, no significant "modality" effect was found for mechanical ventilatory 17 constraint parameters (expFL, VT/IC and IV/FVC). 18

Discussion: We could conclude that neither of the modalities studied induced more mechanical 19 ventilatory constraints than the other, but that exercise intensities specific to each modality might be 20 greater sources of exacerbation for mechanical ventilatory constraints. 21 22

Introduction 1 2

In the literature, it has been reported that children show specific ventilatory adaptations in 3 comparison with adults during exercise. In fact, children present a phenomenon of hyperventilation 4 for a similar level of relative intensity in comparison with adults. This phenomenon of 5 hyperventilation in children is characterized by a higher respiratory equivalent in oxygen (

E V & / 2 O V & ) 6
in children than in adults [1][2][3][4] and could result from a specific breathing pattern. The latter is 7 characterized by rapid and shallow breathing in children, defined by higher values of respiratory 8 frequency and lower values of tidal volume than in adults 5 . 9 10 Moreover, as already shown in adults during incremental exercise, mechanical ventilatory constraints 11 were also highlighted in children during exercise [6][7][8][9][10][11][12] . Such constraints represented the balance 12 between ventilatory demand and ventilatory capacity 6 and could easily be identified by estimating 13 breathing reserve (BR) 6 . However, BR provides no information on breathing pattern, or on the 14 source and type of ventilatory constraints. This type of information could be obtained by using the 15 exercise flow/volume loops method, consisting in plotting tidal flow/volume loops within maximal 16 flow/volume loops during exercise 6 . By using this method, some studies have assessed mechanical 17 ventilatory constraints in children during incremental exercise and reported several cases of 18 expiratory flow limitation during high intensities of the incremental exercise 13,14 . Thus, Nourry et al 19 reported an "intensity" effect for mechanical ventilatory constraint occurrence and severity during a 20 maximal graded test in trained or untrained prepubescent children 13,14 . From these results, we have 21 hypothesized that intermittent exercise could induce higher mechanical ventilatory constraints than 22 continuous exercise, as the exercise intensities prescribed for intermittent exercises are higher than 23 those prescribed for continuous exercises. The answers to this question could enlighten us on 24 specific ventilatory adaptations in children during two exercise modalities frequently encountered 25 The aim of this study was 1) to evaluate and compare breathing pattern and tidal flow/volume loops 4 between continuous and intermittent exercises in healthy prepubescent children, so as to determine 5 if both exercise modalities could induce mechanical ventilatory constraints and 2) to determine the 6 effects of the exercise modality on the severity of potential ventilatory constraints, in order to 7 determine if one exercise modality was more restrictive than the other on the children's ventilatory 8 system. 9 Twelve 7 -11 year old healthy prepubescent children volunteered to participate in this study, which 4 had previously received approval from the local ethics committee. Parents and children were 5 informed as to the modalities of the study and then gave their written, informed consent prior to any 6 involvement in it. All the children were free of respiratory or cardiac diseases and of orthopedic 7 counter indications for physical exercise, performed less than 3h/week of extra academic physical 8 activity and their sum of pubertal stages was lower than 3. Sexual maturity was evaluated from 9 pubertal stages, according to Tanner's method: indices of breast, pubic hair and genital 10 development 15 . The same physician made all the observations for all the children. 11 12

Study design 13

All the children came to the laboratory three times within a one-month period, with at least 48 hours 14 between two evaluation days. At the beginning of each visit, a resting ECG and a clinical examination 15 were performed. During the first visit, spirometry and sexual maturity were evaluated, followed by a 16 period of familiarization and realization of a maximal graded test on a treadmill. Measurement of 17 pulmonary gas exchanges allowed the determination of peak oxygen uptake ( At each visit, the children underwent spirometry before each of the exercises performed (Vmax 1 Encore, Sensormedics, Viasys Healthcare, USA). Each child was seated on a chair and fitted with a 2 noseclip. All the lung tests were carried out by the same physician respecting American Thoracic 3 Society recommendations 16 . For each child, three measurements of maximal flow/volume loops 4 (MFVL) were carried out, allowing the determination of forced vital capacity (FVC) and forced 5 expiratory volume in 1 second (FEV 1 ). Among the registered measurements, the MFVL with the 6 greatest combination of FVC and FEV 1 was retained. 7 8 Cardio-respiratory parameters 9 At each visit and during all exercise tests, the children wore a mouthpiece connected to a breath-by-10 breath gas analyzer (V max Encore laboratory system, Sensormedics, Viasys Healthcare, USA), 11 allowing the measurement of oxygen uptake (

2 O V & ), carbon dioxide output ( 2 CO V &
), ventilation 12 ( E V & ), tidal volume (Vt) and respiratory frequency (f). In order to ensure accurate measurements, O 2 13 and CO 2 analyzers were calibrated before each exercise test, using ambient air and two gases of 14 known O 2 concentration (16% and 26%) and CO 2 concentration (4% and 0%). The calibration of the 15 pneumotachograph was performed using a 3-liter syringe following the manufacturer's 16 recommended procedures. 17

Heart rate (HR) was continuously monitored with a heart rate monitor (Polar S810, Polar Electro Oy, 18

Kempele, Finland). The data acquisition frequency for HR was set at 5 sec. 19 Borg's CR-10 scale 17 (ranging from 0 to 10) was used during all the exercises in order to evaluate 20 dyspnea. This scale was explained to the children at the beginning of each visit and was shown to 21 them at the end of each exercise. A period of familiarization on the treadmill was proposed for all the children, consisting in explaining 1 the security procedures and letting them walk and run on it. After explaining the exercise to the 2 children, the test began with a resting period of three minutes in a standing position on the treadmill. 3 Subsequently, a warm-up period of 3 minutes at 3km.h -1 was performed. The exercise began with 4 one minute at 4 km.h -1 , then the speed was increased by 0.5 km.h -1 every minute. During the whole 5 exercise, pulmonary gas exchanges and heart rate were continuously recorded and the values were 6 averaged every 30 seconds. The exercise ended when the children were no longer able to maintain 7 the imposed running speed. It was judged that they had performed a maximal exercise when 3 or 8 more of the following criteria were obtained: visible exhaustion (excessive hyperpnea, facial flushing, 9 sweating, discomfort) 18 , maximal heart rate >90% of predicted maximal heart rate (208-0.7 x age) 19 , a 10 plateau in

O

V & despite increasing running speed or a final respiratory exchange ratio (RER) higher 11 than 1.0 20 . The maximal aerobic speed (MAS), corresponding to the speed at the last completed 12 stage 21 , was used as a reference value for the calculation of the individualized speeds of the 13 continuous and intermittent exercises. 14

15

Continuous and intermittent visits 16

At each of the two other visits, the children performed 3 continuous (CE) or 3 intermittent exercises 17 (IE). Figure 1 shows the design for each exercise-testing visit. First, children rested for three minutes. 18 Then, a 6-minute exercise period was carried out, immediately followed by a 4-minute period of 19 passive recovery, with measurement of gas exchanges. After that, the mouthpiece of the pulmonary 20 gas exchange apparatus was removed until the next exercise. Between two exercises, a 30-minute 21 rest period was observed, without taking any measurements. For the continuous exercises, the 6-22 minute exercise period was performed at an individualized and constant speed. For intermittent 23 exercises, the exercise period was divided into periods of 15s of running followed by 15s of passive 24 recovery, except for the last minute of each intermittent exercise which was divided into one 30s 25 period of running and one 30s period of passive recovery, so as to perform exercise tidal 1 flow/volume loop measurement. In order to avoid the inertia of the treadmill for the two exercise 2 modalities, the treadmill belt was set in motion at the adequate running speed before the beginning 3 of the exercise period. The children were instructed to put their feet on both sides of the belt of the 4 treadmill and to get on the belt at the beginning of the exercise period. For intermittent exercises, as 5 the treadmill was not stopped at the end of each 15-second running period, the children were 6 instructed to get on or off the treadmill after each 15-second period of running or passive recovery. 7

Leaving the treadmill consisted in putting their feet on both sides of it. Hence, the children were 8 familiarized with getting on or off the treadmill before the intermittent exercises and were helped by 9 an adult. 10

Continuous exercises intensities corresponded to 60% (CE60), 70% (CE70) and 80% (CE80) of MAS. 11

These intensities were always performed in this order. The intermittent exercise intensities 12 corresponded to 90% (IE90), 100% (IE100) and 110% (IE110) of MAS and here also, this order was 13 kept for all the subjects. These MAS intensity percentages were chosen as they are frequently used 14 for continuous and intermittent exercises respectively 22 . Three intensity levels could be described: 15 low (CE60 vs. IE90), middle (CE70 vs. IE100) and high (CE80 vs. IE110). 16 For all the continuous and intermittent exercises performed by the children, the mean values of 17

2 O V & , E V & , Vt, f, E V & / 2 O V & , E V & / 2 CO V &
and HR, reported in the "Results" section, were obtained 18 by averaging raw data during the first 30 seconds of the last minute of each exercise. This period of 19 measurement corresponded to the time of exercise flow/volume loop measurement. The MVV was estimated for each exercise performed, with the FEV 1 measured just before the 8 exercise. 9

In order to evaluate mechanical ventilatory constraints and breathing strategy during the exercise, 10 exercise tidal flow/volume loops (tidal F/V loops) were recorded just once, at the end of the first 30 11 seconds of the last minute of each exercise using a pneumotachograph. Tidal F/V loops corresponded 12 to a mean of ten tidal breaths recorded and were computer averaged before a maximal inspiratory 13 capacity (IC) maneuver. This maneuver allows correction of drift phenomena, as described by 14 Johnson et al 6 . During exercise, a drift phenomenon could appear in the volume signal due to 15 electrical or physiological changes over time. This drift must be corrected in order to obtain correctly 16 placed tidal F/V loops within MFVL. Thus, the maximal inspiratory capacity maneuver was performed 17 immediately after tidal F/V loop measurement. This maximal IC maneuver was compared to the 18 maximal IC determined at rest. After the maneuver, the computer could correctly plot tidal F/V loops 19 within the MFVL determined before the exercise. For each tidal F/V loop measurement, the mean 20

values of 2 O V & , E V & , Vt, f, E V & / 2 O V & , E V & / 2 CO V &
and HR were calculated at the moment of 21 measurement (corresponding to the mean of the first 30 seconds of the last minute of exercise). 22 Expiratory flow limitation (expFL) corresponds to the part of tidal F/V loop that meets or exceeds the 23 expiratory boundary of the MFVL and is expressed as a % of tidal volume (%Vt). In order to 24 determine breathing strategy, the evolution of different ratios was analyzed. Hence, inspiratory 25 6 , expiratory reserve volume relative to vital 1 capacity (ERV/FVC), tidal volume relative to inspiratory capacity (Vt/IC) 6 and inspiratory reserve 2 volume relative to vital capacity (IRV/FVC) were computed (Figure 2). 3 4

Statistical analysis 5

All the values were expressed as means ± standard deviation (mean ± SD). For each parameter, data 6 normality was verified with a Kolmogorov-Smirnov test. 7

In order to determine the effect of exercise modality, a two-way repeated measure analysis of 8 variance (ANOVA) was performed for all the parameters. The factors were "exercise intensity" and 9 "exercise modality". When the ANOVA F ratio was significant, the mean values were compared by 10 using pairwise multiple comparison procedures (Bonferroni post-hoc test). 11

For intra-modality comparison, a one-way repeated measure analysis of variance (ANOVA) was 12 performed for all the parameters. The factor was "exercise intensity". When the ANOVA F ratio was 13 significant, the mean values were compared by using pairwise multiple comparison procedures 14 (Bonferroni post-hoc test). 15 For rest versus exercise values comparison within the same modality, a "paired t-test" was carried 16 out. 17 Respiratory data from one subject were unworkable. Thus, anthropometric, spirometric and maximal 4 exercise values of the eleven children were analyzed (8 boys and 3 girls; Table 1). For each child, 3 5 criteria of maximality at least were fulfilled, showing that the graded exercise was maximal. 2. For

2 O V &
only, a significant "intensity x 9 modality" interaction and an "intensity" effect were highlighted. However post-hoc tests revealed no 10 significant difference between continuous and intermittent exercises within the three different 11 intensity levels (ie. CE60 vs. IE90, CE70 vs. IE100 and CE80 vs. IE110), allowing the comparison of the 12 ventilatory data obtained during each exercise modality. For HR, an significant intensity effect was 13 highlighted for both modalities, without "modality" effect. Finally, no effect of the exercise modality 14 and intensity was found for Respiratory Exchange Ratio (RER). 15 16 Ventilatory response to exercise 17 18

Breathing pattern 19

The mean values of ventilation are shown in Table 3. Two-way ANOVA analysis highlights "intensity" 20 and "modality x intensity" interaction effects for ventilation (p<0.05). An "intensity" effect (p<0.001) 21 was found for E

V & in continuous exercise (p<0.001) but not in intermittent exercise (p=0.08).

22

Following post-hoc tests, for each level of exercise intensity (CE60 vs. IE90, CE70 vs. I100 and CE80 vs. 23 IE110), ventilation and dyspnea scores were not significantly different between continuous and 24 intermittent exercises. For tidal volume (Vt), no "intensity" or "modality" effects were found for 25 either CE or IE modalities. Concerning respiratory frequency (f), an "intensity" effect was found for IE 1 (p<0.05) and a tendency towards significant effect for CE (p=0.06). No "modality" effect was found 2 for this parameter. 13 between FL and NFL groups for IE90 (38.8 ± 1.2 for FL and 50.9 ± 10.2 for NFL), IE100 (33.9 ± 7.9 for 1 FL and 45.2 ± 12.2 for NFL) and IE110 (24.2 ± 22.2 for FL and 46.7 ± 13.9 for NFL). The mean values of resting and exercise pulmonary parameters are recapitulated in Table 4. IRV 5 decreased significantly from rest to exercise for all continuous or intermittent exercises (p<0.05). ERV 6 increased significantly from rest to exercise for CE70, IE100 and IE110 (p<0.05). For IC, a significant 7 difference was observed between rest and exercise for CE70, IE100 and IE110 (p<0.05). No significant 8 differences were found for IC/FVC and ERV/FVC between rest and exercise values, except for IE110 9 (p<0.05). IRV/FVC decreased significantly between rest and exercise for all the exercises. In the same 10 way, Vt/IC was significantly higher than at rest for all the exercises except for CE70 and IE100 11 (p<0.05). 12 13 For all the parameters of mechanical ventilatory constraints (expFL, Vt/IC, IC/FVC, IRV/FVC and 14 ERV/FVC), there were no significant "modality" and "intensity" effects (Table 4 and Figure 3.a). 15 However, an "intensity" effect tended to be significant for expFL during continuous exercise 16 (p=0.065). 17 The purpose of the study was to evaluate the ventilatory response during continuous and 2 intermittent exercises and to determine if one of these two exercise modalities could induce greater 3 mechanical ventilatory constraints than the other. Our results showed that both exercise modalities 4 (continuous and intermittent) could induce mechanical ventilatory constraints in healthy 5 prepubescent children, as several children presented expiratory flow limitation (expFL) during 6 continuous and intermittent exercises with no influence due to exercise modality. 7 8 Some limits should be considered for this study, mainly concerning recorded exercise flow/volume 9 loops. As mentioned by some authors 6,13 , expFL assessment is highly dependent on the placement of 10 exercise flow/volume loop within maximal flow/volume loop. This limit has been taken in account 11 with the realization of inspiratory capacity (IC) maneuvers after exercise flow/volume loop 12 measurement, allowing drift correction. The IC maneuver has already been used to enhance 13 reproducibility of the flow/volume loop method 6 . 14 15 As described above, the aim of the study was to compare mechanical ventilatory constraints 16 (expiratory flow limitation and dynamic hyperinflation) and breathing pattern (ventilation, tidal 17 volume and breathing frequency) during continuous and intermittent exercises with exercise 18 intensities inducing similar metabolic demand. Even if a significant interaction for oxygen uptake was 19 highlighted, it appears that, within the different levels of exercise intensity (low, medium and high), 20 the metabolic demand (

Effects of exercise intensity on breathing pattern 1

As expected, ventilation for the continuous exercise modality increased with greater exercise 2 intensity. However, during intermittent exercises, ventilation increased slightly, but not significantly, 3 with the increase in exercise intensity. These specific ventilation evolutions according to exercise 4 modality could be due to the fact that, during 15s -15s intermittent exercises performed by the 5 children in this study, the time of running was rather short. Hence, the ventilatory system would not 6 seem to have enough time to adapt to the ventilatory demand induced by the exercise intensity. 7

However, within the three intensity levels (i.e. low, medium or high), no significant difference was 8 highlighted between continuous and intermittent exercises. These results could logically be 9 explained since the intensities of intermittent exercises were specifically chosen in order to be 10 matched with continuous exercise intensities in terms of energy expenditure (i.e. oxygen uptake), as 11 already reported by Berthoin et al. 24 The absence of significant difference for ventilation between 12 both exercise modalities was also required in order to compare breathing pattern components (Vt 13 and f) correctly. In fact, several studies have reported that for a particular subject, the regulation of 14 the breathing pattern at a given level of ventilation is largely independent of the cause of the 15 increased ventilation 25,26 . 16 During all the continuous and the intermittent exercises, our results showed that the increase in 17 ventilation was mainly due to an increase in respiratory frequency. This result supports the findings 18 in the literature 5 and revealed that, whatever the exercise modality, children presented rapid and 19 shallow breathing. However, differences in ventilatory regulation according to exercise modality 20 seemed to exist, as a "modality" effect was found for ventilatory equivalent in carbon dioxide 21

( E V & / 2 CO V &
). Post-hoc analysis revealed that, in general, intermittent exercise modality induced 22

higher E V & / 2 CO V &
values than continuous exercise modality. This could be explained firstly, by a 23 chemical regulation due to a higher acidosis level during intermittent exercise and, secondly, by a 24 As described by Johnson et al 6 , the assessment of breathing reserve (BR) could be an indicator of the 5 occurrence of mechanical ventilatory constraints during exercise. In all the previous studies reporting 6 BR values in children, the exercise performed has been incremental 13,14,[28][29][30] . This difference in 7 protocols makes it impossible to compare our results with the previous studies, which is normal, 8 since this is the originality of our study. In our study, BR significantly decreased during CE and IE 9 whereas there was no "intensity" effect for E

V & during IE. This result is surprising and reinforces the 10 work of Johnson et al. 6 asserting that breathing reserve is a general and inappropriate parameter for 11 the study of mechanical ventilatory constraints during exercise. 12 The study of mechanical ventilatory constraints with exercise flow/volume loops brings more 13 information on the adaptation of the ventilatory system during exercise. 14

In our study, the number of expiratory flow limitation cases increased with exercise intensity in both 15 exercise modalities. For the highest intensity exercises, more than 50% of the whole population (6 of 16 the 11 children studied) showed an expiratory flow limitation. Individual expiratory flow limitation 17 ranged between 10 -90% of Vt. These values are in accordance with values of expFL found in the 18 literature. In fact, Nourry et al. 13 reported an increase in the number of observations of expFL in non-19 athlete prepubescent children (8 children out of the 18 studied) with the increase in intensity during 20 the maximal graded test, ranging between 16 -78% of Vt. The occurrence of expFL during sub-21 maximal (CE60, CE70, CE80 and IE90) and supra-maximal (IE100, IE110) exercises is not surprising, 22 since a previous study has reported the case of expFL from 30% of maximal aerobic power 13 and 23 intensities prescribed in our study were all higher than this threshold value. As with ventilatory 24 response during exercise, the occurrence of expFL in our population was heterogeneous as some 25 children experienced expFL during IE90 and IE110 but not during IE100 for example. The 1 heterogeneity of expFL occurrence has also previously been described in children 13 and could be 2 explained by the different levels of ventilatory system maturation. 3 Thus, for both exercise modalities, the increase in ventilation was generally associated with a 4 significant decrease in IRV and IRV/FVC and an increase in Vt/IC from rest to exercise without 5 significant modification of ERV/FVC and IC/FVC ratios, whatever the exercise performed. These 6 results attested to the absence of dynamic hyperinflation. The absence of significant modification of 7 ERV/FVC and IC/FVC ratios during exercise attested to a specific breathing pattern for children, 8 whatever the exercise modality, in comparison with an adult population. 9 10

Effects of exercise modality on mechanical ventilatory constraints 11

The exercise modality had no effect on breathing reserve, as similar values were found during 12 continuous and intermittent exercises for the three intensity levels. Tidal volume regulation was not 13 influenced by a "modality" effect either. Following statistical analysis, exercise modality was noted to 14 have no effect on the occurrence and/or the severity of expFL. This absence of "modality" effect 15 invalidated our hypothesis. In fact, as higher intensities are frequently used during intermittent 16 exercises, we had hypothesized that the higher ventilatory demand induced by the high-intensity 17 intermittent exercises would induce greater mechanical ventilatory constraints. The absence of a 18 "modality" effect for ventilation could be an explanation for the absence of a "modality" effect on 19 mechanical ventilatory constraints. 20 21 In the present study, the assessment of the ventilatory response corresponded to an intermittent 22 exercise design comprising 15s of exercise followed by 15s of passive recovery. However many 23 modalities of intermittent exercises exist, such as 30s -30s, with passive or active recovery 24 . Our 24 results may only be relevant for this intermittent exercise design. In fact, any change in parameters 25 defining intermittent exercises could modify the ventilatory response values and so alter the 1 breathing pattern, breathing strategy and the occurrence of mechanical ventilatory constraints 2 during intermittent exercises. Yet, on the other hand, the 15s -15s design was chosen as this 3 intermittent exercise design has previously been used quite often in the literature and has generally 4 been well accepted by children 24,31 . 5 6 In conclusion, exercise modality does not seem to be a predominant factor in mechanical ventilatory 7 constraint occurrence during exercise. Nevertheless, the exercise intensity seems to be an important 8 factor in inducing mechanical ventilatory constraints, as the number of occurrences and the severity 9 increased with intensity increased. We could conclude that neither of the modalities studied induced 10 more or greater mechanical ventilatory constraints than the other, but exercise intensities specific to 11 each modality might be greater sources of exacerbation for mechanical ventilatory constraints. 12 We thank all the children and their parents who took part in this study for their participation and 16 their devotion. Data quality assurance was assumed by Clinical Investigation Center of Lille (CIC-9301-17 Inserm-CHRU). Regulatory aspects were performed by the designed ''Good Clinical Practices 18 sponsor'' as University Regional Centre Hospital (CHRU). Values are expressed as mean ± SD. Graph 3.a : mean expiratory flow limitation for the whole population. Graph 3.b : mean expiratory flow limitation for the flow-limited group. ExpFL corresponds to expiratory flow limitation expressed in percentage of tidal volume that meets or exceeds maximal flow/volume loop. Thus, the flow limit group was composed of the children who showed an expFL during exercise. CE60, CE70 and CE80 correspond to continuous exercise at 60%, 70% and 80% of maximal aerobic speed; IE90, IE100 and IE110 to intermittent exercise at 90%, 100% and 110% of maximal aerobic speed. Benoit BOREL [1][2] ; Erwan LECLAIR 1-2 ; Delphine THEVENET 3 ; Laurent BEGHIN [4][5] , Frédéric GOTTRAND 5 and Claudine FABRE 1-2 .

:

Univ Lille Nord de France, F-59000 Lille, France. Results: An "intensity" effect was found for E

V & during CE (p<0.001) but not during IE (p=0.08). The 13 increase in E V & was predominantly assumed by an increase in f for both exercise modalities. During 14 each exercise, several children heterogeneously experienced expFL ranging between 10 -90%Vt. For 15 all exercises, Vt was predominantly regulated by an increase in Vt/IC with no change in IC/FVC from 16 rest to exercise. Finally, no significant "modality" effect was found for mechanical ventilatory 17 constraint parameters (expFL, VT/IC and IV/FVC). 18

Discussion: We could conclude that neither of the modalities studied induced more mechanical 19 ventilatory constraints than the other, but that exercise intensities specific to each modality might be 20 greater sources of exacerbation for mechanical ventilatory constraints. 21

Introduction 1 2
In the literature, it has been reported that children show specific ventilatory adaptations in 3 comparison with adults during exercise. In fact, children present a phenomenon of hyperventilation 4 for a similar level of relative intensity in comparison with adults. This phenomenon of 5 hyperventilation in children is characterized by a higher respiratory equivalent in oxygen (

E V & / 2 O V & ) 6
in children than in adults [1][2][3][4] and could result from a specific breathing pattern. The latter is 7 characterized by rapid and shallow breathing in children, defined by higher values of respiratory 8 frequency and lower values of tidal volume than in adults 5 . 9 10 Moreover, as already shown in adults during incremental exercise, mechanical ventilatory constraints 11 were also highlighted in children during exercise [6][7][8][9][10][11][12] . Such constraints represented the balance 12 between ventilatory demand and ventilatory capacity 6 and could easily be identified by estimating 13 breathing reserve (BR) 6 . However, BR provides no information on breathing pattern, or on the 14 source and type of ventilatory constraints. This type of information could be obtained by using the 15 exercise flow/volume loops method, consisting in plotting tidal flow/volume loops within maximal 16 flow/volume loops during exercise 6 . By using this method, some studies have assessed mechanical 17 ventilatory constraints in children during incremental exercise and reported several cases of 18 expiratory flow limitation during high intensities of the incremental exercise 13,14 . Thus, Nourry et al 19 reported an "intensity" effect for mechanical ventilatory constraint occurrence and severity during a 20 maximal graded test in trained or untrained prepubescent children 13,14 . From these results, we have 21 hypothesized that intermittent exercise could induce higher mechanical ventilatory constraints than 22 continuous exercise, as the exercise intensities prescribed for intermittent exercises are higher than 23 those prescribed for continuous exercises. The answers to this question could enlighten us on 24 specific ventilatory adaptations in children during two exercise modalities frequently encountered 25 The aim of this study was 1) to evaluate and compare breathing pattern and tidal flow/volume loops 4 between continuous and intermittent exercises in healthy prepubescent children, so as to determine 5 if both exercise modalities could induce mechanical ventilatory constraints and 2) to determine the 6 effects of the exercise modality on the severity of potential ventilatory constraints, in order to 7 determine if one exercise modality was more restrictive than the other on the children's ventilatory 8 system. 9 had previously received approval from the local ethics committee. Parents and children were 5 informed as to the modalities of the study and then gave their written, informed consent prior to any 6 involvement in it. All the children were free of respiratory or cardiac diseases and of orthopedic 7 counter indications for physical exercise, performed less than 3h/week of extra academic physical 8 activity and their sum of pubertal stages was lower than 3. Sexual maturity was evaluated from 9 pubertal stages, according to Tanner's method: indices of breast, pubic hair and genital 10 development 15 . The same physician made all the observations for all the children. 11 12

Study design 13

All the children came to the laboratory three times within a one-month period, with at least 48 hours 14 between two evaluation days. At the beginning of each visit, a resting ECG and a clinical examination 15 were performed. During the first visit, spirometry and sexual maturity were evaluated, followed by a 16 period of familiarization and realization of a maximal graded test on a treadmill. Measurement of 17 pulmonary gas exchanges allowed the determination of peak oxygen uptake ( At each visit, the children underwent spirometry before each of the exercises performed (Vmax 1 Encore, Sensormedics, Viasys Healthcare, USA). Each child was seated on a chair and fitted with a 2 noseclip. All the lung tests were carried out by the same physician respecting American Thoracic 3 Society recommendations 16 . For each child, three measurements of maximal flow/volume loops 4 (MFVL) were carried out, allowing the determination of forced vital capacity (FVC) and forced 5 expiratory volume in 1 second (FEV 1 ). Among the registered measurements, the MFVL with the 6 greatest combination of FVC and FEV 1 was retained. 7 8

Cardio-respiratory parameters 9

At each visit and during all exercise tests, the children wore a mouthpiece connected to a breath-by-10 breath gas analyzer (V max Encore laboratory system, Sensormedics, Viasys Healthcare, USA), 11 allowing the measurement of oxygen uptake (

2 O V & ), carbon dioxide output ( 2 CO V &
), ventilation 12 ( E V & ), tidal volume (Vt) and respiratory frequency (f). In order to ensure accurate measurements, O 2 13 and CO 2 analyzers were calibrated before each exercise test, using ambient air and two gases of 14 known O 2 concentration (16% and 26%) and CO 2 concentration (4% and 0%). The calibration of the 15 pneumotachograph was performed using a 3-liter syringe following the manufacturer's 16 recommended procedures. 17

Heart rate (HR) was continuously monitored with a heart rate monitor (Polar S810, Polar Electro Oy, 18 Kempele, Finland). The data acquisition frequency for HR was set at 5 sec. 19 Borg's CR-10 scale 17 (ranging from 0 to 10) was used during all the exercises in order to evaluate 20 dyspnea. This scale was explained to the children at the beginning of each visit and was shown to 21 them at the end of each exercise. A period of familiarization on the treadmill was proposed for all the children, consisting in explaining 1 the security procedures and letting them walk and run on it. After explaining the exercise to the 2 children, the test began with a resting period of three minutes in a standing position on the treadmill. 3 Subsequently, a warm-up period of 3 minutes at 3km.h -1 was performed. The exercise began with 4 one minute at 4 km.h -1 , then the speed was increased by 0.5 km.h -1 every minute. During the whole 5 exercise, pulmonary gas exchanges and heart rate were continuously recorded and the values were 6 averaged every 30 seconds. The exercise ended when the children were no longer able to maintain 7 the imposed running speed. It was judged that they had performed a maximal exercise when 3 or 8 more of the following criteria were obtained: visible exhaustion (excessive hyperpnea, facial flushing, 9 sweating, discomfort) 18 , maximal heart rate >90% of predicted maximal heart rate (208-0.7 x age) 19 , a 10 plateau in

O

V & despite increasing running speed or a final respiratory exchange ratio (RER) higher 11 than 1.0 20 . The maximal aerobic speed (MAS), corresponding to the speed at the last completed 12 stage 21 , was used as a reference value for the calculation of the individualized speeds of the 13 continuous and intermittent exercises. 14 15

Continuous and intermittent visits 16

At each of the two other visits, the children performed 3 continuous (CE) or 3 intermittent exercises 17 (IE). Figure 1 shows the design for each exercise-testing visit. A resting period of three minutes was 18 firstly observed First, children rested for three minutes. Then, a 6-minute exercise period was carried 19 out, immediately followed by a 4-minute period of passive recovery, with measurement of gas 20 exchanges. After that, the mouthpiece of the pulmonary gas exchange apparatus was removed until 21 the next exercise. Between two exercises, a 30-minute rest period was observed, without taking any 22 measurements. For the continuous exercises, the 6-minute exercise period was performed at an 23 individualized and constant speed. For intermittent exercises, the exercise period was divided into 24 periods of 15s of running followed by 15s of passive recovery, except for the last minute of each 25 intermittent exercise which was divided into one 30s period of running and one 30s period of passive 1 recovery, so as to perform exercise tidal flow/volume loop measurement. In order to avoid the 2 inertia of the treadmill for the two exercise modalities, the treadmill belt was set in motion at the 3 adequate running speed before the beginning of the exercise period. The children were instructed to 4 put their feet on both sides of the belt of the treadmill and to get on the belt at the beginning of the 5 exercise period. For intermittent exercises, as the treadmill was not stopped at the end of each 15-6 second running period, the children were instructed to get on or off the treadmill after each 15-7 second period of running or passive recovery. Leaving the treadmill consisted in putting their feet on 8 both sides of it. Hence, the children were familiarized with getting on or off the treadmill before the 9 intermittent exercises and were helped by an adult. 10

Continuous exercises intensities corresponded to 60% (CE60), 70% (CE70) and 80% (CE80) of MAS. 11

These intensities were always performed in this order. The intermittent exercise intensities 12 corresponded to 90% (IE90), 100% (IE100) and 110% (IE110) of MAS and here also, this order was 13 kept for all the subjects. These MAS intensity percentages were chosen as they are frequently used 14 for continuous and intermittent exercises respectively 22 . Three intensity levels could be described: 15 low (CE60 vs. IE90), middle (CE70 vs. IE100) and high (CE80 vs. IE110). 16 For all the continuous and intermittent exercises performed by the children, the mean values of 17

2 O V & , E V & , Vt, f, E V & / 2 O V & , E V & / 2 CO V &
and HR, reported in the "Results" section, were obtained 18 by averaging raw data during the first 30 seconds of the last minute of each exercise. This period of 19 measurement corresponded to the time of exercise flow/volume loop measurement. The MVV was estimated for each exercise performed, with the FEV 1 measured just before the 8 exercise. 9

In order to evaluate mechanical ventilatory constraints and breathing strategy during the exercise, 10 exercise tidal flow/volume loops (tidal F/V loops) were recorded just once, at the end of the first 30 11 seconds of the last minute of each exercise using a pneumotachograph. Tidal F/V loops corresponded 12 to a mean of ten tidal breaths recorded and were computer averaged before a maximal inspiratory 13 capacity (IC) maneuver. This maneuver allows correction of drift phenomena, as described by 14 Johnson et al 6 . During exercise, a drift phenomenon could appear in the volume signal due to 15 electrical or physiological changes over time. This drift must be corrected in order to obtain correctly 16 placed tidal F/V loops within MFVL. Thus, the maximal inspiratory capacity maneuver was performed 17 immediately after tidal F/V loop measurement. This maximal IC maneuver was compared to the 18 maximal IC determined at rest. After the maneuver, the computer could correctly plot tidal F/V loops 19 within the MFVL determined before the exercise. For each tidal F/V loop measurement, the mean 20

values of 2 O V & , E V & , Vt, f, E V & / 2 O V & , E V & / 2 CO V &
and HR were calculated at the moment of 21 measurement (corresponding to the mean of the first 30 seconds of the last minute of exercise). 22

Expiratory flow limitation (expFL) corresponds to the part of tidal F/V loop that meets or exceeds the 23 expiratory boundary of the MFVL and is expressed as a % of tidal volume (%Vt). In order to 24 determine breathing strategy, the regulation evolution of different ratios was analyzed. Hence, 25 6 , expiratory reserve volume relative 1 to vital capacity (ERV/FVC), tidal volume relative to inspiratory capacity (Vt/IC) 6 and inspiratory 2 reserve volume relative to vital capacity (IRV/FVC) were estimated computed (Figure 2). 3 4

Statistical analysis 5

All the values were expressed as means ± standard deviation (mean ± SD). For each parameter, data 6 normality was verified with a Kolmogorov-Smirnov test. 7

In order to determine the effect of exercise modality, a two-way repeated measure analysis of 8 variance (ANOVA) was performed for all the parameters. The factors were "exercise intensity" and 9 "exercise modality". When the ANOVA F ratio was significant, the mean values were compared by 10 using pairwise multiple comparison procedures (Bonferroni post-hoc test). 11

For intra-modality comparison, a one-way repeated measure analysis of variance (ANOVA) was 12 performed for all the parameters. The factor was "exercise intensity". When the ANOVA F ratio was 13 significant, the mean values were compared by using pairwise multiple comparison procedures 14 (Bonferroni post-hoc test). 15 For rest versus exercise values comparison within the same modality, a "paired t-test" was carried 16 out. 17 Respiratory data from one subject were unworkable. Thus, anthropometric, spirometric and maximal 4 exercise values of the eleven children were analyzed (8 boys and 3 girls; Table 1). For each child, 3 5 criteria of maximality at least were fulfilled, showing that the graded exercise was maximal. 2. For

2 O V &
only, a significant "intensity x 9 modality" interaction and an "intensity" effect were highlighted. However, post-hoc tests revealed 10 no significant difference between continuous and intermittent exercises within the three different 11 intensity levels (ie. CE60 vs. IE90, CE70 vs. IE100 and CE80 vs. IE110), allowing the comparison of the 12 ventilatory data obtained during each exercise modality. For HR, an significant intensity effect was 13 highlighted for both modalities, without "modality" effect. Finally, no effect of the exercise modality 14 and intensity was found for Respiratory Exchange Ratio (RER). 15 16 Ventilatory response to exercise 17 18

Breathing pattern 19

The mean values of ventilation are shown in Table 3. Two-way ANOVA analysis highlights "intensity" 20 and "modality x intensity" interaction effects for ventilation (p<0.05). An "intensity" effect (p<0.001) 21 was found for E

V & in continuous exercise (p<0.001) but not in intermittent exercise (p=0.08). either CE or IE modalities. Concerning respiratory frequency (f), an "intensity" effect was found for IE 1 (p<0.05) and a tendency towards significant effect for CE (p=0.06). No "modality" effect was found 2 for this parameter. No significant difference was highlighted between the two groups for breathing pattern (Vt and f) in 21 any of the exercises. 22 Breathing reserve was significantly lower in the FL than the NFL group during continuous exercise, for 23 CE70 (30.8 ± 11.6 for FL and 48.8 ± 10.2 for NFL; p=0.02) and CE80 (21.8 ± 10.1 for FL and 42.1 ± 10. 9 24 for NFL; p=0.01). During all intermittent exercises, no significant difference for BR was found 25 13 between FL and NFL groups for IE90 (38.8 ± 1.2 for FL and 50.9 ± 10.2 for NFL), IE100 (33.9 ± 7.9 for 1 FL and 45.2 ± 12.2 for NFL) and IE110 (24.2 ± 22.2 for FL and 46.7 ± 13.9 for NFL). The mean values of resting and exercise pulmonary parameters are recapitulated in Table 4. IRV 5 decreased significantly from rest to exercise for all continuous or intermittent exercises (p<0.05). ERV 6 increased significantly from rest to exercise for CE70, IE100 and IE110 (p<0.05). For IC, a significant 7 difference was observed between rest and exercise for CE70, IE100 and IE110 (p<0.05). No significant 8 differences were found for IC/FVC and ERV/FVC between rest and exercise values, except for IE110 9 (p<0.05). IRV/FVC decreased significantly between rest and exercise for all the exercises. In the same 10 way, Vt/IC was significantly higher than at rest for all the exercises except for CE70 and IE100 11 (p<0.05). 12 13 For all the parameters of mechanical ventilatory constraints (expFL, Vt/IC, IC/FVC, IRV/FVC and 14 ERV/FVC), there were no significant "modality" and "intensity" effects (Table 4 and Figure 3.a). 15 However, an "intensity" effect tended to be significant for expFL during continuous exercise 16 (p=0.065). 17 The purpose of the study was to evaluate the ventilatory response during continuous and 2 intermittent exercises and to determine if one of these two exercise modalities could induce greater 3 mechanical ventilatory constraints than the other. Our results showed that both exercise modalities 4 (continuous and intermittent) could induce mechanical ventilatory constraints in healthy 5 prepubescent children, as several children presented expiratory flow limitation (expFL) during 6 continuous and intermittent exercises with no influence due to exercise modality. 7 8 Some limits should be considered for this study, mainly concerning recorded exercise flow/volume 9 loops. As mentioned by some authors 6,13 , expFL assessment is highly dependent on the placement of 10 exercise flow/volume loop within maximal flow/volume loop. This limit has been taken in account 11 with the realization of inspiratory capacity (IC) maneuvers after exercise flow/volume loop 12 measurement, allowing drift correction. The IC maneuver has already been used to enhance 13 reproducibility of the flow/volume loop method 6 . 14 15 As described above, the aim of the study was to compare mechanical ventilatory constraints 16 (expiratory flow limitation and dynamic hyperinflation) and breathing pattern (ventilation, tidal 17 volume and breathing frequency) during continuous and intermittent exercises with exercise 18 intensities inducing similar metabolic demand. Even if a significant interaction for oxygen uptake was 19 highlighted, it appears that, within the different levels of exercise intensity (low, medium and high), 20 the metabolic demand (

Effects of exercise intensity on breathing pattern 1

As expected, ventilation for the continuous exercise modality increased with greater exercise 2 intensity. However, during intermittent exercises, ventilation increased slightly, but not significantly, 3 with the increase in exercise intensity. These specific ventilation evolutions according to exercise 4 modality could be due to the fact that, during 15s -15s intermittent exercises performed by the 5 children in this study, the time of running was rather short. Hence, the ventilatory system would not 6 seem to have enough time to adapt to the ventilatory demand induced by the exercise intensity. 7

However, within the three intensity levels (i.e. low, medium or high), no significant difference was 8 highlighted between continuous and intermittent exercises. These results could logically be 9 explained since the intensities of intermittent exercises were specifically chosen in order to be 10 matched with continuous exercise intensities in terms of energy expenditure (i.e. oxygen uptake), as 11 already reported by Berthoin et al. 24 The absence of significant difference for ventilation between 12 both exercise modalities was also required in order to compare breathing pattern components (Vt 13 and f) correctly. In fact, several studies have reported that for a particular subject, the regulation of 14 the breathing pattern at a given level of ventilation is largely independent of the cause of the 15 increased ventilation 25,26 . 16 During all the continuous and the intermittent exercises, our results showed that the increase in 17 ventilation was mainly due to an increase in respiratory frequency. This result supports the findings 18 in the literature 5 and revealed that, whatever the exercise modality, children presented rapid and 19 shallow breathing. However, differences in ventilatory regulation according to exercise modality 20 seemed to exist, as a "modality" effect was found for ventilatory equivalent in carbon dioxide 21

( E V & / 2 CO V &
). Post-hoc analysis revealed that, in general, intermittent exercise modality induced 22

higher E V & / 2 CO V &
values than continuous exercise modality. This could be explained firstly, by a 23 chemical regulation due to a higher acidosis level during intermittent exercise and, secondly, by a 24 As described by Johnson et al 6 , the assessment of breathing reserve (BR) could be an indicator of the 5 occurrence of mechanical ventilatory constraints during exercise. In all the previous studies reporting 6 BR values in children, the exercise performed has been incremental 13,14,[28][29][30] . This difference in 7 protocols makes it impossible to compare our results with the previous studies, which is normal, 8 since this is the originality of our study. In our study, BR significantly decreased during CE and IE 9 whereas there was no "intensity" effect for E

V & during IE. This result is surprising and reinforces the 10 work of Johnson et al. 6 asserting that breathing reserve is a general and inappropriate parameter for 11 the study of mechanical ventilatory constraints during exercise. 12 The study of mechanical ventilatory constraints with exercise flow/volume loops brings more 13 information on the adaptation of the ventilatory system during exercise. 14

In our study, the number of expiratory flow limitation cases increased with exercise intensity in both 15 exercise modalities. For the highest intensity exercises, more than 50% of the whole population (6 of 16 the 11 children studied) showed an expiratory flow limitation. Individual expiratory flow limitation 17 ranged between 10 -90% of Vt. These values are in accordance with values of expFL found in the 18 literature. In fact, Nourry et al. 13 reported an increase in the number of observations of expFL in non-19 athlete prepubescent children (8 children out of the 18 studied) with the increase in intensity during 20 the maximal graded test, ranging between 16 -78% of Vt. The occurrence of expFL during sub-21 maximal (CE60, CE70, CE80 and IE90) and supra-maximal (IE100, IE110) exercises is not surprising, 22 since a previous study has reported the case of expFL from 30% of maximal aerobic power 13 and 23

intensities prescribed in our study were all higher than this threshold value. As with ventilatory 24 response during exercise, the occurrence of expFL in our population was heterogeneous as some 25 children experienced expFL during IE90 and IE110 but not during IE100 for example. The 1 heterogeneity of expFL occurrence has also previously been described in children 13 and could be 2 explained by the different levels of ventilatory system maturation. 3 Thus, for both exercise modalities, the increase in ventilation was generally associated with a 4 significant decrease in IRV and IRV/FVC and an increase in Vt/IC from rest to exercise without 5 significant modification of ERV/FVC and IC/FVC ratios, whatever the exercise performed. These 6 results attested to the absence of dynamic hyperinflation. The absence of significant modification of 7 ERV/FVC and IC/FVC ratios during exercise attested to a specific breathing pattern for children, 8 whatever the exercise modality, in comparison with an adult population. 9 10

Effects of exercise modality on mechanical ventilatory constraints 11

The exercise modality had no effect on breathing reserve, as similar values were found during 12 continuous and intermittent exercises for the three intensity levels. Tidal volume regulation was not 13 influenced by a "modality" effect either. Following statistical analysis, exercise modality was noted to 14 have no effect on the occurrence and/or the severity of expFL. This absence of "modality" effect 15 invalidated our hypothesis. In fact, as higher intensities are frequently used during intermittent 16 exercises, we had hypothesized that the higher ventilatory demand induced by the high-intensity 17 intermittent exercises would induce greater mechanical ventilatory constraints. The absence of a 18 "modality" effect for ventilation could be an explanation for the absence of a "modality" effect on 19 mechanical ventilatory constraints. 20 21 In the present study, the assessment of the ventilatory response corresponded to an intermittent 22 exercise design comprising 15s of exercise followed by 15s of passive recovery. However many 23 modalities of intermittent exercises exist, such as 30s -30s, with passive or active recovery 24 . Our 24 results may only be relevant for this intermittent exercise design. In fact, any change in parameters 25 defining intermittent exercises could modify the ventilatory response values and so alter the 1 breathing pattern, breathing strategy and the occurrence of mechanical ventilatory constraints 2 during intermittent exercises. Yet, on the other hand, the 15s -15s design was chosen as this 3 intermittent exercise design has previously been used quite often in the literature and has generally 4 been well accepted by children 24,31 . 5 6 In conclusion, exercise modality does not seem to be a predominant factor in mechanical ventilatory 7 constraint occurrence during exercise. Nevertheless, the exercise intensity seems to be an important 8 factor in inducing mechanical ventilatory constraints, as the number of occurrences and the severity 9 increased with intensity increased. We could conclude that neither of the modalities studied induced 10 more or greater mechanical ventilatory constraints than the other, but exercise intensities specific to 11 each modality might be greater sources of exacerbation for mechanical ventilatory constraints. 12 We thank all the children and their parents who took part in this study for their participation and 16 their devotion. Data quality assurance was assumed by Clinical Investigation Center of Lille (CIC-9301-17 Inserm-CHRU). Regulatory aspects were performed by the designed ''Good Clinical Practices 18 sponsor'' as University Regional Centre Hospital (CHRU). Values are expressed as mean ± SD. Graph 3.a : mean expiratory flow limitation for the whole population. Graph 3.b : mean expiratory flow limitation for the flow-limited group. ExpFL corresponds to expiratory flow limitation expressed in percentage of tidal volume that meets or exceeds maximal flow/volume loop. Thus, the flow limit group was composed of the children who showed an expFL during exercise. CE60, CE70 and CE80 correspond to continuous exercise at 60%, 70% and 80% of maximal aerobic speed; IE90, IE100 and IE110 to intermittent exercise at 90%, 100% and 110% of maximal aerobic speed.

F o r P e e r R e v i e w

  life, contrary to current data from the literature, which is only based on incremental 1 exercise. 2 3

2 OV

 2 & peak ) and maximal 18 aerobic speed (MAS). During the last two visits, the children performed three continuous (continuous 19 visit) and three intermittent (intermittent visit) exercises on the treadmill in a random order. Prior to 20 the exercises, spirometry was performed and, subsequent to them, measurements of pulmonary gas 21 exchanges, heart rate and exercise flow/volume curves were carried out.22

22

 22

20 21Determination of ventilatory constraints 22 For

 2022 each continuous and intermittent exercise, ventilatory constraints were firstly determined with 23 the calculation of breathing reserve (BR). BR was calculated based on maximal E V & reached during 24 the exercise and the estimated maximal voluntary ventilation (MVV). It corresponded to how closely 25 determined according to the following equation 6,23 : MVV = FEV 1 x 35 and E V & max 6 corresponded to the maximal value of E V & reached during each continuous or intermittent exercise. 7

  ) relative to forced vital capacity (IC/FVC)

For 1 2

 1 all the tests, significance was set at the 0.05 level. 18Anthropometric, spirometric and maximal values at incremental exercise 3

6 7Mean cardiorespiratory values ( 2 O

 62 V & , HR, RER) measured during continuous and intermittent 8 exercises for the whole population are shown in Table

3 4ventilatory constraints 5 - 13 -

 3513 Mechanical Expiratory flow limitation 6 Expiratory flow limitation (expFL) was experienced by 1 child during CE60, 6 children during CE70 and 7 6 children during CE80. Concerning intermittent exercise, expFL was observed for 3, 4 and 6 children 8 during IE90, IE100 and IE110 respectively (Figure 3.b). Individual values of ExpFL ranged between 10 9 -90% of Vt. Only one child showed expFL during all the continuous and intermittent exercises. Mean 10 values of expFL for the whole population ranged from 5% to 29% of Vt and from 13% to 23% of Vt for 11 the continuous and intermittent exercises respectively (Figure 3.a). 12 Flow-limited (FL) vs. non flow-limited (NFL) comparison 14 Within each exercise, the population was divided into two groups according to the presence or 15 absence of an expiratory flow limitation (expFL) during exercise. The children who presented expFL 16 made up the flow-limited group (FL group). The non flow-limited group (NFL) was composed of the 17 other children. 18 For continuous exercise at 60% of MAS, only one child experienced an expFL versus 10 children 19 making up the NFL group, which made comparison impossible. 20 No significant difference was highlighted between the two groups for breathing pattern (Vt and f) in 21 any of the exercises. 22 Breathing reserve was significantly lower in the FL than the NFL group during continuous exercise, for 23 CE70 (30.8 ± 11.6 for FL and 48.8 ± 10.2 for NFL; p=0.02) and CE80 (21.8 ± 10.1 for FL and 42.1 ± 10.9 24for NFL; p=0.01). During all intermittent exercises, no significant difference for BR was found 25

2 3-

 2 Flow volume loops and tidal volume regulation. 4

2 OV

 2 & ) was matching for the pair of tasks concerning the two exercise 21 modalities. In other words, no significant difference was highlighted for IE100 and CE80 vs. IE110, allowing the comparison of the breathing pattern data and 23 mechanical ventilatory constraints data obtained during each exercise.24

2 3

 2 during intermittent exercise which could induce a greater 1 ventilatory response27 .Effects of exercise intensity on mechanical ventilatory constraints 4

  Financial support was assumed by a grant 19 from the French Ministry of Health (regional PHRC n° 1904 -2005). 20

Figure 1 :

 1 Figure 1: Design of continuous and intermittent exercise tests.

Figure 2 :

 2 Figure 2: Exercise tidal volume plotted within maximal flow/volume loop

Figure 3 :

 3 Figure 3: Expiratory flow limitation during the six performed exercises.

  of mechanical ventilatory constraints between continuous and intermittent exercises in healthy prepubescent children.

2 : 4 : 5 : 1 Background:

 2451 UDSL, EA 4488, Faculté des Sciences du Sport et de l'Education Physique, F-59790 Ronchin, France. 3 : M2S, ENS Cachan -Antenne de Bretagne, Campus de Ker-Lann, France. Centre d'Investigation Clinique, CIC-9301-CHRU-INSERM de Lille IFR 114 ; Institut de Médicine Prédictive et Thérapeutique de Lille, Lille -France. Interrelations nutrition, inflammation, et infection au cours des maladies respiratoires chroniques de l'enfant (EA 3925), Université de Lille 2 Droit et Santé, Faculté de médecine, Lille -France. Corresponding author : Claudine Fabre, EA 3608,Faculté des Sciences du Sport et de l'Education Physique, 9 rue de l'université 59790 RONCHIN -FRANCE Tel : +33 (0)3 20 88 73 76 / Fax : +33 (0)3 20 88 73 63 / Email : benoit.borel-2@univ-lille2.fr / Running title: Mechanical ventilatory constraints in children The aim of this study was to evaluate the occurrence and severity of mechanical 2 ventilatory constraints in healthy prepubescent children during continuous and intermittent exercise. 3 Methods: Twelve prepubescent children (7 -11 years old) performed 7 exercises on a treadmill: one 4 graded test for the determination of maximal aerobic speed (MAS), three continuous exercises (CE) 5 at 60, 70 and 80% of MAS and three intermittent exercises (IE), alternating 15s of exercise with 15s of 6 passive recovery, at 90, 100 and 110% of MAS. During each CE and IE, tidal flow/volume loops were 7 plotted within a maximal flow/volume loop (MFVL) measured at rest before each exercise. Expiratory 8 flow limitation (expFL expressed in %Vt) was defined as the part of exercise tidal volume (Vt) meeting 9 the boundary of MFVL. Breathing strategy was estimated by measuring inspiratory capacity relative 10 to forced vital capacity and tidal volume relative to inspiratory capacity. Other breathing pattern 11 parameters (ventilation VE, Vt, respiratory frequency f) were continuously recorded during exercise. 12

  life, contrary to current data from the literature, which is only based on incremental

3 Twelve 7 -

 37 11 year old healthy prepubescent children volunteered to participate in this study, which 4

2 OV

 2 & peak ) and maximal 18 aerobic speed (MAS). During the last two visits, the children performed three continuous (continuous 19 visit) and three intermittent (intermittent visit) exercises on the treadmill in a random order. Prior to 20 the exercises, spirometry was performed and, subsequent to them, measurements of pulmonary gas 21 exchanges, heart rate and exercise flow/volume curves was were carried out.

20 21Determination of ventilatory constraints 22 For

 2022 each continuous and intermittent exercise, ventilatory constraints were firstly determined with 23 the calculation of breathing reserve (BR). BR was calculated based on maximal E V & reached during 24 the exercise and the estimated maximal voluntary ventilation (MVV). It corresponded to how closely 25 determined according to the following equation 6,23 : MVV = FEV 1 x 35 and E V & max 6 corresponded to the maximal value of E V & reached during each continuous or intermittent exercise. 7

  IC) relative to forced vital capacity (IC/FVC)

For 1 2

 1 all the tests, significance was set at the 0.05 level. 18Anthropometric, spirometric and maximal values at incremental exercise 3

6 7Mean cardiorespiratory values ( 2 O

 62 V & , HR, RER) measured during continuous and intermittent 8 exercises for the whole population are shown in Table

3 4ventilatory constraints 5 - 13 -

 3513 Mechanical Expiratory flow limitation 6 Expiratory flow limitation (expFL) was experienced by 1 child during CE60, 6 children during CE70 and 7 6 children during CE80. Concerning intermittent exercise, expFL was observed for 3, 4 and 6 children 8 during IE90, IE100 and IE110 respectively (Figure 3.b). Individual values of ExpFL ranged between 10 9 -90% of Vt. Only one child showed expFL during all the continuous and intermittent exercises. Mean 10 values of expFL for the whole population ranged from 5% to 29% of Vt and from 13% to 23% of Vt for 11 the continuous and intermittent exercises respectively (Figure 3.a). 12 Flow-limited (FL) vs. non flow-limited (NFL) comparison 14 Within each exercise, the population was divided into two groups according to the presence or 15 absence of an expiratory flow limitation (expFL) during exercise. The children who presented expFL 16 made up the flow-limited group (FL group). The non flow-limited group (NFL) was composed of the 17 other children. 18 For continuous exercise at 60% of MAS, only one child experienced an expFL versus 10 children 19 making up the NFL group, which made comparison impossible. 20

2 3-

 2 Flow volume loops and tidal volume regulation. 4

2 OV

 2 & ) was matching between the pair of tasks concerning the two exercise 21 modalities. In other words, no significant difference was highlighted for IE100 and CE80 vs. IE110, allowing the comparison of the breathing pattern data and the 23 mechanical ventilatory constraints data obtained during each exercise.24

2 3

 2 during intermittent exercise which could induce a greater 1 ventilatory response27 .Effects of exercise intensity on mechanical ventilatory constraints 4

  Financial support was assumed by a grant 19 from the French Ministry of Health (regional PHRC n° 1904 -2005). 20

Figure 1 :

 1 Figure 1: Design of continuous and intermittent exercise tests.

Figure 2 :

 2 Figure 2: Exercise tidal volume plotted within maximal flow/volume loop

Figure 3 :

 3 Figure 3: Expiratory flow limitation during the six performed exercises.

  Values are expressed as mean ± SD. Graph 3.a : mean expiratory flow limitation for the whole population. Graph 3.b : mean expiratory flow limitation for the flow-limited group. ExpFL corresponds to expiratory flow limitation expressed in percentage of tidal volume that meets or exceeds maximal flow/volume loop. Thus, the flow limit group was composed of the children who showed an expFL during exercise.CE60, CE70 and CE80 correspond to continuous exercise at 60%, 70% and 80% of maximal aerobic speed; IE90, IE100 and IE110 to intermittent exercise at 90%, 100% and 110% of maximal aerobic speed.

Table 1 :

 1 Anthropometric, spirometric and maximal values at incremental exercise.With BMI : Body Mass Index ; FVC : Forced Vital Capacity ; FEV 1 : Forced Expiratory Volume in 1 Second ; VO 2 peak : peak oxygen uptake ; HRmax : maximal heart rate ; RERmax : maximal respiratory exchange ratio ; MAS : maximal aerobic speed.

	Parameter (units)	Mean ± SD
	Age (years)	8.6 ± 1.4
	Height (cm)	135.2 ± 9.4
	Body mass (kg)	29.0 ± 5.8
	BMI (kg.cm -2 )	15.7 ± 1.5
	FVC (L)	2.1 ± 0.4
	FVC (% of theoritical value)	99.6 ± 11.5
	FEV 1 (L)	1.8 ± 0.4
	FEV1 (% of theoritical value)	101.8 ± 12.3
	VO 2 max (L.min -1 )	1.5 ± 0.4
	VO 2 max (mL.min -1 .kg -1 )	50.3 ± 8.2
	HR max (bpm)	186.5 ± 10.6
	RER max	1.0 ± 0.1
	MAS (km.h -1 )	10.0 ± 1.6

Table 2 :

 2 Cardio-respiratory data during continuous and intermittent exercisesValues are expressed in mean ± SD. CE60, CE70 and CE80 represented continuous exercises at 60%, 70% and 80% of maximal aerobic speed (MAS). IE90, IE100 and IE110 represented intermittent exercises at 90%, 100% and 110% of MAS. VO 2 : oxygen uptake; HR : heart rate; RER : Respiratory exchange ratio.

							Page 44 of 49
	Intensity level	Low		Medium		High
	Exercise	CE60	IE90	CE70	IE100	CE80	IE110
	VO 2 (L.min -1 )	0.92 ± 0.27 $μ	1.03 ± 0.22 μ	1.10 ± 0.27 §	1.19 ± 0.31	1.27 ± 0.33	1.17 ± 0.35
	HR (bpm)	135.0 ± 11.9 $μ	146.4 ± 17.3 μ	152.1 ± 10.6 §	151.2 ± 8.1	163.6 ± 16.8	158.2 ± 11.3
	RER	0.94 ± 0.05	0.94 ± 0.07	0.96 ± 0.04	0.92 ± 0.06	0.98 ± 0.04	0.96 ± 0.06
	$ : p<0.05 for CE60 vs. CE70
	§ : p<0.05 for CE70 vs. CE80
	µ : p<0.05 for CE60 vs. CE80 or IE90 vs. IE110 comparisons

Table 3 :

 3 Respiratory parameters for the continuous and intermittent exercises.Values are expressed in mean ± SD. CE60, CE70 and CE80 represented continuous exercises at 60%, 70% and 80% of maximal aerobic speed (MAS). IE90, IE100 and IE110 represented intermittent exercises at 90%, 100% and 110% of MAS. VO 2 : oxygen uptake; mean VO 2 : mean oxygen uptake of the last two minutes of exercise; VE: ventilation at tidal FV loop measurement; Vt: tidal volume; f: respiratory frequency; VE/VO 2 and VE/VCO 2 : respiratory equivalent in oxygen and in carbon dioxide; BR: breathing reserve. $ : p<0.05 for CE60 vs. CE70 or IE90 vs. IE100 comparisons § : p<0.05 for CE70 vs. CE80 or IE100 vs. IE110 comparisons µ : p<0.05 for CE60 vs. CE80 or IE90 vs. IE110 comparisons

	Intensity level	Low			Middle	High
	Exercise	CE60	IE90	CE70	IE100	CE80	IE110
	V E (L.min -1 )	28.6 ± 7.5 $µ	33.1 ± 6.5	33.7 ± 7.8 §	36.6 ± 8.7	38.7 ± 8.4	36.8 ± 9.7
	Vt (mL)	616.8 ± 230.4	665.6 ± 224.8	665.5 ± 317.5	603.1 ± 145.5	687.1 ± 350.0	585.1 ± 194.8
	f (cycles.min -1 )	49.8 ± 16.5	51.8 ± 12.2 µ	57.4 ± 19.5	63.0 ± 18.4	64.4 ± 22.7	67.4 ± 23.6
	V E /VO 2	31.8 ± 3.3	32.3 ± 3.0	31.2 ± 2.9	31.0 ± 3.4	31.6 ± 3.3	33.2 ± 4.8
	V E /VCO 2	32.4 ± 2.6	32.9 ± 2.9	32.1 ± 3.1	33.2 ± 3.0	31.6 ± 2.7	33.0 ± 2.9
	BR (%)	51.4 ± 12.4 $µ	46.9 ± 9.7 µ	39.0 ± 14.0 §	41.1 ± 11.8	31.1 ± 14.5	34.5 ± 21.5
	Dyspnea	1.1 ± 1.2 $	1.1 ± 1.5 µ	3.1 ± 2.4	2.2 ± 2.2	3.6 ± 2.7	3.5 ± 2.2

F o r P e e r R e v i e w

Table 4 :

 4 Ventilatory response at rest and during exercise.Values are expressed by mean ± SD. CE60, CE70 and CE80 correspond to continuous exercise at 60%, 70% and 80% of maximal aerobic speed; IE90, IE100 and IE110 to intermittent exercise at 90%, 100% and 110% of maximal aerobic. Vt: tidal volume (liters), IRV: inspiratory reserve volume (liters), ERV: expiratory reserve volume (liters), IC: inspiratory capacity (liters), FVC: forced vital capacity (liters). * : p<0.05, ** : p<0.01.

	Continuous exercise

John Wiley & Sons, Inc.Pediatric Pulmonology

John Wiley & Sons, Inc.

Acknowledgements 14

Acknowledgements 14


Mariusz Jasiński

Miroslaw Dors

Helena Nowakowska

Gerietta V Nichipor

Mariusz Jasi Ski

Mirosław Dors

Jerzy Mizeraczyk

Production of hydrogen via

Keywords: microwave plasma, hydrogen production, conversion of hydrocarbons

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Introduction

Recently hydrogen has gained in importance as fuel in fuel cell applications, combustion engines or gas turbines with the goal to achieve more efficient exploitation of energy sources and to reduce noxious emissions. The main substrate for hydrogen production is methane from the natural gas. Methane or natural gas reforming is widely used in industry to obtain hydrogen or synthesis gas (H 2 +CO), which are utilized in industry, for example as source materials for the production of raw chemicals (e.g. methanol and ammonia), as well as hydrogenation agents in oil refinery and reducing gases in steel industry [1][2][3].

Usual reforming of methane is carried out thermally with steam and oxygen where oxidation of methane takes place to provide reaction heat because the methane reforming reaction using steam is endothermic. The main reaction in the steam reforming of methane is the oxidation with steam, yielding a mixture of hydrogen and carbon monoxide [1,3]: CH 4 + H 2 O ↔ CO + 3 H 2 , ∆H 298 =206 kJ mol -1 .

(1)

Since that process is highly endothermic, to decrease activation energy, it requires catalyst, which is usually Ni/Al 2 O 3 working at temperature 1100-1150 K. Hence the reforming system is sensible to impurities in substrates which deactivates catalysts [2,3].

Conventional technologies of hydrogen production, i.e. coal gasification, hydrocarbon reforming and water electrolysis, are too expensive or not applicable for specific applications (e.g. for fuel cells) due to technical reasons. Thus, new methods are under development, like water photolysis, biological and plasma methods [4][5][6][7].

One of attracting methods for conversion hydrocarbons to produce hydrogen is the use of plasmas [1][2][3][5][6][7][8][START_REF] Reveau | Proc. HAKONE VII[END_REF][START_REF] Czernichowski | Proc. 16 th Int. Symp. on Plasma Chemistry[END_REF][START_REF] Sekiguchi | [END_REF][START_REF] Sekiguchi | Proc. 16 th Int. Symp. on Plasma Chemistry[END_REF][START_REF] Kappes | Proc. 8 th Int. Symp. on High Pressure Low Temperature Plasma Chemistry[END_REF][START_REF] Kabashima | [END_REF][START_REF] Futamura | Proc. 4 th Int. Symp. on Non-thermal Plasma Technology for Pollution Control and Sustainable Energy Development[END_REF][START_REF] Pietruszka | [END_REF][17][START_REF] Pietruszka | 16 th Int. Symp. on Plasma Chemistry[END_REF][START_REF] Mutaf-Yardimci | [END_REF][START_REF] Chang | Proc. 4 th Int. Symp. on Non-thermal Plasma Technology for Pollution Control and Sustainable Energy Development[END_REF][START_REF] Kappes | Proc. 4 th Int. Symp. on Non-thermal Plasma 15 Technology for Pollution Control and Sustainable Energy Development[END_REF][START_REF] Deminsky | [END_REF][23]. The plasma contains reactive radicals, ions and highenergetic electrons. High reactivity shown by these species enhances the chemical reaction rates whereby expensive and impurity vulnerable catalysts can be avoided.

These advantages as well as its high energy density ensure the compactness of the plasma reformer. Besides, the plasma system can be adapted for reforming various hydrocarbons, like natural gas, gasoline, heavy oils and biofuels. Fast response time can be also achieved because the plasma is operated by electricity. When steam is used as the plasma supporting gas, reductive and oxidative radicals such as H, OH, and O are produced in the plasma, enabling the plasma to be effective for reforming different hydrocarbons.

Refrigerants based on fluorinated hydrocarbons may be another source of hydrogen. They have been used for many years in air-conditioned systems. Formerly, chlorofluoromethanes (CFCs) with different number of chlorine and fluorine atoms were used in these systems. Unfortunately, these CFCs are potential ozone depleting substances (ODSs). The use of CFCs has been prohibited by the Montreal Protocol since 1987. CFCs used as refrigerants have been replaced by hydrocarbons consisting of carbon, hydrogen and fluorine atoms, i.e. having no chlorine atoms. These chemicals, the so-called HFCs, are considered to be less harmful to the ozone layer. However, it is already foreseen that in the near future also HFCs will be out of use due to their particularly high global warming potential. Thus, efficient methods of destruction of the stored refrigerants, which have been used before, as well as those currently emitted to the atmosphere are in great demand. Since HFCs have hydrogen atoms in their structures, their destruction may be combined with hydrogen production.

Recently developed microwave plasma sources (MPSs) operated at atmospheric pressure [START_REF] Sekiguchi | [END_REF][START_REF] Sekiguchi | Proc. 16 th Int. Symp. on Plasma Chemistry[END_REF][START_REF] Deminsky | [END_REF][23][24][25][26][27][28][29][30][31] seem to have a high potential for hydrogen production via hydrocarbon conversion. The microwave plasma at atmospheric pressure is one of the plasma techniques providing the electron temperature of 4000-10000 K, and the heavy particle temperature of 2000-6000 K [26][27][28][29][30][31].

Our previous investigations in methane conversion to hydrogen [23] were carried out using methane (17.5-175 l min -1 ) and nitrogen swirl (50-100 l min -1 ) at relatively high absorbed microwave powers (3000-5000 W). The best conditions corresponded to the absorbed microwave power of 3000 W and methane flow rate of 175 l min -1 , where the hydrogen mass yield rate and the corresponding energetic hydrogen mass yield were 222 g[H 2 ] h -1 and 74 g [H 2 ] per kWh of microwave energy absorbed by the plasma.

Since the hydrogen production presented in [23] was carried out in the presence of nitrogen without any oxygen carriers, the main chemical reaction producing hydrogen was methane pyrolysis.

Presence of nitrogen as well as methane conversion by-products, i.e. acetylene and unconverted methane, necessitate to separate hydrogen from other gaseous components using a gas separator unit. In order to avoid or to facilitate separation process, when high hydrogen purity is not required, we propose to use hydrocarbon as the only gas entering the plasma generator. Results of methane CH 4 and tetrafluoroethane C 2 H 2 F 4 (HFC-134a) conversion to hydrogen without any other gaseous admixture are presented in this paper.

Experimental setup

The main parts of the experimental setup used in this investigation were: a microwave generator (magnetron), microwave plasma source (MPS), microwave supplying and measuring system, and gas supplying system (Figure 1). The microwave power (2.45 GHz, 6 kW) was supplied from the magnetron to the MPS via a rectangular waveguide (WR-430) having a reduced-height section.

The absorbed microwave power P A (0.6-5 kW), i.e. microwave power delivered to the discharge was calculated as P I -P R , where P I and P R are the incident and reflected microwave powers, respectively. The incident and reflected microwave powers P I and P R were directly measured using directional coupler (MEGA IND. 0G9Y7-66078-704) equipped with HP 478A bolometric heads and HP 432A power meters (Figure 1).

For hydrogen production via hydrocarbon conversion we used the similar waveguide-based nozzleless cylinder-type MPS as in our previous work (Figure 2, [23]). In contrary to the previous work, the processed gaseous hydrocarbons (50-212 l min -1 ) were introduced to the plasma not by the central duct of MPS but in the form of a swirl. The processed hydrocarbons were introduced to the plasma by the four gas ducts (Figures 3) creating the swirl flow in the discharge cylinder. The swirl concentrated near the quartz cylinder wall and stabilized plasma generation. The swirl held the discharge in the centre of the cylinder and thus protected the cylinder wall from overheating. The inner diameter of the used quartz discharge tube was 26 mm.

The diameter of the copper shielding cylinder is 46 mm, so microwave at frequency of 2.45 GHz cannot be guided along the copper shielding cylinder (operation below the cut-off frequency). This causes lower losses of microwave energy, i.e., the higher microwave power is delivered to the unit volume of the plasma. Moreover, the copper shielding cylinder placed coaxially around the quartz discharge cylinder protected the personnel and instrumentation from the electromagnetic radiation.

Important advantages of the presented waveguide-based nozzleless cylinder-type MPS are: stable operation in various gases (including CO 2 , air, nitrogen, methane, HFCs) at high flow rates (hundreds l min -1 , which is similar to other plasma methods operating at high gas flow rate, [2,8,[START_REF] Czernichowski | Proc. 16 th Int. Symp. on Plasma Chemistry[END_REF]), easy initiation of the discharge in various gases without any admixture of noble gases, no need for any special cooling system and for sophisticated impedance matching (e.g., no need for a three-stub tuner).

On-line diagnostics of the working gas composition before and after the microwave plasma processing of hydrocarbons was carried out using gas chromatograph (SRI 8610C, sensitivity towards H 2 was 0.01%), Fourier Transform Infrared (FTIR) spectrophotometer (Nicolet 380 with a gas cell of 10 cm path length, sensitivity towards CH 4 and C 2 H 2 F 4 was 55 ppm and 16 ppm, respectively) and hydrogen detector (Crowcon XGard, sensitivity 0.1%). Gas chromatograph was fixed for the hydrogen measurements using silica gel packed 1 m column of temperature 60ºC and TCD. Calibration was carried out with hydrogen of purity 99.999% mixed with nitrogen of 99.99% purity and injected to the column with 10 port gas sampling valve.

Calibration curve was prepared from the hydrogen peak area measured at the TCD for the hydrogen concentrations in the range of 0-100%. FTIR spectrophotometer was calibrated for CH 4 and C 2 H 2 F 4 separately. Mixture of nitrogen of 99.99% purity and methane of the same purity was flowing through the gas cell when FTIR spectra were recorded. Calibration curve was made from values of peak height at 2885 cm -1 . The same procedure was used for C 2 H 2 F 4 (purity of 98%) calibration using the peak area measured from 3048 cm -1 to 2938.6 cm -1 .

In order to avoid high pressure drop at the outlet of the plasma device, diagnostics of the gas composition was carried out in the by-pass (Figure 2). Production of hydrogen was calculated from hydrocarbons and their conversion products mass balance.

Results

Production of H 2 from CH 4

Diagnostics of the exit gas composition showed that unprocessed methane was found as the only gaseous by-products resulting from methane conversion in microwave plasma. The methane decomposed to hydrogen H 2 and carbon (soot). The soot deposit could be easily noticed on the reactor walls. The soot deposition started just after plasma ignition, however not all soot deposited on the reactor walls. Major part of the soot was blown off the reactor by the high gas flow. As a result, the thickness of the soot layer deposited on the reactor walls has not exceeded 2 mm and did not influence the reactor lifetime.

Concentrations of H 2 and CH 4 in the exit gas depend on the absorbed microwave power. At the power of 1.5 kW concentrations of hydrogen and methane were 99.88% and 0.12%, respectively, whereas at the power of 5 kW they were 99.94% and 0.06%, respectively. Thus, the total methane conversion degree [(CH 4 ) converted / (CH 4 ) initial ×

100 %] was 99.76-99.88% (Figure 4), where (CH 4 ) initial is the total mass of CH 4 , and (CH 4 ) converted is the converted mass of CH 4 . The selectivity of methane conversion to hydrogen [H 2 / 2 (CH 4 ) converted × 100 %] was 100%. Such a high selectivity as well as lack of other gaseous by-products and significant production of soot show that reaction of methane pyrolysis was the main path of methane conversion into hydrogen.

The energetic parameters of the hydrogen production via the methane pyrolysis, i.e. the hydrogen mass yield rate (Figure 5a) and energetic hydrogen mass yield (Figure 5b) were up to 866 g[H 2 ] h -1 and 600 g [H 2 ] per kWh of microwave energy absorbed by the plasma, respectively. The best energetic mass yield of hydrogen corresponds to the absorbed microwave power of 1 kW (Figure 5b). At higher absorbed microwave power, the energetic mass yield of hydrogen is lower due to the fact, that hydrogen mass yield rate is almost independent on the absorbed microwave power (Figure 5a). At absorbed microwave power of 5 kW, the energetic mass yield of hydrogen reached about 150 g [H 2 ] per kWh of microwave energy absorbed by the plasma.

In our experiment, the plug efficiency of the microwave magnetron generator was higher than 66 %, so taking into account this efficiency, the energetic hydrogen mass yield is up to about 400 g[H 2 ] per kWh of electrical energy used.

Comparison of the energetic hydrogen mass yields for different methods in which electric energy is directly used for methane conversion into hydrogen, is given in Table 1.

It must be pointed out that the energetic hydrogen mass yields shown in Table 1 take into account only the electrical energy used in the reforming (in some cases it is not clear either the total electric energy used or absorbed by the plasma is considered). In the plasma methods presented in Table 1, the energy equivalent of methane used in the reforming was not considered.

It is seen from Table 1 that the plasma methods (except the electron beam [START_REF] Kappes | Proc. 8 th Int. Symp. on High Pressure Low Temperature Plasma Chemistry[END_REF] and dielectric barrier discharge [17]) exhibit higher energetic hydrogen mass yield than the conventional water electrolysis [32]. However, when the energy equivalent of methane used in the conversion is taken into account, the energetic hydrogen mass yields for the plasmatron with catalyst [2] and our method, which exhibit the highest yields, are comparable with that of the conventional water electrolysis.

Considering both the cost of methane and the total energy consumption (including losses in power supplies), nowadays, among the hydrogen production methods, it seems that the conventional steam reforming of methane [33] ensures the lowest cost of hydrogen production. However, the conventional steam reforming of methane is a large volume hydrogen production method. When the distributed hydrogen production method are considered, the microwave plasma method presented in this paper seems to be attractive.

Production of H 2 from C 2 H 2 F 4 (HFC-134a)

The on-line analysis of the exit gas composition using FTIR spectrophotometer showed that unprocessed C 2 H 2 F 4 was found as the only gaseous compound detectable by FTIR after the conversion of C 2 H 2 F 4 in the microwave plasma However, since the C 2 H 2 F 4 was decomposed and soot (carbon black) was produced as a solid by-product, it is obvious that other gaseous by-products, i.e. F 2 and H 2 , which are not detectable by FTIR, also had to be in the exit gas. We monitored the presence of H 2 in the exit gas using the hydrogen detector, whereas F 2 in the exit gas was detected (qualitative detection), observing changes of the colour of a paper wetted with potassium iodide and starch solution. The soot deposits could be easily noticed on the reactor walls.

Two series (series 1 and series 2) of measurements were carried out. In series per kWh of microwave energy absorbed by the plasma (Figure 7b). This value corresponds to the C 2 H 2 F 4 flow rate of 97 l min -1 and absorbed microwave power of 0.6 kW. At higher absorbed microwave power, the energetic mass yield of hydrogen is lower due to the fact, that conversion degree of C 2 H 2 F 4 is almost independent on the absorbed microwave power (Figure 6).

The energetic parameters of tetrafluoroethane destruction obtained in the present experiment are superior to those when other plasma methods (e.g. corona [34], barrier discharges [35,36], gliding [37] and arc [38,39,40] discharges, low-power [41][42][43] or moderated-power [27] Our method ensured the best mass yield rate and energetic mass yield. Due to the absence of oxygen/air in the initial gas in our method, resulting in the lack of oxidation reactions, the destruction degree (84 %) of the processed tetrafluoroethane was lower than in our previous investigations and when using DC arc plasma torches.

Conclusions

The results of this investigations show that hydrogen may be produced by pyrolysis of both methane and tetrafluoroethane.

When methane flow rate was 87.5 l/min, the total methane conversion degree was The conversion degree of both hydrocarbons and H 2 mass yield rate are almost independent on the absorbed microwave power. When increasing the microwave power delivered to the discharge, the electron density and electron temperature increase, but the gas temperature is constant. This confirms that thermal pyrolysis is the main path of hydrocarbon conversion.

The proposed atmospheric pressure microwave plasma system for hydrogen production via hydrocarbons pyrolysis is expected to be of low cost and effective, and thus promising for applications in the distributed hydrogen production.

The absence of oxygen compounds as by-products in the off-gas is important

Hydrogen production method

Initial composition

Figure 1 .

 1 Figure 1. Photo of the experimental setup with the waveguide-based nozzleless

Figure 2 .

 2 Figure 2. Sketch of the waveguide-based nozzleless cylinder-type MPS.

Figure 3 .

 3 Figure 3. Bottom view on the plasma (a) with a swirl flow and four ducts creating swirl

Figure 4 .

 4 Figure 4. Conversion degree of methane resulting from methane conversion using the

Figure 5a. H 2

 2 Figure 5a. H 2 mass yield rate as a function of absorbed microwave power. CH 4 flow rate: 50 -212 l min -1 . H 2 mass yield rate resulted from the H 2 concentration

Figure 5b .

 5b Figure 5b.Energetic mass yield rate of H 2 as a function of absorbed microwave power. CH 4 flow rate: 50 -212 l min -1 .

Figure 6 .

 6 Figure 6. Conversion degree of tetrafluoroethane as a function of absorbed microwave

Figure 7a. H 2

 2 Figure 7a. H 2 mass yield rate as a function of absorbed microwave power. HFC-134a flow rate: 50 -212 l min -1 . H 2 mass yield rate resulted from the H 2

Figure 7b .

 7b Figure 7b.Energetic mass yield rate of H 2 as a function of absorbed microwave power. HFC-134a flow rate: 50 -212 l min -1 .

Figure 1 (Figure 2 (Figure 3 (Figure 4

 1234 Figure 1 (Fig1_Jasinski.tif)

  1 the C 2 H 2 F 4 treatment was performed at constant flow rate (50 and 97 l min -1 ) for various absorbed microwave powers. In series 2 the C 2 H 2 F 4 treatment was carried out for selected values of C 2 H 2 F 4 flow rate and absorbed microwave power. The conversion degree of C 2 H 2 F 4 after plasma treatment as a function of absorbed microwave power is The mass yield rate of H 2 is also almost independent of the absorbed microwave power P A at constant C 2 H 2 F 4 flow rate. The reason of such behaviour is the constant gas temperature in the plasma versus the microwave power delivered to the discharge at constant C 2 H 2 F 4 flow rate. The best energetic mass yield of hydrogen reached 670 g[H 2 ]

	shown in figure 6. At a power of 0.6 -3 kW and C 2 H 2 F 4 flow rate of 50 -212 l min -1 ,
	total C 2 H 2 F 4 conversion degree [(C 2 H 2 F 4 ) converted / (C 2 H 2 F 4 ) initial × 100 %] was 63.3 -
	84.1 %.
	The selectivity of C 2 H 2 F 4 conversion to hydrogen [H 2 / (C 2 H 2 F 4 ) converted × 100 %]
	was 99.99% in all cases. Such a high selectivity as well as lack of other gaseous
	hydrogen containing by-products and significant production of soot show that thermal
	dissociation of C 2 H 2 F 4 was the main path of C 2 H 2 F 4 conversion into the hydrogen and
	fluorine.
	The H 2 mass yield rate and energetic mass yield calculated from the mass balance,
	are shown in figures 7a and 7b. The mass yield rates of products reached 0.7 kg[H 2 ] h -1
	for H 2 .

  microwave torch discharges) were employed for destruction of HFCs. Among the other plasma methods, the best energetic parameters (C 2 H 2 F 4 destruction degree, C 2 H 2 F 4 mass yield rate and C 2 H 2 F 4 energetic mass yield) of C 2 H 2 F 4 destruction were achieved by Watanabe et al.[39] and Ohno et al.[40] using DC arc plasma torches. There destruction degree, mass yield rate and energetic mass yield of C 2 H 2 F 4 were up to 100 %, 2 kg[C 2 H 2 F 4 ] h -1 and 0.7 kg[C 2 H 2 F 4 ] kWh -1 , respectively.

  99.76-99.88%, depending on microwave power delivered. When methane flow rate was doubled then only half of CH 4 was converted. The energetic parameters of the hydrogen production via methane conversion, i.e. the H 2 mass yield rate and energetic H 2 mass yield were up to 866 g[H 2 ] h -1 and 640 g [H 2 ] per kWh of microwave energy absorbed by the plasma.When C 2 H 2 F 4 was used as a source of H 2 , the total tetrafluoroethane conversion degree was 63.3-84.1 %, depending on tetrafluoroethane flow rate. In this case, the energetic parameters of the H 2 production, i.e. the H 2 mass yield rate and energetic H 2

	mass yield were up to 700 g[H 2 ]/h and 670 g[H 2 ] per kWh of microwave energy
	absorbed by the plasma, respectively.

Table and figure captionsTable 1 .

 1 Comparison of the energetic hydrogen mass yields for different methods in which electric energy is directly used for methane conversion into hydrogen.

Acknowledgements

This research was supported by the Ministry of Science and Higher Education (MNiSW) under the programmes NR14-0091-10 and PB 2595/B/T02/2008/35.

advantage of the presented method.