Brigitte Fauroux
email: brigitte.fauroux@trs.aphp.fr

PhD Karl Leroux

Gilbert Desmarais

PhD Daniel Isabey

PhD Annick Clément

PhD Frédéric Lofaso

PhD Bruno Louis

Gilbert Raa

Desmarais

Brigitte Pr

MD Fauroux

Performance of ventilators for noninvasive positive

Keywords: bench study, child, lung model, pressure support, trigger, volume targeted ventilation

est destinée au dépôt

Introduction

Noninvasive positive pressure ventilation (NPPV) is increasingly used at home in children [START_REF] Fauroux | Long-term noninvasive mechanical ventilation for children at home: A national survey[END_REF]. NPPV may improve respiratory failure in children with neuromuscular disease [START_REF] Simonds | Impact of nasal ventilation on survival in hypercapnic Duchenne muscular dystrophy[END_REF][START_REF] Mellies | Long-term noninvasive ventilation in children and adolescents with neuromuscular disorders[END_REF], upper airway obstruction and sleep apnea [START_REF] Marcus | Adherence to and effectiveness of positive airway pressure therapy in children with obstructive sleep apnea[END_REF], and lung diseases such as cystic fibrosis [START_REF] Fauroux | In vivo physiological comparison of two ventilators used for domiciliary ventilation in children with cystic fibrosis[END_REF]. These diseases concern both infants as well as older children, which implies that the ventilator is able to adapt to a large range of patient demands.

Children with respiratory failure, and especially the youngest ones, may develop "extreme" breathing patterns, which may represent a "challenge" for a ventilator [START_REF] Essouri | Noninvasive positive pressure ventilation in infants with upper airway obstruction: comparison of continuous and bilevel positive pressure[END_REF].

Indeed, home ventilators may not be able to adequately synchronise with the patient respiratory effort [START_REF] Fauroux | The effect of back-up rate during non-invasive ventilation in young patients with cystic fibrosis[END_REF][START_REF] Fauroux | Setting of pressure support in young patients with cystic fibrosis[END_REF], leak compensation may be insufficient, and the triggers of assist modes and alarms are not always adapted for young children. This is explained by the fact that most ventilators have not been specifically developed for paediatric patients. However, in practice, the clinician has to deal with the available devices.

Although some studies have tested or compared home ventilators in young patients with cystic fibrosis [START_REF] Fauroux | The effect of back-up rate during non-invasive ventilation in young patients with cystic fibrosis[END_REF][START_REF] Fauroux | Setting of pressure support in young patients with cystic fibrosis[END_REF] or upper airway obstruction [START_REF] Essouri | Noninvasive positive pressure ventilation in infants with upper airway obstruction: comparison of continuous and bilevel positive pressure[END_REF], no study has evaluated different types of ventilators in children with various causes of chronic respiratory insufficiency. In France, 17 ventilators are proposed for home ventilation in children. The choice of the most appropriate ventilator for a specific patient is thus a real challenge for the clinician. Indeed, the testing of several ventilators in every single patient is unrealisable in practice.

The aim of the present study was to evaluate the performance of the 17 ventilators available for home ventilation in France with the most common "paediatric profiles", namely neuromuscular disease, upper airway obstruction, and cystic fibrosis. Therefore, we used a bench lung model that simulated the mechanical respiratory characteristics and the pattern of breathing of six "typical" paediatric patient profiles.

Material and methods

Patients profiles

In our clinical practice, about one third of the children treated with NPPV at home have neuromuscular diseases, one third has upper airway obstruction, and the last third have lung diseases or other causes of chronic hypercapnic respiratory insufficiency [START_REF] Fauroux | Long-term noninvasive mechanical ventilation for children at home: A national survey[END_REF]. We selected thus from our NPPV cohort, 6 patient profiles who represent approximately 90% of the patient profiles (Table 1).

During routine initiation of NPPV and follow up, breathing pattern in baseline condition, respiratory mechanics and respiratory output were recorded using a pneumotachograph (Fleisch # 3, Lausanne, Switzerland) and a catheter mounted pressure transducer system with two integral transducers (Gaeltec, Dunvegan, Isle of Skye, UK). Breathing pattern in baseline condition, i.e., when the patient was not connected to a ventilator and was spontaneously breathing, was inferred by measuring the patient flow rate. The tidal volume (VT) and the inspiratory time (Tinsp) were directly deduced from this flow tracing (Table 1).

The patient's respiratory mechanics was inferred when the patient was connected to the ventilator by measuring transdiaphragmatic pressure (Pdi) and esophageal pressure (Pes) as previously described [START_REF] Fauroux | In vivo physiological comparison of two ventilators used for domiciliary ventilation in children with cystic fibrosis[END_REF]. Briefly, dynamic lung compliance (C L dyn) was calculated as the ratio of VT to the Pes difference between the beginning and the end of inspiration during quiet breathing. Individual values indicated in Table 1 were averaged on the basis of 10-20 consecutive cycles during air breathing.

Airway and lung resistance (R) were calculated according to the formula based on the technique of Mead and Whittenberger: R = [(Pes 0 -Pes) -(V/C L dyn)/ ]V' where Pes 0 is the Pes value at the start of inspiratory flow, V is the instantaneous volume, C L dyn is calculated for the same breath, and V' is the instantaneous airflow [START_REF] Mead | Physical properties of the human lung measured during spontaneous respiration[END_REF]. Mean values over the inspiration were used as estimates of inspiratory airway and lung resistance (Table 1).

The analysis of the patient's profile was approved by the hospital ethics committee and patients and parents gave their informed consent.

Testing of the ventilators

Seventeen ventilators were tested (Table 2). Twelve ventilators were pressure targeted ventilators, one ventilator was a volume targeted ventilator, and 4 ventilators had the 2 modes. Every ventilator was tested with the 6 different patient profiles with the recommended circuits. When assist-control ventilation (ACV) and pressure support ventilation (PSV) were available, both modes were tested.

The setting of the ventilator (targeted pressure/volume, positive end-expiratory pressure (PEEP)) was different for each patient profile (Table 1). The 2 first patients were patients with neuromuscular disease, a 4-yr old boy with spinal muscular atrophy and a 17-yr old boy with Duchenne muscular dystrophy. Because both ACV and PSV may be used in these patients, ventilators able to deliver one or both modes were tested. For patient 3 with cystic fibrosis, ventilators able to deliver PSV and/or ACV were tested. For these first three patients, PSV or ACV with zero end-expiratory pressure of zero (ZEEP) was chosen because of the absence or a low (< 2 cm H 2 O) level of intrinsic PEEP [START_REF] Fauroux | In vivo physiological comparison of two ventilators used for domiciliary ventilation in children with cystic fibrosis[END_REF][START_REF] Hart | Changes in pulmonary mechanics with increasing disease severity in children and young adults with cystic fibrosis[END_REF]. Patient 4 was an infant with laryngomalacia in whom only ventilators able to deliver PS with PEEP were tested. PSV ventilators with PEEP were tested in patient 5 having obstructive sleep apnea due to vocal cord paralysis.

All the PSV ventilators able to deliver ZEEP were tested in patient 6 with central apnea.

All ventilators were studied at their most sensitive inspiratory trigger that did not induce auto-triggering. When possible, the highest inspiratory flow was used. In the majority of the ventilators, the expiratory trigger was set automatically. In 4 ventilators (GK 425ST, KNIGHTSTAR 330, VIVO 40, VPAP 3STA), it was possible to modify the sensitivity of the expiratory trigger. In this case, we used the most sensitive level that did not induce a Tinsp inferior to the spontaneous Tinsp. When available on the same ventilator, pressure and flow-triggering were tested. In case of an optional integrated humidification system, the ventilator was tested with and without the humidification system. For all the ventilators, the most recent model (year 2006) was used.

Experimental bench study

Each tested ventilator was connected via its standard circuit to the first chamber of a two-chamber Michigan test lung MII Vent Aid TTL; Michigan Instrument, Grand Rapids, MI) (Figure 1). The second chamber of the test lung (driving chamber) was connected to a flow-rate generator that could produce various wave forms previously stored in a microcomputer. The two chambers were physically connected to each other by a small metal component that allowed the driving chamber to lift the testing chamber. The flow rate generator, developed by the laboratory as previously described, was built by associating pressurised air, flow rate measurement and a servo valve driven by a microcomputer [START_REF] Lofaso | Bench evaluation of flow limitation detection by automated continuous positive airway pressure devices[END_REF]. This allows a continuous adjustment of the servo valve in order to produce for each patient profile the desired flow, as indicated in the paragraph "patient profiles". Moreover, in order to simulate the mechanical characteristics of respiratory system of each patient, the compliance of the testing chamber was adjusted and a resistance was added between the testing chamber and the tested ventilator. The compliance of the testing chambers (compliance of the respiratory system Crs) was set according to the formula 1/Crs = 1/Cw + 1/ CL, where Cw was the chest wall compliance theoretical value which represents about 4% of the patient's predicted value of vital capacity per cm H 2 O and CL was the lung compliance corresponding to the patient's CLdyn. The resistance was a parabolic airway resistor (Pneuflo® Airway resistor Rp5, Rp20, Rp50 or Rp200; Michigan Instrument, Grand Rapids, MI). For each profile, the resulting breathing effort generated in the bench test was characterised by the inspiratory airway occlusion pressure at 0.1 second (P 0.1 ), and by the volume (V 0.1 ) and the flow (V' 0.1 ) at 0.1 second after initiation of a spontaneous breath (Table 1). P 0.1 was inferred when the tested ventilator and its circuit (Figure 1) were replaced by a rigid stopper, while V 0.1 and V' 0.1 were inferred when the lung test was opened to the atmosphere. A leak valve was added to simulate leaks that could occur through a mask during NPPV, which allowed the testing of an increasing leak.

Airway pressure (Paw) and flow were measured at the end of the ventilator circuit using respectively a pressure differential transducer (Validyne DP 45  56 cm H 2 O, Northridge, CA) and a pneumotachograph (Fleish n°2, Lausanne, Switzerland) associated with a pressure differential transducer (Validyne DP 45  3.5 cm H 2 O).

The leak flow was also measured with a second pneumotachograph. Calibration of pressure and flow was performed before each test. Signals were digitised at 200 Hz by an analogic/digital system (MP100, Biopac Systems, Goleta, CA) and recorded on a microcomputer for further analysis.

As classically done, the following parameter were computed from each pressure and/or flow trace: PEEP, pressure support for PSV, measured tidal volume (VTm) and the tidal volume indicated by the ventilator (VTv). The sensitivity of the inspiratory trigger was evaluated on the trigger time delay (T: time between the onset of inspiratory effort to the point of minimum airway pressure), and the trigger pressure (P: pressure swing between the baseline pressure and the minimum airway pressure) [START_REF] Fauroux | The effect of back-up rate during non-invasive ventilation in young patients with cystic fibrosis[END_REF]. The sensitivity of the expiratory trigger was evaluated as the difference between the patient's inspiratory time during spontaneous breathing (Tinsp) and the inspiratory time during NPPV (TI). The pressurisation slope was calculated from the time of the minimum airway pressure up to this time +150 ms.

Each parameter was averaged on the base of 30 respiratory cycles.

In order to facilitate the interpretation of the results and guide the reader, the performances of the ventilators are presented qualitatively as follows. The inspiratory trigger was considered "appropriate" if T ≤ 100 ms and P ≤ -1 cmH 2 O [START_REF] Richard | Bench testing of pressure support ventilation with three different generations of ventilators[END_REF],

"acceptable" if T ≤ 150 ms and P ≤ -1.5 cmH 2 O or if T ≥ 100 ms or P ≤ -1 cmH 2 O, and "inappropriate" if the ventilator did not detect the inspiratory effort or in case of autotriggering. The coping of the ventilator with leaks was ranked as follows:

(1) relatively insensitive to a leak (no triggering or auto-triggering for a leak ≥ 40 l/mn), (2) moderately sensitive to a leak (no triggering or auto-triggering for a leak >10 and < 40 l/mn), (3) very sensitive to a leak (no triggering or auto-triggering for a leak ≤ 10 l/mn). The results of the performance of each ventilator are also given qualitatively as follows: "appropriate": for ACV, VTm < required VT ± 10%, and for PSV, measured PS (PSm) < required PS ± 10%, and pressurisation slope ≥ 60 cm H 2 O/s, "acceptable": for ACV, VTm < required VT ± 15 %, and for PSV: PSm < required PS ± 15 %, and pressurisation slope ≥ 40 cm H 2 O/s , "inappropriate": does not detect the inspiratory effort or autotriggering or, for ACV: VTm ≥ VT ± 15 %, or for PSV, PSm ≥ required PS ± 15 %, and pressurisation slope < 40 cm H 2 O.

Results

Except for 3 cases; SMARTAIR in the patient with cystic fibrosis (profile # 3), VIVO 40 in the patient with vocal cord paralysis (profile # 5), and VS ULTRA double circuit pressure trigger in the patient with central apnea (profile #6), we found very close results with and without humidification system. The results given are thus the average obtained with and without humidification system.

The complete data concerning the performance of each ventilator with the 6 different patient profiles are given in the online supplement (Online Tables 1 to 6).

For the patient with spinal muscular atrophy, all the 7 ventilators that had a compatible mode had inappropriate triggers (Table 3). For the adolescent with Duchenne muscular dystrophy, only 2 ventilators (the EOLE 3 with a flow trigger and the LEGENDAIR) had an appropriate inspiratory trigger in the ACV mode. However, the EOLE was very sensitive and the LEGENDAIR moderately sensitive to leaks. Of the 5 other ventilators that had a compatible mode, all had an inappropriate trigger.

For the patient with cystic fibrosis, only 4 ventilators had an appropriate trigger (the EOLE 3 with a flow trigger, the LEGENDAIR with both modes, the SMARTAIR with a simple circuit, the VS ULTRA in the PSV mode with a pressure trigger and a double circuit, and in the ACV mode with a simple circuit). The VS ULTRA in the PSV mode with a simple circuit and in the ACV mode with a double circuit and a pressure trigger had an acceptable trigger. But the ELYSEE 150, the NEFTIS, and the VS INTEGRA had all inappropriate triggers. None of the 16 PSV ventilators was able to detect the inspiratory effort of the infant with laryngomalacia. For the patient with vocal cord paralysis, 4 ventilators (the iSLEEP 22, the KNIGHTSTAR 330, the VS INTEGRA with a simple circuit and the VS SERENA) had an appropriate trigger. However, these ventilators were either moderately sensitive (the iSLEEP 22 and the VS SERENA) or very sensitive (the KNIGHTSTAR 330 and the VS INTEGRA with a simple circuit) to leaks. Eight ventilators had an acceptable trigger with this patient profile (the GK 425ST, the HARMONY 2, the LEGENDAIR, the NEFTIS 2, the SYNCHRONY, the SYNCHRONY 2, the VPAP 3STA, and the VS ULTRA with a simple circuit or a double circuit with a pressure trigger). Only 4 ventilators had an inappropriate trigger (the ELYSEE 150, the SMARTAIR, the VIVO 40, and the VPAP 3STA). Four ventilators were relatively insensitive to leaks (the GK 425ST, the HARMONY 2, the SYNCHRONY, and the SYNCHRONY 2), 3 were moderately sensitive to leaks (the iSLEEP 22, the VPAP 3STA, and the VS SERENA), the 5 others being very sensitive to leaks. None of the 6 ventilators that had a compatible mode had an appropriate trigger for the patient with central apnea. Three ventilators had an acceptable trigger (the LEGENDAIR, the SMARTAIR with a simple circuit, and the VS ULTRA), but none of these ventilators coped adequately with leaks. The ELYSEE 150, the NEFTIS 2, and the VS INTEGRA had inappropriate triggers.

The quality of the expiratory triggers is represented online in Table 7. The major observation is that the performance of the expiratory triggers varies according to the ventilator but also the patient profile. Only the KNIGHTSTAR 330 and the LEGENDAIR were able to detect the expiratory effort of patient 4, who was the infant with laryngomalacia. The expiratory trigger of the ELYSEE was good in patient 2 (Duchenne muscular dystrophy) but much less in patient 3 (cystic fibrosis) and patient 5 (vocal cord paralysis).

Concerning the performance of the ventilators, for patient 1 with spinal muscular atrophy, only the ELYSEE 150 in the ACV mode with a simple circuit had an appropriate performance. The performance of the NEFTIS 2 and the VS ULTRA with a double circuit and a flow trigger were acceptable. Four ventilators had an inappropriate performance (the EOLE 3, the LEGENDAIR, the SMARTAIR, and the VS INTEGRA). The ELYSEE 150, the LEGENDAIR in the ACV mode, and the VS ULTRA in the PSV mode with a simple circuit or a double circuit with a flow trigger had an appropriate performance. The performance of the VS ULTRA in the ACV mode with a double circuit and a flow trigger was acceptable, whereas the performance of the NEFTIS 2, the SMARTAIR and the VS INTEGRA was inappropriate. For the patient with cystic fibrosis, only 3 ventilators had an appropriate performance (the EOLE 3, the NEFTIS 2, and the VS ULTRA with the 2 modes with a simple circuit or a double circuit with a flow trigger). The KNIGHTSTAR 330 was the only ventilator having an acceptable performance in the infant with laryngomalacia. For the patient with vocal cord paralysis, 2 ventilators had a acceptable performance (the VPAP 3STA and the VS ULTRA with a simple circuit or with a double circuit and a pressure trigger), all the other devices having a compatible mode had inappropriate performances. For the patient with central apnea, the ELYSEE 150 with a double circuit and the VS ULTRA with a double circuit and a flow trigger had appropriate performances, whereas the 4 other ventilators that had a compatible mode had inappropriate performances.

Discussion

The current study is the first to provide a strictly protocolised bench test evaluation of the performance of a broad range of home ventilators, all not primarily developed for children, for 6 different paediatric patient profiles. The major findings of the study can be summarised as follows: 1) no ventilator is perfect and able to adequately ventilate the 6 different patient profiles, 2) the performance of the ventilators was very heterogeneous and depended on the type trigger and circuit, and most importantly, on the characteristics of the patient, 3) the sensitivity of the inspiratory triggers of most of the ventilators was insufficient for infants.

Paediatric specificities

The present study confirms the limitations of the currently available ventilators for home ventilation in children. Numerous ventilators were not able to adequately respond to the patient's demands. Several paediatric specificities may explain these difficulties. First, the patient's inspiratory effort may be too low, or lower than those of adults, which reduces the ability of the ventilator to detect the onset of the inspiration.

For the 6 patient profiles, P 0.1 in the lung model ranged between 0.4 cm H 2 O and 4.3 cm H 2 O. These values are in agreement with the values reported in the literature for adults [START_REF] Tobin | Respiratory monitoring in the intensive care unit[END_REF]. A recent study observed that the inspiratory effort, evaluated by P 0.1 , was higher in children with neuromuscular disease than in healthy controls [START_REF] Mulreany | Noninvasive measurement of the tension-time index in children with neuromuscular disease[END_REF]. However, when we compared for each patient his P 0.1 value with the number of ventilators detecting the patient's inspiratory effort, we observed that the patients who had the lowest P 0.1 were also those in whom the majority of the ventilators were not able to detect the patient's inspiratory effort. Indeed, only 39% of the ventilators were able to detect the inspiratory effort of patient 1 (Online Table 1), who has a P 0.1 of 0.94 cm H 2 O and only 9% of the ventilators were able to detect the inspiratory effort of patient 4 (Online Table 4), who has a P 0.1 of 0.4 cm H 2 O. This suggests that the inspiratory effort generated by the youngest children may be too small to be detected by the majority of the ventilators. Moreover these two patients had also the smallest V 0.1 and V' 0.1 during spontaneous breathing (with a V 0.1 of 5.8 ml and 1.3 ml, and a V' 0.1 of 17ml/s and 17 ml/s, for patient 1 and 4, respectively). This implies that a ventilator with a trigger based upon a flow signal should be able to detect a flow and/or a volume inferior to these values in order to generate an adequate T. In practice, the use of a high back up rate, i.e. equivalent or 2 or 3 breaths below the patient's spontaneous breathing rate, may overcome problems associated with an inadequate inspiratory trigger. Such a setting is recommended for patients with neuromuscular disease [START_REF]Clinical indications for noninvasive positive pressure ventilation in chronic respiratory failure due to restrictive lung disease, COPD, and nocturnal hypoventilation -A Consensus Conference Report[END_REF].

The patient with central apnea should theoretically be ventilated with a controlled mode. But, these patients may have some spontaneous breaths. Thus, in order to increase the comfort of NPPV and favour the synchronisation of the patient with the ventilator, a spontaneous mode with a back up rate slightly below the spontaneous breathing rate of the patient may be used, allowing the evaluation of the inspiratory trigger is this patient. These limitations of the ventilators observed in the present study with simulated paediatric patterns were not completely unexpected since few devices have been specifically developed for children. Also, the majority of the manufacturers (12 out of 17) do not implicitly recommend to ventilate the youngest children with their ventilator ("adult + child", "not for newborn", "> 30 kg"). The quality of the inspiratory triggers may also limit the performance of ventilators. Nevertheless due to the lack of information disclosed by the manufacturers concerning the principle and the algorithms used for the inspiratory trigger, it is difficult to understand why a ventilator seems to have a better trigger than another. With a classical pressure trigger a closed system is mandatory in order to facilitate the generation of a differential pressure. For example in the case of the "EOLE 3 pressure trigger" we did not observe a large decrease of the Paw during the patient's inspiratory effort while a inspiratory flow signal is detected. This confirms an open system which is one explanation of the lack of detection of the inspiratory effort observed with this ventilator. With a trigger based upon flow signal the system should be open. One of the major problems of such a trigger is the take up of the leak. Nevertheless our results do not suggest that a simple circuit + leak allowed a better or a worse inspiratory trigger than a ventilation without leak (with a simple or double circuit). In case of a flow trigger, the ventilator should be able to detect very low flows especially in young children who have the smallest VT. Significant differences with regard to the expiratory triggers were also observed. These results are in agreement with our clinical results which showed that the sensitivity of the expiratory triggers may be insufficient for infants requiring NPPV for severe upper airway obstruction [START_REF] Essouri | Noninvasive positive pressure ventilation in infants with upper airway obstruction: comparison of continuous and bilevel positive pressure[END_REF].

Characteristics of the ventilators

Ventilators become more sophisticated and tend to integrate continuously new options and measures. A large number of ventilators are able to deliver different ventilatory modes, such as PSV, with or without PEEP, as well as ACV. Different circuits (simple, double or leak circuit) and triggers (pressure or flow triggers) may be available on the same ventilator. The present study clearly shows that the performance of a ventilator may vary according to the ventilatory mode or the type of trigger and circuit. Indeed, the quality of the inspiratory trigger varied among the different ventilators and also for a specific ventilator, according to the patient profile.

As example, the T of the NEFTIS was shorter with patient 6 (0.15 s) who had a high inspiratory effort than with patient 1 who had a low inspiratory effort (0.28 s) (Online Tables 1 and6). It is important that the clinician who will choose the device is aware of these differences; which are rarely specified by the manufacturer. Some ventilators, such as the LEGENDAIR, had a low pressurisation slope and stability index, which signifies that the ventilator is not able to reach the preset pressure within a minimal time frame. Most ventilators "measure" physiological variables such as VT or Paw. Significant differences were observed for almost all the ventilators between the results shown on the ventilator and the values measures on the bench. This may be explained by the fact that most of these variables are estimated by a software incorporated inside the ventilator. Because NPPV is a "leak" ventilation, the VTv represents the volume of air generated by the device. On the bench, the VTm was measured by a pneumotachograph inserted between the circuit and the interface. This measure was thus closer to the patient and reflects thus more accurately the VT received by the patient in case of a calibrated leak ventilation. But differences between the VT set on the ventilator, the VT measured by the ventilator and by a pneumotachograph have also been observed previously with other ventilatory modes [START_REF] Lofaso | Inaccuracy of tidal volume delivered by home mechanical ventilators[END_REF]. Of note, less discrepancy was observed for Paw.

The ability of a ventilator to compensate for additional leaks is important in case of NPPV. The effect of an additional leak in the inspiratory circuit was thus tested for every ventilator. Most of the ventilators were not able to cope with additional leaks, which resulted in autotriggering or the inability to detect the patient's inspiratory effort.

Advantages and limitations of the study

We compared the responses of several devices to identical mechanical properties of the respiratory system and patterns of inspiratory flow contour, which was not possible in a clinical study given the variability of these respiratory parameters. In addition, given the number of ventilator available to test, one could not reasonably ask to children to perform this study.

One limitation of the bench is that the resistance added by the test system may be more representative of upper airway obstruction, as encountered in the patients with laryngomalacia and vocal cord paralysis, than small airway disease, such as encountered in the patient with cystic fibrosis.

One other limitation of our study was that our 6 patients were recorded during wakefulness and not during sleep. Sleep may be associated with both upper airway and inspiratory effort instability. Thus, the mechanical output during spontaneous respiratory drive, i.e. the inspiratory flow or airway depression that the ventilator has to detect in order to synchronise the ventilator assistance to the patient inspiratory effort, may be less easy to detect during sleep. We refrained recording the patients during sleep, although NPPV is generally performed during sleep, considering that NPPV is initially started and adapted during wakefulness, before being tested during sleep. In addition, we used "typical" patient profiles but in clinical practice, the addition of several factors favouring nocturnal hypoventilation is a common situation, such as the association of obesity and upper airway obstruction in patients with Duchenne muscular dystrophy. We were not able to integrate such mixed pathologies in the present bench model. Neither were we able to include dynamic modifications like upper airway obstruction and decrease of respiratory drive during sleep as well as. If we are confident that the ventilators which were not able to detect the simulated respiratory efforts would not be able to detect the respiratory efforts in real life condition, we cannot ascertain that the ventilators considered as appropriate by our bench study were effectively appropriate in real life condition. Therefore, our study only allows to "preselect" ventilator devices which can be reasonably tested in a paediatric patient, and cannot exclude a clinical evaluation before considering that a ventilator is really appropriate for a child.

Nevertheless, systematic comparison of bench data with in vivo data is lacking. However, for most of the typical situations, the in vitro results are in agreement with the in vivo patient tracings. Indeed, the lack of detection of the patient's inspiratory and expiratory effort by the majority of the bilevel devices in infants and young children has been previously observed by our group [START_REF] Essouri | Noninvasive positive pressure ventilation in infants with upper airway obstruction: comparison of continuous and bilevel positive pressure[END_REF]. The insufficient sensitivity of the inspiratory trigger of the EOLE 3XLS has also been observed in young patients with cystic fibrosis [START_REF] Fauroux | The effect of back-up rate during non-invasive ventilation in young patients with cystic fibrosis[END_REF]. Moreover, the majority of the problems encountered during the bench testing with the different ventilators have been observed on patients [START_REF] Essouri | Noninvasive positive pressure ventilation in infants with upper airway obstruction: comparison of continuous and bilevel positive pressure[END_REF].

Practical recommendations

Our results underline the necessity of a systematic bench evaluation of all ventilators proposed for NPPV in children. This evaluation should ideally include the assessment of the quality of the inspiratory (pressure and/or flow) trigger, the ability of the ventilator to reach and maintain the preset volume or pressure, as well as the coping with leaks. However, for some patients, such as patients with neuromuscular disease or central apnea, effort independent modes are recommended, precluding the evaluation of the inspiratory trigger. This bench evaluation should be followed by a clinical evaluation in the patients for whom the ventilator has shown good or acceptable performances, as defined by specific criteria (for example as those proposed in the present work).

The choice of a ventilator for a specific patient depends on the patient's characteristics (underlying disease, age and weight), the ventilatory mode that will be used, and the performance of the ventilator. Other ventilator characteristics, not evaluated in the present work, such as the accuracy of the alarms, the possibility of humidification or additional oxygen therapy, have also to be taken in account. Finally, the ergonomics, such as transportability and internal battery are important in clinical use. But ultimate efficacy must be checked in each individual case by daytime performance and comfort, associated with overnight control.

Conclusion

This bench study, which evaluated for the first time 17 home ventilators with the 6 most common paediatric profiles, shows that the performance of the ventilators varied not only according to ventilator characteristics (type of circuit and type of trigger) but most importantly also according to the patient profile, including his age and weight as well as his underlying disease. Even if different modes and different ventilators may be used in a specific patient, we recommend a systematic bench evaluation, coupled to a clinical in vivo evaluation for all the ventilators proposed for home NPPV in children. Abbreviations: na: not applicable. This parameter was only calculated for pressure-targeted ventilators.

Table 1 :

 1 Patient profiles and ventilatory modes used for the bench lung model study.

	Patient*	Pathology	Age (year)	Weight (kg)	VT (ml)	Tinsp (s)	CLdyn (L/cm H 2 O)	R(cmH 2 O/L.s) /Rp	P 0.1 (cm H 2 O)	V 0.1 (ml)	V' 0.1 (ml/s)	Tested ventilatory mode
	1	spinal muscular amyotrophy	4	13	150	1.3	0.038	52/50	0.94	5.8	71	PSV10 ZEEP ACV250 ZEEP
		Duchenne										PSV16 ZEEP
	2	muscular	19	70	293	0.9	0.024	17/20	2.8	9.5	142	ACV500 ZEEP
		dystrophy
	3	cystic fibrosis	14	27	188	1.1	0.019	57/50	2.5	11.0	135	PSV16 ZEEP ACV500 ZEEP
	4	laryngomalacia 0.25	4	18	1.0	0.024	200#	0.4	1.3	17	PSV14 PEEP8
	5	vocal cord paralysis	4	12	250	1.3	0.064	6/5	1.4	14.7	180	PSV14 PEEP8
	6	central apnea	13	42	296	1.1	0.153	7/5	4.3	19.7	273	PSV12 ZEEP

Abbreviations: VT: tidal volume, Tinsp: inspiratory time, CLdyn: dynamic lung compliance, Rp: resistance of the respiratory system, # the measure of the Rp was not possible in this patient, PSV: pressure support ventilation, ZEEP: zero positive end-expiratory pressure, PEEP: positive end-expiratory pressure, ACV: assist control ventilation, P 0.1 : inspiratory airway occlusion pressure at 0.1 sec after the initiation of spontaneous breath, V 0.1 : inspired volume at 0.1 sec after initiation of spontaneous breath, V' 0.1 : flow at 0.1 sec after initiation of spontaneous breath. P 0.1 , V 0.1 and V' 0.1 are measured on the bench test. * The respiratory mechanics were measured during wakefulness.

Table 2 :

 2 Ventilators tested ZEEP: zero end-expiratory pressure, CPAP: continuous positive airway pressure, P: pressure targeted mode, V: volume targeted mode, B: bilevel positive pressure ventilation, S: simple circuit with expiratory valve, D: double circuit, S +leak; simple circuit with leak, nctt: no choice of trigger type, NPPV: noninvasive positive pressure ventilation.

	Ventilator	Manufacturer	Modes	Circuit	Trigger	ZEEP	CPAP	Humidifier	Remarks
	ELYSEE150	ResMed SA, Saint Priest, France	P / V	S / D	nctt	yes	no	no	adult + child
	EOLE3	ResMed SA, Saint Priest, France	V	S / D	flow or pressure	yes	no	no	not for newborns
	GK425ST	Tyco Healthcare, Elancourt, France	P	S + leak	nctt	no	yes	yes	> 30 kg
	HARMONY2	Respironics France, Carquefou, France	P	S + leak	nctt	no	yes	yes
	iSLEEP22	ResMed SA, Saint Priest, France	B	S + leak	nctt	no	yes	yes
	KNIGHSTAR330	Tyco Healthcare, Elancourt, France	B	S + leak	nctt	no	yes	no	> 30 kg
	LEGENDAIR	Airox, Pau, France	P / V	S	nctt	yes	no	no	adult + child
	NEFTIS2	TAEMA, Anthony, France	P / V	S	nctt	yes	no	no	invasive + NPPV adult + child
	SMARTAIR+	Airox, Pau, France	P	S / S +leak	nctt	yes	no	no	invasive + NPPV
	SYNCHRONY	Respironics France, Carquefou, France	P	S + leak	nctt	no	yes	no
	SYNCHRONY2	Respironics France, Carquefou, France	P	S + leak	nctt	no	yes	yes	> 30 kg
	VIVO40	Breas Medical, Saint Priest, France	P	S + leak	nctt	no	yes	yes	adult + child
	VPAP3ST	ResMed SA, Saint Priest, France	P	S + leak	nctt	no	yes	yes
	VPAP3STA	ResMed SA, Saint Priest, France	P	S + leak	nctt	no	yes	yes
	VS INTEGRA	ResMed SA, Saint Priest, France	P	S / S +leak	nctt	no:S+leak yes:S	no	no	adult + child
	VS SERENA	ResMed SA, Saint Priest, France	P	S + leak	nctt	no	no	no	adult + child
	VS ULTRA	ResMed SA, Saint Priest, France P / V S / D / S + leak	flow or pressure	no:S+leak yes:S D	no	no	adult + child

Table 3 :

 3 Trigger performances of the ventilators according to the 6 patient profiles.

		spinal muscular amyotrophy (PSV + ACV)	Duchenne muscular dystrophy (PSV + ACV)	cystic fibrosis (PSV + ACV)	laryngomalacia (PSV + PEEP)	vocal cord paralysis (PSV + PEEP)	central apnea (PSV + ZEEP)
	ELYSEE 150	inappropriate	inappropriate	inappropriate	inappropriate	inappropriate	inappropriate
	EOLE 3	inappropriate	acceptable ACV-a (3) appropriate ACV-a (3) no compatible mode no compatible mode no compatible mode
	GK 425ST	no compatible mode no compatible mode	no compatible mode	inappropriate	acceptable (1)	no compatible mode
	HARMONY2	no compatible mode no compatible mode	no compatible mode	inappropriate	acceptable (1)	no compatible mode
	iSLEEP 22	no compatible mode no compatible mode	no compatible mode	inappropriate	appropriate (2)	no compatible mode
	KNIGHTSTAR 330	no compatible mode no compatible mode	no compatible mode	inappropriate	appropriate (3)	no compatible mode
	LEGENDAIR	inappropriate	acceptable ACV (2)	appropriate PSV (1) ACV (3)	inappropriate	acceptable (3)	acceptable (3)
	NEFTIS 2	inappropriate	inappropriate	inappropriate	inappropriate	acceptable (3)	inappropriate
	SMARTAIR	inappropriate	inappropriate	appropriate b (1)	inappropriate	inappropriate	acceptable b (3)
	SYNCHRONY	no compatible mode no compatible mode	no compatible mode	inappropriate	acceptable (1)	no compatible mode
	SYNCHRONY 2	no compatible mode no compatible mode	no compatible mode	inappropriate	acceptable (1)	no compatible mode
	VIVO40	no compatible mode no compatible mode	no compatible mode	inappropriate	inappropriate	no compatible mode
	VPAP 3ST	no compatible mode no compatible mode	no compatible mode	inappropriate	inappropriate	no compatible mode
	VPAP 3STA	no compatible mode no compatible mode	no compatible mode	inappropriate	acceptable (2)	no compatible mode
	VS INTEGRA	inappropriate	inappropriate	inappropriate	inappropriate	appropriate b (3)	inappropriate
	VS SERENA	no compatible mode no compatible mode	no compatible mode	inappropriate	appropriate (2)	no compatible mode
				acceptable PSV-b
	VS ULTRA	inappropriate	inappropriate	ACV-c (3) appropriate PSV-c	inappropriate	acceptable b-c (3)	acceptable c (2)
				ACV-b (3)

Table 1 :

 1 Patient 1 with spinal muscular amyotrophy T: trigger time delay, P: trigger pressure, PS: pressure support, Pmax: maximal inspiratory pressure delivered by the ventilator, VTm: measured tidal volume, VTv: tidal volume measured by the ventilator, ZEEP: zero end-expiratory pressure, ACV: assist control ventilation. Values reported in this table were obtained when the leak valve (Figure1) was closed. Effects of a leak simulated with the leak valve are indicated in the footnotes * does not detect the inspiratory effort or auto-triggering.

	Setting	PEEP (cmH 2 O)	T (s)	P (cmH 2 O) (cmH 2 O) PS	Pmax ven tilator (cmH 2 O)	Pressurisation slope (cmH 2 O/s)	VTm (L)	VTv (L)

1 auto-triggering for a leak of 3L/min, the ventilator requires a back up rate slightly below the spontaneous breathing rate

Table 2 :

 2 Patient 2 with Duchenne muscular dystrophy

	Ventilators	Setting	PEEP (cmH 2 O)	T (s)	P (cmH 2 O) (cmH 2 O) PS	Pmax ventilator (cmH 2 O)	Pressurisation slope (cmH 2 O/s)	VTm (L)	VTv (L)
	ELYSEE150 double circuit 1	PSV16 ZEEP 0.36 0.20 -1.52	16.1	16.0	142.9	0.357 0.384
	ELYSEE150 simple circuit 2	PSV16 ZEEP 0.47 0.19 -1.33	16.0	16.0	137.2	0.354 0.373
	LEGENDAIR 3	PSV16 ZEEP 0.64 0.20 -0.59	17.3	16.0	12.6	0.351 0.323
	NEFTIS2 4	PSV16 ZEEP 1.00 0.21 -2.04	14.4	17.0	36.0	0.370 0.300
	SMARTAIR simple circuit	PSV16 ZEEP	*	*	*	*	*	*	*	*
	VS INTEGRA simple circuit	PSV16 ZEEP	*	*	*	*	*	*	*	*
	VS ULTRA double circuit flow trigger 5	PSV16 ZEEP 1.35 0.17 -1.31	15.9	16.0	100.5	0.370 0.450
	VS ULTRA simple circuit 6	PSV16 ZEEP 0.89 0.17 -1.37	16.0	17.0	109.9	0.378 0.460
	VS UTRA double circuit pressure trigger PSV16 ZEEP	*	*	*	*	*	*	*	*
	ELYSEE150 double circuit 7	ACV500 ZEEP 0.40 0.32 -2.08	25.3	26.5	10.4	0.482 0.498
	ELYSEE150 simple circuit 8	ACV500 ZEEP 0.54 0.29 -1.81	25.1	26.0	7.2	0.476 0.499
	EOLE3 pressure trigger	ACV500 ZEEP	*	*	*	*	*	*	*	*
	EOLE3 flow trigger 9	ACV500 ZEEP -0.04 0.15 -0.48	27.7	29.0	10.5	0.504 0.585
	LEGENDAIR 10	ACV500 ZEEP 0.66 0.12 -0.38	21.9	22.5	32.6	0.473 0.498
	NEFTIS2 11	ACV500 ZEEP 1.01 0.21 -2.09	23.0	23.0	37.8	0.474 0.420
	VS ULTRA double circuit flow trigger 5	ACV500 ZEEP 1.38 0.17 -1.30	23.2	23.5	69.3	0.431 0.530
	VS ULTRA simple circuit	ACV500 ZEEP	*	*	*	*	*	*	*	*
	VS UTRA double circuit pressure trigger ACV500 ZEEP	*	*	*	*	*	*	*	*
	Abbreviations: PEEP: positive end-expiratory pressure, T: trigger time delay, P: trigger pressure, PS: pressure support, Pmax: maximal inspiratory pressure delivered by the ventilator, VTm: measured tidal volume, VTv: tidal volume measured by the ventilator,
	ZEEP: zero end-expiratory pressure, ACV: assist control ventilation.
	Values reported in this table were obtained when the leak valve (Figure1) was closed. Effects of a leak simulated with the leak valve
	are indicated in the footnotes
	* does not detect the inspiratory effort or autotriggering.

Table 3 :

 3 Patient 3 with cystic fibrosis T: trigger time delay, P: trigger pressure, PS: pressure support, Pmax: maximal inspiratory pressure delivered by the ventilator, VTm: measured tidal volume, VTv: tidal volume measured by the ventilator, ZEEP: zero end-expiratory pressure, ACV: assist control ventilation. Values reported in this table were obtained when the leak valve (Figure1) was closed. Effects of a leak simulated with the leak valve are indicated in the footnotes * does not detect the inspiratory effort or autotriggering.

	Ventilators	Setting	PEEP (cmH 2 O)	T (s)	P (cmH 2 O) (cmH 2 O) PS	Pmax ven tilator (cmH 2 O)	Pressurisation slope (cmH 2 O/s)	VTm (L)	VTv (L)
	ELYSEE150 double circuit 1	PSV16 ZEEP	0.17 0.33 -1.24	18.5	18.5	170.2	0.263 0.236
	ELYSEE150 simple circuit 2	PSV16 ZEEP	0.42 0.19 -1.34	18.8	17.5	170.9	0.258 0.268
	LEGENDAIR 3	PSV16 ZEEP	0.64 0.06 -0.53	16.6	16.0	7.1	0.240 0.190
	NEFTIS2 4	PSV16 ZEEP	0.97 0.16 -1.75	15.1	16.0	74.4	0.298 0.260
	SMARTAIR simple circuit 5	PSV16 ZEEP	0.24 0.10 -0.20	16.7	17.0	22.3	0.318 0.340
	VS INTEGRA simple circuit	PSV16 ZEEP	*	*	*	*	*	*	*	*
	VS ULTRA double circuit flow trigger 6	PSV16 ZEEP	1.43 0.16 -1.41	16.3	16.0	92.6	0.272 0.340
	VS ULTRA simple circuit 7	PSV16 ZEEP	0.66 0.13 -1.15	16.2	16.0	119.1	0.286 0.315
	VS UTRA double circuit pressure trigger 6 PSV16 ZEEP	0.30 0.07 -0.69	16.2	16.0	69.0	0.293 0.295
	ELYSEE150 double circuit 7	ACV500 ZEEP 0.27 0.34 -1.67	27.8	29.0	60.6	0.447 0.446
	ELYSEE150 simple circuit	ACV500 ZEEP	*	*	*	*	*	*	*	*
	EOLE3 pressure trigger	ACV500 ZEEP	*	*	*	*	*	*	*	*
	EOLE3 flow trigger 8	ACV500 ZEEP	0.0 0.07 -0.42	32.9	33.0	29.8	0.488 0.415
	LEGENDAIR 9	ACV500 ZEEP	0.7 0.06 -0.50	27.3	27.0	33.3	0.488 0.500
	NEFTIS2 10	ACV500 ZEEP 1.16 0.16 -1.76	28.4	29.0	106.6	0.472 0.420
	VS ULTRA double circuit flow trigger 11	ACV500 ZEEP 1.40 0.15 -1.20	32.3	31.5	173.4	0.434 0.530
	VS ULTRA simple circuit 12	ACV500 ZEEP 0.80 0.10 -1.0	32.1	32.5	108.7	0.552 0.500
	VS UTRA double circuit pressure trigger 13	ACV500 ZEEP 0.44 0.14 -0.84	32.1	31.5	102.7	0.454 0.525
	Abbreviations: PEEP: positive end-expiratory pressure,

Table 4 :

 4 Patient 4 with laryngomalacia T: trigger time delay, P: trigger pressure, PS: pressure support, Pmax: maximal inspiratory pressure delivered by the ventilator, VTm: measured tidal volume, VTv: tidal volume measured by the ventilator.Values reported in this table were obtained when the leak valve (Figure1) was closed. Effects of a leak simulated with the leak valve are indicated in the footnotes * does not detect the inspiratory effort or autotriggering.

	Ventilators	Setting	PEEP (cmH 2 O)	T (s)	P (cmH 2 O) (cmH 2 O) PS	Pmax ven tilator (cmH 2 O)	Pressurisation slope (cmH 2 O/s)	VTm (L)	VTv (L)
	ELYSEE150 double circuit	PSV14 PEEP8	*	*	*	*	*	*	*	*
	ELYSEE150 simple circuit	PSV14 PEEP8	*	*	*	*	*	*	*	*
	GK425ST	PSV14 PEEP8	*	*	*	*	*	*	*	*
	HARMONY2	PSV14 PEEP8	*	*	*	*	*	*	*	*
	iSLEEP22	PSV14 PEEP8	*	*	*	*	*	*	*	*
	KNIGTSTAR330 1	PSV14 PEEP8 8.19 0.21 -0.16	14.6	14.0	46.9	0.051 0.074
	LEGENDAIR 2	PSV14 PEEP8 8.23 0.21 -0.48	14.3	14.0	12.0	0.042 0.030
	NEFTIS2	PSV14 PEEP8	*	*	*	*	*	*	*	*
	SMARTAIR leak circuit	PSV14 PEEP8	*	*	*	*	*	*	*	*
	SMARTAIR simple circuit	PSV14 PEEP8	*	*	*	*	*	*	*	*
	SYNCHRONY	PSV14 PEEP8	*	*	*	*	*	*	*	*
	SYNCHRONY2	PSV14 PEEP8	*	*	*	*	*	*	*	*
	VIVO40	PSV14 PEEP8	*	*	*	*	*	*	*	*
	VPAP3ST	PSV14 PEEP8	*	*	*	*	*	*	*	*
	VPAP3STA	PSV14 PEEP8	*	*	*	*	*	*	*	*
	VS INTEGRA leak circuit	PSV14 PEEP8	*	*	*	*	*	*	*	*
	VS INTEGRA simple circuit	PSV14 PEEP8	*	*	*	*	*	*	*	*
	VS SERENA	PSV14 PEEP8	*	*	*	*	*	*	*	*
	VS ULTRA double circuit flow trigger	PSV14 PEEP8	*	*	*	*	*	*	*	*
	VS ULTRA leak circuit	PSV14 PEEP8	*	*	*	*	*	*	*	*
	VS ULTRA simple circuit	PSV14 PEEP8	*	*	*	*	*	*	*	*
	VS UTRA double circuit pressure trigger PSV14 PEEP8	*	*	*	*	*	*	*	*
	Abbreviations: PEEP: positive end-expiratory pressure,

Table 5 :

 5 Patient 5 with vocal cord paralysis Abbreviations: PEEP: positive end-expiratory pressure, T: trigger time delay, P: trigger pressure, PS: pressure support, Pmax: maximal inspiratory pressure delivered by the ventilator, VTm: measured tidal volume, VTv: tidal volume measured by the ventilator.Values reported in this table were obtained when the leak valve (Figure1) was closed. Effects of a leak simulated with the leak valve are indicated in the footnotes * does not detect the inspiratory effort or autotriggering.

	Ventilators	Setting	Peep (cmH 2 O)	T (s)	P (cmH 2 O) (cmH 2 O) PS	Pmax ventilator (cmH 2 O)	Pressurisation slope (cmH 2 0/s)	VTm (L)	VTv (L)
	ELYSEE150 double circuit 1	PSV14 PEEP8 8.6	0.289 -1.94	13.9	14.0	24.4	0.242	0.233
	ELYSEE150 simple circuit 2	PSV14 PEEP8 6.7	0.246 -1.61	14.1	14.0	34.5	0.260	0.262
	GK425ST 3	PSV14 PEEP8 7.5	0.127 -0.21	13.6	14.0	23.7	0.294 No ind.
	HARMONY2 4	PSV14 PEEP8 8.1	0.128 -0.25	14.0	14.0	34.7	0.323 No ind.
	iSLEEP22 5	PSV14 PEEP8 8.3	0.073 -0.54	14.5	15.0	12.2	0.285 No ind.
	KNIGTSTAR330 6	PSV14 PEEP8 8.3	0.061 -0.44	14.9	14.0	25.1	0.349	0.373
	LEGENDAIR 7	PSV14 PEEP8 8.1	0.095 -1.24	14.4	14.0	8.2	0.303	0.278
	NEFTIS2 8	PSV14 PEEP8 7.5	0.073 -1.17	13.4	14.5	17.1	0.294	0.245
	SMARTAIR leak circuit	PSV14 PEEP8	*	*	*	*	*	*	*	*
	SMARTAIR simple circuit	PSV14 PEEP8	*	*	*	*	*	*	*	*
	SYNCHRONY 9	PSV14 PEEP8 8.2	0.119 -0.58	14.2	14.0	36.6	0.305	0.262
	SYNCHRONY2 9	PSV14 PEEP8 8.0	0.138 -1.01	14.3	14.0	36.1	0.330	0.262
	VIVO40 10	PSV14 PEEP8 8.0	0.210 -0.60	13.6	14.0	19.5	0.250	0.160
	VPAP3ST 11	PSV14 PEEP8 7.8	0.170 -0.23	14.0	14.0	39.3	0.313	0.312
	VPAP3STA 12	PSV14 PEEP8 7.8	0.112 -0.21	14.2	14.0	44.4	0.313	0.322
	VS INTEGRA leak circuit	PSV14 PEEP8	*	*	*	*	*	*	*	*
	VS INTEGRA simple circuit 13	PSV14 PEEP8 7.2	0.059 -0.69	13.9	14.0	37.4	0.340	0.405
	VS SERENA 14	PSV14 PEEP8 7.7	0.1	-0.4	14.1	14.0	35.5	0.327	0.320
	VS ULTRA double circuit flow trigger	PSV14 PEEP8	*	*	*	*	*	*	*	*
	VS ULTRA leak circuit	PSV14 PEEP8	*	*	*	*	*	*	*	*
	VS ULTRA simple circuit 15	PSV14 PEEP8 8.8	0.1	-1.1	15.0	14.0	51.7	0.3	0.3
	VS UTRA double circuit pressure trigger 16 PSV14 PEEP8 9.0	0.1	-1.2	14.8	14.0	50.4	0.3	0.3

Table 6 :

 6 Patient 6 with central apnea P: trigger pressure, PS: pressure support, Pmax: maximal inspiratory pressure delivered by the ventilator, VTm: measured tidal volume, VTv: tidal volume measured by the ventilator, ZEEP: zero end-expiratory pressure, ACV: assist control ventilation. Values reported in this table were obtained when the leak valve (Figure1) was closed. Effects of a leak simulated with the leak valve are indicated in the footnotes * does not detect the inspiratory effort or autotriggering. leak of 2.3L/min, the ventilator requires a back up rate slightly below the spontaneous breathing 3 alarm "disconnected circuit, patient compliance" + autotriggering for a leak of 10L/min, autotriggering for a leak of 24L/min and does not detect the patient's inspiratory effort with a humidificator Ventilators which do not have a compatible mode: EOLE3XLS, GK425T, HARMONY2, iSLEEP22, KNIGHTSTAR330, SMARTAIR + leak circuit, SYNCHRONY and SYNCHRONY2, VIVO40, VPAP3ST, VPAP3STA, VS SERENA, VS ULTRA + leak circuit

	Ventilators	Setting	PEEP (cmH 2 O)	T (s)	P (cmH 2 O) (cmH 2 O) PS	Pmax (cmH 2 O) ventilator	Pressurisation (cmH 2 O/s) slope	VTm (L) VTv (L)
	ELYSEE150 double circuit 1	PSV12 ZEEP 0.23 0.18 -1.03	12.3	12.0	83.7	0.26	0.65
	ELYSEE150 simple circuit	PSV12 ZEEP	*	*	*	*	*	*	*	*
	LEGENDAIR 2	PSV12 ZEEP 0.55 0.15 -0.97	12.1	12.0	7.4	0.85	0.83
	NEFTIS2 3	PSV12 ZEEP 0.97 0.15 -1.80	6.2	13.0	16.4	0.36	0.31
	SMARTAIR simple circuit 4	PSV12 ZEEP 0.23 0.08 -0.36	12.1	13.0	7.8	1.02	1.03
	VS INTEGRA simple circuit	PSV12 ZEEP	*	*	*	*	*	*	*	*
	VS ULTRA double circuit flow trigger	PSV12 ZEEP	*	*	*	*	*	*	*	*
	VS ULTRA simple circuit	PSV12 ZEEP	*	*	*	*	*	*	*	*
	VS UTRA double circuit pressure trigger 5 PSV12 ZEEP 0.47 0.13 -1.50	13.0	12.0	96.0	1.12	1.01
	4 autotriggering for a leak of 1.5L/min 5

Abbreviations: PEEP: positive end-expiratory pressure, T: trigger time delay, 1 does not detect the patient's inspiratory effort or autotriggering for a leak of 8L/min 2 autotriggering for a

Table 7 :

 7 Testing of the quality of the expiratory triggers by the difference between the patient's inspiratory time during spontaneous breathing (Tinsp) and the inspiratory time during mechanical ventilation (TI).

	Tinsp-TI (s)

Acknowledgements

The authors gratefully thank Emmanuelle Cohen, Claire Justine, and Chantal Boniface of their excellent technical assistance.

The research of Brigitte Fauroux is supported by the Association Française contre les Myopathies (AFM), Assistance Publique-Hôpitaux de Paris, Inserm, Legs Poix, and Université Pierre et Marie Curie-Paris6.

Abbreviations: PSV: pressure support ventilation ACV: assist control ventilation, PEEP : positive end-expiratory pressure, ZEEP : zero end-expiratory pressure. appropriate: T ≤ 100 ms and P ≤ -1 cmH 2 O acceptable: T ≤ 150 ms and P ≤ -1.5 cmH 2 O or if T ≥ 100 ms or P ≤ -1 cmH 2 O inappropriate: the ventilator does not detect the inspiratory effort or in case of autotriggering [START_REF] Fauroux | Long-term noninvasive mechanical ventilation for children at home: A national survey[END_REF] relatively insensitive to a leak: no triggering or auto-triggering for a leak ≥ 40 l/mn [START_REF] Simonds | Impact of nasal ventilation on survival in hypercapnic Duchenne muscular dystrophy[END_REF] moderately sensitive to a leak: no triggering or auto-triggering for a leak >10 and < 40 l/mn [START_REF] Mellies | Long-term noninvasive ventilation in children and adolescents with neuromuscular disorders[END_REF] very sensitive to a leak: no triggering or auto-triggering for a leak ≤ 10 l/mn a flow trigger b simple circuit c double circuit pressure trigger