This is the first physiological study that demonstrates that NPPV is able to unload the respiratory muscles in a group of selected children admitted to the PICU for acute moderate hypercapnic respiratory insufficiency. This unloading of the respiratory muscles was associated with a significant improvement in alveolar ventilation and gas exchange. Importantly, the efficacy of NPPV set on clinical noninvasive parameters was comparable to a setting based on invasive measurements. The clinical outcome of this group of selected patients was also favourable with no death.
Efficacy of pressure support ventilation
The clinical benefit of NPPV in the PICU in the present study is in agreement with previous clinical studies [3, 4]. More recently, we reported our own 5-year experience of NPPV in a large group of 114 children [2]. The overall NPPV success rate was high (between 67 and 100%), except in the patients with the acute respiratory distress syndrome (22%). The present study included patients with similar primary diagnoses than our descriptive study [2]. We acknowledge that the patients included in the present study represent a small group of patients presenting with moderate hypercapnic respiratory insufficiency due to various diseases. However, almost the uniform benefit of NPPV in this heterogeneous population has to be underscored. Our findings may not be extrapolated to patients admitted with more severe hypercapnic respiratory failure or with acute hypoxemic respiratory failure. Indeed, the benefit of NPPV has been shown to be less obvious in children with severe status asthmaticus [16, 17]. Of note, our results are in agreement with the studies performed in a more stable situation in children with hypercapnic respiratory failure due to cystic fibrosis or severe upper airway obstruction [6, 8–10, 18–20].
Setting of NPPV in the PICU
The ventilatory mode used was PS with PEEP. The levels of PS and PEEP used in our study are comparable to those used in other studies, both in the acute and the chronic setting [3, 4, 9, 17]. In 10 adults presenting an acute lung injury, L’Her et al. [21] found that noninvasive CPAP improved gas exchange but had a minimal effect on respiratory effort compared to PS with PEEP. Other clinical trials suggested that PS might be superior over CPAP in selected patient with acute lung injury with regard to the clinical response and/or the decrease in respiratory effort [22–24].
The sensitivity of the inspiratory and expiratory triggers is of great importance in children, in particular in case of “leak” ventilation such as NPPV. Indeed, the inability of the ventilator to detect the patient’s respiratory effort leads to patient-ventilator asynchrony. In the present study, patient-ventilator asynchrony was more common, occurring in 33% (4/12) of the patients. In adults, ineffective inspiratory efforts and double-triggering are the most common types of asynchrony during PS, with auto-triggering representing less than 1% of the asynchrony events. This was clearly different in our study, with auto-triggering representing the most frequent (3/4) type of patient-ventilator asynchrony. Ineffective triggering may be associated with a high inspiratory pressure, intrinsic PEEP, or a high V
t [25, 26]. Auto-triggering may be caused by a too sensitive inspiratory trigger, air leaks, or cardiac oscillations, particularly in young children [27]. In this physiological study, we used the Pes tracing to detect the patient’s inspiratory effort and his synchronisation with the ventilator, as has been performed in a series of previous studies [6, 9, 28]. Airway pressure (Paw) has also been used in some more recent studies [26, 29]. We recorded the Pes trace to evaluate the benefit of NPPV on the patient’s respiratory muscle output. Because of the accuracy and validity of the Pes signal, we also used this variable to assess the patient-ventilator synchrony. However, in the only patient presenting ineffective inspiratory efforts, ineffective triggering detected by the Pes signal correlated with the Paw signal, as reported by Thille et al. [26].
Most ICU ventilators have a cycling mechanism based on the achievement of a preset flow threshold. Leaks around the mask may prevent the airflow reaching the preset expiratory flow which leads to an abnormal prolongation of the inspiratory time and patient-ventilator mismatching. The excellent sensitivity of the ICU ventilator used in our study allowed the use a spontaneous mode, which contributed to the good adaptation and tolerance of the NPPV.
In practice, NPPV is set on clinical noninvasive parameters such as a decrease in RR and an improvement in gas exchange. These parameters have been shown to be of clinical value. Indeed, in our experience, the decrease in RR and PtcCO2 after 2 h of NPPV was significant predictors of the success of NPPV [2]. In another study, the FiO2 required to maintain SaO2 > 94% after 1 h of NPPV, was also predictive of the clinical outcome with NPPV [1]. In the present study, a clinical setting of NPPV was as efficacious as a physiological setting, as has been previously shown in stable patients with advanced cystic fibrosis lung disease [9]. However, as has been observed in children with cystic fibrosis [9], the synchronisation of the patient with the ventilator was generally better with the physiological setting. As such, a more in-depth physiological measurement could be proposed for patients who have difficulties to tolerate or acclimatise to NPPV.
Tolerance of NPPV
The tolerance of NPPV was excellent and only one patient failed NPPV. First, after a careful acclimatisation, NPPV was applied intermittently for a minimum of 2–4 h, two to four times per day. In our study, four patients developed skin injury, which resolved after the application of colloid dressing on the facial pressure points. Patient 10 used alternatively a face mask and the Helmet which has been shown to be an interesting interface in four children with acute leukaemia [30]. Gastro-oesophageal reflux is commonly observed during acute respiratory insufficiency in children and has also been reported during NPPV [31]. Our patients did not complain of gastric distension or emesis which may be explained by the systematic use of a naso-gastric tube connected to room air.
Limitation of the study
The present study included only 12 patients admitted to the PICU for acute moderate respiratory insufficiency due to heterogeneous diseases. However, these patients represent a small but real activity of a polyvalent PICU and it seems that despite the relative heterogeneity of primary diagnosis and age, these patients had an almost uniform increase in respiratory muscle output, which decreased significantly during NPPV. This observation is important for clinicians and probably rather an advantage than a limitation, by showing that NPPV may be a safe and efficient ventilatory support in various cases of moderate hypercapnic respiratory insufficiency in children.
As this study was a physiological study, no control group was included. This precludes a definite conclusion about the potential effectiveness of NPPV to ameliorate the outcome of these patients.
The recruitment of patients was limited by the difficulty to include the patients within the 12 h after initiation of NPPV, only one physician was able to record the physiological parameters, and written consent from the two parents was sometimes unavailable. Also, for technical reasons, we took the option not to include patients with a weight <10 kg.
Because oxygen therapy was systematically added to maintain a target SaO2 of at least 94%, the benefit of NPPV on oxygenation was difficult to establish. However, a beneficial effect of NPPV on oxygenation has been observed in previous studies [1, 3].
Another limitation of this study was the difficulty to record the flow trace during NPPV. Because of abrupt change of flow delivered by ICU ventilator, the flow trace was not analysable during and we took thus the option to use the V
t
e measured by the ventilator. However, this volume measured by the ventilator and the volume measured at the Y-piece seem to be well correlated in patients [32].