Effect of Helmet CPAP on Tidal Volume Assessed by Bioelectrical Impedance

Introduction

Patients with acute respiratory failure may exhibit an excessive respiratory drive that often results in large tidal volumes ( ${\text{V}}_{\text{T}}$),¹ potentially leading to patient self-inflicted lung injury.^2,3 When noninvasive support strategies are used, clinicians often have limited control over large ${\text{V}}_{\text{T}}$, which have been reported to be independently associated with failure of noninvasive respiratory support.⁴ Therefore, monitoring ${\text{V}}_{\text{T}}$ and minute ventilation holds high clinical relevance for optimizing the management of these patients.

CPAP is a noninvasive respiratory support modality that delivers an adjustable PEEP level that is chosen by the clinician, with free flow and no inspiratory pressure support. Accordingly, previous literature shows that CPAP does not increase ${\text{V}}_{\text{T}}$ in patients with acute respiratory failure.⁵ In addition, in a recently published study, the application of CPAP in a murine model of acute lung injury was shown to be a protective technique at both pulmonary and diaphragmatic levels for patient self-inflicted lung injury.⁶ However, a recent study by Menga et al⁷ reported that both CPAP and pressure-support ventilation delivered by a helmet resulted in a marked increase of ${\text{V}}_{\text{T}}$ when measured as tidal impedance variation with the electrical impedance tomography compared with a high-flow nasal cannula.

We recently validated, in healthy subjects, the ExSpiron (Respiratory Motion, Watertown, Massachusetts), a device that uses bioelectrical impedance technology, which allows noninvasive monitoring of ${\text{V}}_{\text{T}}$ and minute ventilation.⁸ We performed a pilot observational study that evaluated the impact of helmet CPAP on respiratory parameters. Specifically, we used the ExSpiron to verify if the application of helmet CPAP resulted in increased ${\text{V}}_{\text{T}}$ compared with baseline (before helmet CPAP).

Methods

We designed a pilot physiological study that included adult patients with clinical indications for CPAP either during the acute phase of respiratory distress or in the postextubation setting, pre-emptively addressing postextubation respiratory failure.⁹ The study was approved in March 2022 by the institutional review board (reference 3993) All included subjects provided written informed consent. Fresh gas flow was administered by using the DIMAR flow generator equipped with the Easy Flow monitor (DIMAR, Medolla, Italy), which offers real-time data on the fresh gas flow and delivered oxygen fraction. DimAir 500/9666 helmets (DIMAR) were used with the provided adjustable PEEP valve.

The study involved 2 steps:

1. Baseline (before helmet application)

2. Helmet CPAP (1 h after helmet application)

${\text{F}}_{{\text{IO}}_{\text{2}}}$ was maintained constant. ExSpiron tracings were recorded for 10 min during each step and analyzed offline. At the end of the recordings, arterial blood samples were obtained from an indwelling line for gas analysis, and clinical parameters were recorded. To analyze the variation in respiratory parameters ( ${\text{V}}_{\text{T}}$, breathing frequency, and minute ventilation) during the study, we used a restricted maximum likelihood method to fit a general linear mixed model. In this model, the step was considered as the independent factor, whereas subjects were treated as random effects. Clinical parameters and blood gas analyses results recorded at the end of each step were compared by using paired Student t tests. To assess whether subject severity may impact the change in ${\text{V}}_{\text{T}}$ before and after helmet CPAP implementation, we divided subjects into 2 groups based on their baseline ${\text{P}}_{{\text{aO}}_{\text{2}}}$/ ${\text{F}}_{{\text{IO}}_{\text{2}}}$, either below or above the median value. Statistical significance was considered with an alpha level of 0.05 (2 tailed). Statistical analysis was performed with JMP 16.0 (SAS Institute, Cary, North Carolina).

Results

Twenty patients participated in this study, including 6 women (30%) and 14 men (70%). The median (interquartile range [IQR]) age was 59 (55–71) y, and the median (IQR) body mass index was 27 (24–32) kg/m². The most common diagnoses at ICU admission were pneumonia (5 [25%]), septic shock (5 [25%]), lung immunologic disorders (3 [15%]), and polytrauma (2 [10%]). The median IQR) Sequential Organ Failure Assessment score at ICU admission was 3 (2–5). The helmet was used as a noninvasive support after extubation in 15 subjects (75%), who had a median (IQR) duration of invasive ventilation of 5 (2–8) d. The remaining 5 subjects required noninvasive support for acute hypoxemic respiratory failure. The median (IQR) ${\text{F}}_{{\text{IO}}_{\text{2}}}$ was 0.5 (0.4-0.5), the median (IQR) PEEP was 8 (8-8) cm H₂O, and the median (IQR) gas flow into the helmet CPAP was 90 (80–95) L/min. Oxygen therapy at the baseline condition was administered via air-entrainment mask in 11 subjects (55%) when using high-flow nasal cannula in 8 subjects (40%), and via nasal prongs in 1 subject (5%). The study results are displayed in Figure 1 and Table 1.

Fig. 1.

Changes in A: tidal volume (spaghetti plot); B: breathing frequency (bar plot); and C: minute ventilation (bar plot) over the 2 study steps. Data are presented as mean ± SD.

Table 1.

Blood Gas Analysis, Respiratory, and Hemodynamics Parameters in the Study

${\text{V}}_{\text{T}}$ did not change after helmet CPAP implementation (P = .52). Breathing frequency and minute ventilation mildly decreased (P < .001 for both parameters). Arterial blood gas analysis revealed a significant increase in ${\text{P}}_{{\text{aO}}_{\text{2}}}$/ ${\text{F}}_{{\text{IO}}_{\text{2}}}$ after helmet CPAP application, while ${\text{P}}_{{\text{aCO}}_{\text{2}}}$ remained stable. When the subjects were stratified into 2 groups based on their ${\text{P}}_{{\text{aO}}_{\text{2}}}$/ ${\text{F}}_{{\text{IO}}_{\text{2}}}$, in the low ${\text{P}}_{{\text{aO}}_{\text{2}}}$/ ${\text{F}}_{{\text{IO}}_{\text{2}}}$ group, the mean ± SD ${\text{V}}_{\text{T}}$ was 452 ± 85 mL at baseline and 457 ± 9 mL during helmet CPAP. Similarly, in the high ${\text{P}}_{{\text{aO}}_{\text{2}}}$/ ${\text{F}}_{{\text{IO}}_{\text{2}}}$ group, the mean ± SD ${\text{V}}_{\text{T}}$ was 553 ± 238 mL at baseline and 543 ± 186 mL during helmet CPAP.

Discussion

In this pilot physiological study, we analyzed the impact of helmet CPAP on the respiratory pattern and gas exchange. Oxygenation was significantly improved, breathing frequency and minute ventilation were mildly reduced, whereas ${\text{V}}_{\text{T}}$ did not vary. Our findings may have important clinical implications. Helmet CPAP is known to be an effective strategy for improving oxygenation by recruiting and stabilizing collapsed alveoli, ultimately reducing the work of breathing and enhancing ventilation-perfusion matching. The enhancement in oxygenation can be attributed to both the PEEP level and the precise delivery of oxygen fraction.¹⁰ Compared with pressure-support ventilation, CPAP does not provide any inspiratory support. For this reason, PEEP is expected to increase end-expiratory lung volume without causing a rise in ${\text{V}}_{\text{T}}$.¹¹

A previous study by L’Her et al⁵ confirmed this hypothesis by showing that CPAP did not increase ${\text{V}}_{\text{T}}$ during noninvasive support and resulted in lower ${\text{V}}_{\text{T}}$ compared with noninvasive ventilation. Our results corroborate these findings, showing that PEEP delivered through a helmet interface does not trigger an increase in ventilation. Menga et al⁷ reported different findings: in their physiological study, both CPAP and noninvasive ventilation by helmet significantly increased ${\text{V}}_{\text{T}}$ compared with baseline. We may explore the underlying reasons for these conflicting results. A relevant difference among the 3 studies is the level of PEEP delivered noninvasively during CPAP: a moderate PEEP level was used both in our study and in the work by L’Her et al⁵ (8 and 10 cm H₂O, respectively), whereas the PEEP level was very high (14 cm H₂O) in the work by Menga et al.⁷

The method for measuring the ${\text{V}}_{\text{T}}$ also differed. In our study, we used the ExSpiron device for this purpose, which demonstrated excellent trending capability in detecting changes in ${\text{V}}_{\text{T}}$.⁸ L'Her et al⁵ used a pneumotachograph between the mask and the Y-piece, which is not a feasible option in patients on continuous flow helmet CPAP. Menga et al⁷ indirectly estimated ${\text{V}}_{\text{T}}$ by analyzing the tidal impedance variation (arbitrary units), which was doubled during helmet CPAP compared with baseline (high-flow nasal cannula). We must note that the estimated end-expiratory lung impedance was increased by almost 3-fold, which may have biased the ${\text{V}}_{\text{T}}$ estimation. No significant change in inspiratory effort and transpulmonary driving pressure was reported between the 2 modes. In addition, ${\text{P}}_{{\text{aCO}}_{\text{2}}}$ levels did not change, which is inconsistent with the reported doubling of minute ventilation. Also, the 3 studies^7,8 varied in their study settings, including the baseline oxygen delivery method and the CPAP configuration (eg, fresh gas flow, see below). This variability in settings may have partially contributed to the differences observed in their results.

The use of a helmet interface for delivering noninvasive ventilation can lead to carbon dioxide rebreathing.¹² Therefore, its application with a ventilator is generally discouraged unless a high flow-by can be set. Free-flow systems allow for the delivery of high fresh gas flows, minimizing the issue of rebreathing. In our study, our CPAP settings used a very high gas flow (median of 90 L/min), which was particularly advantageous for CO₂ washout, creating conditions similar to those used in the study by L’Her et al⁵ with mask CPAP. A lower free flow in the helmet CPAP system could result in increased minute ventilation due to CO₂ rebreathing, potentially leading to different results compared with the present study (ie, an increase in ${\text{V}}_{\text{T}}$).

The use of the ExSpiron device for noninvasive monitoring allowed the evaluation of alterations in ${\text{V}}_{\text{T}}$ subsequent to the introduction of helmet CPAP. Our findings support the use of helmet CPAP as a valuable therapeutic approach for patients not intubated and experiencing respiratory distress. This technique improves oxygenation without the undesirable effect of increasing ${\text{V}}_{\text{T}}$. Further research and clinical exploration are needed to reinforce and validate these findings.