Respective Effects of Helmet Pressure Support, Continuous Positive Airway Pressure, and Nasal High-Flow in Hypoxemic Respiratory Failure: A Randomized Crossover Clinical Trial

Abstract

Rationale: The respective effects of positive end-expiratory pressure (PEEP) and pressure support delivered through the helmet interface in patients with hypoxemia need to be better understood. Objectives: To assess the respective effects of helmet pressure support (noninvasive ventilation [NIV]) and continuous positive airway pressure (CPAP) compared with high-flow nasal oxygen (HFNO) on effort to breathe, lung inflation, and gas exchange in patients with hypoxemia (Pa_O2/Fi_O2 ⩽ 200). Methods: Fifteen patients underwent 1-hour phases (constant Fi_O2) of HFNO (60 L/min), helmet NIV (PEEP = 14 cm H₂O, pressure support = 12 cm H₂O), and CPAP (PEEP = 14 cm H₂O) in randomized sequence. Measurements and Main Results: Inspiratory esophageal (ΔP_ES) and transpulmonary pressure (ΔP_L) swings were used as surrogates for inspiratory effort and lung distension, respectively. Tidal Volume (Vt) and end-expiratory lung volume were assessed with electrical impedance tomography. ΔP_ES was lower during NIV versus CPAP and HFNO (median [interquartile range], 5 [3-9] cm H₂O vs. 13 [10-19] cm H₂O vs. 10 [8-13] cm H₂O; P = 0.001 and P = 0.01). ΔP_L was not statistically different between treatments. Pa_O2/Fi_O2 ratio was significantly higher during NIV and CPAP versus HFNO (166 [136-215] and 175 [158-281] vs. 120 [107-149]; P = 0.002 and P = 0.001). NIV and CPAP similarly increased Vt versus HFNO (mean change, 70% [95% confidence interval (CI), 17-122%], P = 0.02; 93% [95% CI, 30-155%], P = 0.002) and end-expiratory lung volume (mean change, 198% [95% CI, 67-330%], P = 0.001; 263% [95% CI, 121-407%], P = 0.001), mostly due to increased aeration/ventilation in dorsal lung regions. During HFNO, 14 of 15 patients had pendelluft involving >10% of Vt; pendelluft was mitigated by CPAP and further by NIV. Conclusions: Compared with HFNO, helmet NIV, but not CPAP, reduced ΔP_ES. CPAP and NIV similarly increased oxygenation, end-expiratory lung volume, and Vt, without affecting ΔP_L. NIV, and to a lesser extent CPAP, mitigated pendelluft. Clinical trial registered with clinicaltrials.gov (NCT04241861).

Keywords: acute hypoxemic respiratory failure; acute respiratory distress syndrome; helmet support; noninvasive ventilation.

Figure 1.

Representative figure of two patients (left and right panels) during the trial, showing pendelluft between the ventral and dorsal regions of the lung. Pendelluft percentage was calculated as described in the Methods and the online supplement. In both patients, during muscular inspiration (first panel), air moves from the ventral regions of interest (ROIs) (third panel) toward the dorsal ROIs (fourth panel) and vice versa during muscular expiration. This pendelluft effect is represented as the yellow area that moves from the ventral ROIs (third panel) to the dorsal ROIs (fourth panel). The second panel shows tidal impedance variation (TidalΔZ, a surrogate of Vt) computed as: 1) the difference between the end-expiratory and end-inspiratory impedance during the breathing cycle (dashed line); and 2) the gas inflating the lungs during the breathing cycle, calculated on a pixel-by-pixel basis (solid green line). TidalΔZ, computed as the difference between the end-expiratory and end-inspiratory impedance (dashed line), does not consider the intratidal shift between the ventral and dorsal ROIs. This yields underestimation of the volume of lung distension, especially in the dorsal regions. TidalΔZ, calculated on a pixel-by-pixel basis (solid green line), represents a more accurate estimate of the amount of lung distension. A detailed description of the TidalΔZ calculation is provided in the online supplement. P_ES = esophageal pressure; ROIsΔZ = tidal impedance variation within ROIs; ΔP_ES = inspiratory effort.

Figure 2.

Individual patient values and medians of inspiratory effort (ΔP_ES) swings, pressure–time product of the P_ES (PTP_ES), and quasistatic transpulmonary pressure (ΔP_L) during the three phases of the study. Compared with high-flow nasal oxygen (HFNO) and helmet continuous positive airway pressure (CPAP), helmet NIV reduced inspiratory effort (ΔP_ES) and muscle workload only compared with CPAP (PTP_ES). ΔP_L was not different between treatments. NIV = noninvasive ventilation.

Figure 3.

Individual patient values and medians of Pa_O2/Fi_O2, Pa_CO2, respiratory rate, and visual analog score (VAS)-measured patient dyspnea and discomfort during the three phases of the study. NIV and helmet continuous positive airway pressure (CPAP) yielded higher Pa_O2/Fi_O2 than HFNO. Pa_CO2, respiratory rate, and VAS dyspnea were not different between treatments. Discomfort increased during CPAP. HFNO = high-flow nasal oxygen; NIV = noninvasive ventilation.

Figure 4.

Individual patient values and medians of the impedance-derived measures during high-flow nasal oxygen (HFNO), helmet continuous positive airway pressure (CPAP), and helmet noninvasive ventilation (NIV). Both helmet NIV and helmet CPAP increased the Vt compared with HFNO. Quasistatic respiratory system compliance was similar between HFNO and helmet NIV and showed a trend toward an increase during helmet CPAP. The application of positive end-expiratory pressure caused a significant increase in the end-expiratory lung impedance, which was related to recruitment in the dorsal lung regions, so the dynamic lung strain during helmet NIV and helmet CPAP decreased. Because of increased aeration of the dependent lung regions, both helmet NIV and helmet CPAP reduced the pendelluft effect.

Figure 5.

Intrapulmonary distribution of air in one representative patient. The patient showed a high pendelluft percentage during HFNO (65% of Vt), which progressively decreased with both helmet CPAP (38% of Vt) and NIV (26% of Vt) (top): red pixels represent those deflating during inspiration, with color intensity displaying percentage of Vt contributing to pendelluft. Consistent with the reduction in pendelluft and the increase in end-expiratory lung impedance, there was a reduction in the global and regional dynamic strain (bottom row). In addition, there was a reduction in the amounts of the regions where dynamic strain was >2 (in red in the bottom row). CPAP = continuous positive airway pressure; HFNO = high-flow nasal oxygen; NIV = noninvasive ventilation.