Physiological Effects of Combining High-Flow Nasal Cannula and Helmet CPAP in Acute Hypoxemic Respiratory Failure: A Randomized Crossover Study

Background
 High-flow nasal cannula (HFNC) and helmet continuous positive airway pressure (HCPAP), are commonly used to prevent endotracheal intubation in acute hypoxemic respiratory failure (AHRF). However, while HFNC reduces inspiratory effort, HCPAP may increase it despite improving oxygenation. The combined use of HFNC and HCPAP (HFNCPAP) has not been extensively studied. This study aimed to assess the impact of these modalities on inspiratory effort, respiratory mechanics, and gas exchange. Methods
 In this randomized crossover physiological study, 18 ICU patients with AHRF underwent three 30-minute trials with HFNC, HCPAP, and HFNCPAP in random order. Esophageal pressure swings (ΔPes) and pressure-time product (PTPes) were measured as primary outcomes, along with transpulmonary driving pressure (∆PL), respiratory rate, gas exchange, and patient comfort. Variables were compared using Wilcoxon signed-rank tests with Bonferroni correction for multiple comparisons. Results
 Compared to HFNC, HCPAP significantly increased inspiratory effort, as indicated by higher ΔPes (15.7 [10.8, 18.9] vs. 11.9 [8.2, 16.4] cmH2O, p < 0.001) and PTPes (p < 0.001). When HFNC was added to HCPAP (HFNCPAP), inspiratory effort was mitigated, with ΔPes (14.3 [11.0, 18.6] cmH2O) and PTPes (p = 0.68) similar to HFNC alone. HFNCPAP also reduced respiratory rate (p < 0.001) and improved gas exchange compared to HFNC. Conclusions
 While HCPAP alone increases inspiratory effort, adding HFNC mitigates this effect without compromising gas exchange, potentially optimizing patient comfort. Notably, although HCPAP appears to increase the work of breathing, this did not translate into safety concerns, as transpulmonary pressure swings (ΔPL) remained unchanged.