High flow nasal cannula and low level continuous positive airway pressure have different physiological effects during de novo acute hypoxemic respiratory failure.

Background: Large tidal volumes during de novo acute hypoxemic respiratory failure (AHRF) may promote patient self-inflicted lung injury. Tidal volume assessment under high flow nasal cannula (HFNC) is not routinely feasible at the bedside. Our objective was to determine whether tidal volume during low-level continuous positive airway pressure (CPAP) could predict tidal volume during HFNC and to compare the physiological effects of HFNC and low-level CPAP.

Methods: Prospective, single-center study including 29 de novo AHRF patients treated with HFNC (50 to 60 L.min^- 1). Patients were monitored using electrical impedance tomography during HFNC then CPAP at 4 cmH₂O. Tidal volume during HFNC was calculated based on tidal impedance variation. The ability of tidal volume under low-level CPAP to predict tidal volume under HFNC was explored using Bland-Altman analysis. CPAP and HFNC were compared in terms of tidal volume, minute ventilation, respiratory comfort, dyspnea, oxygenation, ventilation distribution, end-expiratory lung volume, thoraco-abdominal asynchrony and recruitment.

Results: Under HFNC, patients had a tidal volume of 6.6 (5.9-8.7) mL.kg^- 1 PBW. 20 (69%) patients exhibited a tidal volume between 4 and 8 mL.kg^- 1 PBW, while in 5 (17%) patients it exceeded 9 mL.kg^- 1 PBW. Tidal volume under CPAP was higher (9.4 (8.3-11) mL.kg^- 1 PBW, p < 0.001). Tidal volumes under CPAP and under HFNC were modestly correlated (Spearman r = 0.50, p = 0.005). Bland-Altman analysis showed a bias of 2.4 mL.kg^- 1, with limits of agreement ranging from - 1.1 mL.kg^- 1to 5.9 mL.kg^- 1. Nevertheless, a larger (> 11.5 mL.kg^- 1 PBW ) tidal volume under low-level CPAP predicted a larger (> 9 mL.kg^- 1 PBW ) tidal volume under HFNC with 80% sensitivity and 96% specificity. Low-level CPAP was associated with increased minute ventilation, end-expiratory lung volume, and oxygenation as compared to HFNC. It decreased signs of respiratory distress in the most severe patients but was associated with lower comfort compared to HFNC.

Conclusion: Among ICU patients with de novo AHRF, tidal volume under HFNC was mostly protective. Tidal volume during CPAP at 4 cmH₂O did not predict tidal volume during HFNC. Such low-level CPAP was associated with increased tidal volume, minute ventilation, end-expiratory volume, and oxygenation.

Trial registration: ClinicalTrials.gov ID NCT03919331. Registration date: 2019-03-26.