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The following is a summary of “High flow nasal cannula and low level continuous positive airway pressure have different physiological effects during de novo acute hypoxemic respiratory failure,” published in the November 2024 issue of Critical Care by Tuffet et al. 

Large tidal volumes in de novo acute hypoxemic respiratory failure (AHRF) were found to potentially cause patient self-inflicted lung injury, with tidal volume assessment under a high-flow nasal cannula (HFNC) not routinely feasible at the bedside.  

Researchers conducted a retrospective study to specify the tidal volume during low-level continuous positive airway pressure (CPAP) could predict tidal volume during HFNC and compare the physiological effects of both treatments.  

They included 29 patients with de novo AHRF treated with a HFNC at 50 to 60 L/min^-1. Electrical impedance tomography was used to monitor patients during HFNC and later during CPAP at 4 cmH₂O. Tidal volume during HFNC was calculated from tidal impedance variation. Bland-Altman analysis assessed whether tidal volume on low-level CPAP could predict tidal volume on HFNC, CPAP and HFNC were compared in terms of tidal volume, respiratory comfort, minute ventilation, oxygenation, dyspnea, ventilation distribution, end-expiratory lung volume, thoraco-abdominal asynchrony, and recruitment.  

The results showed that under a HFNC, tidal volume was 6.6 (5.9–8.7) mL/kg^-1 predicted body weight (PBW), 20 patients (69%) had tidal volumes between 4 and 8 mL/kg^-1 PBW, while 5 patients (17%) exceeded 9 mL/kg^-1 PBW. Under CPAP, tidal volume was higher at 9.4 (8.3–11) mL/kg^-1 PBW (P < 0.001). A moderate correlation was found between tidal volumes under CPAP and HFNC (Spearman r = 0.50, P = 0.005). Bland-Altman analysis exhibited a bias of 2.4 mL/kg^-1, with limits of agreement from −1.1 mL/kg^-1 to 5.9 mL/kg^-1. A tidal volume greater than 11.5 mL/kg^-1 PBW under CPAP predicted a tidal volume greater than 9 mL/kg^-1 PBW under HFNC with 80% sensitivity and 96% specificity, CPAP resulted in increased minute ventilation, end-expiratory lung volume, and oxygenation compared to HFNC. It also reduced respiratory distress in the most severe cases but was associated with lower comfort levels than HFNC.  

Investigators concluded that, among patients admitted to ICU with de novo AHRF, tidal volume under HFNC was primarily protective, and tidal volume during CPAP at 4 cmH₂O did not predict tidal volume during HFNC. 

Source: annalsofintensivecare.springeropen.com/articles/10.1186/s13613-024-01408-w