Positioning and Flow Effects on Lung Volume in Asymmetric High-Flow Nasal Cannula

High-flow nasal cannula (HFNC) therapy improves oxygenation and increases end-expiratory lung volume (EELV). Asymmetric cannulas are a novel interface, but their interaction with body position and breathing route (mouth open vs closed) on EELV is unclear. This study used electrical impedance tomography (EIT) to assess the impact of these factors on end-expiratory lung impedance (EELI), a surrogate for EELV, in healthy subjects.

This prospective, randomized crossover physiological study included 16 healthy volunteers who received HFNC via an asymmetric cannula. EELI was measured across different body positions (Semi-Fowler's, High Fowler's, flat [180°] prone, and reverse Trendelenburg, flows (40 and 60 L/min), and mouth conditions (open/closed). A linear mixed-effects model compared EELI changes from baseline.

The greatest significant increase in global EELI occurred in the reverse Trendelenburg at 60 L/min with the mouth closed (difference = 2.07, P < .001). This condition was significantly superior to most other combinations. Mouth closure was a critical factor for increasing EELI. The reverse Trendelenburg specifically enhanced EELI in dorsal lung regions at high flow with a closed mouth.

Body position, flow setting, and mouth condition significantly impacted EELV during asymmetric HFNC. The combination of the reverse Trendelenburg, high flow (60 L/min), and mouth closure resulted in the greatest increase in EELV and promoted more homogeneous lung ventilation in healthy subjects. Encouraging nasal breathing and optimizing positioning could enhance the physiological benefits of HFNC.