629: Effect of Flow Settings on Lung Impedance: A Randomized Crossover Study in Healthy Subjects

Introduction: High-flow nasal cannula (HFNC) oxygen therapy reduces work of breathing and improves oxygenation by providing flows that meet a patient’s peak inspiratory flow during tidal breathing. Despite its widespread use, the optimal flow setting remains unclear. Studies suggest a positive correlation between HFNC flows and end-expiratory lung impedance (EELI), the findings have been inconsistent. This study evaluated the effect of various flows on EELI using a novel HFNC system (HFT 750, MEKICS Ltd) and Electrical Impedance Tomography (EIT) device in healthy adults. Methods: Upon approval from ethic committee (No.23101708-IRB01), this randomized crossover trial enrolled healthy adults to receive HFNC at different flow settings (20, 40, and 60 L/min) in a random order. Some participants also received additional testing with two Bi-flow settings (20-20 and 20-40 L/min). Each setting was used for 20 minutes, followed by a 5-minute wash out period during which HFNC was removed. Before each flow, tidal volume and respiratory rate (RR) were obtained by a respiratory monitor. Throughout the study periods, EELI and tidal volume variation were monitored using EIT. Results: 26 subjects were enrolled between May and October 2024. All completed the standard flow interventions, and 12 also underwent Bi-flow. Compared to baseline, EELI significantly increased at 40 and 60 L/min, with increments of 74±16 and 131±20 units, respectively (both p< 0.001), but not 20 L/min (p=0.551). Flow at 60 L/min yielded higher EELI than both 20 and 40 L/min (p=0.002 and p< 0.001). HFNC reduced RR by 2±0.6, 4±1, and 5±0.8 breathes/min at 20, 40 and 60 L/min, respectively (p=0.020, < 0.001, < 0.001). Only 60 L/min increased tidal volume variation (22±5 units, p=0.001). Both Bi-flow settings (20-20 L/min and 20-40 L/min) increased EELI compared to baseline (78±59 and 67±60 units; p=0.001 and 0.003), but did not change tidal volume variation (both p>0.05). Conclusions: In healthy volunteers, HFNC significantly increased EELI only at flow of 40 L/min or higher. RR decreased with increasing flows, but tidal volume variation increased only at 60 L/min. Both Bi-flow settings enhanced EELI without significantly altering tidal volume variation.