Effect of High-Flow Nasal Cannula on Expiratory Muscle Activity in Critically Ill Subjects

Background: High-flow nasal cannula (HFNC) is commonly used for acute hypoxemic respiratory failure and for subjects at risk of postextubation failure. HFNC reduces inspiratory work, but its effects on expiratory muscle activity, including the abdominal wall muscles, are uncertain. We tested whether HFNC decreases expiratory effort in a flow-dependent manner. Methods: In this single-center, randomized, crossover trial, we enrolled 30 ICU subjects requiring HFNC. After baseline low-flow oxygen, subjects received HFNC at 30, 40, and 50 L/min in random order for 15 min each. We quantified expiratory muscle activity by ultrasound-derived thickening fractions of the lateral abdominal muscles (TF ABD ; external oblique + internal oblique + transversus abdominis) and rectus abdominis (TF RA ). We also recorded diaphragm thickening fraction (DTF) and hemodynamic variables. Results: Overall, HFNC did not change TF ABD , TF RA , or respiratory and hemodynamic variables versus baseline. DTF decreased at 50 L/min (mean 13% vs 25% at baseline; P = .042). In exploratory analyses, subjects with higher baseline expiratory activity (TF ABD > 10%) showed reduced TF ABD at 40 and 50 L/min, whereas those with baseline TF ABD ≤ 10% did not. Baseline TF ABD correlated inversely with flow-related changes at all flows (eg, r = −0.867 at 40 L/min; P < .001). Conclusions: HFNC did not uniformly reduce expiratory muscle activity in this relatively stable ICU cohort. Subjects with higher baseline expiratory activity showed abdominal muscle unloading at higher flows. Bedside abdominal ultrasound may help identify subjects more likely to benefit from higher HFNC flow settings.