Effect of High-Flow Nasal Cannula and Body Position on End-Expiratory Lung Volume: A Cohort Study Using Electrical Impedance Tomography

BACKGROUND: Electrical impedance tomography measures changes in lung impedance, which are mainly related to changes in lung volume. We used electrical impedance tomography to investigate the effects of high-flow nasal cannula (HFNC) and body position on global and regional end-expiratory lung impedance variation (ΔEELI). METHODS: Prospective study with 20 healthy adults. Two periods were defined: the first in supine position and the second in prone position. Each period was divided into 3 phases. In the first and the third phases the subjects were breathing ambient air, and in the second HFNC was implemented. Four regions of interest were defined: 2 ventral and 2 dorsal. For each respiratory cycle, global and regional ΔEELI were measured by electrical impedance tomography and were expressed as a function of the tidal variation of the first stable respiratory cycle (units). RESULTS: HFNC increased global EELI by 1.26 units (95% CI 1.20–1.31, P < .001) in supine position, and by 0.87 units (95% CI 0.82–0.91, P < .001) in prone position. The distribution of ΔEELI was homogeneous in prone position, with no difference between ventral and dorsal lung regions (−0.01 units, 95% CI −0.01 to 0, P = .18), while in supine position a significant difference was found (0.22 units, 95% CI 0.21–0.23, P < .001) with increased EELI in ventral areas. CONCLUSIONS: HFNC increased global EELI in our population, regardless of body position, suggesting an increase in functional residual capacity. Prone positioning was related to a more homogeneous distribution of ΔEELI, while in supine position ΔEELI was higher in the ventral lung regions.