Fig. 1.

Bench study and physiological measurements. Changes in pressure and volume related to the oxygen flow rate. a Mean (bar chart) ± standard deviation (error bar) of end expiratory pressure recorded on the oro-nasal mask of Boussignac FF-CPAP assembled with the DAR filter (see text), obtained at five constant oxygen flow rates (10, 15, 20, 25, and 30 L/min) at four simulated inspiratory efforts (from weak to strong). Irrespective of the simulated respiratory effort, the observed end expiratory pressure on the mask was at 2, 4, 6, 8, and 10 cm H₂O approximately, for oxygen flow of 10, 15, 20, 25, and 30 L/min, respectively. Green circles denote mean values recorded in four COVID-19 patients. b Tidal volume recorded for each combination of simulated inspiratory effort and constant oxygen flow. Tidal volume variations depended on simulated respiratory effort variations but not on oxygen flow (thus not on end expiratory pressure) variations