Fig. 2
Patient response to different flow ratios. On the top 2 figures, X-axis is the flow ratio of setting HFNC flow to patients’ peak inspiratory flow during tidal breathing and the flow ratios are divided into four groups (≤ 1, 1.01–1.33, 1.34–1.67, and ≥ 1.68). On the Y-axis, the ratio of patients’ SpO2/FIO2 (left top) or ROX (right top) at the flow setting to SpO2/FIO2 or ROX at their matching flow are shown. As the flow ratio increased, the SpO2/FIO2 ratio increased. Similarly, compared to ROX ratio with flow ratios ≤ 1.33, ROX ratio was higher with flow ratios ≥ 1.34–1.67 and 1.68, but ROX ratio did not increase beyond flow ratios of 1.34–1.67. RR ratio was lower with flow ratios of 1.34–1.67 than with flow ratios ≤ 1 (left bottom). No significant differences of comfort score ratios were found at different flow ratios (right bottom). *p < 0.05 compared to flow ratios ≤ 1. #p < 0.05 compared to flow ratios of 1.01–1.33. &p < 0.05 compared to flow ratios of 1.34–1.67. SpO2, pulse oximetry; FIO2, fraction of inspired oxygen; RR, respiratory rate; ROX = (SpO2/FIO2)/RR