Delivered Oxygen Concentrations Using Low-Flow and High-Flow Nasal Cannulas

INTRODUCTION: Nasal cannulas are commonly used to deliver oxygen in acute and chronic care settings; however, there are few data available on delivered fraction of inspired oxygen (F_IO2). The purposes of this study were to determine the delivered F_IO2 on human subjects using low-flow and high-flow nasal cannulas, and to determine the effects of mouth-closed and mouth-open breathing on F_IO2.

METHODS: We measured the pharyngeal F_IO2 delivered by adult nasal cannulas at 1–6 L/min and high-flow nasal cannulas at 6–15 L/min consecutively in 10 normal subjects. Oxygen was initiated at 1 L/min, with the subject at rest, followed by a period of rapid breathing. Gas samples were aspirated from a nasal catheter positioned with the tip behind the uvula. This process was repeated at each liter flow. Mean, standard deviation, and range were calculated at each liter flow. F_IO2 during mouth-open and mouth-closed breathing were compared using the dependent t test for paired values, to determine if there were significant differences.

RESULTS: The mean resting F_IO2 ranged from 0.26–0.54 at 1–6 L/min to 0.54–0.75 at 6–15 L/min. During rapid breathing the mean F_IO2 ranged from 0.24–0.45 at 1–6 L/min to 0.49–0.72 at 6–15 L/min. The mean F_IO2 increased with increasing flow rates. The standard deviation (±0.04–0.15) and range were large, and F_IO2 varied widely within and between subjects. F_IO2 during mouth-open breathing was significantly (p < 0.05) greater than that during mouth-closed breathing.

CONCLUSIONS: F_IO2 increased with increasing flow. Subjects who breathed with their mouths open attained a significantly higher F_IO2, compared to those who breathed with their mouths closed.