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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(IO(2))). The purposes of this study were to determine the delivered F(IO(2)) 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(IO(2)). METHODS: We measured the pharyngeal F(IO(2)) 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(IO(2)) during mouth-open and mouth-closed breathing were compared using the dependent test for paired values, to determine if there were significant differences. RESULTS: The mean resting F(IO(2)) 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(IO(2)) ranged from 0.24-0.45 at 1-6 L/min to 0.49-0.72 at 6-15 L/min. The mean F(IO(2)) increased with increasing flow rates. The standard deviation (+/- 0.04-0.15) and range were large, and F(IO(2)) varied widely within and between subjects. F(IO(2)) during mouth-open breathing was significantly (p < 0.05) greater than that during mouth-closed breathing. CONCLUSIONS: F(IO(2)) increased with increasing flow. Subjects who breathed with their mouths open attained a significantly higher F(IO(2)), compared to those who breathed with their mouths closed.
Wettstein et al. (Sun,) studied this question.