BACKGROUND: A review of the literature reveals that research on the effects of mouth vs nose breathing and resting (or 'normal') vs hyperventilation on oxygen concentrations delivered by nasal cannula has been limited. MATERIALS & METHODS: 15 healthy subjects fitted with hypopharyngeal catheters for gas sampling and nasal cannulas at 2 L/min breathed at resting and hyperventilation levels through valved systems that allowed only mouth breathing or only nose breathing. RESULTS: Oxygen concentrations were significantly different with mouth-open vs mouth-closed at resting and hyperventilation levels and with mouth-closed resting vs mouth-closed hyperventilation. Mean FIO2 with mouth-open resting ventilation at 2 L/min was only 0.24 and with mouth-open hyperventilation only 0.23. CONCLUSION: The data suggest that guidelines for the use of the nasal cannula and adjustment of flowrates may need to be re-examined. Breathing through the nose should be encouraged for maximum FIO2 at a given oxygen flowrate.
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