To the Editor,The continuing professional development article by Drs. Tanoubi, Drolet, and Donati regarding optimizing preox-ygenation addresses a very important aspect of anesthesia. Unfortunately, their explanation of the related respiratory physiology overlooks one important point. The alveolar fraction of oxygen is diluted not only by the alveolar CO_2 but also by the saturated water vapor. Even with complete denitrogenation, there is about an 87 mmHg dilution (PaCO_2 40 mmHg and saturated water vapor pressure 47 mmHg at 37°C). Thus, even ideally, the end-tidal O_2 is not more than about 89%.
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