Functional residual capacity (FRC) is the volume remaining in the lungs at exhalation following normal tidal volume breathing. FRC is approximately 2,500 mL in a healthy adult and is reduced as a patient is moved from an upright to a supine or prone position. FRC is additionally reduced by 15% to 20% following induction of anesthesia. During preoxygenation, the patient inspires 100% oxygen via a face mask before induction, replacing nitrogen with oxygen in the patient’s FRC. Normal oxygen consumption in a healthy adult is approximately 250 mL/min. Oxygen desaturation may occur as rapidly as 30 to 60 seconds in a healthy adult with an FRC of 21% oxygen following induction of anesthesia and subsequent apnea, despite normal initial oxygen saturation. Denitrogenation during spontaneous breathing is 95% complete within 3 minutes when a patient is breathing a normal tidal volume of 100% oxygen. This increases the margin of safety to approximately 4 to 6 times during periods of apnea following induction of anesthesia. Preoxygenation with eight maximum deep breaths over 60 seconds results in arterial oxygenation that is not different from tidal volume breathing for 3 minutes. This technique increases minute ventilation above
FRC and minimizes nitrogen rebreathing, ensuring washout of FRC; additionally, taking eight deep breaths may open collapsed airways, increasing FRC oxygen store. Four maximum breaths over 30 seconds also increases arterial oxygenation, but the time for hemoglobin desaturation is shortened compared with patients breathing normal tidal volume for 3 minutes or taking eight maximum breaths over 60 seconds.