Hypoxemia during apnea in normal subjects: mechanisms and impact of lung volume.

Seven normal awake males were studied to define the mechanisms and impact of lung volume on the hypoxemia occurring during apnea. During repeated 30-s voluntary breath holding, these subjects were studied at different lung volumes, during various respiratory maneuvers, and in the sitting and supine body positions. Analysis of expired gases and arterial O2 saturation during these repeated breath holdings yielded the following conclusions. Apnea of 30-s duration at low lung volumes is accompanied by severe arterial O2 desaturation in normal awake subjects. Initial lung volume is the most important determinant of hypoxemia during apnea. The hypoxemia of apnea at most lung volumes can be explained by simple alveolar hypoventilation in a uniform lung. The lung does not behave as a single-compartment model at lung volumes at which dependent airways are susceptible to closure.