Hematocrit variation during sleep apnea in elephant seal pups

Hematocrit (Hct) levels were found to increase throughout long-duration sleep apnea in the northern elephant seal pup. Breath holds ranging from 4 to greater than 11 min were sampled. Blood collected from the extradural intravertebral vein showed an increase in Hct from 55.6 to 63.1% with the peak value occurring within 1 min of the end of apnea. Plasma lactate did not change throughout the breath hold. Previous observations of changing Hct levels in seals were made on animals diving naturally, and consequently the effects of exercise could not be separated from those of apnea. The results of this study demonstrate that the rise in Hct is produced quickly upon apnea, occurs without significant levels of hypoxic metabolism, and does not depend on exercise.