Overnight decrease in hematocrit after nasal CPAP treatment in patients with OSA.

To clarify the paradox of a decrease in urine and sodium excretion occurring along with the elimination of peripheral edema when patients with obstructive sleep apnea (OSA) are treated with nasal continuous positive airway pressure (CPAP), we investigated the immediate effects of this treatment on the hematocrit and red blood cell count in eight patients with OSA. The hematocrit decreased in all patients, from a mean of 45.6 +/- 1.2 percent to 43.0 +/- 1.4 percent, with a parallel decrease in the red blood cell count from 4.777 +/- 0.168 millions/cu mm to 4.577 +/- 0.174 millions/cu mm (p less than 0.0005, one-tailed, in both cases). These results suggest that nasal CPAP treatment causes a hemodilution in patients with OSA, and are compatible with the hypothesis of an atrial natriuretic peptide-induced fluid shift from the intravascular to the extravascular volume in untreated patients with OSA. The reversal of these changes with CPAP treatment could explain the simultaneous decrease in sodium and urine excretion and the reduction of peripheral edema.