Although right heart failure is a recognized complication of obstructive sleep apnea, the incidence and pathogenesis of this complication have not been established. We therefore studied 50 consecutive patients with obstructive sleep apnea to determine the incidence of right heart failure and the factors involved in its development. Six patients (12%) were found to have right heart failure. There were no differences in the number of apneas between those with right heart failure (mean +/- SE, 30 +/- 10 per h sleep) and those without right heart failure (33 +/- 4 per h sleep). In contrast, mean nocturnal oxygen saturation was lower in patients with right heart failure (76 +/- 3%) than in those without right heart failure (90 +/- 1%; p less than 0.001). Furthermore, patients with right heart failure also had a substantially lower awake arterial PO2 (52 +/- 4 mmHg versus 75 +/- 2 mmHg; p less than 0.001) and a higher PCO2 (51 +/- 2 mmHg versus 36 +/- 1 mmHg; p less than 0.001) than those without right heart failure. Severe nocturnal hypoxemia in the absence of diurnal hypoxemia was not associated with right heart failure. Daytime hypoxemia in the patients with right heart failure was associated with a higher residual volume (p less than 0.001) and lower forced expiratory volume in one second (p less than 0.001) than in the patients without right heart failure. The findings suggest that sustained hypoxemia and/or hypercapnia over a 24-h period is a necessary prerequisite for the development of right heart failure in patients with obstructive sleep apnea, and that diffuse airway obstruction plays a major role in causing such hypoxemia.