During sleep, relatively major respiratory physiological changes occur in healthy subjects. The contributions and interactions of voluntary and metabolic breathing control systems during waking and sleep are quite different. Alterations of ventilatory control occur in chemosensitivity, response to mechanical loads. and stability of ventilation. The activities of intercostal muscles and muscles involved in regulating upper airway size are decreased during sleep. These respiratory physiological changes during sleep compromise the nocturnal ventilatory function, and sleep is an important physiological cause of the nocturnal alveolar hypoventilation. There are several causes of chronic alveolar hypoventilation including cardiopulmonary, neuromuscular diseases. Obstructive sleep apnea syndrome(OSAS) is an important cause of nocturnal hypoventilation and hypoxia. Coexistent cardiopulmonary or neuromuscular disease in patients with OSAS contributes to the development of diurnal alveolar hypoventilation, diurnal hypoxia and hypercapnia.? The existing data indicates that nocturnal recurrent hypoxia and fragmentation of sleep in patients with OSAS contributes to the development of systemic hypertentsion and cardiac bradytachyarrhythmia, and diurnal pulmonary hypertension and cor pumonale in patients with OSAS is usually present in patients with coexisting cardiac or pulmonary disease. Recent studies reported that untreated patients with OSAS had high long-term mortality rates, cardiovascular complications of OSAS had a major effect on mortality, and effective management of OSAS significantly decreased mortality.