The respiratory control system normally is able to respond to changes in oxygen consumption and carbon dioxide (CO₂) production to maintain homeostasis. That is accomplished by changes in minute ventilation (V_E) that maintain arterial partial pressure of oxygen (Pa o₂) and CO₂ (Pa co₂) within a narrow range. Alveolar hypoventilation, an elevation in the Pa co₂ to levels greater than 45 mm Hg, can occur with a number of disorders, referred to as the hypoventilation syndromes. Associated with hypercapnia is the development of hypoxemia, which adds to the clinical manifestations and morbidity found with the disorders. In addition, hypoventilation becomes more profound during sleep and can significantly worsen pre-existing hypercapnia and hypoxemia. In some, clinically significant hypercapnia and hypoxemia may develop only during sleep and may be unsuspected based on awake values.

Mechanisms responsible for the development of hypoventilation include: (1) a decrease in central respiratory drive, (2) chest wall and lung parenchymal deformities, and (3) respiratory muscle weakness. In many disorders that provoke hypoventilation, more than one mechanism is responsible.

This article reviews the normal control of breathing, including the changes that occur during sleep. In addition, it discusses the diagnostic work-up of hypoventilation, including the assessment of ventilatory control. Finally, individual hypoventilation syndromes, including the pathophysiologic mechanisms and their treatment, are reviewed.