Pathophysiology of hypoventilation during sleep

Alveolar hypoventilation defined by an increase in PaCO2 occurs due to either reduced minute ventilation and/or increased dead space. Mild alveolar hypoventilation may be observed in healthy subjects during sleep. Sleep hypoventilation is accentuated in disease states with potential carryover to the daytime, producing chronic hypercapnia during wakefulness due to failure of compensation during sleep and/or during wakefulness. Elevation of blood bicarbonate concentration, although appropriate to defend blood pH, provides a mechanism for perpetuation of a chronic hypercapnic state due to blunting of respiratory drive.