Definitions: Apnea and Hypopnea

Apnea is commonly defined as airflow cessation for at least 10 seconds. Apneas may be central, mixed, or obstructive (Fig. 67-1). During central apneas, no effort is made to breathe, the diaphragm is inactive, and ventilation ceases. Mixed apneas begin with a pause in respiratory activity followed by efforts to breathe against an obstructed airway. During obstructive apneas, airflow is impeded by an obstructed upper airway despite continued efforts to breathe.

The term hypopnea is applied when airflow decreases enough to lower oxygen saturation. Hypopneas may be obstructive, mixed, or central. During obstructive hypopneas, the upper airway partially collapses. The patient does not fully compensate for the added breathing resistance, and hypoventilation results in oxyhemoglobin desaturation.

Apneas and hypopneas are referred to as sleep-disordered breathing events. They cause asphyxia and provoke arousal from sleep. The severity of asphyxia depends on baseline gas exchange and the duration of the apnea or hypopnea. Apneas and hypopneas are terminated by arousal, which serves a protective function but also disrupts sleep. Duration of the apnea or hypopnea depends on the patient's arousal threshold, which can vary. Apneas, hypopneas, and sustained periods of hypoventilation may occur in any sleep stage. They tend to be most severe during rapid eye movement (REM) sleep, when ventilation is most irregular.