hypopnea

If an apnea is the airway shutting off, a hypopnea is the airway pinching down — breathing does not stop, it just shrinks. During a hypopnea, airflow drops substantially but not to zero, like turning a tap down to a trickle. It still costs the body something, because that shallow breathing is usually paired with a fall in blood oxygen, a brief arousal from sleep, or both.

By common scoring rules, a hypopnea is a reduction in airflow of at least about 30 percent lasting ten seconds or more, accompanied by a drop in oxygen saturation (often defined as 3 or 4 percent) or by an arousal on the brain-wave recording. The exact thresholds vary between scoring guidelines, which is one reason the same study can yield slightly different numbers under different rules.

Hypopneas matter because they are counted alongside apneas to grade the severity of sleep-disordered breathing. A night dominated by hypopneas can be just as fragmenting and just as harmful as one dominated by full apneas; the body is still being repeatedly nudged awake and dipped in oxygen. For practical purposes, clinicians often treat apneas and hypopneas together rather than fixating on the label.

Most hypopneas in adults are obstructive — partial collapse of a narrowing throat — but they can also be central, reflecting a partial drop in respiratory drive. Distinguishing the two on a tracing can be subtle, and the choice of scoring rule directly changes the apnea–hypopnea index, so the threshold used should be reported alongside the result.