Sleep-disordered breathing describes conditions where normal breathing is interrupted during sleep, leading to fragmented rest and reduced oxygen in the blood. During a sleep study (polysomnography), a hypopnea is recorded as a partial collapse of the airway that results in shallow breathing. Understanding the precise definition of this event is important for interpreting diagnostic results.
Defining the Hypopnea Event
A hypopnea is a period of abnormally shallow or slow breathing that lasts for at least ten seconds. It involves a partial collapse or blockage of the upper airway, which prevents the full volume of air from reaching the lungs. The physical mechanism behind an obstructive hypopnea involves the muscles in the throat and neck relaxing during sleep, causing soft tissues to narrow the breathing passage. While the brain signals the body to continue breathing effort, the constricted airway limits the movement of air. This restricted airflow results in a decrease in the oxygen concentration in the blood, known as oxygen desaturation.
Hypopnea Compared to Apnea
Hypopneas and apneas differ fundamentally in the degree of airway compromise. A hypopnea involves only a partial blockage of the airway, resulting in a reduction of airflow, often described as shallow breathing. In contrast, an apnea is characterized by the complete cessation of airflow, or a near-complete blockage, typically lasting ten seconds or longer. While the hypopnea represents a reduction in breathing, the apnea is a full pause. Both events are counted together to gauge the overall severity of a sleep disorder.
Measuring and Scoring Hypopneas
A hypopnea must meet specific scoring criteria established by the American Academy of Sleep Medicine (AASM). The first requirement is a measurable reduction in airflow by at least 30% from the pre-event baseline, sustained for a minimum duration of ten seconds. The second requirement is a corresponding physiological change: a drop in the blood oxygen saturation level. The standard requires an oxygen desaturation of at least 3% or an associated arousal (a brief awakening in the brain waves), though an alternative scoring rule requires a desaturation of 4% or greater. Using the 4% rule tends to produce a lower count of events, potentially leading to an underestimation of the condition’s severity, so sleep centers must document the specific criteria used in the final polysomnography report.
Clinical Significance and the Apnea-Hypopnea Index
Counting hypopneas and apneas forms the basis for diagnosing and grading the severity of sleep apnea. The primary diagnostic tool is the Apnea-Hypopnea Index (AHI), calculated by adding the total number of apneas and hypopneas recorded during the study and dividing that sum by the total hours of sleep. The resulting AHI score classifies the severity of the sleep disorder. For adults, an AHI of five to fewer than 15 events per hour indicates mild sleep apnea; 15 to fewer than 30 events per hour is moderate; and 30 or more events per hour signifies severe sleep apnea. This severity rating directly informs treatment decisions, which can range from lifestyle changes to the use of a continuous positive airway pressure (CPAP) machine.