Sleep-disordered breathing is a common medical condition that disrupts sleep, leading to various health consequences. The most frequently discussed form of this disorder is obstructive sleep apnea, which involves repeated episodes of airway collapse during the night. Understanding the terminology used by medical professionals, specifically the difference between an apnea and a hypopnea, is central to comprehending a sleep disorder diagnosis. These terms describe the types of breathing interruptions experienced and are central to determining the nature and severity of the condition.
The Definitional Difference: Apnea Versus Hypopnea
The distinction between an apnea and a hypopnea rests primarily on the degree of airflow reduction during a sleep event. An apnea is defined as a near-complete cessation of breathing, representing an almost total stop in airflow. To be counted as an apnea during a sleep study, the reduction in airflow must be at least 90% of the baseline breathing rate. This drop must be sustained for a minimum duration of 10 seconds.
A hypopnea, in contrast, is a partial reduction in airflow, resembling a significant slowdown rather than a full stop. It is characterized by a reduction in breathing defined as a decrease of 30% or more from the baseline airflow. Similar to an apnea, this partial obstruction must also last for at least 10 seconds to be recognized as a hypopnea.
These definitions categorize the two distinct ways the airway can become compromised during sleep. Apneas represent a full collapse of the upper airway, while hypopneas indicate a partial collapse or significant narrowing. Both events disrupt normal breathing, but the severity of the obstruction is what separates the two terms in a diagnostic context.
Physiological Consequences of Breathing Events
An airflow reduction event must produce a measurable physiological effect to be considered clinically significant. The two main consequences are oxygen desaturation and arousals. Oxygen desaturation refers to a drop in the blood’s oxygen level, which is recorded using a pulse oximeter during a sleep study.
For a hypopnea to be counted, it must be accompanied by an oxygen desaturation of at least 3% or 4% from the baseline. Alternatively, the event is counted if it causes a brief awakening, known as an arousal. These arousals fragment sleep architecture and prevent the body from entering restorative deep sleep stages.
Both apneas and hypopneas cause the brain to initiate a brief arousal to restore muscle tone to the airway and prompt breathing to resume. This reaction attempts to mitigate the oxygen drop and the buildup of carbon dioxide. The body’s reaction—either a drop in blood oxygen or an arousal—makes the airflow reduction clinically relevant to a diagnosis. The repeated cycles of oxygen drops and sleep fragmentation drive the long-term health risks associated with the disorder.
Quantifying Sleep Disorder Severity
The total count of these breathing events is used to calculate the Apnea-Hypopnea Index (AHI), the primary metric for diagnosing and classifying the severity of sleep apnea. The AHI is calculated by dividing the total number of apneas and hypopneas recorded during a sleep study by the total number of hours spent asleep. This index provides the average number of breathing events per hour of sleep.
Physicians use the AHI score to categorize the severity of the sleep disorder.
- An AHI of fewer than five events per hour is considered normal or minimal.
- Mild sleep apnea is diagnosed when the AHI falls between five and fewer than 15 events per hour.
- A moderate diagnosis is assigned for an AHI score ranging from 15 to fewer than 30 events per hour.
- An AHI score of 30 or more events per hour indicates severe sleep apnea.
The AHI connects the individual breathing events to an overall measure that guides treatment decisions.