Sleep-disordered breathing (SDB) describes conditions where normal breathing is interrupted during sleep, most commonly known as sleep apnea. This disorder is characterized by repetitive events where breathing either stops completely or becomes significantly shallow. Healthcare providers rely on specific definitions of two distinct events, apnea and hypopnea, to accurately diagnose and determine the severity of a patient’s condition. Understanding the precise difference between an apnea and a hypopnea is fundamental to grasping how sleep specialists assess a person’s sleep health.
Apnea: The Complete Pause
An apnea is defined as a full or near-full cessation of airflow through the nose and mouth during sleep. This temporary pause in breathing must last for a minimum of ten seconds to be officially recorded as an event in a sleep study.
This complete stop in breathing can occur due to two primary mechanisms. The most common form, Obstructive Apnea, happens when throat muscles relax too much, causing soft tissue to collapse and physically block the upper airway. Despite the blockage, the chest and diaphragm muscles continue to attempt to breathe, which is a distinguishing feature noted during monitoring.
A different type is Central Apnea, a neurological event where the brain temporarily fails to send the necessary signal to the breathing muscles. In this case, there is no physical obstruction, and the body makes no effort to inhale or exhale. Regardless of the cause, an apnea results in a sharp drop in blood oxygen levels, prompting the brain to briefly arouse the person from sleep to restart breathing.
Hypopnea: The Partial Reduction
A hypopnea, in contrast to an apnea, represents a significant reduction in breathing, not a complete halt. This event is an episode of abnormally shallow breathing where the airway is only partially collapsed, allowing some airflow to continue.
During a hypopnea, the volume of air moving in and out of the lungs is substantially decreased, meaning the body is not receiving adequate ventilation. The insufficient exchange of gases leads to poor oxygenation, even though the person is technically still breathing.
The physiological consequence of a hypopnea is similar to an apnea: a drop in blood oxygen saturation and an arousal from sleep. Although the breathing reduction is less severe, the resulting repeated oxygen desaturation and lack of restorative sleep contribute significantly to the diagnosis of sleep apnea.
The Measurable Metric: Airflow Reduction Criteria
The defining difference between an apnea and a hypopnea is based on the quantitative reduction in airflow measured during a polysomnography (sleep study). An apnea is clinically scored when the airflow signal shows a reduction of 90% or more from the baseline breathing rate, sustained for at least ten seconds.
Hypopnea criteria require a less severe drop in the airflow signal. The current standard from the American Academy of Sleep Medicine (AASM) defines a hypopnea as a reduction of 30% or more in airflow, lasting for a minimum of ten seconds. Crucially, this partial reduction must also be accompanied by a specific physiological consequence.
The event must cause either a 3% or 4% drop in the blood oxygen saturation level, or it must result in an arousal from sleep (a change in brain activity). These associated factors confirm that the partial reduction was severe enough to disrupt the body’s oxygen supply or the quality of sleep.
Calculating Sleep Apnea Severity (AHI)
The practical application of distinguishing between these two events is calculating the Apnea-Hypopnea Index (AHI), the primary metric for diagnosing sleep apnea severity. The AHI is determined by adding the total number of apneas and hypopneas recorded during a sleep study and dividing that sum by the total hours of sleep, yielding an average number of breathing disturbances per hour. Both apneas and hypopneas are counted together because the resulting oxygen deprivation and sleep fragmentation contribute equally to the disorder’s health consequences.
AHI Severity Levels
For adults, AHI scores are classified as follows:
- Fewer than five events per hour is considered normal.
- A score between five and 15 indicates mild sleep apnea.
- A score between 15 and 30 events per hour is classified as moderate sleep apnea.
- Any score of 30 or greater signifies severe sleep apnea.
Clinicians use this index to determine the most appropriate course of treatment for the patient.