Sleep-disordered breathing is marked by repeated interruptions in airflow during sleep, affecting millions globally. These events fragment sleep and cause drops in blood oxygen levels, leading to significant health consequences if left unmanaged. Understanding the difference between an apnea and a hypopnea is the first step toward grasping a diagnosis and subsequent treatment plan. These terms quantify the nature and severity of breathing disruptions that occur overnight.
Defining Apnea and Hypopnea
An apnea is defined as the complete or near-complete cessation of airflow through the nose and mouth. For an event to be categorized as an apnea, the reduction in airflow must be at least 90% from the baseline. This stoppage must last for a minimum of ten seconds to be counted as a diagnostic event during a sleep study.
In contrast, a hypopnea describes a partial reduction in airflow, representing a severe slowdown rather than a full cessation. Standard criteria require a reduction of at least 30% in airflow, persisting for ten seconds or longer. For the event to be counted, it must be accompanied by a physiological consequence. This consequence is either an oxygen desaturation of three to four percent or greater, or an arousal from sleep.
Quantifying Severity Using the AHI
Both apneas and hypopneas are counted together to generate the Apnea-Hypopnea Index (AHI). The AHI is a single metric that determines the severity of sleep-disordered breathing. It is calculated by adding the total number of apnea and hypopnea events recorded during a sleep study and dividing that sum by the total number of hours the patient spent asleep. This provides the average number of breathing disruptions that occur each hour.
The AHI score is the primary method used by clinicians to classify the severity of the condition and guide treatment recommendations. An AHI of fewer than five events per hour is considered normal breathing during sleep for adults.
AHI Severity Classifications
- Mild sleep apnea: 5 to 14 events per hour.
- Moderate sleep apnea: 15 to 29 events per hour.
- Severe sleep apnea: 30 or more events per hour.
Underlying Causes of Breathing Events
The mechanism behind the breathing events determines whether the condition is classified as Obstructive Sleep Apnea (OSA) or Central Sleep Apnea (CSA). OSA is the far more common type, accounting for approximately 84% to 90% of all cases. This type occurs when the muscles and soft tissues in the throat relax during sleep, causing a physical collapse or blockage of the upper airway. The patient’s chest and diaphragm are still trying to breathe, but the air cannot get past the obstruction.
CSA, however, is a neurological issue, not a physical one. In this scenario, the brain temporarily fails to send the necessary signals to the muscles that control breathing, such as the diaphragm. The respiratory effort stops completely because the central nervous system has faltered in its command. This type is less common and is often associated with conditions like heart failure or certain neurological disorders. Both the physical obstruction of OSA and the neurological failure of CSA can result in either a complete apnea or a partial hypopnea event.
Long-Term Health Consequences
The repeated drops in blood oxygen levels (hypoxemia) and the continuous sleep fragmentation caused by these events have serious systemic effects on the body. Each apnea or hypopnea triggers a stress response, causing blood pressure to surge and increasing the strain on the cardiovascular system. Untreated sleep-disordered breathing significantly raises the risk of developing chronic hypertension.
These chronic physiological stresses are associated with an increased likelihood of heart attack, stroke, and irregular heart rhythms (arrhythmias). The condition is also linked to metabolic dysfunction, contributing to insulin resistance and increasing the risk for Type 2 diabetes. Disruption of normal sleep architecture also impairs cognitive functions like concentration and memory. Both apnea and hypopnea events contribute equally to the AHI, driving these serious long-term health risks.