The Apnea-Hypopnea Index (AHI) is a standardized metric utilized by medical professionals to diagnose and classify the severity of sleep apnea. This index provides a single numerical score that reflects the average number of breathing interruptions an individual experiences per hour during sleep. The AHI helps to identify whether a person’s breathing patterns during sleep are within a typical range or if they indicate a sleep-related breathing disorder.
Defining the Components of the Index
The Apnea-Hypopnea Index is composed of two distinct types of breathing events: apneas and hypopneas. An apnea event refers to a complete or near-complete cessation of airflow through the nose and mouth for at least 10 seconds.
A hypopnea event, in contrast, involves a partial reduction in airflow. This is characterized by shallow or abnormally slow breathing, where airflow decreases by at least 30% for 10 seconds or longer.
The AHI is calculated by summing the total number of apnea events and hypopnea events recorded during a sleep study, then dividing that sum by the total number of hours the individual was asleep. For instance, if a person experiences a combined total of 42 apnea and hypopnea events over 7 hours of sleep, their AHI would be 6 events per hour.
The AHI Severity Scale
Medical professionals use the AHI score to classify the severity of sleep apnea in adults. A normal AHI, indicating no sleep apnea, is defined as fewer than 5 events per hour.
For individuals diagnosed with sleep apnea, the severity is categorized into three levels. Mild sleep apnea is indicated by an AHI ranging from 5 to fewer than 15 events per hour. Moderate sleep apnea is diagnosed when the AHI falls between 15 and fewer than 30 events per hour. Severe sleep apnea is characterized by an AHI of 30 or more events per hour.
How the AHI Is Measured
The data required to calculate the AHI is primarily collected through a medical test known as a sleep study. The most comprehensive method is in-lab polysomnography (PSG), often considered the “gold standard” for diagnosing sleep apnea. During a PSG, multiple body functions are monitored overnight in a specialized sleep laboratory. This includes tracking brain waves, heart rate, breathing patterns, and blood oxygen levels. This detailed information allows for a precise determination of sleep stages and the occurrence of apnea and hypopnea events.
Home sleep apnea tests (HSATs) offer a more convenient alternative for data collection. These portable devices are used in a person’s home and typically measure fewer variables compared to in-lab PSG. HSATs usually monitor breathing effort, airflow, and blood oxygen levels, providing sufficient data to calculate an AHI. While HSATs are practical, they may sometimes underestimate the AHI because they calculate events based on total recording time rather than actual sleep time. If an at-home test yields inconclusive results, an in-lab PSG may still be recommended for a more definitive diagnosis.
Medical Significance of AHI Scores
Untreated moderate and severe AHI scores are associated with a range of significant health risks. The repeated breathing interruptions and subsequent drops in blood oxygen levels during sleep can strain the cardiovascular system. This can lead to conditions such as high blood pressure, an increased risk of heart disease, and a heightened likelihood of stroke.
Individuals with elevated AHI scores also face an increased risk of developing type 2 diabetes due to impacts on the body’s response to insulin and glucose. Beyond these physical health concerns, fragmented sleep caused by frequent apneas and hypopneas often results in excessive daytime sleepiness, impaired concentration, and a higher risk of accidents, including those involving motor vehicles.
Following a diagnosis based on AHI scores, healthcare providers recommend tailored treatment pathways. For mild sleep apnea, lifestyle adjustments such as weight loss, avoiding alcohol and sedatives before bedtime, and sleeping on one’s side may be suggested. For moderate to severe sleep apnea, continuous positive airway pressure (CPAP) therapy is a common treatment, involving a machine that delivers pressurized air through a mask to keep airways open during sleep.
Other options include oral appliances that reposition the jaw or tongue, and in some cases, surgical interventions to address airway obstructions. The goal of these treatments is to reduce the number of breathing events per hour, aiming for an AHI below 5, thereby mitigating health risks and improving overall sleep quality.