The Apnea-Hypopnea Index (AHI) is the primary measurement used by clinicians to diagnose and assess the severity of sleep apnea, a disorder characterized by repeated interruptions in breathing during sleep. This single numerical score quantifies how often a person’s breathing is disrupted throughout the night. The AHI allows healthcare providers to determine the required level of intervention and track the effectiveness of treatments like Continuous Positive Airway Pressure (CPAP) therapy. The index is derived from data collected during a polysomnography, commonly known as a sleep study.
Defining the Components
The AHI is a composite score that accounts for two distinct events: apneas and hypopneas. An apnea is defined as a complete or near-complete cessation of airflow, where breathing stops for a minimum of ten seconds. This pause occurs when the upper airway muscles relax to the point of total collapse, preventing air from entering the lungs.
A hypopnea is a partial obstruction of the airway that results in significantly shallow breathing. For an event to be classified as a hypopnea, the airflow must be reduced by a minimum of 30% for at least ten seconds. This reduction must also be accompanied by a drop in blood oxygen saturation or a brief awakening from sleep.
Calculating the Index
The AHI is calculated by transforming the total number of breathing disruptions into an hourly average. Technicians count the total number of apneas and hypopneas recorded during the sleep study. This combined count represents all the times the patient’s breathing was interrupted.
To arrive at the final index, this total event count is divided by the total number of hours the patient was actually asleep. For example, 60 combined events over six hours of sleep results in an AHI of 10 events per hour. The index uses total sleep time in the denominator, ensuring the score accurately reflects events per hour of sleep.
Interpreting the Score
The numerical result of the AHI calculation is used to classify the severity of sleep apnea in adults, guiding treatment decisions. An AHI score of fewer than five events per hour is considered within the normal range, indicating minimal or no sleep apnea. Many adults experience a few brief breathing lapses, so this minimal number is not considered a disorder.
A score between five and 14 events per hour indicates mild sleep apnea. Individuals in this category may experience symptoms like excessive daytime sleepiness or loud snoring, and treatment often begins with lifestyle changes or oral appliances. While seemingly low, a score of ten means the person’s breathing is being disrupted at least once every six minutes throughout the night.
Moderate sleep apnea is diagnosed when the AHI falls between 15 and 29 events per hour. At this level, the frequency of breathing interruptions can lead to more significant health consequences, including increased risks for cardiovascular problems. Patients with moderate scores frequently require continuous positive airway pressure (CPAP) therapy to maintain an open airway during sleep.
A score of 30 or more events per hour signifies severe sleep apnea. In this range, the patient is experiencing a breathing disruption at least every two minutes, leading to chronic sleep fragmentation and low blood oxygen levels. Untreated severe sleep apnea carries the highest risk for associated conditions such as hypertension, heart disease, and stroke, making CPAP treatment often required.