The Apnea-Hypopnea Index (AHI) is the primary metric used by clinicians to diagnose and quantify the severity of sleep-related breathing disorders, most notably obstructive sleep apnea. Derived from data collected during a diagnostic sleep study, this single number provides a standardized measure of how frequently a person’s breathing is disrupted during sleep. The AHI represents the average number of breathing events, specifically apneas and hypopneas, that occur every hour the person is asleep.
Defining Apnea and Hypopnea Events
The AHI is a combined count of two distinct types of respiratory events: apneas and hypopneas. An apnea event is defined as the complete or near-complete cessation of airflow (typically a reduction of 90% or more) through the nose and mouth. For this event to be counted, the breathing pause must persist for a minimum of ten seconds, often triggering the brain to briefly wake the sleeper to restore breathing.
A hypopnea, by contrast, refers to a partial reduction in airflow. Clinical scoring criteria define a hypopnea as a reduction in airflow of 30% or more for at least ten seconds. This partial obstruction must also be accompanied by a drop in the blood oxygen saturation level (usually 3% or 4%) or an arousal from sleep. Both apneas and hypopneas reflect a physiological struggle to move air, leading to fragmented sleep.
How the Apnea-Hypopnea Index is Calculated
The calculation of the AHI is a straightforward division, resulting in a number that reflects an average rate of events. Technicians conducting a sleep study (polysomnography) first tally the total number of all recorded apnea events and all hypopnea events over the course of the night. This cumulative total of breathing disruptions is the numerator in the AHI formula.
The denominator is the total number of hours the patient was actually asleep, a metric known as Total Sleep Time (TST). The final AHI score is calculated by dividing the total number of events by the TST. For instance, if a person experiences 150 total events over five hours of sleep, the AHI would be 30 events per hour.
The data used for this calculation is collected through various sensors, including airflow monitors and pulse oximeters, either during an in-lab polysomnogram or a simplified home sleep apnea test. While both tests provide the necessary data points, in-lab studies are generally considered the gold standard because they offer a more comprehensive measurement of actual sleep time.
Understanding the AHI Severity Scale
The resultant AHI score is used to place a patient onto a standardized severity scale, which helps determine the degree of sleep apnea present. For adults, an AHI of fewer than five events per hour is considered normal or minimal. An AHI ranging from five to fewer than 15 events per hour signifies mild sleep apnea.
The classification shifts to moderate sleep apnea when the AHI falls between 15 and fewer than 30 events per hour. At this level, the frequent interruptions correlate more strongly with daytime symptoms and increased health risks. An AHI of 30 or more events per hour is classified as severe sleep apnea.
Individuals in the severe category experience a high frequency of respiratory events that severely fragments sleep architecture and can lead to significant drops in blood oxygen saturation. The severity classification guides the choice of treatment, with higher AHI scores generally necessitating more aggressive interventions, such as continuous positive airway pressure (CPAP) therapy.
Clinical Context and Related Metrics
While the AHI is a foundational number for sleep apnea diagnosis, it is not the only piece of information used in clinical decision-making. Physicians always consider the AHI in conjunction with a patient’s reported symptoms, such as excessive daytime sleepiness or unrefreshing sleep, to create a complete picture.
To gain a broader understanding of respiratory disruption, clinicians also look at companion metrics derived from the sleep study.
Respiratory Disturbance Index (RDI)
The Respiratory Disturbance Index (RDI) expands upon the AHI by including Respiratory Effort-Related Arousals (RERAs). RERAs are breathing disturbances that do not meet the strict criteria for an apnea or hypopnea but still cause the brain to briefly wake up, further fragmenting sleep quality.
Oxygen Desaturation Index (ODI)
The Oxygen Desaturation Index (ODI) focuses specifically on the physiological impact of the events. The ODI counts the number of times per hour that the blood oxygen saturation level drops by a certain percentage, typically 3% or 4%, from the baseline. A high ODI combined with a moderate AHI can indicate that the breathing events are more severe in their effect on oxygen levels, necessitating careful treatment planning.