Continuous Positive Airway Pressure (CPAP) therapy is the most common method used to manage Obstructive Sleep Apnea (OSA), a disorder characterized by repeated pauses in breathing during sleep. The success of this treatment hinges on delivering pressurized air to keep the airway open. The Apnea-Hypopnea Index (AHI) is the most important metric to monitor effectiveness. This index, often displayed on the machine’s screen or app, provides an objective measure of how often breathing interruptions still occur while using the device.
Understanding AHI: Apneas, Hypopneas, and Calculation
The AHI quantifies the number of respiratory disturbances experienced during one hour of sleep. It counts two specific types of events: apneas and hypopneas. An apnea is the complete cessation of airflow, or a reduction of at least 90%, lasting for a minimum of ten seconds.
A hypopnea is a partial collapse of the upper airway, resulting in a reduction in airflow of 30% or more, lasting at least ten seconds. This partial blockage must also be accompanied by a drop in blood oxygen saturation or an arousal from sleep to be counted. Both types of events disrupt sleep quality and oxygen levels, which is why they are combined into a single index.
The AHI is calculated by dividing the total number of apneas and hypopneas recorded by the total hours of monitoring time. For example, 60 total events over 8 hours of sleep results in an AHI of 7.5 events per hour. This standardized measure reflects the frequency and severity of breathing interruptions.
What the AHI Score Indicates for Sleep Apnea Severity
Before treatment, sleep specialists use the AHI to diagnose and classify the severity of Obstructive Sleep Apnea. This initial score is derived from a full diagnostic sleep study (polysomnography) conducted without CPAP. A higher AHI score indicates more frequent nightly breathing disturbances and a greater degree of severity.
The medical community uses a standardized scale for adults based on the untreated AHI score:
Severity Classification
- Fewer than five events per hour is considered within the normal range.
- 5 to 15 events per hour classifies the condition as mild sleep apnea.
- 15 to 30 events per hour is diagnosed as moderate sleep apnea.
- 30 events per hour or greater is classified as severe sleep apnea.
These categories establish a baseline that guides the necessary intensity of treatment.
Interpreting AHI on a CPAP Machine (The Goal Score)
The AHI reported on a CPAP machine represents the residual events that still occur while the device is actively providing therapy. The goal of CPAP treatment is to reduce this number to a clinically acceptable level, demonstrating that the pressurized air is successfully holding the airway open. An AHI of less than five events per hour is the accepted benchmark for successful treatment, as this falls within the normal range.
Sleep specialists often encourage patients to aim for an even lower AHI, ideally less than two events per hour. The AHI reported by the CPAP machine is an estimate; the device uses internal algorithms to detect airflow fluctuations, which differs from the comprehensive data collected during a formal lab sleep study. While this machine-generated AHI is useful for monitoring effectiveness at home, it is not a substitute for polysomnography.
Small daily fluctuations in AHI, such as variations between 0.5 and 1.5, are normal and can be influenced by sleep position or health. However, if the CPAP-reported AHI is consistently higher than five, the treatment is not fully optimized. The first step is checking for a mask leak, as escaping air decreases effective pressure and increases the event count. If the mask fit is secure, a consistently elevated AHI indicates the need to contact the prescribing physician for pressure adjustments.