What Is the Definition of the Apnea-Hypopnea Index (AHI) in Sleep?

Measure Commonly Used to Categorize Sleep Apnea Severity

The AHI definition is important to understand sleep apnea severity and response to treatment
The AHI definition is important to understand sleep apnea severity and response to treatment. Science Picture Co/Getty Images

What is the definition of the apnea-hypopnea index (AHI)? Learn how the AHI is determined in sleep studies and what it means for the severity of your sleep apnea and the selection of treatment options, including determining the effectiveness of continuous positive airway pressure (CPAP) therapy.

Definition of Apnea-Hypopnea Index (AHI)

The apnea-hypopnea index, or AHI, is an index used to assess the severity of sleep apnea based on the total number of complete cessations (apnea) and partial obstructions (hypopnea) of breathing occurring per hour of sleep.

By definition, these pauses in breathing must last for at least 10 seconds. Hypopneas must be associated with a decrease in blood oxygen levels by 3 or 4 percent or an arousal or awakening from sleep.

You man wonder how these events are measured during a sleep study. Whether the study occurs overnight at a testing center or with home sleep apnea testing, similar measurements are used to determine the AHI. Airflow through the nose or mouth is measured, often with an oxygen cannula that records pressure variations or with a sensor called a thermistor that detects temperature changes. As you breathe in, cool air enters the nose, and as you breathe out, warm air leaves. This difference can create a signal that can be monitored. Pressure differences are often used for hypopnea identification while temperature differences are used to identify apnea events.

Beyond variability in airflow or temperature, there must be a consequence of these changes.

For sleep apnea to be diagnosed, there must be clinical consequences to the observation. When the change in breathing is more modest, it must be associated with a oxygen drops. Alternatively, if sleep stages are recorded via EEG, an arousal from deep to light sleep or an awakening associated with the sleep-disordered breathing identifies meaningful events.

The Classification of Sleep Apnea Based on AHI

In general, the AHI can be used to classify the severity of disease. The following categories are routinely used for adults:

  • Normal: fewer than 5 events per hour
  • Mild sleep apnea: 5-14.9 events per hour
  • Moderate sleep apnea: 15-29.9 events per hour
  • Severe sleep apnea: greater than 30 events per hour

Children are considered to have sleep apnea if they have more than 1 abnormal breathing event per hour of sleep. It is also abnormal for them to chronically snore.

Using AHI to Determine CPAP Effectiveness

With most modern CPAP machines, the AHI can be reported with nightly therapy. Though this is not measured in the same way, it can be a useful proxy measure to ensure that CPAP use is therapeutic. It measures resistance within the airway. Mask leak may compromise the measurement. If the AHI remains elevated, it may suggest the CPAP is not working well. It is possible that an adjustment in the pressure may be required. Sometimes a change in therapy is necessary.

If you have further questions about what AHI means for you, speak with your sleep specialist.

Also Known As: Apnea-Hypopnea Index, Respiratory Disturbance Index or RDI

Examples: My AHI was quite elevated, but my sleep apnea has greatly improved with CPAP treatment.

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