The Apnea-Hypopnea Index (AHI) in Sleep

Understanding what it assesses in relation to sleep

The AHI definition is important to understand sleep apnea severity on testing and response to treatment
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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 breathing changes. For sleep apnea to be diagnosed, there must be clinical impacts to the observation. When the change in breathing is more modest, it must be associated with a drop in the blood oxygen levels.

Alternatively, if sleep stages are recorded via EEG, an arousal from deep to light sleep or a full awakening associated with the sleep-disordered breathing identifies meaningful events.

The Classification of Sleep Apnea Severity Based on AHI

In general, the AHI can be used to classify the severity of the disease.

The following categories are routinely used to classify sleep apnea severity in 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: equal or 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 Select Therapy and Determine CPAP Effectiveness

The AHI will be used to help select the most appropriate treatment. The use of continuous positive airway pressure (CPAP) is appropriate for mild, moderate, or severe sleep apnea. In contrast, the use of an oral appliance may be limited to mild or moderate sleep apnea. Surgery may be selected based on risk factors related to your anatomy. Positional therapy may be suggested if your sleep apnea is worsened by sleeping on your back. Other treatments may be recommended, depending on the level of AHI that is observed with your sleep study.

Beyond the initial selection of treatment, ongoing therapy efficacy may be judged by monitoring the AHI. With most modern CPAP machines, the AHI can be reported with nightly therapy by the device itself.

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. (It is noteworthy that 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 the mask interface or the type of therapy is necessary.

If surgery is performed, or an oral appliance is fabricated, a test may be repeated to ensure the treatment has been effective in resolving the AHI.

If you have further questions about what AHI means for you in regards to the diagnosis, severity, and treatment of sleep apnea, 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.