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What Is AHI?

Apnea-Hypopnea Index Used to Assess Sleep Apnea Severity


Updated May 20, 2014

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

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If you have a sleep study such as a polysomnogram, you may receive a report from your doctor that describes the severity of your sleep apnea according to an AHI, but what is the AHI?

What Is AHI?

AHI, or the apnea-hypopnea index, is a numerical measure that accounts for the number of pauses in your breathing per hour of sleep. It is used to assess the severity of an individual’s sleep apnea. The AHI overlaps with the respiratory disturbance index (RDI), though the latter differs as it includes other minor breathing difficulties. The AHI is part of the report from a standard sleep study for sleep apnea.

AHI Measurement During a Sleep Study

A sleep study called a polysomnogram is typically used to diagnose sleep apnea. A lot of information is collected, and part of the study consists of tracking your breathing patterns through the night. This is accomplished with a sensor that sits in the nostril as well as two belts that stretch across the chest and stomach. In addition, a sensor called an oximeter measures oxygen by shining a light through your fingertip.

All of this information is analyzed to determine how many times you breathe shallowly or stop breathing altogether during the night. Any partial obstruction of the airway is called a hypopnea and a complete cessation in breathing is called apnea. In order to count in the AHI these pauses in breathing must last for 10 seconds and be associated with a decrease in the oxygen levels of the blood or an awakening called an arousal. The AHI is the total number of pauses that occur per hour of sleep.

How AHI Is Used

The AHI is used to classify the severity of your sleep apnea, according to the following criteria:

  • Mild: 5-15 events per hour of sleep
  • Moderate: 15-30 events per hour of sleep
  • Severe: greater than 30 events per hour of sleep

This classification is useful in determining the likelihood of associated symptoms (including excessive daytime sleepiness), the risk of developing serious consequences (such as high blood pressure, diabetes, stroke, etc.), and the likely response to treatment with CPAP. For example, only 30 percent of people with mild obstructive sleep apnea will tolerate and respond to CPAP treatment. In addition, you may discover that your AHI is higher when sleeping on your back or during REM sleep, which may have therapeutic implications.


American Academy of Sleep Medicine. "International classification of sleep disorders: Diagnostic and coding manual." 2nd ed. 2005.

Giles, TL et al. "Continuous positive airways pressure for obstructive sleep apnoea in adults." Cochrane Database Syst Rev. 2006; 3:CD001106.

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