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

This Measure Is Commonly Used to Assess Sleep Apnea Severity

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The apnea-hypopnea index (AHI) is a measure used in sleep apnea studies and CPAP treatment. Credit: Science Picture Co/Collection Mix: Subjects/Getty Images

If you have a sleep study such as a polysomnogram or even home sleep testing, you may receive a report from your doctor that describes the severity of your sleep apnea according to the apnea-hypopnea index (AHI), but what is the AHI? Learn what the AHI is and how the measure is used to assess the severity of sleep apnea and your response to therapy.

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.

These breathing disturbances are typically associated with either a brief arousal or awakening from sleep or a 3 to 4 percent drop in the blood oxygen levels, called a desaturation. 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 often 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. It is also possible for the condition to be diagnosed based on home testing. A lot of information is collected, and part of the purpose of these studies consists of tracking your breathing patterns through the night. This is accomplished with a sensor that sits in the nostril as well as a respiratory belt that stretches across the chest and often stomach.

In addition, a sensor called an oximeter measures your continuous oxygen and pulse rate by shining a laser light through your fingertip via a clip.

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.

 Hypopnea refers to a transient reduction of airflow (often while asleep) that lasts for at least 10 seconds, shallow breathing or an abnormally low respiratory rate.

A complete cessation in breathing is called apnea. Hypopnea is less severe than apnea (which is a more complete loss of airflow). It may likewise result in a decreased amount of air movement into the lungs and can cause oxygen levels in the blood to drop. It more commonly is due to partial obstruction of the upper airway.

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 cause an awakening called an arousal. The AHI is the total number of pauses that occur as averaged 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 for adults:

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

In children, it is considered abnormal if there is more than one abnormal breathing event per hour of sleep as measured by AHI, and children should never chronically snore.

This classification is useful in determining the best treatment options as well as the likelihood of associated symptoms, including: excessive daytime sleepinesshigh blood pressure, diabetes, stroke, and other complications. If the condition is mild or moderate, an oral appliance may be appropriate.

For all levels of severity, continuous positive airway pressure (CPAP) can be considered. Many CPAP machines are able to provide a daily proxy measure of AHI as a way to ensure proper response to therapy. 

It is important for your doctor to consider your risk factors for sleep apnea in selecting your treatment. For example, one study estimated that only 30 percent of people with mild obstructive sleep apnea will tolerate 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.

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

Sources:

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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