Apnea-Hypopnea Index

The apnea-hypopnea index (AHI) is a measurement of how many times per hour your breathing fully or partially stops during sleep. An AHI score is calculated during a sleep study. This score helps determine if you have sleep apnea and how severe your sleep apnea is. 

The AHI provides important information related to the two types of sleep apnea, called obstructive sleep apnea (OSA) and central sleep apnea (CSA). Both conditions involve disrupted breathing during sleep, but OSA is caused by blockage of airflow while CSA involves a failure by the brain to properly control breathing.

You may find your AHI score on the test report from a sleep study, or a health professional may share this number with you when reviewing the results of a sleep study. We cover the factors involved in measuring AHI, why this number matters, and some of the limitations to interpreting an AHI score.


What Is the Apnea-Hypopnea Index?

The AHI is a way of measuring the frequency of breathing problems during sleep. Apneas and hypopneas are types of breathing disruptions. The AHI is calculated based on how many of these disruptions occur per hour. 

Doctors and sleep specialists use the apnea-hypopnea index when diagnosing sleep apnea. Sleep apnea cannot be diagnosed by symptoms alone, so the AHI score plays an important role in determining whether your breathing disruptions meet the criteria for a diagnosis of sleep apnea.

The AHI is one of the primary ways of evaluating the severity of sleep apnea. A higher AHI means more reductions in breathing, which generally reflects more severe sleep apnea and a greater risk of health complications.

When detected, breathing disruptions are classified based on whether they are caused by an obstruction in the airway or a reduction in the central nervous system’s drive to breathe. As a result, an AHI score can be broken down into an obstructive AHI and a central AHI.


How is the Apnea-Hypopnea Index Score Determined?

The apnea-hypopnea index is determined during an overnight sleep study in a specialized sleep laboratory. This type of study is the most dependable way of diagnosing sleep apnea as it tracks a range of data including your sleep stages, breathing, brain activity, muscle and eye movement, oxygen levels, and heart rate. 

During your sleep study, three key pieces of information collected by the sleep technician are used to calculate your AHI score. 

  • Apneas: An apnea is a near or total stoppage in breathing. During a sleep study, apneas are identified when there is a 90% or greater reduction in breathing that lasts for 10 seconds or longer. 
  • Hypopneas: A hypopnea is a partial reduction in breathing. Hypopneas are detected when you have a 30% to 90% drop in airflow lasting 10 seconds or longer, along with either a reduction in your blood oxygen level or an arousal from sleep detected in your brain activity.
  • Total sleep time: The sensors used during a sleep study enable a precise determination of how long you were actually asleep during the night, which is known as your total sleep time. 

The apnea-hypopnea index is calculated by adding together the number of detected apneas and hypopneas and dividing that number by your total sleep time. In this way, the AHI shows your average number of breathing disruptions per hour of sleep. 

This total AHI number is further divided into an AHI for obstructive apneas and another for central apneas. By examining various aspects of the data from the sleep study, a sleep technician can distinguish the cause of apneas and hypopneas, which allows for separate AHI scores to be reported. 

Home Sleep Apnea Tests

Not all sleep apnea testing is done in a sleep laboratory. In many cases, home sleep apnea tests are now an option for diagnostic testing for obstructive sleep apnea. However, at-home tests for sleep apnea often underestimate a person’s AHI.

For these reasons, after a home sleep apnea test, your doctor may recommend a follow-up sleep study conducted in a sleep laboratory. In-lab sleep studies are an important follow-up test if your doctor suspects a different sleep disorder or needs to further evaluate the severity of your sleep apnea.


What Do the Numbers Mean in the Apnea-Hypopnea Index?

To understand your apnea-hypopnea index score, it is important to know how it is calculated and the significance of different AHI levels. 

It’s common to have a few pauses in breathing when you sleep. In fact, up to five apneas or hypopneas per hour is considered normal. More frequent breathing disruptions can be a sign of sleep apnea. 

If you have an AHI score of 15 or higher, you are considered to have obstructive sleep apnea. If you have an AHI score from 5 to 14, you are only diagnosed with OSA if you also have notable symptoms or complications of the condition. 

If you are diagnosed with OSA, the AHI score helps determine whether the condition is mild, moderate, or severe. 

AHI ScoreSleep Apnea Severity
5-15Mild obstructive sleep apnea
15-30Moderate obstructive sleep apnea
30 or higherSevere obstructive sleep apnea

Having more severe OSA usually means that you have an elevated risk of health problems that can develop if the condition is left untreated. However, a higher AHI score does not always mean that you have more severe symptoms of OSA. 

In addition, there can be variation in how sleep laboratories classify hypopneas, so it is important to ask your doctor or sleep specialist about your specific AHI score and what it means about the severity of OSA in your case. 

The AHI score also plays an important role in detecting central sleep apnea. In most cases, an AHI score of five or higher is one of several criteria used to diagnose CSA. 


Limitations of the Apnea-Hypopnea Index

While the apnea-hypopnea index is a valuable tool for measuring breathing disruptions associated with sleep apnea, it can have limitations and must be carefully interpreted alongside other considerations.

  • Symptoms: The AHI does not perfectly correlate with sleep apnea symptoms, and your doctor or a sleep specialist will take your symptoms into account when evaluating your sleep apnea.
  • Breathing variability: The primary AHI score reflects an average from your entire sleep study, but the rate of apneas and hypopneas may not be consistent throughout the night. For example, if your AHI is higher when you sleep on your back, your treatment plan may involve changing your sleeping position.
  • Other sleep study measurements: The sleep study collects diverse information, including about blood oxygen levels, awakenings, and sleep stages, that may provide useful context for understanding your AHI score. 
  • Overall health: Some people are more prone to having sleep apnea or experiencing health complications from the condition. For this reason, doctors review your medical history and overall health when diagnosing and treating sleep apnea. 

Your doctor can discuss your AHI score with you and explain its significance in the context of these and other factors relevant to your situation.


Other Measurements Similar to the Apnea-Hypopnea Index

The apnea-hypopnea index is not the only way of measuring breathing problems during sleep. While an AHI score is commonly included on test reports from sleep studies, some laboratories use different measurements, such as the respiratory distress index (RDI). 

Like the AHI, the RDI is calculated during a sleep study and provides a score that reflects an average number of breathing disruptions per hour. The key difference is that the RDI uses a wider definition of breathing disruptions. 

While the AHI counts only apneas and hypopneas, the RDI counts apneas, hypopneas, and respiratory effort-related arousals (RERAs). RERAs do not involve a major reduction in breathing. Instead, RERAs are changes in breathing patterns that last at least 10 seconds and cause you to briefly wake up from sleep. 

Because the RDI includes a wider range of breathing issues, it is always higher than the AHI score. Sleep specialists have not reached a consensus about whether the AHI or RDI better reflect the severity of obstructive sleep apnea, but some experts believe that the two numbers may convey different information about how sleep apnea affects sleep and health.


Treatment After an Elevated Apnea-Hypopnea Index Score

You may benefit from treatment if your AHI score is indicative of sleep apnea. After a diagnosis of sleep apnea, your doctor may suggest treatment to reduce your AHI score, improve your sleep, and prevent potential health effects of sleep apnea.

For obstructive sleep apnea, treatment frequently involves using a continuous positive airway pressure (CPAP) device that helps keep the airway open with a stream of pressurized air. Consistent use of a CPAP usually reduces the AHI, and even when the AHI doesn’t go down, CPAP devices may still resolve symptoms of OSA. 

Another aspect of treatment for OSA can involve lifestyle changes, such as losing weight or altering your sleeping position, which for some people can result in a lower AHI score.  

Treatment for central sleep apnea may also try to reduce AHI through the use of a CPAP device or supplemental oxygen. CSA is often caused by another health condition, and treating that underlying issue may be a key focus of treatment. 


Apnea-Hypopnea Index Scores in Children

The apnea-hypopnea index is measured when children have a sleep study to check for sleep apnea. However, both the criteria for detecting apneas and hypopneas and the interpretation of the AHI score are different in children compared to adults. 

For example, in children, breathing disruptions do not need to last as long to be considered apneas and hypopneas. In addition, the scale for using AHI to assess the severity of obstructive sleep apnea is quite different than in adults.

AHI ScoreSleep Apnea Severity
1-4.9Mild obstructive sleep apnea
5-9.9Moderate obstructive sleep apnea
10 or higherSevere obstructive sleep apnea

As in adults, the AHI score is considered along with other factors like a child’s oxygen levels during sleep, awakenings from sleep, and sleep position.

For teenagers, doctors and sleep specialists have the option of applying either the adult or childhood AHI scoring criteria. Because this choice can impact whether the child or teen are diagnosed with OSA, doctors carefully review and interpret a teenager’s sleep study results. 

It is important to talk with your child’s doctor about their symptoms and medical history as well as their AHI score, how it was calculated, and what it means for their health.