Obstructive sleep apnea (OSA) is a common sleep disorder, believed to affect at least 2% to 4% of people1. During sleep, people with OSA experience multiple partial or complete obstructions of the airway, termed hypopneas and apneas. Additional symptoms may include loud snoring, gasping or choking, and daytime sleepiness.
If you have recently received a diagnosis of sleep apnea or taken a sleep study, you may have noticed an AHI reading on your results. AHI is short for apnea-hypopnea index. This is a scale doctors use when diagnosing obstructive sleep apnea (OSA).
What Is the Apnea-Hypopnea Index (AHI)?
The apnea-hypopnea index (AHI) is a diagnostic tool for determining the presence and severity of obstructive sleep apnea (OSA).
People with OSA experience a collapse of their airways during sleep. When this causes their breathing to completely stop or reduce to 10% of normal levels for at least 10 seconds, it is called an apnea. Hypopneas occur when your airways partially collapse, resulting in shallow breathing. If your airflow decreases by more than 30% for at least 10 seconds, it can be considered a hypopnea. Apneic and hypopneic events disrupt sleep and lead to lower blood oxygen levels, contributing to long-term health complications.
The AHI sleep apnea scale helps doctors calculate the severity of your symptoms. The AHI is different from the central apnea-central hypopnea index (CAHI) used for central sleep apnea, a form of sleep apnea that develops when the brain fails to tell the respiratory muscles to breathe.
How Is AHI Measured?
The apnea-hypopnea index (AHI) represents the average number of apneas and hypopneas you experience each hour during sleep. To measure it, doctors divide the total number of apneic and hypopneic events by the total number of hours you were asleep. To register as an event, an apnea or hypopnea must last at least 10 seconds or longer.
Doctors typically calculate AHI during a sleep study, or polysomnogram, which monitors your brain waves, blood oxygen levels, heart rate, and breathing while you sleep. Polysomnography generally takes place at a sleep laboratory, but some may be able to perform a simplified version at home.
While the AHI is the primary measurement for diagnosing OSA, your doctor may review other metrics to better understand the severity of your OSA. For example, the oxygen desaturation index (ODI) measures how many times per hour, on average, your blood oxygen levels fall below normal for 10 seconds or longer. Another important metric, especially for children, is the level of carbon dioxide in the blood. A high level of carbon dioxide may arise due to a long period of breathing at less than full capacity, even if the airway is not completely blocked.
Understanding the Apnea-Hypopnea Index for Adults and Children
The AHI is measured on a numeric scale. Scores for adults are divided into three categories, which correspond to different levels of OSA severity:
- Mild: An AHI of at least five events per hour, but fewer than 15.
- Moderate: An AHI of at least 15 events per hour, but fewer than 30.
- Severe: An AHI of at least 30 events per hour.
While five is the cutoff for adults, an AHI of one or above is sufficient to diagnose obstructive sleep apnea in children. Children breathe faster than adults in order to support their faster metabolism and smaller lung capacity. This is why even one apneic event can have more of an impact for a child.
Although the categories are not as standardized as they are for adults, most sleep experts consider childhood sleep apnea to fall into three categories:
- Mild: Children with an AHI of one to five events per hour may be diagnosed with mild sleep apnea.
- Moderate: Children with an AHI of six to 10 events per hour may be diagnosed with moderate sleep apnea.
- Severe: Children with an AHI of more than 10 events per hour may be diagnosed with severe sleep apnea.
Adolescents may be diagnosed using either the child or adult AHI scale.
Sleep Apnea AHI Chart
||≥ 5 to < 15 events per hour
||≥ 1 to ≤ 5 events per hour
||≥ 15 to < 30 events per hour
||> 5 to ≤ 10 events per hour
||≥ 30 events per hour
||> 10 events per hour
Drawbacks of the AHI
While the AHI can help doctors diagnose OSA, it does not take into account all factors which may point to the severity, or existence, of OSA.
Hypopneas May Be Measured Differently
Most experts agree on the standard definition of an apnea as a reduction in airflow of at least 90%. Hypopneas are more subjective, since they occur when your airways partially collapse. As a result, there is no standard measurement for what counts as a hypopnea.
Experts have experimented with defining hypopneas according to a certain percentage of decreased airflow, coupled with associated changes in blood oxygen levels or arousals from sleep. However, there is no set definition, and as a result, different definitions of hypopnea can lead to different AHI scores.
The AHI Exclusively Measures the Number of Respiratory Events
The AHI only tells you how often you experience a pause in breathing during sleep. It does not reveal other important elements about that breathing event which could point to the severity of your OSA. For example, it does not show how that pause in breathing affects your blood oxygen levels, which, when repeatedly decreased over time, may increase your risk of related conditions like hypertension and diabetes.
The AHI also does not measure how long an apnea or hypopnea lasts; only that it occurs for at least 10 seconds. People with apneas lasting 30 seconds may experience greater consequences than people whose apneas last 10 seconds.
Since the AHI represents an average taken across the night, it does not reveal patterns in breathing from hour to hour, or connections between sleep position and apneic events. Moreover, since the AHI is calculated during one night in a sleep lab, it might not be accurate for someone whose AHI changes from night to night.
Home Sleep Tests Underestimate the AHI
Home sleep tests calculate the AHI based on the total recording time, as opposed to the more precise total sleep time measured in a polysomnogram. As a result, at-home sleep tests often underestimate AHI by about 15%.
It is important to note these shortcomings of the AHI, because they can affect treatment. If doctors rely solely on AHI when recommending treatment, it may lead them to overlook other aspects of the person’s health history and their related symptoms. For example, for people with a high AHI but no daytime sleepiness, common OSA treatments may be less effective at reducing the risk of hypertension or related cardiovascular conditions. Scientists are still debating how best to incorporate other diagnostic criteria, such as daytime sleepiness, blood oxygen levels, and blood pressure, for a fuller picture of OSA.
The Impact of OSA Treatment on the AHI
Continuous positive airway pressure (CPAP) therapy to keep the airway open is the recommended treatment option for obstructive sleep apnea. Studies have found that sleeping with a CPAP device can decrease the AHI by 73%. When people with severe OSA use their CPAP devices for at least six hours, their AHI returns to normal levels (below five events per hour).
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