Obstructive sleep apnea (OSA) is a common sleep disorder. It involves several episodes of apnea throughout the night, or episodes of complete cessation in breathing. To receive this diagnosis, one must undergo a sleep study where one of the parameters is apnea-hypopnea index, or AHI. It's important to understand what this is and what is measures so you're more knowledgeable about your sleep study results. 

Apnea-Hypopnea Index (AHI) Overview 

The AHI is a measurement of the average number of apnea and hypopnea events that you experience during each hour of sleep. Doctors divide the total number of apneic and hypopneic events by the total number of hours that you were asleep. In order for the event to be counted, it has to last for a minimum of 10 seconds. 

Doctors make this calculation during a polysomnography, or sleep study. During sleep studies, all of your vitals and brain waves are being measured. The AHI is the main measurement used to diagnose OSA. But, there are additional measurements that are used to better understand how severe your OSA actually is. 

One of these is the oxygen desaturation index which is a measurement of how many times, per hour, on average your blood oxygen levels fall below normal for ten seconds or longer. This is coupled with another measurement of your carbon dioxide levels in the blood. The lower your oxygen goes, the higher your carbon dioxide rises, so this relationship is important. 

Understanding AHI in children and adults

The AHI is measured on a scale and used to categorize the severity of your apnea. Mild sleep apnea is characterized by at least 5 events per hour, but less than 15. Moderate is at least 15 events per hour, but fewer than 30. And severe is a count of 30 apnea events or more per hour. 

This scaling primarily applies to adults. For kids, the cutoff is one because kids breathe so much faster than adult. Therefore having one ten second episode is severe enough to have great consequences for them. 

Children also have three categories for the severity of sleep apnea, even though the are not standardized the same as adults. Mild is one to five events per hour. Moderate is six to ten events per hour. Severe is a count of ten or more events per hour. 

Adolescents fall in the middle and can be diagnosed by either the adult or child scale. 

Problems with AHI

AHI is a great tool to help with the diagnosis of OSA, but it is not without its shortcomings. One of those issues is that hypopneas may be measured differently. Hypopneas are more subjective than apneas, so they may not be counted the same in the AHI. 

Another issue is that AHI exclusively measures the number of respiratory events. It only shows the pause in breathing, but not the effect on blood gas levels, the length of the breathing event, and how your breathing patterns change from hour to hour since it is an average.

Another issue is that home tests underestimate AHI. Home sleep tests only measure AHI on the total recording time as opposed to the more precise total sleep time that is measured in a polysomnogram. They can underestimate AHI by about 15%.

Keep these shortcomings in mind if you receive AHI results but don't have normal symptoms of OSA, or have conflicting findings. AHI is very helpful, but is not a flawless design.  

Impact of CPAP treatment on AHI 

Continuous positive airway pressure (CPAP) is the gold standard treatment for sleep apnea. This is because it is a machine that blows air into the back of the throat to keep it open so the muscles and tonsils don't collapse and block the airway during an exhale. 

Sleeping with a CPAP can decrease the AHI by 73% but when individuals with severe OSA use their CPAP for at least six hours, their AHI returns to normal levels. 

If you are struggling with sleep apnea or interpreting your sleep study results, please click the orange button to take a free online sleep test and talk with one of our sleep health experts. 

https://www.sleepfoundation.org/sleep-apnea/ahi