What Is AHI?
AHI stands for Apnea-Hypopnea Index. An apnea is a pause in breathing – it is when your breathing stops completely for a time. On the other hand, a hypopnea is an episode of shallow breathing.
The Apnea-Hypopnea Index measures the severity of your sleep apnea. It is the total number of apneas and hypopneas that occur within an hour. AHI is calculated by dividing the number of apnea and hypopnea events by the number of sleeping hours.
What Is a Good AHI Score on CPAP?
An AHI score of less than 5 is considered normal. It means that you’ve had fewer than five apnea/hypopnea events within one hour of sleep.
How Accurate Is CPAP AHI?
The AHI score that your CPAP machine reveals is not always accurate. One study revealed that home CPAPs tend to overreport hypopneas which are part of the equation (source).
In addition, to get a full and accurate picture of the severity of your sleep apnea, it is necessary to look at more than just AHI. For example, how your apneas and hypopneas affect your oxygen levels on an hourly basis (Oxygen Desaturation Index) is another critical value to consider (source).
In general, if your CPAP consistently shows a high AHI that cannot be easily explained, it is probably time to take another sleep study. Here is our detailed guide to sleep studies.
What Is Normal AHI on a Sleep Study?
Whether revealed by your CPAP or a sleep study, an AHI score of less than 5 is considered normal.
What Is Normal REM AHI?
According to a study that examined REM AHI vs Non-REM AHI in terms of clinical features, there is little variation between these two types of AHI when it comes to the score (source).
Clinically, REM AHI isn’t associated with as much daytime fatigue and other OSA symptoms as Non-REM AHI. So, it is up to the sleep doctor to decide if a patient with high REM AHI but a low overall AHI (NREM and REM AHI) needs therapy.
Thus, it makes sense to concern yourself primarily with your overall AHI because REM AHI is the AHI score during REM sleep (which is roughly 20 to 25% of your sleep cycle), and the normal score remains the same as for the overall score at <5 events/hr (see table below).
What Level of AHI Requires CPAP?
AHI is a major but not the only determinant in the diagnosis of OSA. A sleep study is ordered when the patient presents with other symptoms, such as excessive daytime sleepiness, headache, obesity, and cardiac conditions, among others.
In other words, a person would be concerned about some medical symptoms; the doctor would screen for OSA based on those concerns; and the diagnosis would be sealed by the AHI.
What Is a High AHI Score?
An AHI score of 5 or higher is considered abnormally high and indicates the presence of sleep apnea. Please see the chart below.
Sleep Apnea AHI Score Chart
| AHI | Sleep Apnea Rating |
| Less than 5 | Normal (no sleep apnea) |
| 5 – 15 | Mild Sleep Apnea |
| 15 – 30 | Moderate Sleep Apnea |
| Greater than 30 | Severe Sleep Apnea |
What Causes AHI to Increase?
If you’re wondering why your AHI is still high while on CPAP, note that one or more of four factors can increase your AHI:
- Mask leak
- Mask off event
- Consuming alcohol, narcotics, or medication
- Developing central or complex sleep apnea.
Mask leak
If your CPAP mask leaks air, then not enough air pressure is applied to keep the airway open. This raises the AHI. We suggest adjusting your CPAP mask fit for a proper seal. If no amount of mask adjustment helps reduce the leak, you many need to consider a different type of CPAP mask.
Learn about the different types of CPAP masks and how to choose the best one here.
Mask off events
It is possible to unconsciously take off your mask during the night, especially when you first start CPAP therapy. If this happens, your CPAP will register a higher AHI.
Substances
The use of such substances as a narcotic, a medical drug, or alcohol may increase airway obstruction. That, in turn, results in a higher AHI.
CSA or Complex Sleep Apnea
Central Sleep Apnea (CSA) is different from Obstructive Sleep Apnea (OSA). CSA is a sleep disorder in which your brain is not signaling your body to breathe. This is a central nervous system issue and requires a different CPAP prescription and therapy.
How Does a CPAP Machine Know and Measure AHI?
CPAP machines measure AHI using built-in algorithms that use changes in airflow to determine apneas and hypopneas. When the machine detects a decrease in airflow, it compares this to the average airflow in the previous two minutes to determine if a hypopnea or an apnea has occurred.
CPAP machines estimate the AHI as the number of apnea and hypopnea events per hour of use rather than per hour of sleep because they do not know when you are asleep. You can be wide awake when you’re using your CPAP, and you would still get your AHI reading in the morning.
How Do I Improve My AHI on CPAP?
You can try three things to lower your AHI score.
1. Change your sleeping position
Sleeping on your back will produce the highest AHI because in this position the airway is the most likely to become obstructed. Simply switching to sleeping on your side can improve your AHI score and quality of sleep.
2. Reduce mask leak
Since a mask leak results in a higher AHI score, reducing the amount of air that leaks through your mask can help raise the AHI score.
Try to replace your mask cushion and your headgear first. If no significant reduction in air leak results, then consider replacing the entire mask.
3. Exercise
Studies show that exercise improves factors that contribute to sleep apnea, such as airway muscle tone and fluid accumulation in the neck. Regular exercise, mostly aerobic, has resulted in reduced severity of OSA and lower AHI (source).
What AHI Is Considered Severe Sleep Apnea?
An AHI score of greater than 30 is considered severe sleep apnea. Please see the chart above.
References
Andrade, F. M., & Pedrosa, R. P. (2016). The role of physical exercise in obstructive sleep apnea. Jornal Brasileiro de Pneumologia, 42(6), 457–464. Retrieved from: https://doi.org/10.1590/S1806-37562016000000156
Liu, Y., Su, C., Liu, R., Lei, G., Zhang, W., Yang, T., Miao, J., & Li, Z. (2011). NREM-AHI greater than REM-AHI versus REM-AHI greater than NREM-AHI in patients with obstructive sleep apnea: Clinical and polysomnographic features. Schlaf & Atmung, 15(3), 463–470. Retrieved from: https://doi.org/10.1007/s11325-010-0358-z
Meadows, A. & Singh, A. (2022). Apnea-Hypopnea Index (AHI). Sleep Foundation. Retrieved from: https://www.sleepfoundation.org/sleep-apnea/ahi
Peters, B. (2022). What Is the Goal AHI for CPAP Treatment of Sleep Apnea? Retrieved from: https://www.verywellhealth.com/sleep-apnea-what-is-my-goal-ahi-with-cpap-treatment-3015054
Rey de Castro, J., Huamaní, C., Escobar-Córdoba, F., & Liendo, C. (2015). Clinical factors associated with extreme sleep apnoea [AHI>100 events per hour] in Peruvian patients: A case-control study-A preliminary report. Sleep Science (São Paulo, Brazil), 8(1), 31–35. Retrieved from: https://doi.org/10.1016/j.slsci.2015.03.002
Stepnowsky, C., Zamora, T., Barker, R., Liu, L., & Sarmiento, K. (2013). Accuracy of Positive Airway Pressure Device—Measured Apneas and Hypopneas: Role in Treatment Followup. Sleep Disorders, 314589, 1-6. Retrieved from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3767059/