What Does the AHI on Your CPAP Machine Mean?

The AHI on a continuous positive airway pressure (CPAP) machine’s app or display is a residual apnea–hypopnea index: its estimate of how many times an hour your breathing still stopped or became shallow while you used it. It shows how well treatment is working, not how severe your sleep apnea is. It’s useful for spotting problems, but it isn’t measured the way a sleep study is, and each manufacturer counts events its own way.

This article is general information, not medical advice. Don’t change your machine’s settings without your care team.

How the Machine Counts Events

A CPAP machine has no sensors on your body. It infers apneas and hypopneas from changes in the airflow through the mask, and divides them by the hours the machine ran, not the hours you slept. It can’t see your oxygen levels or brief awakenings, which a sleep study uses to score hypopneas. So the residual AHI is an approximation of the AHI a laboratory would measure on the same night.

An official American Thoracic Society statement on CPAP tracking (Schwab and colleagues, 2013) found that machines reliably record how many hours you use them, but that residual events and leak are harder to interpret, and that their definitions differ between manufacturers. It also concluded that the ends of the range, very high or very low residual events or leak, are clinically meaningful.

Reading It Well

• Look at the trend over weeks. A single night’s residual AHI moves with your sleeping position, alcohol, and a blocked nose.
• Check the leak too. A poorly fitting mask leaks, and high leak makes the machine’s event detection less reliable and the treatment less effective.
• Hours of use matter most. Treatment works only while you wear it. Using CPAP at least 4 hours a night on 70% of nights is a common definition of adherence, which the ATS statement discusses, and the statement notes the link between adherence and outcomes.
• Raise a persistent rise. A residual AHI that stays high, or creeps up over months, can mean the pressure needs adjusting, the mask needs changing, or something else has changed, such as weight. That’s a conversation for your sleep clinic.

The AASM’s guideline on positive airway pressure (Patil and colleagues, 2019) strongly recommends it for adults with obstructive sleep apnea and excessive sleepiness, which is why getting the hours in matters.

Following It in Kaizora

Residual Apnea–Hypopnea Index is on the Sleep page, separate from your diagnostic Apnea–Hypopnea Index, since one measures treatment and the other the condition. Log your machine’s nightly values with Log, with the machine as its device. Put it beside Dietary Alcohol or Weight on Correlations to see what moves it. For what the diagnostic numbers mean, see What Is the Apnea–Hypopnea Index?

The Short Version

• A CPAP machine’s AHI is a residual estimate during treatment, from airflow alone.
• It’s less exact than a sleep study, and each brand counts differently.
• Follow its trend with leak and hours of use, and raise a persistent rise with your clinic.

Sources

• Schwab RJ et al., “An official American Thoracic Society statement: continuous positive airway pressure adherence tracking systems: the optimal monitoring strategies and outcome measures in adults,” American Journal of Respiratory and Critical Care Medicine 188, no. 5 (2013): 613–620. doi:10.1164/rccm.201307-1282ST
• Patil SP et al., “Treatment of adult obstructive sleep apnea with positive airway pressure: an American Academy of Sleep Medicine clinical practice guideline,” Journal of Clinical Sleep Medicine 15, no. 2 (2019): 335–343. doi:10.5664/jcsm.7640