AHI (apnea-hypopnea index)

The apnea-hypopnea index counts how many times an hour your breathing stops or shrinks while you sleep. An apnea is a pause of at least ten seconds; a hypopnea is a partial reduction with a drop in blood oxygen or an arousal from sleep. A sleep study counts both, divides by hours asleep, and produces one number that determines whether you are diagnosed with obstructive sleep apnea and how severe it is called.

That single number is the endpoint SURMOUNT-OSA was built around. Malhotra and colleagues ran two parallel trials in adults with moderate-to-severe OSA and obesity — one in people not using CPAP, one in people who were — and published the results in the New England Journal of Medicine in 2024. Participants started severe: mean AHI was 51.5 events per hour in the first trial and 49.5 in the second.

After 52 weeks, AHI fell by 25.3 events per hour on tirzepatide versus 5.3 on placebo in the first trial, and by 29.3 versus 5.5 in the second. That is a treatment difference of 20 to 24 events per hour — enough, for many participants, to move them down a severity category. The result supported the Zepbound label indication to treat moderate-to-severe obstructive sleep apnea in adults with obesity.

The common confusion is treating AHI as a measure of how you feel. It is not. Two people with the same index can have very different daytime sleepiness, and someone whose index improves substantially may still need CPAP. AHI is a count of respiratory events, not a score for sleep quality, and the trials measured both separately for that reason.

Worth asking your clinician: what your baseline AHI actually was, whether a repeat sleep study is planned after meaningful weight loss, and whether your CPAP pressure needs re-titrating as your weight changes — a mask set for your old body can become uncomfortable and less effective. Our SURMOUNT-OSA deep dive and sleep-apnea patient guide cover the practical sequence.