Why does my apnea–hypopnea index (AHI) change?

If you’re using a CPAP machine to treat your sleep apnea, you may be wondering about the term apnea-hypopnea index (AHI). What is AHI? How is it calculated? Why does it change?

To break it down, the term apnea means that you stop breathing for at least 10 seconds. Hypopnea is when you have a partial blockage of your airway so your breath is more shallow than normal. When you look at your apnea-hypopnea index, it tells you how many apneas and hypopneas you have per hour while you sleep and gives you an average. For example, if your AHI is 2, you have approximately two incidents of hypopnea or apnea per hour for every hour that you’re asleep.

An AHI less than 5 is considered normal, and some patients with severe sleep apnea may be told by their doctor that they can accept even higher numbers so long as they’re feeling more rested each morning, experiencing fewer symptoms and their AHI is progressively decreasing.

Why does my AHI change?

A woman with sleep apnea messaged us after she noticed that over the previous week her AHI had fluctuated between 1.9 and 5. She said she hadn’t had a “5” in 10 years, but now they were coming every other night even though her CPAP machine was reporting a good mask fit each morning. Concerned, she asked if it’s normal for AHI to go up and down from night to night.

The short answer is: Yes; it is normal for AHI to vary within reason. However, if your AHI was stable, but it is suddenly increasing over the past few days or weeks, you should report this to your equipment provider and/or sleep specialist.

Causes of rising apnea-hypopnea index

In this case, the woman in our example had AHIs fluctuating between 1-5, which was normal and still within a safe range. If your nightly AHIs are rising above what’s considered safe, you may be experiencing:

• Mask leak: If air is escaping your CPAP mask, you’re not getting all the air pressure you need to keep your airway open. The three most common causes of mask leak are a poorly fitting mask, inadequate cleaning or mouth leak. Mouth leak is most common if you use a bilevel PAP machine or you tend to breathe through your mouth and are using nasal pillows or a nasal mask. Here are some common fixes to each of these causes.
• Mask off events: It’s common for some people to remove their mask during the night, either consciously or unconsciously, due to the initial foreign feeling or discomfort of wearing a mask. If your CPAP machine is reporting this happening or you suspect that it is, don’t worry, it’s normal. But do talk to your doctor about ways to reduce these events so you can get the full benefit of your therapy.
• Alcohol, medicine and other drugs: The periodic use of certain medication, alcohol or narcotics may cause your AHI to go up. Ask your doctor if anything you’re taking could be causing your fluctuation, and how you can compensate for it.
• Central or complex sleep apnea: A rising AHI could also be a sign that while CPAP is treating your obstructive apneas, your brain might not always be telling your body to breathe. This can cause central sleep apneas, “central” because your apneas are causes by a dysfunction in the central nervous system, not a physical obstruction. Most newer CPAP machines will report if you’re experiencing central apneas or you can’t identify what is causing your rising apnea-hypopnea index, talk to your doctor or equipment provider as soon as possible.

The ResMed myAir^TM app helps you track your sleep apnea therapy for Air10^TM devices. Download the myAir app for your Apple® iPhone® on the App Store® or for your compatible Android^TM smartphone on the Google Play^TM store. ¹