CPAP users are frequently perplexed by the numbers that their CPAP machine or CPAP app spits out. Probably the most important metric that your CPAP machine tracks is the AHI. Read on to find out if your AHI is good or bad and what you can do to lower it.
What does AHI mean on CPAP?
Many CPAP machines show data from the previous night's use, including an AHI, on their displays when you take your mask off in the morning. I prefer to use the term "calculated AHI" or "cAHI" to differentiate this machine-calculated number from the AHI that is determined by the gold-standard, a sleep study.
Some CPAP machines have smartphone apps where you can get more longitudinal data to see trends in your numbers. To see ALL the possible data, you have to get your CPAP machine set up in a clinicians' system. As a sleep doctor, I can get all the data possible, including a breakdown of the AHI in terms of obstructive apneas, central apneas, and hypopneas as well as snoring.
What is a good AHI for CPAP?
A sleep study derived AHI of <5 is indicative of the absence of sleep apnea so this is typically the minimum target for the cAHI. Based on my experience, a cAHI <5 is good but a cAHI <3 is what you need to achieve for really great control of your sleep apnea. Please note, I have never seen an AHI of 0.0. The machine will always calculate a few residual breathing events, this is nothing to fret over.
A rule of thumb in medicine is "treat the patient not the numbers." If an AHI is high but the patient is telling me that they're sleeping great and feeling terrific during the day, I'll usually check a nighttime oxygen level study. If the oxygen levels are normal, I won't make any changes to the CPAP settings.
How does a CPAP machine measure AHI?
CPAP machines estimate AHI based on proprietary algorithms. We sleep specialists suspect that the technology involves sensing resistance to the airflow that they are trying to deliver and "pinging" your airway with small packets of air to see if it is open or not.
For instance, if a CPAP machine senses no airflow for more than 10 seconds it will ping your airway. If the air packet bounces back, it means that your airway was closed and this is registered as an obstructive apnea. If the air packet doesn't return, it means your airway was open and this was a central or "clear airway" apnea and will be recorded as such. It is believed that hypopneas are determined by a significant increase in the resistance to the airflow delivered by the CPAP for at least 10 seconds.
It is important to understand that CPAP machines are not performing diagnostic sleep studies on you every night so you need to take the cAHI with a grain of salt.
How accurate is CPAP AHI?
Not very. Think of the cAHI as providing a reasonable overview of how well you're doing. Also, it is also important to look at the trends of the data, not just one night, before any conclusions can be reached. AHI can vary from night to night depending on a number of factors including: mask seal, alcohol intake, medications, body position, and altitude. I like to get at least two weeks of continuous data before deciding if a CPAP machine needs to be adjusted based on the cAHI.
When I've had the opportunity to compare the cAHI from the CPAP machine to the AHI on a sleep study performed while the same patient was using the CPAP, I have observed a trend where the AHI on the sleep study tended to be significantly higher than the cAHI from the CPAP. My suspicion is that the cAHI significantly underestimates the actual frequency of residual abnormal breathing events.
Here is a download from a patient of mine who had noticed that his sleep quality had deteriorated after having a stroke. Prior to the stroke, he had reported good quality sleep while using his BiPAP. The cAHI had dramatically increased since we had last checked it in 2017:
Suspecting complex sleep apnea (strokes are one common cause of central sleep apnea), we had him repeat a home sleep study while wearing his BiPAP. The study confirmed that he had significant residual sleep apnea despite using his BiPAP. The majority of events were central. His true AHI was almost 50% higher than the cAHI reported by the BiPAP machine.
How can the AHI on a CPAP be improved?
The most common factor causing a high cAHI is high leak. If air is escaping out of the system, it is not getting to your throat where you need it and your airway will keep collapsing. There are two types of leak:
- Mask leak - if this is happening, it's time for a new CPAP mask, cushion, or headgear. You might also need a special pillow for CPAP users who sleep on their sides.
- Mouth leak - this most common symptom is a chronic dry mouth on waking up. If this is happening, you'll need a full face mask or chin strap.
This is an example of a patient who's leak and cAHI were very high. For a ResMed machine, like the one below, the 95th percentile pressure should be under 24:
This patient was a man with a beard who required a full face mask. We switched him to an AirTouch full face mask which we've found tends to offer the best seal for bearded gentlemen who need a full face mask. His leak and cAHI dramatically improved as seen below:
Other issues that can cause a high CPAP AHI include:
- "Complex sleep apnea" - where you started out with obstructive sleep apnea but starting CPAP caused you to have central sleep apnea.
- Your machine's pressure needs to be adjusted. There are a number of reasons that this can happen including:
- weight loss
- weight gain
- significant amount of time passing since your machine's pressure was sent, i.e., age
- moving to a significantly different altitude
- new medications that can affect muscle tone or the diameter of your airway including testosterone
- new medical problems that can make sleep apnea worse like hypothyroidism, PCOS, atrial fibrillation, heart attacks, and stroke
If you are chronically seeing a trend towards high AHIs and it does not seem to be leak related, schedule an appointment with a qualified sleep doctor to review your data.
Joseph Krainin, M.D., FAASM is the founder of Singular Sleep, the world's first online sleep center. He is a Fellow of the American Academy of Sleep Medicine and board-certified in both sleep medicine and neurology. He has been practicing medicine for over 10 years.