81: High CPAP AHI? Here are the reasons why!

So you’ve been waking up and looking at the “AHI” number on your CPAP machine.  It will faithfully give you an estimate of your sleep time and how effective the therapy is.  The key word here is estimate.  I’m going to break it down so you can understand why the machine’s AHI may bounce around or go up from time to time.  

AHI stands for Apnea Hypopnea Index, or how many “events” (usually airway obstructions) per hour of sleep you are averaging on a sleep study in the sleep lab.   It’s the total number of breathing pauses (apneas) and partial obstructions (hypopneas) tallied up over the course of the night and then divided by your objectively measured sleep time (sleep is objectively measured with sensors on your scalp and face).  

You probably know that on a sleep study, an AHI of less than 5 is normal.  So if your CPAP is doing its job well, your AHI on a sleep study (while using CPAP) would be less than 5 events per hour of sleep.  

When you’re using CPAP at home, you don’t have sensors on your scalp and face to measure sleep time objectively.  Your breathing isn’t monitored the same way as it is on a sleep study.  So the “AHI” reported by your CPAP machine isn’t a *TRUE* AHI at all– both the breathing events and sleep time are estimated.  

All these estimates mean the “AHI” reported by your CPAP machine is vulnerable to error.  

When I see a patient with CPAP, the very first thing I want to know is:  How good is their sleep quality and daytime alertness?  THEN I will look at the CPAP AHI.  I generally don’t get too excited for CPAP AHI numbers less than 10 because of the estimation factor I described above.  Here are some possible situations:

• Patient feels sleep/wake quality is good and CPAP “AHI” is averaging less than 10 = Great!  No change to treatment.  High fives all around.
• Patient feels sleep/wake quality is good and CPAP “AHI” is averaging more than 10 = Hmm, depending on how high the AHI is (and other factors), a change to CPAP treatment *may* be needed.  But I am happy the patient is sleeping well, high five for that.  
• Patient feels sleep/wake quality is poor and CPAP “AHI” is averaging less than 10 = Hmm, CPAP is probably working okay but something is not right.  More information is needed to help the patient sleep better.
• Patient feels sleep/wake quality is poor and CPAP “AHI” is averaging more than 10 = Hmm, more information is needed to help the patient sleep better.  A change to the CPAP pressure *may* be needed.

Side note: Changing your CPAP pressure by yourself is NOT recommended.  It is part of the PRESCRIBED therapy and you can make things worse for yourself by taking on this game of whack-a-mole.  

There is something to be said for “AHI” PATTERN when it comes to your CPAP report.  Your AHI, even if it was measured in the sleep lab, is not going to be exactly the same from one night to the next.  There are many reasons for this, and all of them apply when you consider the “AHI” your CPAP machine shows you in the morning:  

• Sleeping with mouth open or closed
• More or less REM sleep
• More or less sleep on your back (vs. left side or right side)
• More or less total sleep time or time slept with CPAP mask in place
• Medications, sedatives (or dose changes)
• Alcohol, caffeine, or other substances - how much and how recently consumed before bed
• Stress, pain, or something in the environment affecting your sleep
• Mask leak, mask fit (often related to cleanliness or age of mask)
• Fluid balance, recent salt intake
• Allergies or a cold causing nasal congestion
• Change in your sleep schedule or sleep deprivation

Bottom line is, the CPAP “AHI” is a new metric in your life.  It’s just one component of healthy, restorative sleep.  For CPAP users, it’s worth it to pay attention to the machine’s “AHI” and see what range is typical for you.  You may be able to correlate a change in the number with one of the factors listed above.  And knowing your “AHI” range will help you collaborate with your sleep doctor when something about your sleep or sleep apnea treatment gets out of line.