Apnea hypopnea index (AHI)

AHI	Rating
<5	Normal (no Sleep Apnea)
5-15	Mild Sleep Apnea
15-30	Moderate Sleep Apnea
>30	Severe Sleep Apnea

The Apnea-Hypopnea Index (AHI) is used as a measure of the severity of the condition known as sleep apnea. The AHI is the sum of the number of apneas (pauses in breathing) plus the number of hypopneas (periods of shallow breathing) that occur, on average, each hour. To count in the index apneas and hypopneas, collectively called events, must have a duration of at least 10 seconds. Apnea events may be further classified as Obstructive sleep apnea (OSA) or Central sleep apnea (CSA), abbreviated OA and CA respectively.

The AHI, as with the separate Apnea Index and Hypopnea Index, is calculated by dividing the number of events by the number of hours of sleep.

Therefore,

         AHI = (OA + CA + H) ÷ # of hours of sleep

ie. A person sleeps 8 hours. In 8 hours the individual experiences 2 obstructive apnea, 5 central apnea and 10 hypopneas.

         AHI = (2 OA + 5 CA + 10 H) ÷ 8 hours of sleep
               (17) ÷ 8 = 2.125
         AHI = 2.13

As we can see, AHI is proportional to the number of apnea events and inversely proportional to the number of hours logged as sleep. Naturally, if a person experiences any combination of apnea events then they will see a corresponding increase in their AHI, while an increase in hours slept will cause the AHI to decrease if the number of events stays the same.

This poses another problem. While an increase in sleep hours should cause a lower AHI, the increase in sleep hours will also increase the time that apnea events have the potential to occur.

For instance, a patient who sleeps for 6 hours on night 1, and experiences 5 obstructive apnea, 2 central apnea and 7 hypopneas.Their AHI will look like:

         AHI = (5 OA + 2 CA + 7 H) ÷ 6 hours of sleep
               (14) ÷ 6 = 2.33
         AHI = 2.33

If the same patient sleeps 8 hours on night 2 there are 2 more hours of sleep that the potential for apnea may occur. So the patient may experience 6 obstructive apnea, 4 central apnea, and 9 hypopneas. The patient will have an AHI on night 2 that looks like this:

         AHI = (6 OA + 4 CA + 9 H) ÷ 8 hours of sleep
               (19) ÷ 8 = 2.375
         AHI = 2.38

While the patient achieved 2 more hours of logged sleep on night 2, the patient also experienced more apnea events because of the 2 extra hours that apnea events have the potential to occur. So the AHI only varied slightly even though the patient slept 25% more on night 2 versus night 1.

Although the reduction of AHI is a good indicator of CPAP efficacy, AHI alone is not necessarily an indicator of sleep quality. Many new CPAP patients will report feeling tired even though they report AHI numbers consistently in the normal range (AHI 0-5). There are many factors that affect sleep quality. AHI is certainly one aspect of sleep quality but should not be considered as the empirical value of sleep. While less than 5 AHI is considered treated, many feel "the lower the better" as long as comfort is considered and you are not chasing unrealistic numbers such as zero.

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