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Apnea Hypopnea Index

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Contents

Definition

AHI (Apnea Hypopnea Index) is the number of apneas and hypopneas per hour. Or an index for sleep apnea. 5-20 mild, 21-50 moderate, above 50 severe.

The Greek word “apnea” means “without breath.” You stop breathing during sleep for ten seconds or longer.

During a Hypopnea there is airflow through your throat but at a much reduced level, which leads to not getting enough oxygen. It’s abnormally shallow breathing lasting at least ten seconds.

Good AHI vs. Bad AHI

Less than 5 events (apnea or hypopnea) per hour is considered normal. 5 or more events per hour is considered Mild sleep apnea 15+ considered Moderate 30+ considered Severe (from T. S. Johnson MD, Sleep Apnea - The Phantom of the Night, page 211)

Flow limitation or Upper Airway Resistance Syndrome (UARS) is another important, subtle form of SBD. The airflow meets resistance in the nose or mouth, causing the brain to waken the sleeper. The American Academy of Sleep Medicine advocates counting apneas, hypopneas, and flow limitations/UARS for the Respiratory Disturbance Index, RDI, which is more comprehensive than the Apnea-Hypopnea Index.

Measuring AHI

To determine AHI add the total number of Apnea events, plus Hypopnea events and divide by the total number of minutes of actual sleep time, then multiplied by 60.

Example

Apnea + Hypopnea divided by actual sleep time, then multiply by 60

200 Apnea 200 Hypopnea

400 Total Events

420 Actual Sleep time (7 hours)

Divide 400 by 420 =.95 x 60 = 57 AHI (Severe OSA)

Two measurements of Sleep Disordered Breathing Apnea(SDB)

Remember AHI is an Average

The AHI of 14.3 is an index -- an AVERAGE calculated for the entire night. It doesn't mean you stopped breathing literally 14.3 times each and every hour. You could be having no apneas or hypopneas during some hours of the night, and then be slammed with a ton of them for scattered spots through the night, when REM was trying to happen.

Most people are hit hardest with apneas and hypopneas during REM sleep. REM -- rapid eye movement -- the type of sleep when we are thought to dream the most. People normally have 4 or 5 cycles into REM during the night, with each REM cycle getting longer than the last one. First REM cycle usually happens about an hour and a half after a person goes to sleep and might last just 4 or 5 minutes -- what if your husband happens to have drifted off to sleep briefly then? You could have tons of apneas and hypopneas each time you're starting into REM. Apneas/hypopneas or UARS arousals preventing you from settling into sufficient REM can certainly lead to daytime sleepiness.

In other words, sleep disordered breathing isn't always a matter of noticeably "stopping breathing" or "gasping" or even of repeated wake-ups that are long enough for a person to be aware of being awake.

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