ahi index

What Is The Apnea Hypopnea Index (AHI)?

When suffering from sleep apnea, it is important to understand the difference between an apnea and a hypopnea. An apnea occurs during the stopping of breath; the subject suffering from this disorder stops breathing completely for ten seconds or longer. A hypopnea relates to reduced airflow; breathing is occurring, but at a severely reduced level, for ten seconds or more. In both cases, the flow of air, specifically the flow of oxygen, into the body in dangerously compromised.

The AHI, or the Apnea Hypopnea Index, is a system used to differentiate between levels of sleep apnea hypopnea. The index itself is a numerical system used to record the number of apneas and hypopneas that occur during a subject’s sleep per hour. AHI is also, informally, used to index the severity of a case of sleep apnea.

Generally, if a subject experiences, during an hour-long period, less than five apneas or hypopneas, that subject is rated below a five on the AHI and is considered normal. Once the number of events rises above five an hour, there is reason for concern. People suffering from five to twenty apneas an hour are considered to have mild cases; those suffering from more than twenty, but less than fifty, are considered to have moderate cases; people who suffer more than fifty apneas an hour have severe cases. The numerical ranges of severity differ from doctor to doctor.

There is a very specific formulaic process in determining the Apnea Hypopnea Index of a subject. To determine AHI, add the total number of apnea events and hypopnea events that occurred during a full night of sleep. Once that sum is acquired, divide by the total number of minutes that actual sleep took place. After dividing the sum of events, by the minutes of sleep, the outcome of that equation is then multiplied by sixty (the number of minutes in an hour). The formula would read: (Apnea events + Hypopnea events / total number of minutes slept) * 60.

For Example:

Person A experienced 150 Apnea events, and 150 Hypopnea events during the night.
Person A slept for a total of seven hours during the night.
Person A experienced a total of 300 events, and slept for a total of 420 minutes.

To determine AHI for Person A, place those numbers into the formula: (A + H / mins) * 60.
(150 + 150 / 420) * 60 = (300 / 420) * 60 = 0.71 * 60 = 42.9

The product, 42.9, would be Person A’s AHI. In most instances, this would be considered a pretty severe case of sleep apnea.

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