AHI stands for apnea-hypopnea index. It is a measure of how often an individual suffering from sleep apnea stops breathing over a certain amount of sleep time. Types of apnea that factor into this calculation include obstructive sleep apnea, central sleep apnea and mixed sleep apnea. According to the American Academy of Sleep Medicine Task Force, an AHI under 5 is normal, an AHI between 5 and 15 is mild, an AHI between 15 and 30 is moderate and an AHI above 30 is severe.
Add the number of apnea and hypopnea events during the sleep cycle. Hypopnea is an episode in which the patient undergoes shallow breathing due to a constricted airway. For example, if there are 150 apnea events and 100 hypopnea events, add 150 and 100 to yield 250.
Multiply the total number of hours of sleep by 60 to yield the number of minutes. For example, if the study was conducted over 6 hours, multiply 6 by 60 to yield 360 minutes.
Divide the total number of events calculated in Step 1 by the total number of minutes calculated in Step 2. In this example, 250 events divided by 360 minutes yields 0.694 events per minute.
Multiply the result from Step 3 by 60 to yield the AHI number. In this example, 0.694 multiplied by 60 yields 41.62. This patient would be categorized as having severe sleep apnea.