Apnea, hypopnea, and arousal
An apnea is a near-total stop in airflow lasting at least ten seconds. A hypopnea is the milder cousin: airflow drops substantially (commonly by about 30% or more) for at least ten seconds, paired with either a dip in oxygen saturation or a brief brain awakening called a respiratory arousal. Both end the same way — the airway reopens and breathing resumes.
There is an even subtler version. Sometimes the airway never fully blocks and oxygen barely dips, but the effort of breathing against a narrow throat keeps jolting the brain awake. That is upper airway resistance syndrome — fragmented sleep without dramatic numbers, which is why a normal oxygen trace alone does not prove all is well.
Turning a night into a number
The apnea–hypopnea index, usually shortened to AHI, is simply the total number of apneas plus hypopneas divided by the hours you actually slept. It answers one plain question: on average, how many times an hour did your breathing break down?
Worked example — calculating AHI
During the sleep study the technician counts:
Apneas .................. 42 events
Hypopneas ............... 96 events
Total respiratory events 138 events
Actual sleep time ......... 6.0 hours
AHI = total events / sleep hours
= 138 / 6.0
= 23 events per hour
Severity bands (adults, events/hour):
Normal ............... < 5
Mild ................... 5 to 14
Moderate .............. 15 to 29 <-- this patient (AHI 23)
Severe ................ 30 or more
Interpretation: moderate obstructive sleep apnea.The oxygen side of the story
AHI counts events, but it says nothing about how far oxygen actually fell. That is where the oxygen desaturation index (ODI) comes in: the number of times per hour oxygen saturation dropped by a set amount (often 3% or 4%). Two people can share an AHI of 25 yet have very different ODIs — one barely desaturating, the other plunging deeply. The depth and duration of those dips matter for the body's stress, so doctors read both numbers together.