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What Is a Hypopnea?
A hypopnea is a partial reduction in breathing during sleep. Under the AASM adult rule, airflow falls by at least 30% for at least 10 seconds, and the event only counts if it comes with a 3% or greater drop in blood oxygen or an arousal from sleep. An alternative rule still allowed by the scoring manual requires a 4% oxygen drop and ignores arousals. An apnea is a near-complete stop in airflow, a drop of 90% or more. Hypopneas and apneas are added together to make the apnea-hypopnea index (AHI). Which hypopnea rule a lab uses can change a person's AHI a lot.
Hypopnea definition
A hypopnea is a shallow-breathing event: air still moves, just much less of it. On its own a flow drop is not enough. Scoring rules tie it to a consequence, either a dip in blood oxygen or a brief arousal seen on the EEG.
The AASM Sleep Apnea Definitions Task Force set the adult rule in its 2012 update of the scoring manual (Berry et al., Journal of Clinical Sleep Medicine, 2012). A hypopnea is scored when:
the nasal pressure signal (or the PAP flow signal, or an alternative sensor) drops by at least 30% from the pre-event baseline,
the drop lasts at least 10 seconds, and
it comes with a 3% or greater oxygen desaturation or an arousal.
In children the drop has to last at least the length of 2 breaths instead of 10 seconds, with the same 30% threshold and the same 3% or arousal condition.
Apnea vs hypopnea
| Apnea | Hypopnea | |
|---|---|---|
| Airflow drop (adults) | 90% or more from baseline | 30% or more from baseline |
| Sensor (diagnostic study) | Oronasal thermal sensor | Nasal pressure |
| Minimum duration (adults) | 10 seconds | 10 seconds |
| Needs an oxygen dip or arousal? | No | Yes: 3% dip or arousal (recommended rule) |
| Obstructive vs central | Classified by breathing effort | Optional; scored only with a good effort signal |
| Counts toward AHI | Yes | Yes |
Thresholds are from Berry et al., 2012. Apneas are split into obstructive, central, and mixed by whether breathing effort is present; central sleep apnea covers that split. The same update made classifying a hypopnea as obstructive or central optional.
The 3% vs 4% hypopnea rule
The scoring manual carries two hypopnea definitions, and that is where most of the confusion comes from. An AASM position statement describes them this way (Malhotra et al., Journal of Clinical Sleep Medicine, 2018):
Recommended rule: reduced airflow with either an arousal or a 3% or greater desaturation.
Acceptable rule: reduced airflow with a 4% or greater desaturation. Arousals alone do not count.
The position statement says the recommended rule should be used to calculate AHI. If a lab has to use the 4% rule because of payer policy, it says hypopneas under the recommended rule should be scored as well, or a respiratory disturbance index (RDI) reported. Its reasoning: breathing events that end in an arousal can cause real sleep apnea symptoms even without an oxygen drop.
| Rule | Airflow drop | Also required | Arousal-only events counted? |
|---|---|---|---|
| Recommended (AASM) | 30% or more, 10 s or more | 3% desaturation or arousal | Yes |
| Acceptable (AASM) | 30% or more, 10 s or more | 4% desaturation | No |
How the hypopnea rule changes AHI
The rule can move the whole index. Ruehland and colleagues re-scored in-lab sleep studies from 328 consecutive patients at two Australian hospital sleep labs under three AASM hypopnea definitions (Ruehland et al., Sleep, 2009). The definitions they compared were older versions:
The 1999 "Chicago" criteria: a flow drop over 50%, or a smaller drop with a desaturation over 3% or an arousal.
The 2007 manual's then-recommended rule: a drop of 30% or more with a desaturation of 4% or more.
The 2007 alternative rule: a drop of 50% or more with a desaturation of 3% or more or an arousal.
The median AHI under the 4% rule was about 30% of the median Chicago AHI. Under the 3%-or-arousal alternative it was about 60%. Differences between patients were large. If the same 5, 15, and 30 cut points were kept, about 40% of patients called positive for OSA under the Chicago criteria would have been negative under the 4% rule, and about 25% under the alternative rule.
The 2012 update then made the 3%-or-arousal definition the recommended one, with a 30% flow drop. The 2017 AASM diagnostic guideline puts the effect plainly: depending on which hypopnea definition is used, AHI may be considerably different in the same person (Kapur et al., Journal of Clinical Sleep Medicine, 2017). When you compare two sleep reports, or a study against a wearable result, check which rule each one used. What is AHI covers the index and its severity bands.
Hypopneas on a home sleep apnea test
Most home sleep apnea tests do not record EEG, so they cannot see arousals. They can only score hypopneas by oxygen drop. The 2018 position statement notes this and says a sleep study in the lab should follow when someone at higher risk for OSA has a negative home test. The home sleep apnea test vs polysomnography guide explains the trade-offs and why a home test reports the REI instead of the AHI.
What wearables can and cannot see
A watch or ring with red and infrared PPG can estimate blood oxygen and track pulse rate and pulse amplitude through the night. That is enough to spot repeated oxygen dips and the heart-rate swings that often follow breathing events. It is not enough to score a hypopnea. A wearable has no nasal pressure signal for the 30% airflow drop and no EEG for arousals, so arousal-only hypopneas are invisible to it.
Counting oxygen dips gives the oxygen desaturation index, which is related to AHI but is a different number; see wearable ODI and overnight oximetry vs a sleep study. How PPG models build an AHI-like estimate is covered in wearable AHI estimation, and published results sit on the sleep apnea AHI leaderboard. For what oxygen usually does overnight, see normal SpO2 while sleeping. Sensor Bio keeps a review of wearable sleep apnea studies, and the sleep apnea condition page covers PPG detection more broadly.
This page explains what a hypopnea is and how it is scored. It is not a diagnosis and not treatment advice. Hypopneas are scored on a sleep study and interpreted by a sleep clinician.
Frequently Asked Questions
What is a hypopnea?
A hypopnea is a partial reduction in breathing during sleep. In adults, AASM rules score it when airflow drops by at least 30% for at least 10 seconds together with a 3% or greater oxygen desaturation or an arousal (Berry et al. 2012, doi:10.5664/jcsm.2172).
What is the difference between apnea and hypopnea?
An apnea is a near-complete stop in airflow, a drop of 90% or more for at least 10 seconds in adults. A hypopnea is a partial drop of 30% or more that only counts when it comes with an oxygen dip or an arousal. Both are added together in the apnea-hypopnea index (AHI).
What is the 3% vs 4% hypopnea rule?
The AASM scoring manual has two hypopnea definitions. The recommended rule counts a flow drop with a 3% or greater desaturation or an arousal. The acceptable rule counts only a flow drop with a 4% or greater desaturation and ignores arousals. An AASM position statement says the recommended rule should be used to calculate AHI (Malhotra et al. 2018, doi:10.5664/jcsm.7234).
Does the hypopnea rule change my AHI?
It can, by a lot. In a 2009 study that re-scored 328 sleep studies, the median AHI under a 4% desaturation rule was about 30% of the median under the older Chicago criteria, with large differences between patients (Ruehland et al. 2009, doi:10.1093/sleep/32.2.150). Check which rule a report used before comparing numbers.
Can a home sleep test detect hypopneas?
It can score hypopneas that come with an oxygen drop. Most home tests do not record EEG, so they miss hypopneas that only cause an arousal. The AASM recommends an in-lab sleep study when someone at higher risk for OSA has a negative home test.
Can a smartwatch detect hypopneas?
Not directly. A smartwatch or ring can estimate oxygen dips and pulse changes, but it has no airflow sensor and no EEG, so it cannot apply the hypopnea scoring rule. Its sleep apnea numbers are estimates, not a diagnosis.