Plain-language definitions grounded in the clinical and regulatory literature.
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Metric
What it isA measurement of how many times per hour your breathing becomes blocked (apnea) or significantly reduced (hypopnea) during sleep. It’s the main number used to diagnose sleep apnea severity and track treatment success.
Why it mattersAn AHI under 5 is considered normal. Above 30 is severe. Effective treatment should bring this number down to near-normal levels.
Think of it like thisThink of it like counting potholes per mile on a road. More potholes mean a rougher ride and more wear on your car. More breathing interruptions mean more wear on your body.
The Apnea-Hypopnea Index (AHI) is a polysomnographic metric representing the average number of apnea and hypopnea events per hour of sleep. Apnea is defined as cessation of airflow for ≥10 seconds; hypopnea definitions vary but typically require ≥30% reduction in airflow with ≥3% oxygen desaturation or arousal.
MechanismEach apnea or hypopnea event represents partial or complete upper airway obstruction causing interrupted gas exchange, leading to cyclical hypoxemia, hypercapnia, increased sympathetic tone, and cortical arousal. The frequency of these events (captured by AHI) correlates with disease severity and cardiovascular risk.
Scientific ConsensusAHI thresholds for OSA severity classification: normal <5, mild 5-14, moderate 15-29, severe ≥30 events/hour. AHI reduction is the primary endpoint for evaluating treatment efficacy.
Active DebateWhether AHI alone is sufficient for treatment decisions or whether phenotyping approaches incorporating arousal threshold, loop gain, and anatomical factors provide better guidance.
Emerging ResearchWhether AHI fully captures disease burden, given that hypoxic burden, arousal intensity, and sleep fragmentation may be independently important; Development of alternative metrics like oxygen desaturation index (ODI) and hypoxic burden as predictors of cardiovascular outcomes
Key ResearchThe AASM scoring rules (Berry et al., 2012, updated 2020) standardize AHI measurement. Studies by Azarbarzin et al. (2019) have proposed hypoxic burden as a complementary metric.
— ATS research statement on metrics of OSA severity beyond AHI including hypoxic burden and arousal intensity
— AASM rules for scoring respiratory events defining apnea and hypopnea criteria
— AASM clinical practice guideline for diagnostic testing in adult OSA establishing AHI thresholds
— Shows AHI does not correlate well with mood disorders in OSA, questioning its utility as sole severity metric
— Demonstrates PSG indices including AHI are discordant with quality of life and symptoms
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