AHI, ODI, CT90 and hypoxic burden: how sleep apnea severity is measured
Medically reviewed by Dr. Iván Méndez-Benegassi Silva · ENT & Sleep Medicine Specialist · HM Hospitales, Madrid · License No. 282855946
Apnea severity is not just the number of breathing pauses per hour (AHI). What truly predicts health risk is how often oxygen drops (ODI), how long it stays low (CT90) and, above all, the hypoxic burden — which combines frequency, duration and depth of those drops and is the best predictor of cardiovascular events.
How Do We Measure Sleep Apnea Risk?
It's not enough to count how many times you stop breathing. What really matters is how much oxygen your body loses and for how long.
IAH
Apnea-Hypopnea Index
Counts respiratory pauses per hour. The most widely used metric, but it has limitations: two patients with the same score can have very different health risks.
Classification (Adults / Children)
IDO
Oxygen Desaturation Index
Measures how many times per hour blood oxygen drops. It is a better predictor of cardiovascular mortality than the AHI itself.
Classification
CT90
Time below 90% oxygen saturation
Measures how long you spend with dangerously low oxygen levels. Linked to cognitive decline and increased diabetes risk.
Risk thresholds
Hypoxic Burden
The Most Advanced Metric
Integrates the frequency of oxygen drops (ODI) + the duration of those drops (CT90) + how deep those drops go. It is the best predictor of serious cardiovascular events like heart attack or stroke.
Cardiovascular prediction
HR = Hazard Ratio vs. lowest quintile. AHI was not significant in MrOS.
Children: A Different Disease
In children, apnea thresholds are much stricter (an AHI ≥1 is already abnormal). Even mild cases can cause learning difficulties, hyperactivity (often confused with ADHD), growth problems, and cardiovascular changes. Early diagnosis is essential because the consequences are different from adults and can affect their development.
In our practice, we evaluate all these parameters to provide you with a complete and precise diagnosis.
This guide is educational and does not replace a sleep study or a medical diagnosis. The indices and thresholds shown are a general orientation; their interpretation depends on each patient and on the test used.
Reviewed by Dr. Iván Méndez-Benegassi Silva · ENT & Sleep Medicine Specialist · HM Hospitales · Last reviewed: 21 Jun 2026