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The AHI - Apnea-Hypopnea Index: Everything You Need to Know
The AHI is the central measurement in sleep apnea diagnosis. It is the number your doctor will use to assess the severity of your apnea and decide on treatment. Here is a clear explanation of what it means, how it is calculated, and why it matters.
What is the AHI?
The AHI (Apnea-Hypopnea Index) measures the average number of apneas and hypopneas occurring per hour of sleep.
- An apnea: complete cessation of breathing for at least 10 seconds, caused by airway collapse (obstructive) or a brain signaling failure (central).
- A hypopnea: partial reduction in airflow (≥ 30%) for at least 10 seconds, associated with an oxygen desaturation (≥ 3%) or a brain micro-arousal.
Simple example: if you sleep 8 hours and 120 apneas and hypopneas are recorded in total, your AHI is 120 ÷ 8 = 15/hour.
AHI severity thresholds
| AHI (adult ≥ 16 years) | Severity | Clinical interpretation |
|---|---|---|
| < 5 /hour | Normal | No significant apnea |
| 5 to 15 /hour | Mild apnea | Treatment based on symptoms and cardiovascular risk profile |
| 15 to 30 /hour | Moderate apnea | Treatment recommended (CPAP or alternative) |
| > 30 /hour | Severe apnea | Urgent treatment required |
Difference between AHI and IAHO
- AHI (Apnea-Hypopnea Index): the generic international term, counting all apneas and hypopneas (obstructive and central).
- IAHO: the specific term used in Belgian INAMI nomenclature. It counts only obstructive apneas and hypopneas. This is the official criterion used for coverage decisions.
In everyday practice, AHI and IAHO are often used interchangeably for obstructive sleep apnea, which represents the vast majority of cases.
How is the AHI calculated?
- Ambulatory polygraphy (at home): records airflow, thoracic movements, SpO2 and heart rate. Automatically calculates the AHI over recording time.
- Polysomnography (in a sleep center): more comprehensive. AHI is calculated over actual sleep time (excluding waking periods). This is the gold standard.
Clinical importance of AHI
The AHI is never read in isolation. An AHI of 8/hour with disabling daytime sleepiness and hypertension may justify treatment, while an AHI of 12/hour in a totally asymptomatic patient may simply be monitored. Treatment decisions always integrate: AHI value, symptom severity, cardiovascular risk profile, and profession (commercial drivers, pilots).
Residual AHI on CPAP
| Residual AHI | Interpretation |
|---|---|
| < 2 /hour | Excellent treatment |
| 2 to 5 /hour | Satisfactory treatment |
| 5 to 10 /hour | Partial - adjustment recommended (mask, pressure) |
| > 10 /hour | Medical reassessment needed |
AHI in children: a different threshold
In children under 16, an IAHO ≥ 1/hour is considered abnormal and may justify treatment. Pediatric apnea is often caused by enlarged tonsils or adenoids, and the first-line treatment is usually surgical before considering CPAP.
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Frequently asked questions about AHI
AHI measures the average number of apneas and hypopneas per hour of sleep. An apnea is a complete breathing pause of at least 10 seconds. A hypopnea is a reduction of at least 30% in airflow over the same duration, associated with oxygen desaturation or a micro-arousal. AHI is automatically calculated during ambulatory polygraphy or polysomnography. It is the reference measure for classifying sleep apnea severity.
AHI (Apnea-Hypopnea Index) is the generic international term that counts all respiratory events: obstructive and central combined. IAHO (Obstructive Apnea-Hypopnea Index) is specific to the Belgian INAMI nomenclature: it only counts obstructive events. IAHO is the official criterion for CPAP convention eligibility in Belgium. In everyday language, both terms are often used interchangeably.
An AHI of 12 corresponds to mild apnea (threshold 5 to 15). The number alone is not enough: what also matters is symptom intensity and your risk profile. With significant daytime sleepiness, hypertension or diabetes, an AHI of 12 often justifies treatment. Without symptoms and with a healthy cardiovascular profile, monitoring and improved lifestyle may be enough. Discuss with your doctor.
The typical threshold is IAHO greater than or equal to 15 per hour on polysomnography at an accredited sleep centre. Certain specific cases (severe sleepiness, resistant hypertension, comorbidities) may allow eligibility with IAHO between 5 and 15. The sleep centre validates the final decision. If your AHI is below threshold or you want free choice of equipment, out-of-convention direct purchase remains possible: see our page CPAP reimbursement in Belgium.
Yes, and it is perfectly normal. AHI can vary by 30 to 50% between two nights depending on several factors: sleep position (supine = worse), alcohol consumption, nasal congestion, fatigue, sedative medication. For a reliable diagnosis, the test must be performed under standard conditions. Under CPAP, your daily residual AHI is read in myAir or OSCAR: observe the trend over 7 to 30 days rather than a single night.
Yes, it is an excellent result. Under CPAP, a residual AHI below 5 per hour indicates satisfactory treatment, and below 2 indicates excellent treatment. Your 4 falls within the satisfactory zone. Continue monitoring the trend and absence of high leaks. If fatigue persists despite a correct residual AHI, explore other causes: sleep quality, lifestyle or a differential diagnosis to revisit with your doctor.
Ambulatory polygraphy (at home) measures airflow, thoracic movements, SpO2 and heart rate. It calculates AHI over total recording time. Polysomnography (in lab) adds EEG, EMG and EOG, allowing differentiation between actual sleep time and waking phases. PSG is more accurate but more costly. Polygraphy slightly underestimates AHI compared to PSG. PSG remains the gold standard for atypical cases.
Yes, it is one of the most effective levers in overweight patients. A 10% body weight loss can reduce AHI by 20 to 30% on average, sometimes more. Significant lasting weight loss can be enough to bring a patient below the treatment threshold. Combine weight loss with positional therapy (avoid supine) and stopping evening alcohol to maximise effects. CPAP remains the gold-standard treatment during this transition.