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What is the AHI?
The AHI, or apnea-hypopnea index, measures how many apneas and hypopneas you have on average per hour of sleep. This is the number that determines the severity of your sleep apnea and the treatment decision. Below 5 per hour, the index is considered normal.
Apnea and hypopnea: what is the difference?
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
The AHI sorts sleep apnea into 4 levels of severity:
| 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 AHIO
These two terms are sometimes confused:
- AHI (Apnea-Hypopnea Index): the generic international term, counting all apneas and hypopneas (obstructive and central).
- AHIO: 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 AHIO are often used interchangeably for obstructive sleep apnea, which represents the vast majority of cases.
How is the AHI calculated?
The AHI is calculated automatically during a sleep study:
- Ambulatory polygraphy (at home): records airflow, thoracic and abdominal movements, SpO2 and heart rate. The device calculates the AHI over the recording time.
- Polysomnography (in a sleep center): a more comprehensive study that also includes the EEG (to measure sleep stages), the EMG and the EOG. The AHI is calculated over actual sleep time (excluding waking periods).
Clinical importance of AHI
The AHI is never read in isolation. An AHI of 8 an hour with disabling daytime sleepiness and hypertension may justify treatment. Conversely, an AHI of 12 an hour in a totally asymptomatic patient may simply be monitored. The decision to treat always takes in:
- The AHI/AHIO value
- The presence and intensity of symptoms (sleepiness, fatigue, quality of life)
- The cardiovascular risk profile (hypertension, diabetes, cardiac history)
- The profession (commercial drivers, pilots)
Residual AHI on CPAP
When you use a CPAP, the device continuously measures your residual AHI, that is the apneas and hypopneas that still occur despite treatment. It is a key indicator of how well your therapy is working.
ResMed devices show this residual AHI every morning in the myAir app. Our guide explains how to read your myAir results.
| Residual AHI | Interpretation |
|---|---|
| < 2 /hour | Excellent treatment |
| 2 to 5 /hour | Satisfactory treatment |
| 5 to 10 /hour | Partial treatment - adjustment recommended (mask, pressure) |
| > 10 /hour | Medical reassessment needed |
To view and analyse your residual AHI in detail, the free OSCAR software is very useful. The data is also available through the myAir app (ResMed) or DreamMapper (Philips).
AHI in children: a different threshold
In children under 16, the thresholds differ from those for adults. An AHIO of 1/hour or more 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.
Our CPAP devices to treat apnea
At VivaRespire you can buy your CPAP and own it. Our auto devices (APAP) adapt to your AHI night after night:
See all our Auto CPAP devices and CPAP masks.
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. AHIO (Obstructive Apnea-Hypopnea Index) is specific to the Belgian INAMI nomenclature: it only counts obstructive events. AHIO 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 AHIO 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 AHIO 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 considerably from one night to the next, 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 (Peppard et al., JAMA 2000). 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.