The Apnea Hypopnea Index (AHI) and oxygen desaturation levels are used to indicate the severity of obstructive sleep apnea.
Apnea Hypopnea Index (AHI)
The AHI is the number of apneas or hypopneas recorded during the study per hour of sleep. It is generally expressed as the number of events per hour. Based on the AHI, the severity of OSA is classified as follows:
- None/Minimal: AHI < 5 per hour
- Mild: AHI ≥ 5, but < 15 per hour
- Moderate: AHI ≥ 15, but < 30 per hour
- Severe: AHI ≥ 30 per hour
When a home sleep study is done, the AHI is often expressed as the Respiratory Event Index (REI). The REI is the number of apneas or hypopneas recorded during the study per hour of device recording time. Because the recording time is frequently more than the time spent asleep, sometimes the REI is an underestimate of the AHI.
Sometimes the Respiratory Disturbance Index (RDI) is used. This can be confusing because the RDI includes not only apneas and hypopneas, but may also include other, more subtle, breathing irregularities. This means a person's RDI can be higher than his or her AHI.
Reductions in blood oxygen levels (desaturation) are recorded during polysomnography or limited channel monitoring. At sea level, a normal blood oxygen level (saturation) is usually 96 - 97%. Although there are no generally accepted classifications for severity of oxygen desaturation, reductions to not less than 90% usually are considered mild. Dips into the 80 - 89% range can be considered moderate, and those below 80% are severe.