by Alvin Ho, myofunctional therapist, Leeming clinicSleep Study Reports: Why Understanding the Intricacies Matters
In my conversations with patients, there seems to be a huge gap in understanding of their sleep study reports. Many people are very focused on the AHI (Apnoea-Hypopnoea Index) number which is used as the main indicator for presence and severity of Obstructive Sleep Apnoea (OSA).
However, depending on a few other factors, the AHI can give a very misleading view of the extent of OSA. In some cases, the AHI can grossly underestimate the severity of OSA and this may present with dire circumstances for the person due to under-management.
What is the Apnoea-Hypopnoea Index (AHI)?
The AHI indicates the average number of times an apnoeic event or hypopnoeic event occurs per hour of recorded sleep. To qualify as an event, each episode of apnoea or hypopnoea has to last 10 seconds or more. An apnoea refers to complete stopping of breathing, while hypopnoea refers to a partial blockage of at least 30% compared with baseline breathing airflow.
The categorisation of severity of Obstructive Sleep Apnoea (OSA) – a topic I recently covered in detail here – based on the AHI for adults are as below:
- Normal: below 5 events
- Mild OSA: 5 to 14 events
- Moderate OSA: 15 to 29 events
- Severe OSA: 30 or more events
Below I discuss some problems with solely looking at the AHI to determine severity of OSA.
Event Type and Duration Matters
The AHI number alone does not discriminate between the type of event which occurs (i.e., apnoea vs hypopnoea vs mixed). Two people with the exact same AHI can present with completely different symptom severities – if someone has all events occurring as apnoeas, they will have much worse symptoms as opposed to someone with the same AHI having all events occurring as hypopnoeas.
The AHI alone also does not give a good picture of the duration of apnoeic and hypopneoic events – by definition, any event lasting 10 seconds or more qualifies. However, the effects of an event lasting 10 seconds is drastically different from an event lasting 30 seconds; in the second scenario the effects are catastrophic but the same number is reflected in the AHI.
Sleep Position Matters
Most people experience worse apnoea or hypopnoea when they sleep on their back. This is due to tendency for the jaw and tongue to drop backwards blocking the airways.
During the sleep study, someone may be sleeping more on their side – subsequently, the AHI registered will be less severe than having them on their back. This is a common presentation which can underestimate the person’s OSA severity. The AHI can present as mild on average but be dangerously severe while the person is sleeping on their back.
Blood Oxygen Levels and Sleep Stages
The AHI indicates the mechanical stopping or blockage of breathing. However, it does not account for the effect of apnoea or hypopnoea on the body. Some people with a low AHI can still experience significant drops in their blood oxygen levels – this can place the person at a high risk for heart conditions even if their AHI indicates that they have mild OSA.
When the apnoeic or hypopnoiec events happen during sleep also matters. Rapid Eye Movement (REM) sleep is responsible for vital restorative processes including memory consolidation, emotional processing, and brain development. Breathing events happening during the REM stage of sleep can have more damaging effects on the total well-being of the person.
Respiratory Effort-Related Arousals (RERAs)
Some people experience micro-arousals in the form of effortful breathing – enough to drastically reduce the quality of their sleep but not enough to warrant an apnoeic or hypopnoeic event. In these cases, the AHI may indicate that the person is not experiencing OSA but they may still have significantly fragmented sleep that leaves them exhausted the next day. This can lead on to more chronic health issues as well as impact on their level of concentration at work or in school.
What Can Understanding Your Sleep Study Report Do For You?
As you can see, there are more details within the sleep study report which extend from just looking at the AHI. In some cases, certain details being missed can lead on to potential life-threatening events if the person is under-managed.
If you have had a sleep study done before and have yet look through it in detail, I strongly encourage you do so. If you need further help or discussion around this matter, feel free to schedule a session with me at Leeming. You can book online or phone (08) 9313 7433.
