You’ve completed your sleep study with Home Sleep, had the consultation where the sleep scientist has shown you the data and explained the results but by the time you get home with the report, you’ve forgotten half of what was said. Now you’re looking at a bunch of numbers that you’re not sure what to make of. This is the resource for you.
The first number on a Home Sleep report is the AI (Arousal Index) which indicates the total amount of times you woke up (arousals) per hour of sleep during the sleep study. This will be equal to or higher than the next two numbers. Most of the time, the majority of the arousals will be related to respiratory events, but can also include arousals caused by limb movements or unexplained ‘spontaneous’ arousal. You may be surprised to see that you woke up many times, yet you may only recall waking a few times. These awakenings are typically only 5-10 seconds. Too short to remember but enough to make you tired the next day. Typically, we only recall waking if we are awake for at least 5 minutes.
The second number is the RDI (Respiratory Disturbance Index) which is the primary metric we use to quantify the severity of sleep apnoea and indicates the number of arousals per hour of sleep that were caused by some level of obstructed breathing. Someone can have up to 5 respiratory arousals per hour and be considered to have no significant sleep apnoea. 5-15 indicates mild sleep apnoea, 15-30 is moderate, anything over 30 is severe.
Thirdly, we have the AHI (Apnoea Hypopnea Index) which represents the amount of apnoeas or hypopneas per hour of sleep. An apnoea is when there is a complete or very near complete stoppage of airflow through the airway for at least 10 secondsand associated with a drop in blood oxygen levels of at least 3%. A hypopnea is when the measured airflow reduces by at least 30% from baseline and ends in an arousal or blood oxygen drop of at least 3%. The AHI can therefore be thought of as the amount of large, obvious respiratory events per hour of sleep which increases the risk of motor vehicle accidents, heart disease and strokes by 5 to 10 times normal.
These three numbers will often be very similar, especially in particularly severe cases where there are frequent hypopneas and apnoeas throughout the entire study that cause all or most arousals. If the RDI is significantly higher than the AHI, this means that the sleep scientist found a lot of RERAs (Respiratory Related Arousal) which are classed as any reduction in measured airflow that is not enough to qualify as a hypopnea and is immediately followed by an arousal. The RDI is the AHI + however many RERAs per hour. Many sleep clinics don’t look for these RERAs, but it is common in less severe cases for RERAs to be the most common respiratory event, which can make a big difference when it comes to the diagnosis and subsequent treatment plan. They can be a major cause of fatigue but are not strokes or heart disease.
If your study also included leg movement data, there will be a fourth number in the Key Indices section, PLMI (Periodic Limb Movement Index). This is the amount of periodic limb movements detected per hour of sleep and contributes towards the AI, but is independent of the RDI and AHI. Not all limb movements cause an arousal, so the AI is not as simple as RDI + PLMI, but it is likely that some limb movements will cause an arousal. A significant PLMI is likely therefore to lead to the AI being much higher than the RDI or AHI.
Other Important Data
While the frequency of respiratory arousals is the most important metric in determining the relative severity of sleep apnoea, there are other factors to consider. In terms of long-term cardiac health, length of respiratory events and blood oxygen loss are just as important, if not more so.
Longest Apnoea denotes the longest time during the study that you stopped (or were very nearly stopped) breathing. In less severe cases, there may be no apnoeas throughout the study where all respiratory events are hypopneas or RERAs. Apnoeas are at a minimum 10 seconds and can last anywhere up to around 90 seconds.
Minimum SaO2 is the lowest oxygen saturation monitored during your study. Ideally, oxygen saturation remains above 95% at all times, reductions down to 90% are mild, anything 80-90% is moderate and a minimum SaO2 of less than 80% is considered severe and is likely to lead to long-term health complications if left untreated.
‘Supine’ means lying on your back, which is often when obstructed breathing is worse, as the tongue and soft palate are more likely to fall back and block the airflow. REM sleep is the dreaming phase of sleep, when the muscular-skeletal system is switched off as to not act out dreams. This is usually the period of most severe obstructed breathing, so supine REM respiratory events is included as a metric to indicate if there is a significant variability in your sleep apnoea due to sleep stage or position.