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SLEEP STUDY BASICS – understanding your sleep study report
AHI and RDI
The first important numbers to consider in your report are the Apnea Hypopnea Index (AHI), and/or Respiratory Distress Index (RDI). The reason for its importance is the AHI or RDI usually plays a key role in diagnosing or ruling out sleep apnea, and also plays an important role in gauging its severity.
RDI represents the frequency of “respiratory events” such as apneas (no air gets in when you try to take a breath) or hypopneas (some air gets in but it is an inadequate volume). (See the formal definition below)
Although different clinicians use different criteria to define normal, generally speaking, most clinicians would label sleep apnea syndrome to any RDI over 5. We begin to see cardiovascular damage and shorter life expectancy with an RDI greater than 20.
The other important measurement of your sleep test is oxygenation desaturation. Oxygen desaturation measurement is the ratio of oxygenated blood to un-oxygenated blood. When awake, healthy people will have at least 94% of their blood saturated with oxygen. Your blood oxygene saturation level should remain above 90% during sleep. Cardiovascular damage and shorter life expectancy is associated with oxygen saturations below 90%.
You should be aware, not everyone with disordered sleep breathing will have oxygen desaturations. Some people might have quite frequent, but very short duration apneas or hypopneas and very little desaturations will occur. These brief events may however, still disrupt the continuity of sleep, cause daytime sleepiness, and trigger the physiological damage associated with sleep apnea. This is why a simple measurement of oxygen saturation is not appropriate to diagnose sleep apnea. (See the definition of Spontaneous Arousals below)
How to Interpret Sleep Architecture
The EEG defines five stages of sleep (I, II, III, IV, and rapid eye movement or REM) based upon the type of brain waves it identifies. The sequence, time of onset, and duration of these stages are rather predictable in healthy people. The pattern of disruption from the normal healthy pattern of brain waves characterizes the various sleep disorders. (Remember, OSA is just one category of a sleep disorder)
Normal sleep percentages are:
- Stage 1 – 5%
- Stage 2 – 55%
- Stage 3/4 – 20%
- REM – 20%
Stage 1 is only a transition state from wake to stage 2 and has no real rest value. A lot of Stage 1 means you have trouble falling asleep or can not maintain deep, restful sleep.
Stage 3 and 4 and REM sleep are the deep quality sleep phases. Stage 3 and 4 are referred to slow wave sleep. REM sleep is called fast wave sleep because the EEG brain wave tracing are rapid and appear the same as if a person is awake.
Untreated sleep disorders reduce REM and slow wave sleep, leaving predominantly the lighter stages of sleep (1 and 2), as well as reducing total sleep time.
You should be aware there are other causes for alterations in sleep stages. One of the most common is medication of many sorts, especially antidepressants, cold medications such as pseudo ephedrine, caffeine, or too much thyroid medication.
The standard definition of any apnea is a 10-second interval between breaths, accompanied by an arousal identified by the EEG, a 3% drop or blood oxygene desaturation, or both.
An episode of shallow breathing (the airflow reduced by at least 50%) during sleep, usually associated with a fall in blood oxygen saturation or an arousal noted by the EEG.
Normal sleep efficiency is at least 85% (asleep 85% of the night). It is reduced in a number of situations, such as insomnia. Efficiency is essentially the total sleep time multiplied by time in bed.
The ratio of oxygenated blood to un-oxygenated blood. Normal Oxygen Saturation (SaO2) is >94%. Cardiovascular damage and decreased life expectancy are associated with oxygen saturations below 90%.
Rapid Eye Movement sleep is usually 20% to 25% of total sleep. REM sleep is the stage of sleep with the highest brain activity, characterized by enhanced brain metabolism and vivid hallucinatory imagery of dreaming. Muscle activity is suppressed (so you don’t run in your sleep for example), and awakening threshold to nonsignificant stimuli is high.
Sleep Latency is the length of time from ‘lights out, ‘ or bedtime, to the onset of sleep. Normal sleep latency is about 15 minutes, REM latency is 90 minutes.
The simple definition for an arousal is, a three second change in the brain wave pattern measured by the EEG. There are 3 types of arousals reported out on the sleep studies- those attributed to respiratory events, periodic limb movements, and those that are spontaneous. To state the obvious, spontaneous arousals are arousals not related to respiratory events, limb movements, snoring, etc.
If you are over 60 years old you may have as many as 27 arousal per hour. Young and middle aged adults average around 15 arousal per hour.
There are a bunch of things that could create the scenario for the appearance of a large number of spontaneous arousals, such as medications that deter sleep (pseudoephedrine, caffeine, some antidepressants, too much thyroid medication, etc. Depression and narcolepsy can also increase these events.
Upper Airway Resistance Syndrome (UARS) is based on the presence of a large number of spontaneous arousals without the presence of scorable respiratory events such as hypopneas or apneas. These are RERAs (Respiratory Effort-Related Arousals).
If you take the number of RERA, and add it to the AHI, you have the Respiratory Disturbance Index (RDI).
Upper Airway Resistance Syndrome (UARS)
Upper Airway Resistance Syndrome (UARS) is based on the presence of a large number of spontaneous arousals without the presence of scorable respiratory events such as hypopneas or apneas. You may understand UARS as the work to breath during sleep is prevents obstruction and desaturations, but it is so difficult it disrupts the brainwaves. Interestingly, 10% of affected people will not snore.
Mild: Unwanted sleepiness or involuntary sleep episodes occur during activities that require little attention. Examples include sleepiness that is likely to occur while watching television, reading, or traveling as a passenger. These symptoms produce only minor impairment of social or occupational function.
Moderate: Unwanted sleepiness or involuntary sleep episodes occur during activities that require some attention such as while attending activities such as concerts, meetings or presentations. Symptoms produce moderate impairment of social or occupational function.
Severe: Unwanted sleepiness or involuntary sleep episodes occur during activities that require more active attention. Examples include uncontrollable sleepiness while eating, during conversation, walking, or driving. Symptoms produce marked impairment in social or occupational function.»