Sleep Apnea

Determining the Severity of Sleep Apnea

Posted on 06. Dec, 2009 by admin in The Sleep Center: Testing for Sleep Apnea

When is a person’s sleep apnea severe enough to cause concern?
Sleep specialists define “clinical” sleep apnea (sleep apnea that needs medical attention) in the following terms:

1. An apnea event is when breathing stops for more than 10 seconds.

2. A hypopnea event is a partial apnea in which airflow in and out of the lungs is reduced for 10 seconds or more.

3. A person is considered to have “clinical” sleep apnea if he has more than five apnea or hypopnea events per hour.

This definition determines whether the person has sleep apnea, but it tells only part of the story.

The next question is “How severe is the sleep apnea?” How sick is this person? The sleep specialist needs to answer this question to decide on the appropriate treatment.

Sleep specialists use several “yardsticks” to determine the severity of a person’s sleep apnea. The simplest measure, now somewhat outdated, is called the apnea index. This is just the number of apnea events per hour of sleep: a person who has 30 apnea events per hour of sleep would have an apnea index of 30.

Another measurement is the total number of apnea events during an entire night. As an example, 250 apneas in an eight-hour night would not be unusual for a person with moderately serious sleep apnea.

A more accurate measure than the apnea index is the Apnea-plus-Hypopnea Index (AHI), also called the Respiratory Disturbance Index (RDI). The AHI (or RDI) expresses the total number of apneas plus hypopneas. The combined totals are a better measure of the severity of sleep apnea because apneas and hypopneas are equally important in causing the symptoms of sleep apnea.

Even the RDI may not give the full picture of sleep apnea severity. Another important indicator is oxygen saturation — the amount of oxygen present in the blood. Some people, despite a fairly small number of apnea events, may still be quite sick because of a very low oxygen saturation level. So the measurement of oxygen saturation is an important part of the total sleep apnea picture. Oxygen saturation is measured as a percentage. Normal is about 95 percent, and it decreases slightly as we get older. In people with sleep apnea, a blood oxygen content of approximately 80 percent during sleep is fairly common. Levels below 70 percent are considered critically low because of the high probability of irregular heart rhythm at lower blood oxygen levels.

Recently another indicator has been proposed, the respiratoryarousal index (RAI). This is the total number of arousals per hour of sleep from apneas, hypopneas, and all other sleep-disordered breathing events combined.

A further aspect of the severity of sleep apnea and the need for treatment is the question of daytime sleepiness: Is sleepiness interfering with the person’s life? Some people are more sensitive to sleepiness than others, and alertness is more critical for some than for others (e.g., an airplane pilot or a school bus driver). The results of the multiple sleep latency test (MSLT), described earlier, give a good measurement of a person’s tendency to fall asleep during the day.

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Tags: Severity, Sleep Apnea