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Understanding Your Sleep Study Results: RDI, Desaturation, and Sleep Architecture
© Kerrin Leon White
Jan 12, 2001
It is surprising how little patients know about the critical results of a sleep study test. Hardly ever does the patient even possess a copy of the study report.
Obtaining a Sleep Study Report Copy
The best time to get a copy is when you see your sleep doctor to discuss the results. If that occasion has already passed, however, don't wait for the next appointment. Send a letter to the doctor requesting a copy. Mention that your letter represents a consent for release of this information to yourself. You should not only sign and date this, but get a witness to sign and date it as well, and keep a copy for yourself.
If you have access to a fax machine, this is a quicker and more compelling way to transmit the request to your doctor.
Understanding the Report: AHI and RDI
One of the most important numbers on the whole report is the Apnea Hypopnea Index (AHI), or Respiratory Distress Index (RDI). The reason for its importance is the 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 (or AHI) is calculated, by dividing the total sleep time (in hours) into the sum of the total number of apneas plus hypopenas.
RDI is usually a number between 0 and 120, which represents the frequency, per hour of sleep, of "respiratory events" such as apneas (cessation of breathing for 10 seconds or longer) or hypopneas (decrease of breathing for the same duration).
Although different clinicians use different criteria to define normal through severe apnea, you need only get a sense of how this number relates to diagnosis and severity. Some would label sleep apnea syndrome to any RDI over 5; others would use 10 or 15 as a cut-off.
RDI (or AHI) is calculated, by dividing the total sleep time (in hours) into the sum of the total number of apneas plus hypopenas.
Oxygenation Desaturation
Oxygenation desaturation, or reduced blood oxygen, is one effect of prolonged apneas. Desaturation is often summarized with reference to the lowest level of oxygen saturation reached during the night.
Normally, oxygen saturation should not dip below the lower 90's; however, in severe apnea, it has been known to go as low as 40%. This degree of oxygen desaturation is dangerously capable of damaging various organs of the body, and deserves special attention.
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The copyright of the article Understanding Your Sleep Study Results: RDI, Desaturation, and Sleep Architecture in Sleep Disorders is owned by James Foster Robinson. Permission to republish Understanding Your Sleep Study Results: RDI, Desaturation, and Sleep Architecture in print or online must be granted by the author in writing.
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