Definitions and Terms


OBSTRUCTIVE SLEEP APNEA (OSA): OSA is the result of an airway blockage. The main cause of this blockage is a collapse of the soft tissue at the rear of the throat during sleep. OSA is the most common type of apnea.

CENTRAL SLEEP APNEA: Central sleep apnea is caused by a failure on the part of the brain to control breathing. With central apnea, there is no physical blockage but the result is still an inability to breathe.

MIXED SLEEP APNEA: Mixed sleep apnea occurs when the apnea is caused by a combination of physical blockage and by a lack of signal from the brain.

To determine AHI, add the total number of apnea events, plus hypopnea events and divide by the total number of minutes of actual sleep time, then multiply by 60.


Apnea + Hypopnea divided by actual sleep time, then multiply by 60
200 apneas, 200 Hypopneas (400 Total Events)
420 Minutes Actual Sleep Time (7 hours x 60)
Divide 400 by 420 = .95 x 60 = 57 AHI (Severe OSA)

The Severity of OSA’s as defined by the American Academy of Sleep Medicine Task Force (1999) is:

5 = < AHI < 15 MILD
15 = < AHI < 30 MODERATE

AHI – Apnea Hypopnea Index – The # of apneas and hypopneas per hour. This index, as well as oxygen saturation levels (O2 Sat), is used to determine how severe a case of sleep apnea a patient has.

SLEEP DISORDER – A sleep disorder is a physical condition that interferes with the obtaining of adequate and appropriate sleep and requires medical treatment.

AROUSAL – An arousal is an abrupt change from a “deeper” stage of non-REM sleep to a “lighter” stage.

RDI – Respiratory Disturbance Index – Includes apneas and hypopneas, and may also include other respiratory disturbances such as snoring arousals, hypoventilation episodes, desaturation events, etc. The RDI may be larger than the AHI.

BRUXISM – Teeth grinding.

HYPOPNEA – A hypopnea is a decrease in airflow by at least 50% for ten seconds or more, with a 3% desaturation of blood oxygen level.

SLEEP STUDY (POLYSOMNOGRAM) – The gold standard for the diagnosis of several sleep disorders including sleep apnea, restless leg syndrome, and narcolepsy.

TITRATION – Titration means to slowly add a little bit more of something until you reach a desired effect.

CPAP – Continuous Positive Airway Pressure – CPAP therapy provides a constant airflow which holds the airway open so that uninterrupted breathing is maintained during sleep. A CPAP machine blows one constant pressure.

CFLEX – A feature that makes breathing back against CPAP pressure easier to do. This technology was invented by Respironics.

BIPAP – BiPAP therapy alternates blowing two set pressures; a higher pressure for inhalation and a lower pressure for exhalation.




PSG – This is a diagnostic procedure only. This should be checked when the ordering physician does not want any type of treatment done. If this is checked, CPAP will only be added if the patient has sleep apnea that is extremely severe.

SPLIT – NIGHT – This is the most commonly ordered sleep study. If the patient meets predefined criteria (that is set forth by Medicare or their insurance co.) that reveals sleep apnea is present, then CPAP can be applied. The patient must have at least 2 hours of sleep, and during this time have an average of 30 respiratory disturbances per hour, in order to qualify for CPAP. CPAP can then be applied but not later than 2:00am.

CPAP – This test should be ordered only after the patient has been diagnosed with sleep apnea by a sleep study. If we do not perform the baseline sleep study here, then we need a copy of it from the physician.

MSLT – MULTIPLE SLEEP LATENCY TEST – This test is generally ordered when the patient has symptoms of excessive daytime sleepiness or fatigue, and sleep apnea has been ruled out. The patient has this test generally following an all-night PSG. (Dr. Surdulescu wants the MSLT to be done the day after the PSG.) The patient remains in the lab and takes naps every 2 hours for about 20 minutes each. This helps determine the level of their sleepiness and to see if REM sleep occurs too early. This test helps to diagnose Narcolepsy.

The referring physician should only mark one test to be performed, so that there is no confusion on either end.