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Sleep Study Dictionary

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AASM American Academy of Sleep Medicine. The only professional society that is dedicated exclusively to the medical subspecialty of sleep medicine. The AASM is the national accrediting body for sleep disorders centers and laboratories. AASM accreditation is the gold standard by which the medical community and the public can evaluate the services provided by a sleep center or sleep lab. A directory of AASM-accredited sleep centers and labs is available at

AHI apnea-hypopnea index. A measurement of the overall severity of sleep apnea. AHI is an average of the combined episodes of apnea and hypopnea that occur per hour of sleep.  An AHI of 15 or more indicates that you have OSA. If you have symptoms of OSA such as excessive daytime sleepiness or high blood pressure, then a lower AHI of 5 or more confirms that you have OSA.
APAP autotitrating positive-airway pressure. These PAP units have an automatic, self-adjusting mode that raises or lowers the air pressure as needed.
apnea A pause in breathing during sleep that lasts at least 10 seconds. It involves a reduction in airflow of 90% or more. This is measured by thermal sensors.
blood oxygen saturation The level of oxygen in your blood. Also SaO2. It is measured by an oximeter. Episodes of CSA and OSA reduce the oxygen in your blood. This can lead to hypoxemia and hypercapnia.
BPAP bilevel positive-airway pressure. These PAP units provide two levels of air pressure: a higher level when you inhale and a lower level when you exhale.
cannula A flexible tube that is inserted into your nose. It is used with a nasal pressure transducer to measure airflow.
central sleep apnea Also CSA. A sleep related breathing disorder that occurs when the brain fails to tell the lungs to breathe. It is most common in the elderly and in patients who have heart disease or have had a stroke. In contrast to OSA, it is unrelated to an obstructed airway.
CPAP continuous positive-airway pressure. These PAP units provide one fixed level of air pressure.
CPAP study continuous positive-airway-pressure study. A sleep study that evaluates the effectiveness of CPAP therapy. It involves the titration of the CPAP device by a sleep technologist.
CSA central sleep apnea
EEG electroencephalogram. A graphic record of the electrical activity of the brain. The brain waves are recorded by electrodes that are placed on the head. Different types of brain waves occur during the various sleep stages. Each type of brain wave has a different frequency.
EKG electrocardiograph. Also ECG. A graphic record of the electrical activity of the heart. Electrodes are placed below the collarbone and on the ribcage. Episodes of apnea can cause repeated changes in heart rate and blood pressure.
EMG electromyogram. A graphic record of muscle movements. Electrodes are placed on the chin and the legs. Muscles relax during the transition from wake to sleep. Then your brain paralyzes many of your muscles during the stage of R sleep (REM sleep), although muscle twitches may occur. Leg movements may be a sign that you have restless legs syndrome or periodic limb movements.
EOG electrooculogram. A graphic record of eye movements. An electrode is placed on the face near each eye. Slow, rolling eye movements occur during the transition from wake to sleep. Brief, rapid movements occur during the stage of R sleep (REM sleep).

home sleep test This is one way to detect OSA in adults. A home sleep test involves the use of a portable-monitoring system that is small enough for you to take home. It consists of a small recording device, sensors, belts and related cables and accessories. You apply the sensors to your body and turn on the recording device before going to sleep. The sleep recorder gathers important data as you sleep in your own bed. Standard measurements include airflow, respiratory effort, blood oxygen saturation and heart activity (EKG).  Home sleep tests are classified as Type 2, Type 3 or Type 4.

hypercapnia Having a high level of carbon dioxide in the blood. It can be caused by CSA and OSA. 

hypopnea A partial reduction in breathing of at least 30% that lasts at least 10 seconds during sleep. This is measured by a nasal pressure transducer.

hypoxemia Having a low level of blood oxygen saturation. It can be caused by CSA and OSA. Also called hypoxia.
MSLT Multiple Sleep Latency Test. A nap study that measures how quickly you fall asleep during the day. It is used primarily to detect narcolepsy.
MWT Maintenance of Wakefulness Test. Measures how well you are able to stay awake during the day. It is used to evaluate how well treatment improves the daytime symptoms of a sleep disorder.
N sleep  non-rapid eye movement (NREM) sleep. The primary phase for 75% to 80% of an adult’s sleep time. It is made up of three sleep stages.
nasal pressure transducer Measures airflow into and out of your nose. It can detect a hypopnea because it is sensitive even to minor changes. It is used with a cannula.
obstructive sleep apnea Also OSA. A sleep related breathing disorder that involves repeated episodes of hypopnea and apnea despite an ongoing respiratory effort. It occurs when the muscles relax during sleep, causing soft tissue in the back of the throat to collapse and block the upper airway.
OSA obstructive sleep apnea
overnight sleep study A polysomnogram or Type 1 sleep study. It is the standard method of detecting sleep disorders and evaluating treatments in children and adults. You stay in a private room that often has the comforts of a hotel room. As you sleep, electrodes and sensors collect information such as airflow, brain activity (EEG), respiratory effort, eye movements (EOG), leg movements (EMG), blood oxygen saturation and unusual behavior.
oximeter A small device that provides continuous monitoring of your blood oxygen saturation. It can be worn on the ear but most often is placed on a fingertip.
PAP positive airway pressure. This is the most common and effective treatment for obstructive sleep apnea (OSA). It provides a stream of air through a mask that you wear during sleep. This airflow keeps the airway open, preventing pauses in breathing and restoring normal blood oxygen saturation. PAP can be continuous (CPAP), bilevel (BPAP) or autotitrating (APAP).
polysomnogram An overnight sleep study
portable monitoring A home sleep test
PSG A polysomnogram
R sleep rapid eye movement (REM) sleep. The last of the four sleep stages that make up a complete sleep cycle. The name comes from the rapid, twitching eye movements that occur during this stage. Most dreams occur during R sleep.
respiratory effort breathing effort. It can be measured by the movements of your chest and stomach. OSA involves pauses in breathing that occur even though your body continues making an effort to breathe. In contrast, a pause in breathing with no effort to breathe is a sign of CSA. The best method of measuring respiratory effort is respiratory inductance plethysmography.
respiratory inductance plethysmography Also RIP. The most accurate way to measure respiratory effort. You wear belts around your chest and stomach. These belts contain sensors and wires that carry a small electrical current. Chest or stomach movements produce changes in the current.
RIP respiratory inductance plethysmography
RPSGT registered polysomnographic technologist. A professional credential that a sleep technologist earns by passing the national certification examination.
SaO2 blood oxygen saturation
sleep architecture A term that refers to all aspects of your sleep pattern, including total sleep timesleep latencysleep efficiency and sleep stages.
sleep center A sleep disorders center. Provides testing and treatments for all sleep disorders. A sleep center can earn accreditation from the AASM.
sleep cycle A transition through all four sleep stages. Each complete cycle lasts about 90 to 110 minutes. Most adults will go through four to six cycles in a full night of sleep. Children have much shorter sleep cycles than adults. The sleep cycle of a one-year-old may last about 45 minutes. Once a child nears 10 years of age, the length of his or her sleep cycle is similar to that of an adult.
sleep efficiency The percentage of total recording time that you spend asleep.
sleep lab A laboratory for sleep related breathing disorders. Provides testing for all sleep disorders and treatments for sleep-related breathing disorders such as obstructive sleep apnea and snoring. A sleep lab can earn accreditation from the AASM.
sleep latency The length of time from when the lights are turned out to sleep onset.
sleep onset The moment when you fall asleep.
sleep specialist A licensed doctor who is board certified in the subspecialty of sleep medicine. He or she specializes in assessing, testing, diagnosing, managing and preventing sleep disorders.
sleep stages The four unique periods that make up one sleep cycle. The nature of your sleep is different in each stage. A typical sleep cycle consists of three stages of N sleep (N1, N2 and N3) followed by R sleep.
sleep technologist A health-care professional who assists in the evaluation and follow-up care of patients with sleep disorders. A sleep technologist works under the direct supervision of a sleep specialist.
snoring The vibration of tissue in the back of your throat as you breathe, making a noise during sleep. Frequent, loud snoring is a warning sign for obstructive sleep apnea.
split-night study Doing an overnight sleep study and a CPAP study in the same night. During an overnight sleep study, a CPAP study may be performed right away if you show signs of having severe OSA.
thermal sensors These sensors are placed near your nose and mouth to detect episodes of apnea by measuring your airflow. They record changes in temperature as you breathe. Air that you exhale is warmer than the air that you inhale. Two types are thermistors and thermocouples.

titration The process of setting the air-pressure level of a PAP device so that it eliminates episodes of apnea and hypopnea. A sleep technologist raises and lowers the air pressure to find the best setting for you. An APAP unit is autotitrating.

total recording time The length of time from “lights out” to “lights on.

total sleep time The total amount of time you spend asleep during the total recording time.

Type 1 sleep study An overnight sleep study
Type 2 sleep test A home sleep test that records seven or more channels of information. Recorded signals may include airflow, respiratory effort, blood oxygen saturation, brain activity (EEG), heart activity (EKG), leg movements (EMG) and eye movements (EOG).

Type 3 sleep test A home sleep test that records four to seven channels of information. This is the most common type of home sleep test. Recorded signals may include airflow, respiratory effort, blood oxygen saturation and heart activity (EKG).

Type 4 sleep test A home sleep test that records only one to three channels of information. It may record only blood oxygen saturation using an oximeter. Another recorded measure may be airflow. 

WASO Wake after sleep onset. The total amount of time you spend awake in bed after sleep onset. 

Reviewed by David Kuhlmann, MD
March 26, 2008

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