Polysomnography: Equipment and Data

A Visit to a Sleep Lab

Polysomnography is the recording of brain signals [electroencephalograph, EEG , waves], eye movements [electrooculograph, EOG , waves], and muscle tension [electromyograph, EMG , waves] during sleep (1). Emilie visits a sleep lab . Let's explore the sleep lab equipment and procedures.

At the Bayview Medical Center's Sleep Disorders Center of Johns Hopkins University (2), the medical data of the patient undergoing sleep testing are fed through a Grass machine directlyto a computer. Five years ago, this machine would feed the resulting data to ink pens to record information on reams of paper. They still use this recording system for a backup in cases of computer failure. Using the computer makes it easier for the technicians to go through the pages and pages of data. Two pages worth of data is equal to about one minute of sleep. That means about 1000 pages a night for each patient. That's a lot of paper!

These various data show up as different lines which track throughout the sleep either on the computer monitor or on the Grass machine's printout. The machine keeps track of what the patient cannot report for himself, as he is not awake when sleep events happen.

So what do all these data mean? Well, the data provide a lot of information about what is happening to the patient during sleep. Patients are "wired up" shortly after arriving. The electrodes send out these data from their sensing locations on the patient's scalp; near the eyes, nose, and chin; and on the chest and legs.

Data sent from the electrodes on the scalp keep track of the brain waves. Neurons in the brain give off electrical impulses. The EEG waves are measured by the amount of voltage between two electrodes (3). The voltages are amplified and recorded by the Electroencephalogram. The frequency and amplitude of these brain waves determine (and inform the technician monitor) the patient's current stage of sleep. Electrodes are usually placed on both the left and right brain sides, however data is usually only taken from one, and the other is there simply for backup. The reason for this is that the brain is generally "more synchronized" during sleep. (4)

Eye movements, Electrooculograph (EOG) waves, are recorded to inform the technician if the patient is in REM (rapid eye movement) or NREM (non-rapid eye movement) sleep. The electrodes on the legs keep track of the patient's leg movements. Other locations also track muscle tension. These waves are called electromyograph (EMG) waves.

Sensors near the nose keep track of the patient's breathing. A common reason for a patient's being referred to a sleep lab is to check for possible sleep apnea . The line with the data on the patient's breath is what tells the technician how many sleep apneas a patient has during the night. The line should go smoothly up and down as the patient breathes in and out. During an apnea, the line becomes basically straight, as the person isn't breathing at all. The duration of the apnea, the time that the patient stopped breathing, can be easily computed by the length of the line. Meanwhile, the finger probe (a clip that the patient wears on his or her finger) sends back data on the amount of oxygen in the blood.

The technician is collects the data and then passes it on to the doctor for interpretation.

Although there are many indications of sleep recorded in the lab, there are other physiological changes that are associated with or indicate sleep. One of these is "inner ear" movement, associated with the REM-sleep stage. This is not recorded for sleep studies because of the extreme difficulty in capturing that data, as specific, more powerful, test equipment is needed. (4)

(1) Ancoli-Israel, Sonia. All I Want is a Good Night's Sleep. St. Louis, Mosby, 1996. (pp.5-6)

(2) Information in this section is based primarily on:

Nadeaux, Carol, and Lawrence Allen. Interview of sleep lab technicians by Emilie Sutterlin. Baltimore, Maryland, visit to the Bayview Sleep Disorders Center, Johns Hopkins University, Feb. 5, 1999.

(3) Hobson, J. Allan. Sleep. New York, Scientific American Library, 1989. (pp.13-16)

(4) Bootzin, Richard. Interview by Emilie Sutterlin. Tuscon, AZ, Pshychology Dept., University of Arizona, Mar. 30, 1999.