What is monitored during a sleep study?
Although there can be variations in the types of sensors which are used, during most sleep evaluations the following recordings will be made.
- Brain activity (electroencephalogram or EEG). The technician will measure your head and apply electrodes to your scalp using a universal system of measurement for recording from several different parts of your brain. Your scalp will be cleaned and the sensors will be filled with a small amount of electrode jelly for keeping good contact with the scalp. A small gauze pad will be applied over each electrode to hold it in place.
- Eye movements (electro-oculogram or EOG). Eye movements occur not only during waking but they also occur during rapid eye movement sleep (REM sleep) which is the dreaming stage of sleep. Sensors will be placed near both of your eyes for recording these movements in order to detect REM sleep. These electrodes may be attached with surgical tape or small gauze pads.
- Muscle activity (electromyogram or EMG). Activity of the chin muscles is typically recorded from sensors placed on top and underneath your chin. Another special feature of REM sleep or dreaming sleep, besides the presence of eye movements, is that the muscles of the chin and other skeletal muscles in the body (with the exception of the muscles controlling eye movements) lose their tone during this sleep stage. For this reason we are unable to move voluntarily during this sleep stage. These recordings help determine when REM sleep occurs. In addition, the EMG is recorded from muscles in the leg, the anterior tibialis muscles. This muscle activity tells us if periodic limb movements (often abbreviated as PLMS) are present. PLMS are rhythmic twitching of the leg muscles during sleep which can be very disruptive to sleep, but which are most often unknown to the sleeper. In addition, depending upon your symptoms, muscle activity may be recorded from the arms to evaluate twitching or from muscles near the jaw to evaluate tooth grinding, a condition called bruxism.
- There are several different sensors which are used to record your breathing during the study. In order to record the pressure of air movement through your nose, small tubes similar to an oxygen cannula will be placed just inside both your nostrils. A small sensor will also be placed near your nose for measuring the temperature of air moving in and out of your nose. An elastic band will be placed on your upper chest and also on your abdomen for measuring movement of your diaphragm during breathing. Finally, a small sensor may be taped on your chest which senses your body position, that is whether you are sleeping on your back, sides, or stomach. All of these sensors allow us to determine if you have episodes of stopping breathing during sleep, in what body position they occur, and what type of sleep apnea is present.
- Oxygen saturation (SpO2) A small padded clip will be placed on your index finger to measure oxygen levels in your blood. This test is often referred to as oximetry or nocturnal oximetry. With this device a light beam passes from one side of the clip, through your finger, and to the other side of the clip. Nocturnal oximetry allows us to determine if there are significant decreases in your oxygen levels during sleep, particularly if you stop breathing.
- Heart rate (the electrocardiogram or ECG). Changes in heart rate and irregularities in heart rhythms can occur in different sleep stages and with some sleep disorders, most prominently sleep apnea. One or two electrodes will be taped to your chest for recording the activity of your heart.
Some patients, in addition to these sensors, will also have a mask which covers the nose or the nose and mouth, depending upon the circumstance, for further evaluation of sleep apnea. This mask carries pressurized air from a machine and through the mask in order to keep the throat open and breathing regular. This device is a continuous positive airway pressure (CPAP) machine. Your sleep specialist will discuss whether or not CPAP will be used during your evaluation with you before the study.
There is so much equipment attached to me! Is it safe? One concern that you may have is whether the recording sensors and equipment are safe. Nothing is passing into your body from the recording electrodes, sensors, or any other equipment. The equipment is safe. There are electrical safety checks and equipment inspections routinely performed in the Center to ensure patient safety. The tiny electrical potentials generated by your brain and muscles are being magnified and passed through your electrodes into the monitoring room recording devices.
After the sensors are applied, a process which typically takes about an hour, you will probably be allowed to read or relax until just prior to the time that you usually go to bed.
All of the equipment which is used to record the activity of your body during sleep is housed in a monitoring room separate from your bedroom. Your bedroom should be quiet. No radios or television will be left on during the study. This is so that the technician can hear you clearly, and so that your sleep is not disturbed. There is a closed circuit television system which will allow the technician to see you and talk to you from the monitoring room. The technician will be continuously observing you and monitoring your recording throughout the night. Also be aware that the technicians are trained in cardiopulmonary resuscitation (CPR) and the Sleep Center has procedures for implementing emergency medical procedures.