Polysomnography is a health care field with strong technical applications. To be effective, sleep technicians must have a strong health care background and be able to identify technical problems and troubleshoot them quickly and accurately. The signals derived in a sleep study are very minute electrical signals from the head, face, and body, which can easily be interrupted by outside electrical interferences. It is the technician's responsibility to place the sensors in the correct locations, apply them securely to the patient, and identify and correct any intruding signals. All electrode sites are cleaned precisely using the standardized methods described in this lecture and in the required text (Spriggs, 2008).
The International 10/20 System - Locating EEG Electrode Sites
In 1949, Dr. Herbert Jasper developed a standardized method of electroencephalogram (EEG) measurement called the International 10/20 Electrode Placement System. This system uses anatomical features such as the ears and the bridge of the nose to begin measurement. Most EEG electrode sites are located by calculating 10% and/or 20% increments along the measurement lines. The textbook contains details of this measurement system (Spriggs, 2008, pp. 118-132).
A standard sleep study utilizes the sites F3, F4, C3, C4, O1, O2, M1, M2, and FpZ for EEG electrode placement. Certain types of sleep studies require a full EEG hookup, which utilizes the same measurement system but usually includes at least 23 sites. For the purposes of this course, each student must know the measurement system for the standard sleep study EEG electrodes. To pass the sleep technician board examination, technicians must know all EEG locations.
Face and Body Electrode Sites
Electrode sites for the face and body are also standardized (Spriggs, 2008, pp. 133-138). The electro-occugram (EOG), or eye, electrodes are placed just outside each outer canthus, the location at which the upper and lower eyelids meet. E1, the left EOG, is placed 1 cm out and 1 cm down from the left outer canthus. E2, the right EOG, is placed 1 cm out and 1 cm up from the right outer canthus. E1 and E2 are both referenced to the same lead, either M1 or M2.
Three chin electromyogram (EMG) electrodes are used, although only two are actively recording at once and are referenced to each other. The first lead is placed in the middle of the chin, just above the inferior edge of the mandible; this is the exploring electrode. Two reference electrodes (one as a backup) are placed under the chin. One is placed 2 cm under and 2 cm to the left of the middle of the inferior edge of the mandible, while the other is placed 2 cm under and 2 cm to the right. An airflow sensor is placed just under the nose and above the mouth, with a mouthpiece placed in front of the mouth. The snore sensor or microphone is usually placed directly on the center of the patient's throat, where vibrations from snores can be felt the strongest.
Two electrokardiogram (EKG) leads are used as a standard in polysomnography, although a more complete EKG hookup can be utilized if ordered by the physician. The exploring electrode is placed low on the left side of the ribcage, while the reference electrode is placed just under the right collarbone. Two leg EMG electrodes are placed on the anterior tibialis muscle on the outside of the lower half of each leg. Foot and leg movements are best detected from the belly of this muscle. Respiratory effort belts are placed around the thorax and abdomen, and an oximeter finger sensor is placed on any of the three middle fingers of either hand. All electrode sites must be thoroughly cleaned and prepped with an electrically conductive prepping solution.
Sleep technicians must become familiar with the appearance of artifact-free signals. While most sleep systems include built-in impedance meters to check for electrical resistance, the technician still must be able to recognize when a channel appears to have an artifact. An artifact is an extraneous signal appearing in a recording channel on the polysomnograph. Artifacts are undesirable and in most cases should be corrected immediately. Several types of artifacts exist, including EKG artifact, movement artifact, sweat or slow wave artifact (also called sway), snore artifact, 60 Hz interference, electrode popping, and pen blocking. Inappropriate montage settings can also cause the waveforms to appear undesirable, even when no intruding signal exists. Identifying and correcting artifacts is key to performing a quality sleep study.
An important feature of correcting artifacts includes troubleshooting the diagnostic equipment. Sleep technicians must know how the equipment is hooked up, and be able to follow the signal pathway from the patient to the computer. Electrical signals produced by the head, face, and body are sent through an electrode and lead wire. Most artifacts occur because of dirt, debris, or a gap in the bridge between the skin and the electrode. Lead wires are plugged into a head box for easy access and patient mobility. The signals are then sent to the AC amplifier where they are referenced to each other as defined by the montage. DC devices, such as body position sensors, are routed through the DC amplifier. Both amplifiers are sent to the polysomnograph for filtering and viewing.
Polysomnography is a technical field of health care that requires the technician to use precise measurements, make secure electrode placements, and diagnose and troubleshoot artifacts and equipment malfunctions. Learning these skills is crucial to performing a sleep study.