Technical Aspects of Sleep Testing
Chapter from the ACCP Sleep Medicine Board Review Course, 4th Edition Technical Aspects of Sleep Testing
After reading the chapter, test your knowledge with these two questions from ACCP-SEEK® Sleep Medicine: First Edition.
1. A 48-year-old man is referred to the sleep laboratory for a polysomnogram (PSG) to evaluate snoring and apneas witnessed by his wife. His BMI is 35 kg/m2 , and he has a neck circumference of 45 cm. A representative epoch of the PSG is shown in the figure below. Which of the following is the most likely cause of the disturbance seen in the PSG epoch?
A. Change in body position from supine to lateral.
See the Answer.
B. Excessive sweating.
Sweat artifact is a common finding on PSG. Sweating itself is common during sleep and has many causes, including infections, cancers, rheumatologic disorders, and medications. In many cases, nocturnal sweating is idiopathic. Sleep apnea is a recognized cause of nocturnal sweating, and it may be a common cause of nocturnal sweating in sleep laboratories.
When patients undergoing PSG sweat heavily, they may disrupt the contact between the electrode and the skin, resulting in artifacts. The artifacts are usually characterized by large, slow, and somewhat chaotic or dysynchronous waves in the EEG or EOG channels. In the figure above, both the EEG channels and EOG channels are affected and demonstrate the large waves typical of this setting (choice B is correct).
Sweat artifact is corrected by cooling the patient and allowing the sweating to dissipate. In some cases, the electrode artifacts may self-correct. In other cases, a change of electrodes will be required. Sleep laboratory technologists must monitor ambient temperature in the bedroom and make adjustments for patient comfort and signal integrity when sweating is a problem.
Choice A is incorrect because a change in body position would result in movement artifact, which typically involves more than just the eye channels or EEG leads. Choice C is incorrect because slow-wave sleep would not have the fluctuating or wandering baseline seen in sweat artifact. Choice D is incorrect because changing a single electrode would not eliminate the artifacts, which are seen bilaterally.
Mahowald M. Other parasomnias. In: Kryger M, Roth T, Dement W, eds. Principles and Practice of Sleep Medicine. 4th ed. Philadelphia, PA: Elsevier, 2004; 920.
Heller C. Temperature, thermoregulation, and sleep. In: Kryger M, Roth T, Dement W, eds. Principles and Practice of Sleep Medicine. 4th ed. Philadelphia, PA: Elsevier, 2004; 294.
Chokroverty S, Thomas RJ, Bhatt M. Atlas of Sleep Medicine. Philadelphia, PA: Elsevier, 2005; 15, 18-19.
2. Which of the following would correct the artifact shown in the 30-s epoch represented in the figure below?
A. Replace the electrode at the outer canthus of the left eye.
See the Answer.
D. Replace the electrode at the right auricular area.
Eye movements are measured to help stage sleep. The electrical principles behind eye movements are straightforward. The eye functions as an electrical dipole, with the cornea reflecting a negative charge and the iris or sclera possessing a positive charge. As the eye globe moves in its socket, the electrodes placed just above or just below the outer canthus of each eye detect a change in charge. The electrodes are set up so that conjugate eye movements are represented as disconjugate deflections of the pen or computer readout. By convention, the left side of the head has odd numbers and the right side of the head has even numbers. In the figure above, the artifact is seen in the LEOG, C3A2, and O1A2 leads and appears in the first 25 s of the epoch by sharp, high-voltage, conjugate deflections. The first step in identifying the source of the artifact is to look for an electrode common to the leads that show the artifact. In this case, A2 is the likely culprit since it is common to all three of the channels that show the artifact. The A2 (or right auricular) lead is positioned just posterior to the right ear in the mastoid area (choice D is correct).
Replacing A1 (the outer canthus of the left eye), C3 (left side of the vertex), or O1 (left side of the occiput) will not resolve the problem since those electrodes are functioning normally (choices A, B, and C are incorrect). To note, the 2007 American Academy of Sleep Medicine scoring manual renamed the auricular location the mastoid location, so the electrode is designated as M2 rather than A2.
Chokroverty S, Thomas RJ, Bhatt M. Atlas of Sleep Medicine. Philadelphia, PA: Elsevier, 2005; 3-4.
Walczak T, Choroverty S. Electroencephalography, electromyography, and electro-oculography; general principles and basic technology. In: Chokroverty S, ed. Sleep Disorders Medicine. Boston, MA: Butterworth-Heinemann, 1999; 184-186.
Carskadon M, Rechtschaffen A. Monitoring and staging human sleep. In: Kryger M, Roth T, Dement W, eds. Principles and Practice of Sleep Medicine. 4th ed. Philadelphia, PA: Elsevier, 2004; 1359-1367.
Iber C, Ancoli-Israel S, Chesson A, et al, for the American Academy of Sleep Medicine. The AASM Manual for the Scoring of Sleep and Associated Events: Rules, Terminology and Technical Specifications. Westchester, IL: American Academy of Sleep Medicine, 2007.
