From proper electrode application to room setup, sleep technologists can take a number of measures to lessen interference during a sleep study.
In a sleep study, an artifact is an extraneous signal that represents activity other than what is intended for a given channel on a PSG.1 An artifact may arise from a variety of sources, both internal and external to the recording, and timely and accurate correction of the various types of unwanted signals can lead to the acquisition of a high-quality recording.2 The sleep technologist conducting the study plays an essential role in identifying and correcting artifacts.
Dawn Penrod, global product manager, sleep diagnostics, at Philips, says, “Identifying artifacts in a sleep study is an important part of ensuring that all portions of the study are scored despite instances of unreadable data or misleading results. This helps to avoid repeating the study if data quality is poor throughout the test.”
According to Todd Eiken, RPSGT, director of product development for Dymedix Diagnostics, “It is critically important that a sleep disorders technologist, as well as an interpreting physician, be able to identify and eliminate artifact that occurs during a PSG. Artifact can not only mask or reduce the visibility of clinically significant PSG information, but it can also create false-positive and false-negative results.”
Role of the Technologist
“The sleep technologist’s role is to identify artifacts and understand what is causing them. This includes checking at the beginning of the sleep study for accurate and secure lead placement, identifying any high impedances, conducting bio-calibrations, and ensuring equipment is properly grounded and working effectively,” says Penrod. “Throughout the night, the technologist should know when and how to replace dislodged electrodes, know how to re-reference electrodes, apply filters, and change gain settings,” she says.
Dymedix’s Eiken adds: “If artifact is recorded, it is the role of the technologist as well as an interpreting physician to adjust gain, common reference electrodes, or frequency filter settings during the scoring and interpretation phases of the analysis procedure to ensure accurate PSG results are obtained.”
Leah Hanson, REEG/EPT, vice president of global sales for Rhythmlink, says, “This identification is critical as artifacts can obliterate the data of interest, so if artifacts are not recognized…we can never uncover the actual data of interest leaving the testing useless for qualified diagnosis and treatment recommendations.”
Sources of Artifact: Physiological Artifact
The physiological sources of artifact can include a range of causes, including movement, skin, respiration artifact, and muscle activity.
Movement artifact is apparent in all the channels as patients shift normally in sleep or in response to a disruptive breathing event. Although it can often resolve itself, the sleep tech must pay close attention once the patient stops moving that no electrodes have been dislodged or upset.3 If there is artifact once the movement stops, the technologist must isolate the cause and repair or re-reference the problematic channel(s).3 SLP chief technology officer Noam Hadas and director of sales and marketing Sarah Paddock recommend securing wires with a piece of tape several inches from the sensors to prevent movement artifacts.
Eye movement is a specific type of artifact that can occur during a sleep test. In order to prevent this, Penrod says, “Technologists should check that the eye leads are placed properly and the polarity and reference for the EOG channel is correct.”
Penrod also has advice regarding other sources of movement artifact. “Preventing muscle activity can be difficult, but technologists should start by making sure none of the electrodes are broken and that there are no high impedances. Many patients practice relaxation techniques or, as a last resort, change the filters,” she explains. “Addressing respiration artifacts starts with checking effort belts to ensure that none of the electrode leads are trapped underneath a belt. Repositioning the patient or applying a low frequency filter can also help.” To address any artifact caused by sweating or galvanic response, Penrod says techs should verify that impedance values are acceptable, replace or re-reference electrodes, and maintain a cool temperature in the study room.
Respiratory motion affecting a common reference electrode is likely the most common artifact to occur, according to Eiken, and it is seen in most EEG and EOG channels. Eiken suggests sleep techs ensure a stable electrode placement, as well as an electrode location focus of reducing potential contact with the pillow, in order to reduce this artifact. Eiken adds, “Using sensors that are extremely sensitive with a fast response time will help in detecting subtle airflow changes as well as respiratory effort movements that would otherwise go unseen due to slow and less sensitive sensors.”
The integrity of electrode and sensor application is the single largest determinant of the quality and accuracy of recorded data.2 “Your study is only as good as your prep,” says Dana Burger-Dipzinski, vice president of sales and marketing at biomedical service organization and sleep sensor distributor ElectraMed Corp. “Also, for disposable adhesive electrodes, make sure they are not dried out. This can happen if you use expired electrodes, do not close the packages up between uses, or store them where it is too warm/cold.”
Incorrect placement can lead to artifact or improper signals, according to Paddock and Hadas. For example, if RIP/inductive belts are placed too tightly on the patient, belt life can decrease and the over-tightness can be a cause for undue artifact.
Larry Orbeta, RPSGT, sleep product manager at Nihon Kohden, and Sean Gause, support manager for Nihon Kohden subsidiary Neurotronics Inc, say, “Sensor application is an art form. To ensure an artifact-free recording, it is critical technologists receive training and supervision from experienced technologists who are proficient and skilled in their craft. They should also be provided with the proper equipment and supplies to allow them to do their job effectively.” For example, proper snore sensor placement can be achieved by tactile means while the patient is vocalizing.
In order to ensure proper electrode placement, Paddock and Hadas suggest using properly cleaned sensors and using one type of electrode for all locations on one patient. The pair recommend that sleep techs “work carefully and thoughtfully, keep impedances low, and secure sensors and wires properly” to ensure a quality recording. Penrod says the sensors should be cleaned after removal so the conducting cream does not harden. In addition, she adds that cleaning helps ensure sensors have not deteriorated.
Hanson stresses the importance of recognizing the unique needs of patients. “The application process can be unique to each patient, each skin type, hair type, etc. Take the time to get the application right,” Hanson explains. “Use your impedance readings as a guide.”
According to Penrod, a common cause of undue artifact occurs in the chin leads, as they are prone to loosening or coming off. “For patients with facial hair in particular, secure placement is hard to achieve. It’s possible to accommodate this by using a harder paste to secure the electrodes on the chin,” Penrod says.
There are resources available that go further in depth on prep and placement. “Good references for proper prep and placement include the 10 to 20 guidelines for EEG placement, the American Academy of Sleep Medicine recommendations for EOG and EMG placement, and the manufacturer’s guidelines for sensor placement. These include instructions on appropriate belt placement, cannulas, and thermistors,” Penrod says.
Sources of Artifact: Nonphysiological Artifact
Nonphysiological sources of artifact include the surrounding environment and recording instruments and devices. From electric motors to in-wall wiring, a number of elements in the room may cause interference in the study. To avoid this source of artifact, Hadas and Paddock say, devices with electric motors, power cables, and nonmedical electronic devices (such as radios) should be moved as far away from the patient as possible.
Other electrical appliances in the study room, such as overhead lights or fans, can also be a source of artifact. “Extraneous electrical interference being emitted from electrical appliances in the room…can sometimes be a source of artifact. Making sure all of the PSG equipment as well as any ancillary equipment being used is plugged into an electrical source sharing the same ground circuit can sometimes help in preventing electrical interference,” Eiken says. He says sleep-sensing products that incorporate technology and do not require an external power source can help reduce electrical artifact.
Orbeta and Gause say that devices that are not effectively shielded can produce an exorbitant amount of electrical interference. “One basic step to eliminate environmental artifact is to physically move the amplifier away from equipment that emits electrical interference. This can be as simple as moving the offending device to the opposite side of the bed from the amplifier,” they say. “In addition, the amplifier (and mini-input box, if applicable) should be positioned as far from electrical devices, power outlets, and power cords as possible. If the lab has access to a biomedical department, have them perform electrical safety testing on electrical equipment in and around the patient room.”
As electrical interference can pass through the floor and walls, the source of the interference may be anywhere from an adjacent room to a neighboring office. So anything should be suspected and careful work conducted to determine the source, explain Paddock and Hadas. “Using common sense and careful observation is usually sufficient. Try disconnecting…any suspected device. Try to correlate noises with activities in the lab—maybe it’s the microwave in the techs’ room? Try to shake all electrical cables and connectors—does the noise change in rhythm with the shaking? Try rerouting cables and wires—does this change noise levels?” say Paddock and Hadas. “It’s delicate detective work, but it’s usually successful.”
Hadas and Paddock also suggest: not using (or turning off) fluorescent lighting; keeping air humidity at 50% to prevent static charge buildup; making sure beds with metal frames are properly grounded; and arranging a headbox so that sensor wires do not rest on any electrical device or swing freely.
Hanson says, “PSG studies should never be done in a setting where the needs of the PSG equipment aren’t thought through carefully. Protection can be installed by having quality professionals insulate/shield and ground the lines in the walls properly and in a manner that minimizes environmental artifacts.”
Noise may be an additional artifact that can interfere with the quality of a recording. To aid in identification of this type of hindrance, Hanson suggests using an inductance amplifier that can “detect leakage current and will convert the point of concern to a ‘noise’ for identification.”
Orbeta and Gause say, ”When shopping for sleep lab diagnostic equipment, pay particular attention to the manufacturer’s input impedance on their amplifiers. The higher the input impedance, the more likely you will acquire a clean study in a noisy environment. Some amplifiers are better insulated from environmental noise than others.”
Finally, sleep techs may need to simply check signals and materials. “Stop using any sensor that is worn out and may stop during the night. Use fresh materials such as gels and rubbing compounds. Remember that signals will always deteriorate during the night, so never start a recording with less-than-optimal signals.”
The sleep tech has many routes to ensuring an accurate sleep study without unwanted artifacts, which empowers techs to help ensure that patients get accurate assessments of their sleep.
Cassandra Perez is associate editor for Sleep Review. CONTACT [email protected]
1. Ruth S. PSG Artifact Review: Recognition, Source Identification, and Corrective Action. May 2014. www.carolinasleepsociety.org/documents/presentations/2014_may/2014_may_ruth_psg_artifact_review.pdf PowerPoint accessed July 12, 2016.
2. Marshall H, Robertson B, Marshall B, et al. Polysomnography for the Sleep Technologist, Instrumentation, Monitoring, and Related Procedures. Elsevier Health Sciences; 2013.
3. Duffy B. 3 Common Artifacts That a Sleep Technologist Must Manage. American Association of Sleep Technologists blog, March 3, 2016. www.aastweb.org/blog/3-common-artifacts-that-a-sleep-technologist-must-manage Accessed July 12, 2016.