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When patients are being wired for a sleep study, one of the first things they may notice is the pulse oximeter probe being attached to their finger or toe or earlobe. While a pulse oximeter may be a detail to a patient, for a sleep professional, it would be difficult to imagine a modern sleep study without one. When incorporated with other channels of polysomnography, the pulse oximeter becomes a vital component for diagnosing OSA and other sleep disorders in the sleep laboratory. Pulse oximetry is also a key parameter in many home testing devices. Given the recent report in the Annals of Internal Medicine comparing home testing to laboratory polysomnography1 and the Centers for Medicare and Medicaid Services’ current consideration of changing sleep apnea diagnostic testing criteria to allow home testing, SpO2 may become even more significant in the coming years.
Past “Market Trend” articles focusing on polysomnography and oral appliances are available in Sleep Review’s online archives. See the Related Article links at the end of this article.
There are few areas of medicine where pulse oximeters are not used today. Oximetry has become so common that it is often referred to as “the fifth vital sign” along with blood pressure, pulse, respiratory rate, and temperature.
Pulse oximetry’s primary function in polysomnography is to measure and monitor oxygen saturation during data acquisition for sleep scoring. Respiratory events recorded that show a decrease in oxygen saturation can reveal the patient’s irregular or interrupted breathing patterns in OSA and SDB.
“What you’re looking for are deep desaturations,” says Michael Hubbard, RN, market manager at Smiths Medical, Waukesha, Wis. “With obstructive sleep apnea, a patient can undergo many, many of these without even being aware of it. So what physicians are looking for is the depth and duration of the desaturation.”
Oximetry can also detect the typical Cheyne-Stokes breathing in central apnea. A decrease in oxygen saturation levels after a respiratory event may indicate prolonged hypoxemia, which can contribute to cardiac and cerebral ischemia.
MODERN PULSE OXIMETERS IN SLEEP
Modern pulse oximeters have been refined, becoming smaller and more accurate, and have the ability to be incorporated into a variety of multiparameter equipment, including polysomnography systems.
The digital technology used to measure oximetry in these various devices is essentially the same in all platforms. Oximeter sensors flash a combination of red and infrared light through a part of the body, usually a patient’s finger, toe, or earlobe, at a particular rate. When the two types of light travel through the finger, the arterial blood flowing through the tissue absorbs some of the light. Oxygenated hemoglobin absorbs more infrared light and less red light. Conversely, deoxygenated hemoglobin absorbs less infrared light and more red light. Manufacturers use unique proprietary algorithms to calculate the level of oxygen desaturation based on these two light refractions.
|Masimo SET Radical-7 Pulse Oximeter|
The accuracy of a digital pulse oximeter may depend on a number of variables that laboratory technologists must always keep in mind. These factors include:
- The patient moving during sleep (motion artifact)
- Exposure of measuring probe to ambient light during measurement
- Low perfusion states
- Skin pigmentation
- Nail polish or nail coverings if using a finger probe
|Respironics Stardust® II Sleep Recorder|
Daniel Draper, director of acute care marketing, Masimo Corp, Irvine, Calif, says that it is important for sleep technicians to calibrate, or interface, their oximeter correctly to the digital polysomnograph. “[Calibration] is often taken for granted, and it’s not correctly done in some labs,” he says. “Given the critical import that you put on whether a patient is oxygenating during sleep, whether their heart is under duress from myocardial ischemia, oxygenation, or other health limitations, you want to know that right value. To me, that’s not a ballpark figure until your patient’s SpO2 values transition into the 50s and less. It’s an important number that can help a clinician decide whether to intervene with treatment, and as a result, considerable care should be given to proper interface of the pulse oximeter to the digital or analog polysomnograph.”
An oximetry reading may also be affected by patient movement or motion artifact. When a patient’s arm or finger moves during sleep, venous blood complicates the arterial blood so there is no longer a smooth transition of blood through the fingertip, thereby distorting the pulse oximeter’s values. Some oximeters compensate for motion artifact by freezing the last value before motion is recognized by the oximeter and then hold that value until the motion stops. Other oximeters may average some venous blood with the arterial blood, which can give a slightly lower desaturation related to the event.
|Respironics Alice® 5 Diagnostic Sleep System|
Low peripheral profusion, where blood flow to the extremities slows down, can also cause inaccurate results or cause pulse oximeters to freeze on the last valid reading. Both low peripheral profusion and motion artifact can cause either false alarms or missed true events. As a result, manufacturers are constantly refining pulse oximeters to account for motion artifact and perfusion.
Ron Fligge, RRT, global product manager for Sleep Diagnostics at Respironics Inc, Murrysville, Pa, says that his company chose a Masimo oximeter for its Alice 5 polysomnograph because of Masimo’s ability to read through motion artifact, as well as being able to detect weak signals due to low perfusion.
OXIMETRY’S LATEST TECHNOLOGICAL FEATURES
Aside from designing technology that addresses low profusion and motion artifact, pulse oximeter manufacturers are refining their products with other new features and technologies that may be valuable for polysomnography:
Wireless SpO2: For those RPSGTs who would like one less cable or sensor wire, Nonin Medical Inc, Plymouth, Minn, has introduced the Avant® 4000 System with Bluetooth® wireless technology. This wrist-worn patient module wirelessly sends encrypted pulse oximetry data to a small tabletop display, which can then be cable connected to the headboard. There are also ambulatory multichannel polysomnograph systems (from CleveMed, Cleveland; Grass-Telefactor, West Warwick, RI; and Nihon-Kohden America, Foothill Ranch, Calif) that offer wireless channels, including pulse oximetry. (See “What’s New in PSG Diagnostics?“.)
Measuring carboxyhemoglobin: Pulse oximeters typically do not differentiate between carboxyhemoglobin and oxyhemoglobin. Carboxyhemoglobin is seen by two wavelength oximeters as oxyhemoglobin, and may impart a false sense of security, as well as providing misleading oxygenation during sleep values. Consequently, cigarette smokers can artificially elevate oximetry readings because of the extra carbon monoxide in the blood due to smoking before a sleep study. Masimo’s SET system addresses this issue by providing a separate carboxyhemoglobin measurement, which can be used by clinicians to take into account oximetry readings from smokers and more accurately determine the patients’ true oxygenation status.
Faster recording times: Another trend in the industry is faster recording speed of data. Most oximeters have historically performed trending of the data every 6 or 8 seconds, although oximeters are technically capable of sampling data at faster rates. In the past, it was believed that shorter recording times would introduce more motion artifact because of a person’s movement. However, because sleep laboratory patients move relatively little while sleeping and oximeters are increasingly accounting for motion artifact, the new trend is for faster averaging times—as little as 3 seconds—so there are more responses to oximetry data when apnea events occur. (See “The Benefits of Pulse Oximetry During Polysomnography” and read about a study comparing averaging times during sleep studies.) As a result, polysomnography and oximetry companies have designed or adjusted their products so that sleep laboratories are able to customize the recording speed to slower rates.
Oximeter sensors: Oximeter sensors have also evolved over the last few years. Technicians can not only choose a variety of finger sizes for adults and children, but they can also pick from a variety of disposable sensors or reusable ones. A disposable oximetry sensor is essentially an adhesive with red light/infrared light sources built in. As with conventional reusable probes, it is connected to the cable that goes into the oximeter. Hubbard believes that disposable probes may be better for some sleep patients. He says, “For some people, the disposable will give you better contact. For example, for someone who is moving around a lot, you’d have an actual adhesive there that is holding the probe in place. You could also argue that, despite the fact that you can clean the reusables, there is far less chance of cross contamination with the disposable probes.” Disposables are also inexpensive compared to reusable probes; of course, the downside of disposables is their recurring cost.
THE FUTURE OF OXIMETRY: HOME TESTING?
Pulse oximetry has received new attention due to a recent study published in the Annals of Internal Medicine, which reported that polysomnography offers no benefit over ambulatory testing to diagnose OSA in patients with a high probability of moderate to severe OSA. The news may be significant, not only for home testing and ambulatory polysomnography manufacturers, but also for pulse oximetry manufacturers, since the study was conducted using a continuous memory pulse oximeter and an auto-CPAP.
How the Annals of Internal Medicine report and CMS’s reconsideration of OSA diagnostic criteria ultimately influence polysomnography inside or outside the sleep laboratory remains to be seen. Whatever the effects, pulse oximetry will remain an important component of sleep medicine for the foreseeable future.
Tor Valenza is a staff writer for Sleep Review. He can be reached at firstname.lastname@example.org.
- Mulgrew AT, Fox N, Ayas NT, Ryan CF. Diagnosis and initial management of obstructive sleep apnea without polysomnography. Ann Intern Med. 2007;146:157-66.