Technology is advancing toward wireless, ambulatory pulse oximetry

Pulse oximetry is often considered the fifth vital sign, after heart rate, blood pressure, temperature, and respiratory rate. It has served as an important tool for the clinician by providing continuous monitoring of the critically ill patient’s arterial oxygen saturation (SaO2), by calculating an estimate of the SaO2 (known as the SpO2) via an algorithm, and displaying a readout of this estimation. Developing countries still depend heavily on the older pulse oximetry machines; the devices are portable, noninvasive, cost-effective, and easy to use.

Monitoring via pulse oximetry may be particularly useful when a patient is unstable and subject to rapid oxyhemoglobin desaturation. Busy staff may not notice that a patient has become restless, agitated, and confused, with accompanying cyanosis and an increased heart rate. The pulse oximeter’s monitoring of the pulse rate and the SpO2 will theoretically alert the clinician to the patient’s deteriorating status in time to intervene.

Unfortunately, although noninvasive and easy to use, the units have traditionally had problems differentiating between a real change in the patient’s condition and other distractions, such as electronic noise from other equipment, high ambient light, patient movement, and low peripheral perfusion. Or, if the manufacturer claimed to have overcome these problems, the increased cost of the unit made it the Rolls Royce of the oximeter world, an unenviable product position in this era of hospital budget crunching.

More seriously, though, the resulting high frequency of false alarms produced an unsettling tendency on the part of many clinicians to ignore pulse oximetry alarms. In a busy unit, or when RCP staffing was short, a frequently alarming pulse oximeter might be ignored or even turned off. For the safety of the patient, the technology has usually been restricted to areas with a low patient-to-clinician ratio, where the clinician can rapidly respond to an alarm and assess the meaning of changes in the SpO2 readings.

In recent years, most improvements in pulse oximeters have been largely cosmetic modifications to the size of the device and superficial tweaks to the user interface, such as the size and color of the readout. Competition has been fierce as companies fought to gain and hold market share with products that often did not display significant differences. However, according to manufacturers, current innovations are leading to oximeters that are better able to distinguish between motion artifact, low peripheral perfusion, electronic noise, and authentic patient events.

For example, one company claims to have virtually eliminated problems of motion artifact, low peripheral perfusion, and weak signal-to-noise situations. As a result, the pulse oximeter should both alarm less frequently and monitor the patient’s oxygenation more accurately, even under difficult clinical conditions.

If clinical experience supports claims such as these, then clinicians will, happily, have to resensitize themselves to pulse oximeter alarms. In addition, the RCP is likely to see increased usage of pulse oximetry outside of the intensive care units, into areas where there are fewer clinicians available for more patients.

Basics of pulse oximetry remain the same

Despite technical innovations, the basic pulse oximeter continues to be a spectrophotometric device. Two wavelengths of monochromatic light—red and infrared—are shown into the tissue to gauge the absorption of light by the patient’s oxygenated and deoxygenated hemoglobin. Algorithms calculate the SpO2 by comparing the differences between the emitted signal and the absorption of light in the capillary bed. The calculated SpO2 is an estimate of the arterial oxygen saturation, or SaO2. RCPs will recall that SaO2 and arterial oxygen tension (PaO2) are related through the oxyhemoglobin equilibrium curve.

For several years, numerous false alarms and missed true alarms have made SpO2 less useful when monitoring critically ill patients than it could have been had it operated more reliably. The remaining problems (physiologic extremes, skin pigmentation, an arterial saturation of less than 90%, movement, and intense ambient light) seemed to be frankly insurmountable—until recently.

Algorithms and Software
Companies are increasingly clamoring for the clinician’s attention, showing improved models and waving study results that claim to demonstrate the superiority of their unit over previous versions.

But have there been real technological breakthroughs, or are these statements simply marketing hype? According to a leading manufacturer’s technical director, there have indeed been significant improvements, leading to the production of devices that are better than 70%-80% of the oximeters already out in the field.

“Companies are making changes in how they analyze the light and in the electronics that they use,” states an engineering director. Experience has revealed that no single algorithm is sufficient to analyze the light data; manufacturers have therefore developed multiple parallel algorithms in order to cut through the nonspecific noise. Hence, both the algorithms and the related software are improving.

The new performance standard is a pulse oximeter that is resistant to motion and low perfusion, with, ideally, a high sensitivity (probability of hypoxemia detection or true alarm rate) and the lowest possible probability of false alarms. While this has always been the goal, it now appears that manufacturers are very close to achieving this standard, and offering it in an affordable, versatile package.

Motion: From the operating room to home care
Pulse oximetry was originally used in the operating room, ie, with motionless patients. When it began to move out to the neonatal intensive care unit (NICU), adult ICUs, and other locations where patients were sometimes restless and usually free to move about, then motion tolerance emerged as one of the critical technical problems to be solved.

One company advertises its monitor as virtually two devices in one. The technology latches on to a pulse signal, then, through developments in the software and algorithms, the device refines and tracks both the pulse rate and oxygen saturation. In this way, so-called corrupt noise signals may be filtered out, and only the clinically significant information is analyzed.

Companies offering improved filtering of motion signals are now suggesting that monitors can be used effectively in areas where patient movement has traditionally posed an insurmountable barrier to use. According to manufacturers, reliable ambulatory pulse oximetry, perhaps coupled with telemetry (see below) will soon be in routine use by emergency medical services (EMS), in transport, in the emergency department, in the ICU, and in other locations throughout the hospital.

Caution: alarm modifications ahead
Clinicians have always had some degree of control over the alarm settings of pulse oximeters. The newest product modifications, though, claim to combine refined signal processing with programmable alarm sensitivity. Theoretically, with these new products, the clinician may now differentiate between relatively trivial occurrences, such as transient desaturations, and significant patient events. These clinician-programmable alarms are purported to have a built-in “safety net” that can override the alarm settings determined by the clinician, if the alarm condition occurs to a certain degree and with a certain frequency.

However, one note of caution: on the basis of studies conducted under hospital conditions, some manufacturers are said to have produced an apparent decrease in the number of false alarms, but at the cost of missing true hypoxemia and bradycardia events. If true, and the debate continues, then this would clearly be a significant safety issue. Finger-pointing is taking place in the medical journals, and is spreading into the courts, as companies challenge competitors’ patent issues and product claims.

For example, one allegedly independent study, published in the May 2000 issue of the Journal of Critical Care Medicine, attempted to evaluate recent advances in pulse oximeter technology, particularly with regard to the rate of missing genuine hypoxemia and bradycardia events. The study compared three brands of pulse oximeters, and concluded that at least one manufacturer appeared to have produced a pulse oximeter that achieved a reduced false alarm rate at the expense of either an unreliable or a delayed identification of hypoxemia and bradycardia. RCPs should bear this in mind when evaluating new monitors for possible purchase. As one technical director stated, “No one out there has a perfect product yet.”

Problems of skin color and perfusion
Other tenacious obstacles include the effects of skin color and peripheral perfusion on the accuracy and calibration of the device. Initial problems with skin color and accuracy may have stemmed from the fact that early calibration tests were based on tests using Caucasian volunteers. Engineers continue to work on achieving a better understanding of how light interacts with tissue; this work should result in product improvements within the next couple of years.

One company believes that it has improved analysis of oximetry under conditions of poor perfusion by making full use of a “digital”-type pulse signal. The company claims to have the ability to process signals at much higher resolutions and at speeds of 100 times per second, improving patient monitoring even under difficult circumstances.

Another company says that its latest model can distinguish between an actual decrease in and/or loss of the pulse, and a pulse signal that is screened by low peripheral perfusion, patient motion, or electronic noise. Again, however, detractors suggest that these “improvements” may be illusory filtering of real patient events. Competitors have brought patent disagreements before the courts in recent months, declaring that proprietary technology has been copied and that various product claims may be dangerously misleading. The proof will likely come after actual usage on patients.

Telemetry and Electronics
Research departments are changing the electronics of pulse oximetry, not only to design a more portable and reliable device, but also to expand the use of pulse oximeters in telemetry and to decrease power requirements and cost.

Manufacturers operate in an environment that requires them to seek a balance between portability, reliability, telemetry, and cost. It can be like trying to sit comfortably on a one-legged stool. “A manufacturer may use batteries to produce a unit with a very low power requirement, but with a trade-off, in that the device may not be very electronically quiet,” states the technical director of a prominent respiratory therapy equipment company. “Another company may choose more expensive components to build a unit that, while not run off batteries, nevertheless produces less electronic noise.”

Anyone who has ever used a personal computer or a cellular telephone knows that powerful microprocessors with advanced digital signal processing have opened up enormous consumer markets. Increased consumer demand has driven down the price of microprocessors, to the point where their use in pulse oximetry is becoming more cost-effective. Hence, according to industry developers, during the next couple of years, manufacturers will likely leverage wireless and cell phone technology to advance technical developments in pulse oximetry.

One company is already offering an optional infrared link from the pulse oximeter to a personal computer or a printer, to produce a hard copy of the patient’s SpO2 record. Another company recently began to market a remote notification paging system that it says is the first to be based solely on monitoring a patient’s SpO2 and pulse. The manufacturer claims that the device can provide remote alarm notification for up to 144 patients.

Reliable home care pulse oximetry, with transmission of the patient’s data to a central location such as a hospital, is also under development, although industry experts do not expect home pulse oximetry to become a significant market segment. Given the strong trend toward computerized patient records, other companies are certain to be working on similar innovative wireless links of their own.

Sensors and product selection: Offering cost-saving choices
Hospitals have long been familiar with the basic cost-saving advantage of pulse oximetry over the collection and analysis of an arterial blood gas sample. Not only are pulse oximeters noninvasive and therefore less painful for the patient, but their use, despite limitations, has been proven to save money at the hospital’s bottom line. Companies have developed additional arguments and developments that can save money for hospitals, as well as make money for the companies.

A clinician, for example, does not always need the latest model for a patient with normal perfusion. Manufacturers recognize this, and present a portfolio of pulse oximeters, with a selection of attractive features from which to choose. The hospital then has an additional opportunity to save money or to increase efficiency.

The technological advances described above are opening up the opportunities to save money with reliable monitors. One company has devised a pulse oximeter with a three-in-one advantage: it can reportedly be used as a bedside monitor, as a handheld oximeter, or as an element in the upgrade of the hospital’s existing multiparameter monitors, presumably of that company’s brand. In this way, the manufacturer can present potential customers with a pulse oximeter that fits into the hospital’s cost-savings plans while expanding the facility’s oximetry options, and allowing clinicians to choose the machine that is most appropriate.

Manufacturers are also focusing on sensors, with an eye toward improving both sensor function and patient comfort, and helping hospitals to control costs. Among the offerings are latex-free sensors, as well as a wider breadth of sensor line, ranging from flexible band-type sensors for neonates, to ear clips, articulated finger clips, and special forehead sensors, for use when finger sensor placement is inappropriate.

Other firms have developed reusable single-patient sensors designed to last throughout a patient’s monitoring period, thus resulting in a significant cost savings for the hospital. One company claims that use of its reusable sensors, in place of the more common disposable type, can help a hospital save 40%-60% in oximetry operating costs.

Firms continue to pull out the stops in refining and advertising less exciting product improvements, in the hopes that such features, although less flashy than accuracy or alarm issues, will favorably differentiate them from the competition. These include on-screen viewing of the last 48 hours of monitoring; printouts of trends in both graphical and tabular formats; and reduced device weight, especially for handheld models, which may weigh less than 6 oz (162 g).

As long as the health care environment continues to support lower-cost treatment modalities, companies will continue to scramble after and advertise cost-efficient improvements.

Continuing opportunity in pulse oximetry
Despite the significant technical advances already appearing in hospitals, there is still room for improvement in pulse oximetry motion tolerance and accuracy; no one has the perfect solution yet. There is a continuing thrust toward getting the power out, and according to developers, the biggest challenge, and opportunity, still lies in low-power telemetry application—probably the least explored pulse oximetry application at this time.

“The introduction of powerful microprocessors in the consumer market, and the proliferation of wireless technology and cell phones, is driving down the price of micro-processors. During the next couple of years, we are likely to see pulse oximetry leveraging this technology,” remarks a technical director. This will mean a move toward wireless, ambulatory pulse oximetry and telemetric transmission to the nurses’ station.

These advances are significant enough that companies are arguing (and litigating) over whose patents are valid, and which manufacturer was the first to come up with the best technological advances. Regardless of who invented what first, at the end of the day, the patient and the clinician will be the winners.

Aileen Hill, RRT, is a contributing writer for RT Magazine.