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CE Article

New-Generation Pulse Oximetry in the Care of Critically Ill Patients

	Abstract

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• Objective To review the published, peer-reviewed studies to date on use of the new-generation pulse oximeters, which are manufactured with algorithms to filter out patients’ body motions, and describe the application of new-generation pulse oximetry in clinical practice in critical care.

• Methods MEDLINE was used to locate appropriate articles on pulse oximetry for the years 1995 to 2003. Each article was examined for scientific merit, content, and applicability to clinical practice.

• Results A total of 17 relevant articles on the clinical performance of the new-generation pulse oximeters were reviewed, and the data were organized into a table.

• Conclusions The combination of studies done in both clinical and laboratory settings did not provide any strong and convincing evidence that the performance of any single new-generation device was superior to that of any other new-generation device. However, the clinical performance of all the new-generation pulse oximetry devices was better than that of earlier devices.

Notice to CE enrollees: A closed-book, multiple-choice examination following this article tests your understanding of the following objectives: Compare and analyze new-generation pulse oximeters with conventional devices Describe the devices that showed superior clinical performance in the studies involving adults in the postanesthesia care unit, critical care unit, and operating room Describe the devices that showed superior clinical performance in the studies involving children and neonates

Continuous monitoring of oxygen saturation via pulse oximetry is a standard of care for use in the operating room, postanesthesia care unit (PACU), critical care unit, emergency department, and environments in which conscious sedation is used.^3,4 As the acuity level of hospitalized patients increased during the past 10 years, so too did the need for oxygen saturation monitoring outside critical care areas.

The first commercially available pulse oximeter was developed by Takuo Aoyagi and was marketed by Nihon Koden in 1974.⁵ However, both this first pulse oximetry device and the Oximet marketed by Minolta in 1977 were considered primarily research devices. The first pulse oximetry device manufactured for routine clinical use was the Nellcor N-100. It was initially marketed in 1982 and became the standard conventional pulse oximeter.⁵ Those first conventional oximeters made it possible to measure oxygen saturation as a continuous parameter and, as a result, were rapidly adopted for use in surgery to prevent the estimated 40% of accidental deaths that were due to decreases in oxygen saturation.⁵ However, outside the operating room, patients’ body motions became a major technical challenge, because the motions interfere with the ability of conventional pulse oximeters to obtain an accurate measurement.

Motion artifact occurs either when a patient’s movements cause the pulse oximeter to incorrectly interpret the movements as a pulse signal or when the motion artifact prevents accurate detection of the patient’s true pulse signal. The resultant increase in false alarms and erroneous measurements can desensitize clinicians to the alarms and increase the chance of missing a clinically important true alarm.^6–8

Patients’ motion decreases accuracy of conventional pulse oximeters and has led to the development of "motion tolerant" oximeters.

 

Manufacturers of pulse oximeters have sought to reduce motion artifact in the pulse signal through improvements in the algorithms. "Motion tolerant" pulse oximetry devices are now commercially available from several different manufacturers, and as a group are referred to as "new generation" pulse oximeters. To date, peer-reviewed scientific publications on evaluations of these new-generation, motion-tolerant devices are few and are primarily limited to 3 of the devices currently on the market: FAST SpO₂ (Fourier artifact suppression technology SpO₂; Philips Medical Systems, Andover, Mass; first marketed in 1999), SET (signal extraction technology; Masimo Corp, Irvine, Calif; first marketed in 1998), and Oxismart (Nellcor, Pleasanton, Calif; first marketed in 1994).

	Review of the Literature

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MEDLINE was used to identify research studies on the new-generation motion-tolerant pulse oximetry products published in English during the period 1995 through 2004. The search terms "oxygen saturation," "pulse oximetry," "SpO₂," and "oxygen measurement" were used. Even though many articles on pulse oximetry in general have been published, the focus of most of the articles was conventional pulse oximetry. In this state-of-the-science review, we focus solely on databased studies on the performance of the new-generation pulse oximetry devices during patients’ movements. Although we also reviewed some data on the performance of the devices in patients with low perfusion, that aspect was not our focus. We found 17 research studies in which investigators specifically examined the performance of new-generation devices; these studies are listed and described in the Table.

	Laboratory Studies

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The performance of new-generation pulse oximetry devices was evaluated in a laboratory setting in 5 studies.^{9,14,16,17,19} The advantages of doing any kind of clinical study in a laboratory rather than a real clinical setting are generally lower cost and more control over the variables. The major problem, however, is the difficulty of replicating the characteristics of a clinical setting,¹⁶ especially when the clinical setting is as complex as critical care. Additionally, in 4 of the 5 studies, the subjects were healthy volunteers, whose clinical conditions and oxygen saturation measured by pulse oximetry (SpO₂) often bear little resemblance to those of critically ill patients.^9,14,16,19

One approach usually referred to as the "steady-state reference hand" was used in 2 of the studies.^9,17 With this approach, the subject keeps one hand still and the other hand is strapped to a motion arm that is programmed to move in various patterns. The SpO₂ readings of the moving hand are then compared with the readings of the nonmoving reference hand. The disadvantage of this approach is that passive motion is used, a kind of motion that has little relevance to clinicians because these simulated passive motions do not mimic the active motion most common in patients in actual clinical settings.²⁷ Therefore, the use of this type of testing is a significant flaw when used in studies to compare the performance of pulse oximetry devices, a flaw that has been recognized by other researchers.¹⁶

Approaches in which active motion is used to test pulse oximetry devices have also been described.^14,16,17 Both Jopling et al¹⁶ and Gerhing et al¹⁴ used healthy volunteers but devised active-motion protocols of tapping and scratching motions with a wide variety of amplitudes and velocities. A noteworthy weakness of these studies is that both had small sample sizes (Jopling et al, N = 8; Gehring et al, N = 10). In addition to motion, the devices in these studies were tested during conditions of hypoxemia and low perfusion. Jopling et al induced hypoxemia in the healthy subjects and thus obtained a range of oxygen saturations of 70% to 100% during the active-motion testing, which provided a much closer approximation to the range of SpO₂ values that might occur in a real clinical setting. Gehring et al simulated low perfusion by compressing the brachial artery. The overall findings for the detection of hypoxemia of both of these studies indicated that the performance of the new-generation devices was better than that of the conventional devices. When the new-generation devices were compared with one another, differences in the detection of hypoxemia were not significant.

In a lab setting, all new-generation pulse oximeters outperformed conventional devices in their ability to tolerate motion.

 

Kastle and Konecny¹⁷ used an approach referred to as noise-mix-composition. In this approach, 136 different combinations of raw patient-generated signals from the operating room, the intensive care unit, and the PACU were used. The range of oxygen saturation in the signals was 39% to 100% (mean 89.9%; SD 10.7%), and the range of pulse rates was 49/min to 201/min (mean 108.2/min; SD 45.9/min). These signals were then fed digitally into each of the pulse oximeters being studied. The advantage of this approach is that real, not simulated, signals were used. In addition, a wide range of both oxygen saturations and pulse rates were used to test the pulse oximeters. This range of values more closely approximated the types of clinical phenomena encountered in the critical care setting.

In general, the overall results of the 5 laboratory studies were consistent. In studies in which passive motion was used, the Masimo SET device tended to have the best performance, whereas in studies in which active motion was used, the Nellcor and Philips devices tended to have the best performance. For example, in the study by Kastle and Konecny,¹⁷ the Masimo device had the best performance (2.6-fold improvement over conventional pulse oximeters compared with 1.6-fold improvement for both Philips and Nellcor) during the steady-state reference hand, passive-motion part of the testing. In contrast, during the noise-mix-composition, active-motion part of the study, the Philips device had the best performance (1.6 fold improvement compared with 1.5-fold for the Masimo device and 1.3-fold for the Nellcor device). However, none of these differences were clinically significant.

In 4 of the 5 laboratory studies, conventional devices were used as one means of comparison.^9,14,17,19 A consistent and important finding of these 4 studies was that regardless of the differences in measurement criteria, the performance of all the new-generation pulse oximetry devices was better than the performance any of the conventional devices to which they were compared.

	Clinical Studies in Adults

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Real clinical environments and adult patients were used to assess the performance of new-generation pulse oximeters in 6 studies.^{12,13,20,23–25} The new-generation devices included the Masimo SET, Nellcor N-3000, and Philips FAST SpO₂. The clinical areas included the operating room,^23,25 the PACU,^20,24,25 and the critical care unit.^12,13,20 Because patients’ movements should not be a factor in the operating room, we considered studies done in the operating room separately from those done in the PACU or critical care unit.

The new-generation devices used in the operating room included the Nellcor N-3000 and the Philips FAST SpO₂. In the first study, Rheineck-Leyssius and Kalkman²³ found no difference found between the performance of the N-3000 with a 5- to 7-second signal-averaging time and the performance of a conventional pulse oximetry device with a 21-second signal-averaging time.

None of the new-generation oximeters differed from the others in clinical performance.

 

Signal-averaging time for pulse oximeters represents the amount of time (in seconds) used by the device to calculate the displayed SpO₂ value. The shorter the signal-averaging time, the more sensitive the device will be to changes in the SpO₂ value. However, the device will also be more prone to false alarms due to artifact or error. Conversely, as the signal-averaging time is increased, the number of alarms due to error or artifact is decreased, but the amount of time needed to detect a true hypoxemia is concomitantly increased, because the SpO₂ values are averaged over a longer period.

Hence, the results of the study²³ are not surprising because patients’ movements should not be a factor in the operating room. Furthermore, a 21-second signal-averaging time on the conventional comparison device would obscure most of the signal artifact because the signal is averaged over such a long period. In contrast, in most of the new-generation devices, signal-averaging times are less than 10 seconds. This decrease in time enhances detection of hypoxemia, and, at the same time, the improved algorithms are better able to filter out motion artifacts due to patients’ movements, thus helping to control for the increase in false alarms that accompany decreased signal-averaging times with conventional pulse oximetry.

Factors that influenced oximeters’ degree of error were peripheral temperature, finger thickness, hemoglobin concentration, and skin color.

 

In the second study done in the operating room, Wouters et al²⁵ used a total of 2694 SpO₂ recordings in 1483 patients and compared the measurement accuracy of the Philips FAST SpO₂ and the Nellcor N-3000. Values obtained via co-oximetry were used as the reference SpO₂ measurements. The overall results indicated accurate, comparable, and clinically acceptable performance for both devices, with neither device outperforming the other. In addition, this study was the only one in which multivariate analyses were used to determine other patient-related variables that might affect the performance of pulse oximetry devices. Four variables had a significant effect on the degree of error of both pulse oximeters: peripheral temperature, finger thickness, hemoglobin concentration, and skin color. Although finger thickness was addressed in a few of the studies by rotating the placement of the sensor on the patient’s finger (see Table), none of the other variables was addressed. These variables should be included and analyzed in all future studies on pulse oximeters.

The new-generation Masimo SET, Philips FAST SpO₂, and the Nellcor N-3000 devices were used in the studies done in the PACU and the critical care unit.^{12,13,20,24,25} For these studies as a group, two overall conclusions can be made. First, the performance of the Masimo SET and the Nellcor N-3000 devices was significantly better than that of the comparison conventional devices.^12,13,20,24 Second, when compared with one another, none of the new-generation devices differed significantly in clinical performance.

Lutter et al²⁰ compared the performance of Masimo SET, Nellcor N-3000, and Philips FAST SpO₂ devices during intra-aortic balloon pumping in the intensive care unit. They found that all 3 devices had degradations in clinical performance during pumping, suggesting that SpO₂ measurements in general should be used with caution during pumping. This finding seems reasonable, because intra-aortic balloon pumping creates a rhythmic disturbance in the arterial waveform that is in sync with the cardiac cycle, thereby making it difficult for pulse oximetry devices to distinguish between a true cardiac signal and the "artifact" signals generated by the intra-aortic balloon pump.

	Clinical Studies in Children

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The Masimo SET and the N-295 (Nellcor’s earliest motion-tolerant pulse oximetry device) devices were used in 2 clinical studies done in children.^11,21 Malviya et al²¹ studied a group of 75 healthy children in a PACU and compared the Masimo SET device with a conventional Nellcor N-200 pulse oximeter. No gold standard was used and only a single new-generation pulse oximetry device was used. However, the findings from this study were consistent with the results from all other studies; the clinical performance of the motion-tolerant Masimo SET was better with regard to both true and false alarm conditions than was the comparison conventional device. Brouillette et al¹¹ compared the Masimo SET with the Nellcor N-200 and N-395 in children in a sleep laboratory. Unfortunately, this study was so methodologically flawed that no meaningful conclusions can be made. Significant weaknesses included discrepancies in the reported signal-averaging times of the devices and lack of a gold standard reference SpO₂ value, making it impossible to assess accuracy for either device.

	Clinical Studies in Patients in the Neonatal Intensive Care Unit

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A total of 3 studies^10,15,18 were done in patients in the neonatal intensive care unit (NICU). The motion-tolerant devices used in these studies included the Philips FAST SpO₂, the Masimo SET, and the Nellcor N-3000. One study¹⁸ was so poorly designed and had such ambiguous findings that it was excluded from this review. Bohnhorst et al¹⁰ compared the Masimo SET and the Nellcor N-3000 motion-tolerant devices with each other and with the Nellcor N-200 conventional device. The performance of both of the motion-tolerant devices was better than that of the comparison conventional devices, again consistent with findings from previous studies. In addition, when the 2 motion-tolerant devices were compared with each other, the performance of the Masimo SET was significantly better than that of the Nellcor N-3000 for correct alarms, detection of hypoxia, and detection of bradycardia. These data indicate that the overall alarm rate for the N-3000 may have been achieved at the expense of some missed alarms for hypoxemia. If true, this possibility is clearly something that would be a point of concern for critical care clinicians. However, because the rate of false alarms was not evaluated in this study, fully evaluating the performance of each of the devices in the study is difficult.

More specifically, in a recent study, Hay et al¹⁵ examined the performance of the Philips FAST SpO₂, Masimo SET, and the Nellcor N-395 devices and compared them with conventional devices for detection of hyperoxemia. The number of false alarms (specificity) and data dropouts did not differ significantly between the new-generation devices, and the Masimo SET had the best overall clinical performance. However, the software revision of the Philips device was not provided, again making a meaningful comparison difficult.

An important clinical issue specific to patients in the NICU is the performance of new-generation motion-tolerant pulse oximetry devices in detecting hyperoxemia.²² Hyperoxemia in the NICU is generally defined as a PO₂ greater than 80 mm Hg, and only a few investigators attempted to determine the appropriate alarm level of pulse oximetry devices needed for detection of hyperoxemia. Hyperoxemia is associated with a number of complications in neonates, including retinopathy of prematurity, a complication that is not a clinical concern in children and adults. Poets et al²² reported that at an upper alarm limit of 95%, new-generation pulse oximeters are much better than the older generation devices in detecting hyperoxemia.

	Summary

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A consistent finding among the studies reviewed was the superior performance with regard to reduction in frequency of false alarms and overall accuracy of all the new motion-tolerant devices compared with the performance of the various conventional pulse oximeters. However, more clinical evidence on the performance of new-generation pulse oximetry devices is clearly needed, particularly in the setting of critical care. Although some conclusions can be made, a lack of consistency among study variables and techniques and discrepancies between the results of the studies reviewed make it difficult to make any overall recommendations about which of the new-generation motion-tolerant devices are best suited for use in critical care in any of the populations of patients studied. Clearly, more clinical trials are needed before the most accurate and reliable motion-tolerant pulse oximetry device can be determined for use in populations of critically ill patients. These trials should address the performance of the algorithms used for obtaining SpO₂ values in critically ill patients of all ages in situations in which patients’ movements are a factor. In addition, each trial should clearly specify which software revisions are being used with each device and should include a comparison with a known gold standard (preferably co-oximetry) for measurement of oxygen saturation. Finally, on the basis of the findings from Wouters et al,²⁵ the variables of peripheral temperature, finger thickness, hemoglobin concentration, and skin color should be part of every future study on measurement of oxygen saturation by pulse oximetry devices to further assess the significance of these variables on clinical measurement of SpO₂.

New-generation oximeters have fewer false alarms and higher accuracy than do conventional oximeters.

 

New technology has, without doubt, advanced the science of pulse oximetry. Evidence supports the contention that the performance of the new-generation devices with incorporated enhanced algorithms for SpO₂ monitoring is superior to the performance of conventional pulse oximeters. However, until more evidence is available from clinical studies in which the new-generation oximeters are compared with one another, no valid judgments or decisions can be made about which new-generation device has the best clinical performance in different populations of critically ill patients.

To purchase reprints, contact The InnoVision Group, 101 Columbia, Aliso Viejo, CA 92656. Phone, (800) 809-2273 or (949) 362-2050 (ext 532); fax, (949) 362-2049; e-mail, reprints{at}aacn.org.

Commentary by Mary Jo Grap (see shaded boxes).

	REFERENCES

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This Article Abstract Full Text (PDF) Respond to This Article Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Take the CE Test Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Giuliano, K. K. Articles by Higgins, T. L. Search for Related Content PubMed PubMed Citation Articles by Giuliano, K. K. Articles by Higgins, T. L.