Pulse oximetry provides a simple, continuous, noninvasive assessment of arterial blood oxygen saturation. Introduced by Nicolai [1] in 1932, oximetry did not gain widespread acceptance until the 1970s, when it was first reported that arterial hemoglobin oxygen saturation could be measured by analysis of pulsatile blood flow [2]. The pulse oximeter operates by measuring the attenuation of red (660 nm) and infrared light (940 nm) as it passes through a pulsatile tissue bed. Pulsatile blood flow is essential to allow differentiation of arterial from other absorbers of light such as venous and capillary blood components, as well as overlying tissue and skin [2]. Conceptually, the value of pulse oximetry is in identification and correction of hypoxemic events, which, if untreated, could result in serious complications such as brain injury, cardiac arrest, or death [3]. In a review of anesthetic-related, closed malpractice claims, Tinker et al. [4] stated that 31.5% of cases with adverse outcomes could have been prevented through additional monitoring. Eichhorn [5] in a review of 1,001,000 ASA physical status I and II patients, found 11 major intraoperative accidents that could have been attributed solely to anesthesia; seven were deemed preventable. Unrecognized hypoventilation was the most common intraoperative event associated with severe patient injury. Other extensive clinical series document that pulse oximetry increases the identification of hypoxemic events [6,7]. Although pulse oximetry is now well established, the technology has not been greatly improved since 1988 [8], and a number of limitations persist. Kestin et al. [9] reported that 75% of all auditory alarms during routine anesthesia did not originate from changes in physiological variables and only 3% represented patient risk. Freund et al. [10] reported 11,046 cases, with a 1.12% failure rate, defined as the inability to obtain any pulse oximetry reading for a cumulative period of 30 minutes or more after all mechanical problems had been eliminated. A higher rate of failure was present in elderly patients, those with ASA physical status III, IV, and V, and during long or emergency operations. An algorithm developed by Timcenko and Reich [11] predicted a more frequent rate of pulse oximetry failure depending on patient age, ASA physical status, body temperature (<35 degrees C or >37 degrees C), and type of operation (cardiopulmonary bypass, vascular, and orthopedic operations), while Moller et al. [6], by analysis of data from 20,802 patients, identified a total failure rate of 2.5%, increasing to 7.2% in ASA physical status IV patients. A comprehensive review by Severinghaus and Kelleher [8] determined that failure of pulse oximetry could often be explained in terms of low signal-to-noise ratio due to either inadequate signal (e.g., low perfusion, vasoconstrictor use, improper probe placement) or excessive noise (e.g., motion, ambient light, electrocautery, venous pressure waves). Thus, technological evolution recently has been focused on improving function by optimizing the signal-to-noise ratio [12-21] and decreasing the frequency of false alarms [22-24]. Pan and James [23] found that incorporation of a 60-second delay could reduce the false alarm frequency by 88%. However, during a true hypoxemic event, such a delay would reduce the time available for intervention. Egbert et al. [24] developed a neural network pattern classifier for detecting artifact in pulse oximetric signals. Using factor analysis to differentiate pulse oximetry waveforms, they correctly classified 91.4% of the corrupted waveforms and 72.7% of the clean waveforms used to train the network, suggesting that further improvements in signal analysis could reduce artifact and improve performance. Artifact due to motion also can be reduced by coupling an electrocardiograph to the oximeter signal [25]. In this issue, Dumas et al. [12] report the use of another technological advance, Masimo signal extraction technology (SET Trademark), to differentiate the "true" signal from artifact during low perfusion and motion. Recent studies have demonstrated that this technique may have a significantly lower failure rate and a lower false positive alarm rate [12,18,19,21] than conventional oximetric monitoring. Dumas et al. evaluated a prototype Masimo SET Trademark pulse oximeter in 50 ASA physical status I-IV patients in the postanesthesia care unit and found a lower failure rate and a lower frequency and duration of false alarms when compared with conventional pulse oximetry. A similar prototype also alarmed less frequently than a conventional oximeter during inflation of a blood pressure cuff [17]. At present, Masimo SET Trademark awaits Food and Drug Administration approval, although the manufacturers anticipate that the technique will be available for general marketing in late 1996. As anesthesiologists, should we anticipate that this technological advance will improve pulse oximetry sufficiently to advance clinical practice? The answer may vary, depending upon the circumstances in which oximetry is used. In daily practice, the two most annoying limitations of current technology are signal failure in patients with poor peripheral perfusion and signal artifact in patients who have distal extremity movement. Masimo SET Trademark does little to improve the first problem; however, movement artifact complicates pulse oximetry in the postanesthesia care unit and intensive care unit and during emergence from general anesthesia. Anecdotally, the frequency of false alarms is sufficiently high that many personnel are desensitized; they regard a desaturation on pulse oximetry as artifactual until proven otherwise. Perhaps many of us have witnessed delays in treatment because "we didn't trust the monitor!" The Masimo SET Trademark should increase confidence in the validity of alarms, as it increased the incidence of true positive alarms from 12% to 41%. Perhaps our practice will be a bit improved. How much additional cost is justified? Perhaps a bit. However, any technological improvement in pulse oximetry can represent only a small incremental improvement on an enormous advance--and the enormous advance has been shown to make no apparent difference in clinical outcome [6,7].
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