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Accuracy in Critical Care and Threshold Values for Hypoxemia
There is continued discussion of the accuracy of pulse oximetry in critically ill patients. Although studies have documented accuracy of ± 2% for oxygen saturation as measured by pulse oximetry (SpO2) values between 70% and 100%,1–3 more recently, SpO2 has been found, in a clinical setting, to overestimate arterial oxygen saturation (SaO2). Seguin et al4 compared SpO2 values with SaO2 values, obtained in 111 data pairs in a surgical intensive care unit, to determine the optimal lowest reliable value of SpO2 in ventilator-dependent patients before setting up a nurse-directed protocol of fraction of inspired oxygen titration. The investigators found that SpO2 consistently overestimated SaO2 and concluded that a minimum threshold SpO2 value of 96% is more reliable to ensure an SaO2 value greater than or equal to 90%. In a large study (n = 664) to determine the accuracy of pulse oximetry in the emergency department, Lee et al5 compared SpO2 with SaO2 for accuracy and found that the best pulse oximetry threshold for detecting hypoxia is 92%. In addition, Van de Louw et al6 compared pulse oximetry with SaO2 in critically ill patients (323 data pairs) and found that the accuracy of SpO2 was influenced by the type of oximeter, the presence of hypoxemia, and the requirement for vasoactive drugs. These investigators concluded that large SpO2 to SaO2 differences may occur in critically ill patients with poor reproducibility of SpO2. An SpO2 value above 94% was necessary to ensure an SaO2 value of 90%. In fact, Aoyagi (who invented pulse oximetry) and Miyasaka7 state that the accuracy of SpO2 is best at approximately 85%, and errors for SpO2 increase either above or below this level.
Because pulse oximeters are prone to motion artifact, creating a high rate of false alarms, newer models have been designed to reduce motion sensitivity, which may result in an increased frequency of missed true alarms. In a study8 of 2 new generation oximeters, researchers found, in 17 unsedated preterm infants, that one of the 2 instruments missed 5.4% of hypoxemic episodes and 69% of bradycardias, suggesting that the instrument’s reduced false alarm rate is achieved at the expense of an unreliable and/or delayed identification of hypoxemia and bradycardia. The second instrument identified both conditions as well as or more reliably than a conventional pulse oximeter.
Use of pulse oximetry as a substitute for measurement of arterial blood gas values in some patient populations may be of concern as well. Kelly et al9 compared SpO2 values with SaO2 values in 64 patients with acute exacerbations of chronic obstructive pulmonary disease in the emergency department, and concluded that there was not sufficient agreement for SpO2 to replace arterial blood gas analysis in evaluation of oxygenation in these patients. The authors indicated that pulse oximetry can be used as a screening test for systemic hypoxia, with the screening cut-off of 92%. On the basis of these data, the index of suspicion for hypoxemia may need to be set higher, that is, rather than waiting until the SpO2 is 92% or 93% as is commonly done. An SpO2 value of 95% or 96% should initiate an assessment to identify patients at risk for hypoxemia.
Effect on Outcomes
Use of pulse oximeter in the prehospital environment was shown to provide a cost benefit by reducing the amount of oxygen used.10 In 1907 patients, oxygen consumption was reduced by 26% and excessive oxygen therapy was avoided in 55% of patients transported by ambulance. In addition, 11% of the time, use of pulse oximetry identified suboptimally oxygenated patients.
Digital injury induced by pulse oximeter has recently been reported. Wille et al11 found that the frequency rate of this injury was 5% and that it was most often associated with norepinephrine use. However, the digital injury healed without sequelae.
The use of pulse oximetry to warn healthcare providers of hypoxemia before observable symptoms occur is its greatest advantage. Monitoring during the perioperative period is especially useful. However, data to support improvement in patient outcomes are lacking. In a 2002 Cochrane review12 on the effect of perioperative pulse oximetry monitoring to identify adverse outcomes that might be prevented or improved by the use of pulse oximetry, Pedersen et al found that hypoxemia was reduced in patients with pulse oximetry monitoring in the operating and recovery room. However, there was no evidence that pulse oximetry affected the outcome of anesthesia or length of hospital stay.
Continued use of any technology should only proceed with data that show improvement in patient outcomes; however, the nearly universal use of pulse oximetry makes it difficult to design rigorous trials.
Level of Knowledge and Understanding of Pulse Oximetry
Documented deficiencies in knowledge and understanding of pulse oximetry persist. In a recent report13 of knowledge among nursing and medical staff related to the correct use of the pulse oximeter and what may affect the results, Howell found that there was a deficit in participants’ knowledge on pulse oximetry similar to that found in 1994 by Stoneham.14 However, use of pulse oximetry is even more widespread today, and understanding of its use is apparently no better.
Summary
Pulse oximetry, which can detect hypoxemia well before it is clinically obvious, is an extremely useful tool and will continue to be a valuable asset in the care of acute and critically ill patients, expecially if healthcare providers understand this technology completely.
References
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