The effect of sensor malpositioning on pulse oximeter accuracy during hypoxemia

Abstract

Background: Previous studies have shown that pulse oximeters whose sensors are positioned improperly may yield erroneously low saturation (Sp(O2)) values on normoxemic subjects. The behavior of oximeters with malpositioned sensors during hypoxemia has not been studied. The current study is aimed at determining the behavior of several different pulse oximeters over a wide range of arterial oxygen saturation (Sa(O2)). Methods: In each of 12 healthy volunteers, a radial artery cannula was inserted, and eight different pulse oximeters, five of which had malpositioned sensors, were applied. Subjects breathed controlled mixtures of nitrogen and oxygen to slowly vary their Sa(O2) from 100% to 70%. Arterial blood samples were analyzed and pulse oximeter data were recorded at five stable Sa(O2) values for each subject. Results: The oximeters with malpositioned sensors vary greatly in their behavior, depending on both the actual Sa(O2) and the manufacturer and model. One oximeter underestimated saturation at all Sa(O2) values, while three others underestimated at high Sa(O2) and overestimated at low Sa(O2). Linear regression analysis shows a decrease in the slope of Sp(O2) versus Sa(O2) in most cases, indicating a loss of sensitivity to Sa(O2) changes. Between-subject variation in response curves was significant. Conclusions: The calibration curves of the pulse oximeters studied were changed greatly by sensor malpositioning. At low Sa(O2) values, these changes could cause the oximeter to indicate that a patient was only mildly hypoxemic when, in fact, hypoxemia was profound. It is recommended that sensor position be checked frequently and that inaccessible sensor locations be avoided whenever possible.