A LABORATORY COMPARISON OF 3 PULMONARY-ARTERY OXIMETRY CATHETERS

Background: Measurement of mixed venous hemoglobin oxygen saturation v ia catheters employing reflectance spectrophotometry has been availabl e for more than 10 yr. Despite numerous clinical reports that have pre sented data showing the poor accuracy of these devices when used clini cally, they are still widely used in clinical care. The reason for lac k of agreement with measurements made using bench spectrophotometry is unclear. The purpose of this study is to define the performance limit ations of three hemoglobin oxygen saturation catheters (Oximetrix 3, S AT-2, and HEMOPRO(2)) in a controlled laboratory environment using a b lood now loop primed with fresh whole human blood as a model. Our hypo thesis is that the performance limitations of these devices represent inherent limitations in the technology, not error introduced by patien t anatomy and physiology. Methods: Blood was equilibrated in a flow lo op to four analytic gas mixtures designed to achieve oxygen saturation of approximately 50%, 60%, 70%, and 80%, respectively, with carbon di oxide tension, pH, and temperature held constant. Saturation readings from the catheters were collected on-line by microcomputer. Periodic b lood samples were withdrawn from the flow loop for analysis on a bench spectrophotometer and subsequent comparison with catheter-derived val ues.Results: By all measures, performances of the Oximetrix 3 and SAT- 2 systems were comparable (all data are presented as percent saturatio n unless otherwise noted); bias +/- precision was 3.20 +/- 2.47 and -1 .25 +/- 3.36, respectively, versus -9.97 +/- 7.05 for the HEMOPRO(2). The 95% confidence limits based on intracatheter variability were +/- 3.49, +/- 2.90, and +/- 9.13 for the Oximetrix 3, SAT-2, and HEMOPRO(2 ), respectively. The 95% confidence limits based on total variability, although similar for Oximetrix 3 (+/- 4.83) and SAT-2 (+/- 6.59), wer e larger for the HEMOPRO(2) (+/- 13.82). The 95% confidence intervals for agreement between catheter brands were -2.14, 11.04 (Oximetrix 3 - SAT-2); -0.18, 26.52 (Oximetrix 3 - HEMOPRO(2)) and -5.24, 22.68 (SAT -2 - HEMOPRO(2)). Conclusions: While the Oximetrix 3 and SAT-2 may be acceptable as continuous monitors used to detect changes or trends, no ne of the three systems is equivalent to conventional bench oximetry f or the measurement of hemoglobin oxygen saturation.