A laboratory comparison of three pulmonary artery oximetry catheters

Measurement of mixed venous hemoglobin oxygen saturation via catheters employing reflectance spectrophotometry has been available for more than 10 yr. Despite numerous clinical reports that have presented data showing the poor accuracy of these devices when used clinically, they are still widely used in clinical care. The reason for lack of agreement with measurements made using bench spectrophotometry is unclear. The purpose of this study is to define the performance limitations of three hemoglobin oxygen saturation catheters (Oximetrix 3, SAT-2, and HEMOPRO2) in a controlled laboratory environment using a blood flow loop primed with fresh whole human blood as a model. Our hypothesis is that the performance limitations of these devices represent inherent limitations in the technology, not error introduced by patient anatomy and physiology. Blood was equilibrated in a flow loop to four analytic gas mixtures designed to achieve oxygen saturation of approximately 50%, 60%, 70%, and 80%, respectively, with carbon dioxide tension, pH, and temperature held constant. Saturation readings from the catheters were collected on-line by microcomputer. Periodic blood samples were withdrawn from the flow loop for analysis on a bench spectrophotometer and subsequent comparison with catheter-derived values. By all measures, performances of the Oximetrix 3 and SAT-2 systems were comparable (all data are presented as percent saturation unless otherwise noted); bias +/- precision was 3.20 +/- 2.47 and -1.25 +/- 3.36, respectively, versus -9.97 +/- 7.05 for the HEMOPRO2. The 95% confidence limits based on intracatheter variability were +/- 3.49, +/- 2.90, and +/- 9.13 for the Oximetrix 3, SAT-2, and HEMOPRO2, respectively. The 95% confidence limits based on total variability, although similar for Oximetrix 3 (+/- 4.83) and SAT-2 (+/- 6.59), were larger for the HEMOPRO2 (+/- 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 - HEMOPRO2) and -5.24, 22.68 (SAT-2 - HEMOPRO2). While the Oximetrix 3 and SAT-2 may be acceptable as continuous monitors used to detect changes or trends, none of the three systems is equivalent to conventional bench oximetry for the measurement of hemoglobin oxygen saturation.