Continuous dual oximetry in surgical critical care. Indications and limitations

Continuous dual oximetry combines pulse and venous oximetry to provide real-time information about oxygen utilization and pulmonary function. The authors undertook this study to examine the accuracy, utility, and limitations of the technique in surgical critical care. Twelve critically ill patients underwent placement of a modified pulmonary artery catheter and a pulse oximeter, both connected to an on-line computer. Paired blood gas and oximeter measurements were recorded every 4 to 6 hours, with a minimum of six sets per patient. Blood-gas-derived shunt fraction correlated well with oximeter-derived ventilation-perfusion index (r = 0.78, p less than 0.01). Further, the continuous oxygen extraction ratio (O2EI) and mixed venous oxygen saturation (SpvO2) correlated with the oxygen utilization coefficient (O2EI:r = 0.6, p less than 0.01; SpvO2, r = 0.76, p less than 0.01). Computer modeling of ventilation-perfusion index found limitations in accuracy that occur at high arterial oxygen saturations and when pulse oximetry errors are present. The authors conclude that (1) Continuous dual oximetry offers a significant advantage over routine blood gas analysis for monitoring cardiopulmonary parameters in critically ill patients because of its real-time display; (2) Oximeter-derived determinations parallel traditional parameters; and (3) The most significant technical limitations occur primarily at high arterial saturations and with pulse oximetry errors.