Evaluation of the Gas-STAT® fluorescence sensors for continuous measurement of pH,pCO2andpO2during cardiopulmonary bypass and hypothermia

Abstract

ontinuous measurement of pH, pCO2 and pO2 during extracorporeal circulation has become feasible using disposable fluorescence sensors (optodes). We have evaluated a commercial system: Gas-STAT (American Bentley) by reference to in-vitro measurements on discrete samples using conventional electrochemical sensors (BMS-3, Radiometer). The Gas-STAT measures at the actual temperature of the blood in the extracorporeal circuit. The reference measurements were performed at two fixed temperatures of 25 and 37 °C with interpolation of the values to the actual temperature of the Gas-STAT.10 patients undergoing coronary artery bypass grafting during hypothermic extracorporeal circulation with hemodilution were monitored in the venous as well as the arterial line with the Gas-STAT with 6–9 samplings of arterial and venous blood from each patient, a total of 136 samples.The comparisons revealed a large scatter which was due partly to inter-optode partly to intra-optode variation and partly to a memory effect which rendered the initial measurements unreliable. Omitting the first samplings, standard deviations for inter- and intra-optode variations were 0.018 and 0.030 for pH, 0.026 and 0.034 for lg pCO2, and 0.041 and 0.066 for lg pO2. There was a minor systematic difference between the Gas-STAT and the reference method for pH, the bias being +0.017; for pCO2 and pO2 the average bias was insignificant. The calibration slope was 0.87 for pH, 0.85 for pCO2 and 1.08 for pO2 (all significantly different from 1).We conclude that the calibration procedure for the Gas-STAT should be improved, for example by calibration of the pH sensor with certified pH calibration solutions, and separate calibration of the CO2 and O2 optodes with certified gas mixtures. The low and the high calibration values for the pO2 optode should be farther apart, e.g. about 2 and 50 kPa, respectively. All 3 optodes display a memory effect, and a certain stabilization period must be allowed after establishing extracorporeal circulation. The minimum time needs to be better established. With such minor improvements the Gas-STAT would be a reliable method of continuous monitoring of pH, pCO2 and pO2 during extracorporeal circulation.