Evaluation of the Gas-STAT® fluorescence sensors for continuous measurement of pH, pCO₂ and pO₂ during cardiopulmonary bypass and hypothermia

Abstract

Continuous 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.

Key words:

• Carbon dioxide
• continuous monitoring
• extracorporeal circulation
• hydrogen ion
• optical fluorescence sensors
• optodes
• oxygen