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Continuous measurement of pH, pCO2and pO2during 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 pCO2and pO2the average bias was insignificant. The calibration slope was 0.87 for pH, 0.85 for pCO2and 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 CO2and O2optodes with certified gas mixtures. The low and the high calibration values for the pO2optode 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, pCO2and pO2during extracorporeal circulation.

Blood pH, pCO2and pO2were measured at 37 °C and 25 °C and the temperature coefficients were calculated as δpHδT; δlg pCO2δT, and δln pO2δT.A total of 204 blood specimens were obtained from 10 patients undergoing coronary artery bypass grafting with extracorporeal circulation including hypothermia and hemodilution. Multivariate regression analysis gave the following equations:The constants in the parentheses are the mean values of the variables. There is a significant negative correlation between the two temperature coefficients. Unfortunately pH(37) and pCO2(37) were so closely correlated that it is impossible to separate the individual effects of the two, and the same applies to cHb and the plasma protein concentration. Therefore it may be fortuitous that the pH-temperature coefficient appears to vary with pCO2(not pH) while the pCO2-temperature coefficient appears to vary more with pH than with pCO2.The pO2-temperature coefficient was found to vary with pO2and cHb as previously predicted. At a very high pO2the temperature coefficient drops to an average value of 0.013; at low pO2it increases to about 0.069. Ahigh hemoglobin concentration results in a higher temperature coefficient especially at intermediate pO2values.Accurate correction of pH and blood gas values measured at 37 °C to values referring to the temperature of the patient, or conversion of values measured at the actual temperature (using extracorporeal or catheter tip continuous monitoring) to values referring to 37 °C requires more accurate temperature coefficients than are currently employed in most commercial equipment.

Blood pH, pCO2 and pO2 were measured at 37 °C and 25 °C and the temperature coefficients were calculated as δpH/δT; δlg pCO2/δT, and δln pO2/δT. A total of 204 blood specimens were obtained from 10 patients undergoing coronary artery bypass grafting with extracorporeal circulation including hypothermia and hemodilution. Multivariate regression analysis gave the following equations: [image omitted] The constants in the parentheses are the mean values of the variables. There is a significant negative correlation between the two temperature coefficients. Unfortunately pH(37) and pCO2(37) were so closely correlated that it is impossible to separate the individual effects of the two, and the same applies to cHb and the plasma protein concentration. Therefore it may be fortuitous that the pH-temperature coefficient appears to vary with pCO2 (not pH) while the pCO2-temperature coefficient appears to vary more with pH than with pCO2.The pO2-temperature coefficient was found to vary with pO2 and cHb as previously predicted. At a very high pO2 the temperature coefficient drops to an average value of+ 0.013; at low pO2 it increases to about 0.069. Ahigh hemoglobin concentration results in a higher temperature coefficient especially at intermediate pO2 values.Accurate correction of pH and blood gas values measured at 37 °C to values referring to the temperature of the patient, or conversion of values measured at the actual temperature (using extracorporeal or catheter tip continuous monitoring) to values referring to 37 °C requires more accurate temperature coefficients than are currently employed in most commercial equipment.

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.

pH and blood gases were measured in simultaneous samples of arterial blood from the radial artery and mixed venous blood from the pulmonary artery using an ABL300 and OSM3 (Radiometer A/S, Denmark). Cardiac output was measured by thermodilution. The patients were suffering from chronic obstructive pulmonary disease or adult respiratory distress syndrome. The data indicate that patients respond to a decreased arterial oxygen availability by allowing the mixed venous pO2 to fall rather than by increasing the cardiac output to maintain a normal mixed venous pO2. In other words, the arterial oxygen extraction tension and the oxygen compensation factor were both highly correlated to the mixed venous pO2 but unrelated to the cardiac index. For this reason the arterial oxygen extraction tension appears to be a more relevant parameter of the overall arterial oxygen availability than the oxygen compensation factor.Comparison of the arterial and mixed venous data confirms the accuracy of the Oxygen Status Algorithm for calculating the various oxygen parameters, including the p50, the estimated 2,3-diphosphoglycerate concentration, and the estimated physiological shunt, on the basis of a single arterial blood sample.ABLTM and OSMTM are trademarks of Radiometer A/S, Denmark.