Ionized calcium and acid-base status in arterial and venous whole blood during general anaesthesia.

Ionized calcium, pH, pCO2 and base excess (BE) have been measured in paired samples of anaerobically collected arterial (aB) and venous whole blood (vB) from patients under general anaesthesia and during recovery. A statistically significant decrease was found in the concentration of ionized calcium in anaerobic arterial and venous blood during induction of anaesthesia and during surgery partly due to variations in pH. Statistically significant correlations were found between logcCa2+ and pH (r = -0.67, p less than 0.001), and between cCa2+ and pCO2 (r = 0.66, p less than 0.001) in anaerobic arterial blood. The in vivo slope (dlogcCa2+/dpH) was found to be -0.15 +/- 0.08 and -0.14 +/- 0.06 for arterial and venous blood, respectively (mean +/- SE). The in vitro slope estimated by equilibrating arterial and venous serum at two pCO2 values was -0.22 +/- 0.03, and -0.21 +/- 0.03, respectively (mean +/- SE). Because of a considerable variability in slope we believe that the best clinical evaluation of ionized calcium is achieved by taking measurements from anaerobically collected whole blood. Interpolating or extrapolating ionized calcium to a standard pH of 7.40 on the basis of a standard slope may introduce unnecessary errors.