Clinical reliability of measured and calculated oxygen parameters in surgical patients: influence of hyperventilation.

The acid-base and oxygen status were assessed in patients undergoing abdominal surgery. We determined the measured and calculated parameters described by Siggaard-Andersen in arterial and mixed venous blood before, during and after a controlled hyperventilation. In these dynamic conditions, the mixed venous calculated p50 showed the most interesting variations. Derived values in arterial blood were unreliable since: 1) fitting to hyperbolic tangent function is not possible when SaO2 greater than 0.98; 2) in these patients, the arterio-venous difference in CtO2 is different from 2.3 mmol/L. The acute changes in calculated p50 are in agreement with the effect of hyperventilation on oxygen status.