Changes in the actual ionized calcium level (aCa2+) of serum in arterial and mixed venous blood during CPR were investigated in 22 patients with intrinsic cardiopulmonary arrest. Measurements were carried out at an average of 5.4 +/- 0.6 minutes (group 1) and 30 minutes (group 2) after the beginning of CPR, using simultaneously taken blood samples. The aCa2+ of groups 1 and 2 revealed no difference between arterial and mixed venous bloods. As CPR proceeded, the aCa2+ of both bloods diminished, and group 2 had a significantly lower value than did group 1 (arterial blood: 2.36 +/- 0.04 mEq/L vs 2.09 +/- 0.09 mEq/L, P less than .01; mixed venous blood: 2.37 +/- 0.05 mEq/L vs 2.13 +/- 0.06 mEq/L, P less than .001). Conversely, pH values were significantly higher for arterial blood rather than for mixed venous blood, but there was no significant change in pH between groups 1 and 2 (arterial blood, 7.11 +/- 0.05 vs 7.14 +/- 0.04; mixed venous blood, 6.93 +/- 0.04 vs 6.98 +/- 0.03). Furthermore, an analysis of the aCa2+ serum level measurements was carried out in relation to survivors and nonsurvivors and to the presence or absence of a witness at the time of the cardiac arrest. In all groups, there was no difference in aCa2+ between arterial and mixed venous blood. As CPR proceeded, however, the aCa2+ showed a significant decrease in all groups. This decrease was accompanied by a pH increase in arterial blood of patients with a witnessed cardiac arrest and in mixed venous and arterial blood of survivors.