Relation between pH and ionized calcium in vitro and in vivo in man.

We investigated the in vitro and in vivo relation between pH and ionized calcium during acute respiratory pH changes. The in vitro slope delta 1gcCa2+/delta pH determined in 20 healthy volunteers after equilibration at two different pCO2 values was -0.22 +/- 0.04 (mean +/- 2 SD) for whole blood and -0.24 +/- 0.04 (mean +/- SD) for serum. The in vivo slope delta 1gcCa2+/delta pH during acute respiratory changes in 10 healthy volunteers was -0.17 +/- 0.04 (mean +/- 2 SD) for capillary blood. A mathematical formula which describes the relation between pH and ionized calcium is presented.