In vivo and in vitro ionized calcium variations induced by acute respiratory acid base disturbances.

The effects of acute respiratory acid base changes on the ionized calcium concentration were studied in mechanically ventilated patients and compared to the results which were found using an in vitro model. In both in vivo and in vitro studies, the ionized calcium changes correlated negatively with the pH variations. The good agreement between the in vivo and in vitro investigations showed a mean ionized calcium change of 0.40 mmol/l per pH unit. At the same time, the ionized calcium changes correlated positively with the changes of plasma bicarbonate. The mean ionized calcium change was 0.01 mmol/l per mmol/l bicarbonate variation. During respiratory acid base disturbances, the ionized calcium variations due to the competitive albumin binding between hydrogen and calcium ions are buffered and blunted by the bicarbonate ions.