Changes in plasma total and ionized magnesium concentrations and factors affecting magnesium concentrations during cardiac surgery.

The purpose of this study was to measure blood total and ionized magnesium concentrations ([TMg] and [Mg(2+)], respectively) and to investigate factors that might be affecting their changes during cardiac surgery using hypothermic cardiopulmonary bypass. Eight patients were examined. All the patients received diuretics and predeposited autologous blood during surgery. No drugs containing Mg(2+) were administered. Nine blood samples and eight urine samples were collected from the pre-induction period to the end of surgery. Hematocrit, [TMg], [Mg(2+)], plasma concentrations of calcium ([Ca(2+)]), creatinine, parathyroid hormone (PTH), urinary concentrations of TMg, and creatinine were measured, and the fractional excretion of Mg (FEMg) was calculated. Both [TMg] and [Mg(2+)] decreased significantly in the prebypass period and remained significantly depressed thereafter. The ionized fraction of magnesium ([Mg(2+)]/[TMg]) was decreased during the postbypass period. Hematocrit decreased significantly from the prebypass period, and FEMg increased significantly after aortic cross-clamping. In conclusion, hemodilution and renal loss were main causes of hypomagnesemia, and citrate in predeposited autologous blood may contribute to the decrease in [Mg(2+)]/[TMg] in the postbypass period. These results suggest that magnesium supplementation under close monitoring of [Mg(2+)] should be required during cardiac surgery.