[Serum and urinary magnesium during and after cardiac surgery]

To investigate the effects of cardiopulmonary bypass (CPB) on the serum magnesium (Mg) level, we observed the perioperative changes in serum and urinary Mg in 20 patients undergoing open heart surgery. The serum Mg was in the normal range (mean +/- SE, 1.98 +/- 0.04 mg.dl-1) at the induction of anesthesia, and began to fall during CPB, reaching its lowest level (1.43 +/- 0.05 mg.dl-1) on the first postoperative day, and returned to normal level on the second postoperative day. We found a good correlation between the decrease in serum Mg and increase in urinary Mg excretion, suggesting that the decrease in serum Mg level may be attributed to the increase in urinary excretion of Mg. But, compared to the increase in urinary loss of Mg, the observed decrease in serum Mg on the first operative day was not as great as expected. Furthermore, the level of serum Mg returned to normal spontaneously on the second operative day. These observations suggest that Mg was transferred from the intracellular to the extracellular compartment during and after CPB.