Influence of fluid replacement on serum magnesium concentrations and proper magnesium supplementation during general anesthesia

We have studied the influence of fluid replacement on serum magnesium (Mg²⁺) concentrations, and studied proper Mg²⁺ supplementation during general anesthesia. Thirty eight patients undergoing elective surgery randomly received Mg²⁺-free acetated Ringer solution (Group I ,n=15), acetated Ringer solution containing 0.5 mmol · l^-1 of Mg²⁺ (Group II, n=6), 1.0mmol · l^-1 of Mg²⁺ (Group III, n=7), 2.0mmol · l^-1 of Mg²⁺ (Group IV, n=6), or 4.0 mmol · l^-1 of Mg2+ (Group V, n=4). Measurements were made on serum and urine Mg²⁺ concentrations during anesthesia. In Group I, the serum Mg²⁺ concentrations decreased in correspondence with the water balance. It is suggested that dilution due to the fluid replacement induced the reduction in serum Mg²⁺ concentrations since the observed urine Mg²⁺ concentrations were negligible. In Group II-V, the reduction in serum Mg²⁺ concentrations was inhibited by Mg²⁺ supplementation, and the serum Mg²⁺ concentrations remained unchanged in Group IV. We conclude the Mg²⁺ supplementation is required during anesthesia when a large amount of fluid is infused.