MAGNESIUM FLUX DURING AND AFTER OPEN-HEART OPERATIONS IN CHILDREN

Hypomagnesemia and depletion of the body's magnesium stores is known t o be associated with an increased incidence of both cardiac arrhythmia s and neurological irritability. In a two-part prospective study we ha ve evaluated whether magnesium deficiency is a significant occurrence in children treated in the intensive care unit after open heart operat ions, and subsequently have sought to identify how intraoperative meta bolic changes were related to the resultant findings. In 41 children s tudied after operation the plasma magnesium concentration showed a sig nificant decrease from 0.92 mmol/L (10th to 90th centile, 0.71 to 1.15 mmol/L) immediately after operation to 0.77 mmol/L (0.65 to 0.91 mmol /L) on the following morning. The subsequent change in grouped values was not significant but 14 (34.2%) and 7 (17.1%) possessed values of l ess than 0.7 mmol/L and 0.6 mmol/L, respectively. The occurrence of ca rdiac arrhythmias was not statistically related to the occurrence of h ypomagnesemia. In 21 children perioperative changes in extracellular a nd tissue magnesium, potassium, and calcium content were measured. It was found that hemodilution with a prime low in magnesium caused a red uction from a median of 0.81 mmol/L to 0.61 mmol/L (p < 0.01). Plasma potassium level, however, was elevated from 3.7 mmol/L to 4.15 mmol/L (p < 0.05) and the ionized calcium content from 1.17 mmol/L (1.07 to 1 .25 mmol/L) to 1.49 mmol/L (1.25 to 2.56 mmol/L) (p = 0.0009). The myo cardial content of magnesium did not change significantly but skeletal muscle content was depleted from 6.75 mu mol/g (2.85 to 8.35 mu mol/g ) to 5.65 mu mol/g (2.45 to 7.2 mu mol/g) (p < 0.01). Urinary excretio n ratios of Mg2+/creatinine increased by a median 106% from the preope rative value of 0.62 (0.15 to 1.16) to 1.31 (0.59 to 1.43) postoperati vely (p < 0.05). We conclude that depletion of total body magnesium oc curs during and after open heart operation in children, but cardiac ar rhythmias may occur in the absence of low serum concentrations. We sug gest that a study evaluating the role of routine magnesium replacement in the prevention of postoperative depletion and its influence on sym ptomatology would be beneficial.