Magnesium Flux During and After Open Heart Operations in Children

Hypomagnesemia and depletion of the bodyʹs magnesium stores is known to be associated with an increased incidence of both cardiac arrhythmias and neurological irritability. In a two-part prospective study we have evaluated whether magnesium deficiency is a significant occurrence in children treated in the intensive care unit after open heart operations, and subsequently have sought to identify how intraoperative metabolic changes were related to the resultant findings. In 41 children studied after operation the plasma magnesium concentration showed a significant 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 less than 0.7 mmol/L and 0.6 mmol/L, respectively. The occurrence of cardiac arrhythmias was not statistically related to the occurrence of hypomagnesemia. In 21 children perioperative changes in extracellular and tissue magnesium, potassium, and calcium content were measured. It was found that hemodilution with a prime low in magnesium caused a reduction 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 myocardial content of magnesium did not change significantly but skeletal muscle content was depleted from 6.75 μmol/g (2.85 to 8.35 μmol/g) to 5.65 μmol/g (2.45 to 7.2 μmol/g) (p < 0.01). Urinary excretion ratios of Mg2+/creatinine increased by a median 106% from the preoperative value of 0.62 (0.15 to 1.16) to 1.31 (0.59 to 1.43) postoperatively (p < 0.05). We conclude that depletion of total body magnesium occurs during and after open heart operation in children, but cardiac arrhythmias may occur in the absence of low serum concentrations. We suggest that a study evaluating the role of routine magnesium replacement in the prevention of postoperative depletion and its influence on symptomatology would be beneficial.