Magnesium flux during open heart surgery. The effect of St Thomas' Hospital cardioplegia solution.

The Hearse St Thomas' Hospital cardioplegia infusate is one method of preserving the myocardium in the absence of coronary perfusion, during open heart surgery. The infusate contains 16 mmol magnesium/litre and 20 mmol potassium/litre. Peri-operative plasma magnesium levels and urinary excretion of magnesium have been measured, when the infusate was returned to the circulation in 12 patients. The plasma level (+/- SEM) rose to 1.86 mmol/litre (+/- 0.1) 5 minutes after cardiopulmonary bypass commenced, was 1.57 mmol/litre (+/- 0.09) shortly before termination of cardiopulmonary bypass but was normal on the first day after surgery. Urinary excretion of magnesium was 55% of the administered quantity by Day 1 and 77% by the second day. Two patients excreted less than 40% of the administered magnesium within 24 hours probably indicating magnesium depletion. There were no adverse effects from a magnesium load of 16--32 mmol magnesium given during cardiopulmonary bypass.