Plasma magnesium during epidural anaesthesia. A study in patients undergoing transurethral prostatectomy.

The variations of plasma magnesium concentration in relation to protein concentration and plasma volume were studied in 12 patients during epidural anaesthesia for transurethral prostatectomy. The most significant changes in plasma magnesium concentration were observed during the onset of anaesthesia. From the arrival in the operating theatre to the start of operation, the concentration of plasma magnesium decreased from 0.88 (SD 0.10) mmol/l to 0.82 (SD 0.10) mmol/l. At the same time, the increase of 16 per cent in plasma volume was associated with a rise of 8.4 per cent in the total amount of magnesium in plasma. After commencement of surgery, the plasma magnesium concentration was unchanged throughout the remainder of the study period. The total plasma protein concentration and the erythrocyte volume fraction decreased during the onset of anaesthesia, remained stable during surgery and returned to initial values one hour postoperatively. The pre-operative changes in the plasma magnesium concentration may have been the result of haemodilution and mobilisation of magnesium ion from its bone stores. Postoperatively, an increased urinary loss of magnesium must have followed the injection of frusemide at the end of surgery. The consequences of these variations in plasma magnesium concentration are unlikely to be of clinical significance, except in patients with pre-existing subclinical magnesium deficiency.