Prediction of degree of hypomagnesaemia during general anaesthesia

Sasaki and colleagues have demonstrated a reduction in serum magnesium associated with the administration of magnesium-free crystalloid, and suggest that magnesium supplementation should be considered when large amounts of magnesium-free fluid is administered (Sasaki et al. Anaesthesia 2000; 55: 1137–8). They do not tell us whether they measured total or ionised magnesium. From the quoted serum levels, it has to be assumed that they measured ionised magnesium. They tell us that magnesium levels fell from a mean of 0.51 mmol.l−1 to 0.45 and 0.41 mmol.l−1 following the administration of 1500 ml and 3000 ml of fluid, respectively. Unfortunately, they do not inform the reader of the lower limit of the reference range for ionised magnesium. This has been quoted as 0.44 mmol.l−1[1]. Previous studies have documented changes in serum magnesium in relation to anaesthesia. In a study comparing magnesium-free and magnesium-containing crystalloids, the serum magnesium level fell significantly in both groups [2]. General anaesthetic agents have been suggested as a cause of significant falls in ionised magnesium intra-operatively [3]. The relevance of these changes in serum magnesium is not clear. We feel that it is important to make the distinction between statistically significant and clinically significant changes, and the suggestion that this fall in ionised magnesium may warrant treatment requires comment. Ninety-nine per cent of magnesium is intracellular. Serum magnesium represents only 0.3% of the total body magnesium and serum magnesium may not reflect levels in other body stores [4]. Using a magnesium-loading test as a reference, it has been suggested that ionised magnesium is an insensitive marker of functional hypomagnesaemia [5]. Thus a low ionised magnesium does not necessarily equate with magnesium deficiency. In acutely ill patients, a low serum magnesium has been associated with an increase in mortality [6], but this cannot be extrapolated to well patients undergoing elective surgery. The development of ion selective electrodes allows us to measure the physiologically active component of minerals, but the clinical relevance of these measurements is unclear and requires clarification before we can accept the suggestion of magnesium supplementation in this setting.