Magnesium sulphate and ischaemic heart disease.

Editor—We read with interest Wadhwa and colleagues’ paper about the use of magnesium sulphate to reduce the shivering threshold. We question whether there is any potential ‘real world’ use for this proposed therapy of high-dose magnesium in patients with myocardial ischaemia, either peri-infarction, periangioplasty or perioperative coronary bypass graft surgery. We agree that shivering is undesirable due to the increased oxygen consumption and can increase the risk of myocardial ischaemia. However, putting such a frail population, who are already in a compromised cardiovascular state, at the perils of high-dose magnesium infusion seems in contradiction to their best interests. Magnesium can cause heart block, the risk being higher in patients who are already on calcium or beta receptor antagonists. Treatment with magnesium by continuous infusion can cause severe muscle weakness. This effect is significant when serum levels >2 mg litre , especially patients who are in renal failure. Specifically in the postoperative cardiac surgery population, there are other detrimental effects such as profound recurarization in patients who were treated with magnesium sulphate even at lower doses (60 mg kg ) after 1 h of recovery of vecuronium block. Magnesium can also increase the incidence of postsurgical bleeding by inhibition of platelet function and antagonizing calcium function on the clotting cascade. 5 We use magnesium, given as a single 2–5 g slow i.v. infusion to reduce cardiac irritability after cardiac surgery in most of our patients. In a recent prospective audit of 110 postoperative cardiac surgery patients, we found 25% of postoperative patients were shivering with a mean core temperature of 36.3 (range 34.6–38 C), despite sedation with morphine and low dose propofol. We treat shivering patients with meperidine, and warm them appropriately as required. However, we are describing a population that is routinely mechanically ventilated after operation. Maintaining magnesium levels at twice the normal level may not cause any detrimental effects in young, healthy volunteers. However, in sick patients the use of high-dose magnesium for the prevention of shivering is not practical. We wish the authors all the best with future research, which we suspect may be focused more on the patient at risk of cerebral ischaemia, rather than myocardial ischaemia.