Magnesium sulphate only slightly reduces the shivering threshold in humans

Background. Hypothermia may be an effective treatment for stroke or acute myocardial infarction; however, it provokes vigorous shivering, which causes potentially dangerous haemodynamic responses and prevents further hypothermia. Magnesium is an attractive anti-shivering agent because it is used for treatment of postoperative shivering and provides protection against ischaemic injury in animal models. We tested the hypothesis that magnesium reduces the threshold (triggering core temperature) and gain of shivering without substantial sedation or muscle weakness. Methods. We studied nine healthy male volunteers (18-40 yr) on two randomly assigned treatment days: (1) control and (2) magnesium (80 mg kg^-1 followed by infusion at 2 g h^-1). Lactated Ringer's solution (4°C) was infused via a central venous catheter over a period of approximately 2 h to decrease tympanic membrane temperature by ∼ 1.5°C h^-1. A significant and persistent increase in oxygen consumption identified the threshold. The gain of shivering was determined by the slope of oxygen consumption vs core temperature regression. Sedation was evaluated using a verbal rating score (VRS) from 0 to 10 and bispectral index (BIS) of the EEG. Peripheral muscle strength was evaluated using dynamometry and spirometry. Data were analysed using repeated measures ANOVA; P<0.05 was statistically significant. Results. Magnesium reduced the shivering threshold (36.3 [SD 0.4] °C vs 36.6 [0.3] °C, P = 0.040). It did not affect the gain of shivering (control, 437 [289] ml min^-1 °C^-1; magnesium, 573 [370] ml min^-1 °C^-1; P=0.344). The magnesium bolus did not produce significant sedation or appreciably reduce muscle strength. Conclusions. Magnesium significantly reduced the shivering threshold. However, in view of the modest absolute reduction, this finding is considered to be clinically unimportant for induction of therapeutic hypothermia.