Electrocardiographic changes after intravenous administration of magnesium in rabbits

Magnesium (Mg+2) therapy has a beneficial effect in various cardiovascular diseases. However, overdosage of Mg+2 can be fatal. This study was undertaken to determine the amount of Mg+2 that can be safely administered intravenously to rabbits. Twenty New Zealand White rabbits were allotted to three groups. One hundred milliliters of 0.9% NaCl/kg was infused in group-1 rabbits (n = 10) used as a control group. Rabbits in group 2 (n = 6) and group 3 (n = 4) received MgCl2 (2.5 mmol and 5.0 mmol/kg respectively) in 0.9% NaCl (100 ml/kg). The infusion was delivered intravenously for 30 min (group 2, 8 mg/kg per min; group 3, 16 mg/kg per min). The electrocardiogram was monitored throughout the infusion; the P-R interval, QRS duration, and QTc interval of group-2 and group-3 rabbits were prolonged at dosages of 0.8 mmol/kg, 1.7 mmol/kg, and 5.0 mmol/kg of MgCl2 respectively, compared with readings in group-1 rabbits. We hypothesized that the mean +2 standard deviations of the pre-operative baseline values was the safe limit. On the basis of this hypothesis, the safe limit of the P-R interval is 95.3 milliseconds, which corresponds to 0.6 mmol/kg of infused Mg+2; the QRS duration is 44.4 milliseconds, which corresponds to 1.0 mmol of Mg+2/kg; and the QTc interval is 350.0 milliseconds, which corresponds to 3.6 mmol of Mg+2/kg. These results will provide guidance for avoiding cardiac side effects when researchers investigate the effects of Mg+2 in rabbits.