Hemodynamic and electrocardiographic effects of magnesium sulfate in healthy dogs

We investigated the effects of graded dosages of magnesium given IV to anes thetized dogs on blood pressure, cardiac output, and electrophysiology. Mag nesium was infused at 0.12 mEq/kg/minute until ventricular fibrillation occ urred naturally or was provoked by programmed electrical stimulation or unt il arrest of the sinuatrial node in 8 dogs. Plasma total magnesium concentr ations doubled in 1 minute of that infusion rate, and a hemodynamically saf e plasma concentration of 12.2 mEq/L was achieved after 16 minutes of infus ion. Heart rate, inotropy, lusitropy, and cardiac output increased up to a cumulative infusion dosage of magnesium of 1.0-2.0 mEq/kg, which produced p lasma magnesium concentrations of 8.5-12.2 mEq/L (n = 5). Above the cumulat ive infusion dosage, inotropy decreased and lusitropy increased until death occurred between cumulative infusion dosages of 5.9 mEq/kg and 10.9 mEq/kg . Arterial pressure and vascular resistance decreased, and PQ interval and QRS complex increased, in a dose-dependent fashion. The relationship betwee n ionized and total magnesium was y = 0.624x - 0.542 (r(2) = .986), where y is ionized and x is total magnesium in mEq/L in 3 dogs. In conclusion, a c umulative infusion dosage of 0.1-0.2 mEq/kg of magnesium may be given witho ut changing hemodynamic parameters, but with higher cumulative infusion dos es heart rate accelerates. Hemodynamic parameters except those related to b lood pressure continued to increase to a cumulative infusion dosage of 2.0 mEq/kg. At higher cumulative infusion dosages dogs became hypotensive and t he PQ interval was prolonged. However. dangerous arrhythmias were not provo ked until a total dosage of 3.9 mEq/kg.