Parenteral magnesium sulphate restores regional contractile function in the post-ischaemic canine myocardium

The effects of parenteral magnesium sulphate (MS) on the regional contractile response of stunned myocardium was examined in 45 pentobarbital anaesthetized dogs. The hearts were instrumented to measure left ventricular pressure (LVP), coronary flow velocity (CFV), mean arterial blood pressure (MAP), and regional contractile function (percent segment shortening, %S; and end-diastolic segment length, EDL). Stunning was produced by a 10 min occlusion of the first descending branch of the left circumflex coronary artery. Immediately upon release of the occlusion, either magnesium sulphate or a dextrose vehicle (D5W, n = 15) was infused. Magnesium sulphate was given intravenously (IV-MS, 100 mg/kg, n = 15) or intracoronarily (IC-MS, 1.5 mg/kg, n = 15). Coronary occlusion was consistently associated with significant decreases in coronary flow velocity and %S in all groups. Following IV-MS, heart rate (HR) and mean arterial blood pressure decreased significantly from preocclusion values, whereas end-diastolic segment length tended to increase and left ventricular pressure remained constant. IC-MS did not produce any changes in heart rate, mean arterial blood pressure, end- diastolic segment length or left ventricular pressure. At the end of the magnesium sulphate infusion (IV or IC), and for the next 60 min, %S returned to or above pre-occlusion values (P < 0.05 vs. D5W). Dyskinesia and hypokinesia were abolished in the magnesium sulphate groups, but were still present in the D5W group at the end of the 60 min period (P < 0.05 vs. pre-occlusion). We conclude that parenteral magnesium sulphate significantly improves regional contractile function in the stunned myocardium. Data from the IC-MS group would suggest a direct myocardial effect, independent of changes in preload, afterload, heart rate or flow.