Magnesium sulfate reduces myocardial infarct size when administered before but not after coronary reperfusion in a canine model

Effect of magnesium sulfate administration on subendocardial and subepicardial perfusion

The objective of this study was to determine the effect of systemic MgSO4 infusion on subendocardial and subepicardial perfusion. Seventeen spontaneously breathing piglets were examined. Myocardial perfusion was measured using radiolabeled microspheres at baseline, 30 and 60 min after either MgSO4 (80 mg/kg) or saline infusion. Blood pressure, heart rate, and cardiac output were also measured at these time intervals. Comparison of the magnesium-induced changes in systemic blood pressure and on subendocardial and subepicardial perfusion at 30 and 60 min with values obtained with saline solution at 30 and 60 min, yielded no statistically significant difference (Tables 1-3). The ratio of subendocardial/subepicardial blood flow and subendocardial and subepicardial coronary vascular resistance at 30 and 60 min revealed no statistically significant differences between the magnesium and the control group (Table 3). There were no statistically significant difference in cardiac output and heart rate during any of the measured periods (Table 2). Our results suggest that the administration of MgSO4 does not alter the ratio of subendocardial/subepicardial blood flow and the ratio of subendocardial/subepicardial coronary vascular resistance.

The impact of high-risk patients on the results of clinical trials

The results of clinical trials may not reflect equally the experiences of all their individual participants. By modeling populations where patients have very diverse baseline risks of suffering an event of interest, it can be seen that very sick patients of high risk become the major determinants of how many events occur in the whole population, even though they may represent only a small minority. Human immunodeficiency virus-related trials and trials of magnesium in acute myocardial infarction are analyzed. When the benefit or toxicity from a treatment varies with the baseline risk of each patient, the treatment effect may be markedly different in populations with a different representation of high- and low-risk patients. The results of small clinical trials studying heterogeneous populations with binary outcomes depend on the sampling and outcomes of very few high risk participants. Conversely, mega-trials studying homogeneous populations would miss subgroups or individuals with diverse treatment responses. In both cases, aggregate trial results may be misleading for the care of many individuals.

[Effect of magnesium on infarct size after coronary occlusion. Animal experiment studies]

In addition to its antiarrhythmic and antithrombotic effects magnesium is said to have a beneficial effect in patients with acute myocardial infarction. Magnesium can protect myocardial tissue after coronary occlusion and reduces infarct size in experimental models of ischaemia and reperfusion, though the given doses of magnesium are relatively high and differ from clinically reachable serum concentrations. We tested 2 hypotheses in a dog model of ischaemia-reperfusion: 1. The protective effect may be due to a direct, local influence of magnesium on myocardial reperfusion injury. 2. Systemic magnesium treatment with low doses comparable to clinical study regiments may reduce myocardial infarct size. Anaesthetized open chest dogs underwent 1 h of left anterior descending artery occlusion followed by 6 h of reperfusion. 1. Ten animals received intracoronary magnesium aspartate (Mg i.c.) or vehicle infusion (control i.c.) for the first hour of reperfusion to increase regional Mg-concentration by 2 mmol/l. 2. Fourteen animals received intravenous infusion with magnesium potassium aspartate (Mg-K i.v.) or vehicle infusion (control i.v.), beginning 1 h before occlusion until the end of the 6 h reperfusion period. Regional magnesium concentration in the Mg i.c.-group increased to 2.7 +/- 1.00 mmol/l at 45 min of reperfusion. Intravenous infusion raised serum magnesium from 0.71 +/- 0.03 mmol/l to 1.29 +/- 0.14 mmol/l in the Mg-K i.v. group (5 min of reperfusion, p < 0.01 vs. baseline). Infarct size after 6 h reperfusion (TTC staining) was similar in both groups of intracoronary treatment (Mg i.c., 20.6 +/- 5.0; control, 24.4 +/- 8.7% of area at risk; p = n.s.) and intravenous treatment (Mg-K i.v. 18.1 +/- 14.8; control 14.1 +/- 12.2% of area at risk; p = n.s.). Neither regional nor systemic magnesium leads to a clinically relevant reduction of infarct size in the regional ischaemic-reperfused dog heart when it is given in clinically usable doses. The beneficial action of systemic Mg is probably not due to an early direct protective effect on ischaemic-reperfused myocardium but to its antiarrhythmic and antithrombotic effects. Possibly only to high doses of Mg applied under experimental conditions can reduce infarct size.

The role of magnesium therapy in acute myocardial infarction

Magnesium possesses numerous salutary effects for the treatment of acute myocardial infarction (AMI). It is a coronary vasodilator, calcium antagonist, afterload reducer, antiarrhythmic, and antiplatelet drug that modulates autonomic function and limits reperfusion injury when administered early in infarction. Various clinical trials of magnesium therapy for AMI have proffered conflicting results as to the efficacy of magnesium therapy because of significant differences in the timing of magnesium administration. Additional clinical trials that focus on early administration of magnesium are warranted to delineate the role of magnesium therapy for AMI.

Magnesium in acute myocardial infarction: overview of available evidence

Despite improvements in the outcome of patients with acute myocardial infarction (MI) during the past three decades, room for improvement exists in elderly patients and in patients who are not candidates for thrombolysis. Animal models suggest that magnesium supplementation before reperfusion reduces infarct size. Statistical analysis of the randomized trials of magnesium in MI reveals a gradient of response. When higher risk patients were enrolled, a greater benefit of magnesium was observed; progressively smaller benefits of magnesium occurred as the control group mortality approached 7%, at which point no benefit was detected. Although the ISIS-4 study enrolled more than 58,000 patients, no reduction in mortality was seen, probably as a result of a low control group mortality and relatively late administration of the magnesium. Because the potential benefit of magnesium in MI remains an open question, additional trials are needed before this inexpensive and easily administered therapy is prematurely cast aside.