Magnesium inhibits the hypertensive but not the cardiotonic actions of low-dose epinephrine

Intravenous magnesium supplementation is often used to control cardiac arrhythmias and coronary artery vasospasm resulting from disturbances of magnesium homeostasis after coronary artery bypass surgery. Many such patients also require inotropic drug support of depressed myocardial function. However, increased serum magnesium concentrations directly depress cardiac contractility in animals and may interfere with catecholamine actions. To determine whether small intravenous doses of magnesium sulfate (MgSO₄) interfere with the cardiotonic actions of epinephrine, we examined the hemodynamic effects of MgSO₄ and epinephrine infusion in 17 cardiac surgical patients on their 1st postoperative day in a prospective, controlled study. In 11 patients, infusion of MgSO₄ (7-mg·kg^-1 bolus followed by 10 mg·kg^-1·h^-1 as a continuous infusion) increased serum magnesium concentrations by 44% (mean ± standard error of the mean [SEM] of 0.8 ± 0.1 to 1.2 ± 0.1 mM; P < 0.01) but had no significant effect on heart rate; mean arterial, central venous, or pulmonary arterial occlusion pressures; or cardiac output. Epinephrine infusion (30 ng·kg^-1·min^-1) significantly increased cardiac index (2.7 ± 0.1 to 3.1 ± 0.21·min^-1·m^-2; P < 0.05); this effect was not altered by MgSO₄ administration (n = 11). However, MgSO₄ significantly blunted epinephrine's hypertensive action and prevented a significant increase in mean arterial pressure during concurrent MgSO₄-epinephrine administration. Six placebo control patients were given two sequential infusions of epinephrine separated by a placebo infusion to rule out an effect of time on the hemodynamic response to epinephrine. Mean arterial pressure and cardiac index responses to epinephrine were identical before and after placebo infusion. We conclude that infusion of the small doses of MgSO₄ after coronary surgery to treat arrhythmias and vasospasm attenuates the vasoconstrictor actions of epinephrine but has no effect on epinephrine's cardiotonic activity.