Hemodynamic and Metabolic Responses to Vasodilator Therapy in Acute Myocardial Infarction
Abstract
Hemodynamic effects of vasodilator therapy (phentolamine or nitroprusside) were studied in 38 patients with acute myocardial infarction (AMI). Cardiac metabolism was studied in 19 of the 38 patients. According to the initial level of left ventricular filling pressure (LVFP) and left ventricular stroke work index (SWI), patients were divided into three groups: Group I-nine pa- tients with LVFP 15 mm Hg or less; Group II-14 patients with LVFP > 15 mm Hg and SWI >20 g-m/m2; Group III-15 patients with LVFP > 15 mm Hg and SWI < 20 g-m/ m2. In Group I most patients were clinically uncomplicated. In Group IL most patients had clinical left ventricular failure including one patient who had clinical features of cardiogenic shock. Group III patients all had severe left ventricular failure, with eight patients in clinical shock.
In all groups LVFP, pulmonary artery pressure, right atrial pressure, and systemic and pulmonary vascular resistance decreased significantly with vasodilator therapy with only a slight to moderate decrease in arterial pressure. In Group I patients SVI decreased (-7%) together with an increase in heart rate. Significant improvement in left ventricular performance, however, was observed in Groups II and III as indicated by increased stroke volume index (SVI) and cardiac index (CI) and decreased LVFP. The increase in SVI and CI was of similar magnitude in both Group LI (SVI +18%, CI +24%) and Group III (SVI +28%, CI +29%) patients, a change suggesting that vasodilation thereby may be applicable and beneficial even in the presence of severe depression of cardiac performance.
Improved left ventricular performance in group II and III patients was accompanied by a slight decrease in coronary blood flow, myocardial oxygen consumption, and transmyocardial oxygen extraction. There was no change in myocardial lactate metabolism in any group. In vitro studies in isolated cat papillary muscle preparations showed no direct positive inotropic effect of either phentolamine or nitroprusside. Thus, significant improvement in left ventricular performance occurs during vasodilator therapy in patients with AMI and elevated LVFP, even in the presence of severe depression of cardiac performance. Furthermore, this improvement is not accompanied by increased metabolic cost. Vasodilator therapy, therefore, may have an important role in the treatment of pump failure complicating myocardial infarction.
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