While digitalis and diuretics constitute conventional therapy of congestive heart failure due to acute myocardial infarction, systemic vasodilator drugs offer an innovative approach of decreasing left ventricular systolic wall tension (afterload) by reducing aortic impedance and/or by reducing cardiac venous return. Thus, vasodilators increase lowered cardiac output by diminishing peripheral vascular resistance and/or decreasing increased left ventricular end-diastolic pressure (ventricular preload) by reducing venous tone. Concomitantly, there is a reduction of myocardial oxygen demand thereby potentially limiting infarct size and ischaemia.
The vasodilators produce disparate modifications of cardiac function depending on their differing alterations of preload versus impedance: nitrates principally cause venodilatation (decrease left ventricular end-diastolic pressure); sodium nitroprusside, phentolamine and prazosin produce relatively balanced arterial and venous dilatation (decrease left ventricular end-diastolic pressure while increasing cardiac output, provided upper limits of normal left ventricular end-diastolic pressure are maintained); and hydrallazine solely effects arteriolar dilatation (increases cardiac output). Combined sodium nitroprusside and dopamine therapy synergistically enhances cardiac output and decreases left ventricular end diastolic pressure. In addition, sodium nitroprusside is aided by mechanical counterpulsation which sustains myocardial perfusion pressure in acute myocardial infarction.