Pathophysiology of cardiac failure

The pathophysiologic cycle of heart failure is initiated by myocardial failure that accompanies a reduction in myocardial contractility secondary to ischemic or myopathic heart disease. Reduction in cardiac output and oxygen delivery to the tissues is followed by vasoconstriction that raises systemic vascular resistance to preserve systemic arterial pressure while maintaining regional O2 availability. As a consequence, however, impedance to left ventricular ejection is increased, creating an additional hemodynamic burden for the failing heart. A vicious cycle ensues. Hemodynamic features of acute cardiac failure include decreases in cardiac output and mixed venous O2 saturation, together with increases in left ventricular filling pressure and systemic resistance. If hypotension is present with failure, there is a markedly decreased cardiac output or an inappropriate increase in systemic resistance. If acidosis is also present with hypotension and failure, cardiac output is severely decreased and lactic acid is increased. A major objective of medical therapy in acute heart failure is to enhance ventricular emptying, thereby increasing cardiac output and O2 delivery while decreasing left ventricular filling pressure, pulmonary venous pressure and vascular resistance. Potent intravenous drugs that have a positive inotropic effect on the myocardium, including amrinone and dobutamine, have been shown to increase ventricular emptying in patients with acute heart failure. Intravenous amrinone improves pump performance without adversely raising myocardial O2 consumption, thereby enhancing myocardial efficiency. These drugs also promote a degree of vasodilation through both direct and secondary effects on the systemic circulation.(ABSTRACT TRUNCATED AT 250 WORDS)