A fundamental characteristic of cardiac muscle function is rapid deterioration consequent to interruption of myocardial blood supply and oxygen delivery. Experimental and clinical studies have demonstrated, in association with myocardial ischemia or hypoxia, derangements of cardiac mechanical and pump performance, metabolic processes, electrical activity and structure. Deprivation of oxygen is the major factor producing these alterations, but reduced substrate delivery and accumulation of the products of cell metabolism and breakdown are contributing factors. After the onset of myocardial ischemia, there is a period during which the ensuing functional and subcellular structural alterations are reversible with restoration of oxygen supply. After this interval, structural damage is irreversible and there is permanent loss of myocardium. The degree of functional impairment is most closely related to the quantity of myocardium involved. In patients with stable coronary artery disease, clinical cardiac failure is not usually evident in the absence of previous loss of left ventricular myocardium, as occurs with infarction. However, evidence of transient, reversible impairment of ventricular muscle and pump performance is present during episodes of angina pectoris. Myocardial infarction or, less frequently ischemia, is associated with permanent elimination of normal contractile performance in a localized area of myocardium (asynergy). Evidence of cardiac failure is apparent when 20 to 25 percent of left ventricular muscle is asynergic. The standard electrocardiogram is useful in estimating location and degree of asynergy and also cardiac pump performance by the presence and location of pathologic Q waves and associated electrocardiographic abnormalities. In acute myocardial infarction, the shock syndrome is associated with loss of 40 percent or more of left ventricular myocardium. Since in-hospital prognosis in acute myocardial infarction is closely related to extent of left ventricular damage, estimation of the latter would be helpful in applying the most appropriate forms of therapy in these patients. Recent innovations utilizing serum enzyme analysis and isotopic imaging of left ventricular myocardium have provided promising, noninvasive means of achieving this goal.
ASJC Scopus subject areas
- Cardiology and Cardiovascular Medicine