Precordial electrocardiographic mapping systems comprising a large number of recording sites have been utilized in the Coronary Care Unit for following the natural course of ischemic injury and detecting extension of acute myocardial infarction in patients. These electrocardiographic techniques have also been found useful in evaluating the effects of a number of metabolic, hemodynamic and pharmacologic interventions currently proposed as effective means of reducing the extent of ischemic damage. However, these methods have not been found useful in the study of patients with inferior transmural or nontransmural myocardial infarctions. Bundle branch blocks and pericarditis complicating acute myocardial infarction invalidate the mapping technique. To assess effectiveness of therapeutic interventions, ST-segment changes taken as an index of ischemic injury are correlated with alterations of the QRS complex, used as a parameter of ischemic necrosis.