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Thrombolytic therapy has been convincingly demonstrated to open occluded coronary arteries, limit infarct size, preserve left ventricular function and reduce mortality when given within the first six hours of acute myocardial infarction (1). It is generally accepted that the closer to the onset of symptoms the thrombolytic agent is administered, the greater the amount of myocardium that is salvaged and the greater the clinical benefit (2). Because until recently it was felt that the primary mechanism of the clinical benefit of reperfusion was entirely through myocardial salvage, the maximum length of time following symptom onset that thrombolytic treatment should remain beneficial has been controversial.

Many studies have shown that long-term prognosis after myocardial infarction is related closely to myocardial function, which is determined primarily by the size and location of the infarct. Infarct extension and expansion are two complications occurring early in the postinfarct period that can significantly increase the functional infarct size—the former by increasing the mass of necrotic tissue and the latter by thinning and dilating the infarcted zone. Extension and expansion have been associated with increased mortality and morbidity both early and late after myocardial infarction. This chapter examines these complications in terms of incidence, pathogenesis, time course, consequences, and therapeutic implications.