The Clinical Problem
Coronary heart disease is the leading cause of death in the United States, with myocardial infarction a common manifestation of this disease. In 2006, approximately 1.2 million Americans sustained a myocardial infarction. Of these, one quarter to one third had a myocardial infarction with ST-segment elevation.
Of all patients having a myocardial infarction, 25 to 35% will die before receiving medical attention, most often from ventricular fibrillation. For those who reach a medical facility, the prognosis is considerably better and has improved over the years: in-hospital mortality rates fell from 11.2% in 1990 to 9.4% in 1999. Most of the decline is due to decreasing mortality rates among patients with myocardial infarction with ST-segment elevation, as a consequence of improvements in initial therapy, including fibrinolysis and PCI. In an analysis by the National Registry of Myocardial Infarction, the rate of in-hospital mortality was 5.7% among those receiving reperfusion therapy, as compared with 14.8% among those who were eligible for but did not receive such therapy.
On occlusion of the infarct-related artery, all the myocardium that is supplied by the artery becomes ischemic, resulting in chest pain and electrocardiographic evidence of transmural (full-thickness) ischemia (ST-segment elevation) in the leads reflective of that region of the heart. Subsequently, necrosis begins within minutes and progresses during several hours in a “wavefront” fashion from the endocardial surface to the epicardial surface. If ischemia persists for several hours, transmural infarction results. In contrast, if blood flow is restored during the period of progressive necrosis, the ischemic myocardium is salvaged and the size of the infarct is reduced. Since morbidity and mortality from a myocardial infarction correlate with the size of the infarct, prompt restoration of blood flow would also be expected to improve left ventricular function and survival.
Primary PCI consists of urgent balloon angioplasty (with or without stenting), without the previous administration of fibrinolytic therapy or platelet glycoprotein IIb/IIIa inhibitors, to open the infarct-related artery during an acute myocardial infarction with ST-segment elevation. After the identification on coronary angiography of the site of recent thrombotic occlusion, a metal wire is advanced past the thrombus over which a balloon catheter (with or without a stent) is positioned at the site of the occlusion and inflated, thereby mechanically restoring antegrade flow.
Primary PCI restores angiographically normal flow in the previously occluded artery in more than 90% of patients,11,12 whereas fibrinolytic therapy does so in only 50 to 60% of such patients.