Myocardial Infarction Triage and Intervention Project (MITI)

BACKGROUND:

In the past, patients with acute myocardial infarction were treated by putting them to bed. It was felt that once the symptoms and electrocardiographic signs of acute myocardial infarction had occurred, the process was complete and that the heart and patient at rest would allow the illness to run its course and the heart to heal. The focus of treatment in the Coronary Intensive Care Unit was to prevent complications of acute myocardial infarction, primarily arrhythmias leading to cardiac arrest and the treatment of cardiac arrest itself. Acute phase morbidity and mortality related to loss of myocardium through infarction has not changed. Post-discharge mortality and morbidity is, in large part, related to the occurrence of congestive heart failure, secondary to loss of myocardium to infarction. Efforts to preserve myocardium through pharmacologic intervention to limit demand, have not resulted in significant benefit. It is now recognized that the infarction is not complete on presentation, that it occurs as a wavefront over a period of four to six hours, and that in 80 to 90 percent of patients, infarction is due to thrombus in the infarct-related artery. Therefore, reperfusion of the jeopardized myocardium during this window of four to six hours could result in salvage of myocardium with resultant decrease in acute mortality as well as acute and chronic morbidity and mortality from myocardial failure.

The international streptokinase trials have established the feasibility of intravenous coronary thrombolysis with streptokinase and demonstrated a decrease in acute morality ranging from 18 percent to 50 percent, depending upon the interval between the onset of chest pain and the initiation of thrombolytic therapy. Furthermore, preservation of ventricular function, as reflected in ejection fraction, is greater with early administration.

The TIMI trial of tissue plasminogen activator (TPA), which is more clot-specific than streptokinase, has substantiated the effectiveness. The initiation of thrombolytic therapy with TPA as early as possible after the onset of symptoms offers an advantage, therefore, in terms of both a reduction in acute mortality and salvage of ischemic myocardium.

Although thrombolysis is being performed at community hospitals or enroute to hospitals by specially trained teams using helicopter transport, only one major study has been reported which has systematically evaluated the benefit of early on-the-scene thrombolysis. This study documented significant benefit in terms of salvage of myocardium over those patients in whom thrombolytic therapy was initiated after arrival in the hospital, as a direct function of time. This program used physicians in the ambulance to evaluate indications for and contraindications to thrombolytic therapy in the field. The proposed study would evaluate the ability of paramedics, under physician remote control, to accurately evaluate patients in the field in terms of indications and contra-indications to thrombolytic therapy, and to assess the possible benefit in terms of salvage of myocardium in those patients being transported to the hospital by paramedic squads as opposed to those arriving by ordinary means.