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.

DESIGN NARRATIVE:

In Phase I, the paramedics took electrocardiograms of patients who met the definition of cases and administered an abbreviated questionnaire. This phase tested the feasibility of paramedics making a diagnosis of acute myocardial infarction on the basis of clinical history and with the support of computerized electrocardiogram, coming to an appropriate decision to initiate thrombolytic therapy on the basis of the field database, and recognizing appropriate contraindications to thrombolytic therapy with and without remote physician supervision. An analysis was conducted of the pre-hospital selection of patients for thrombolytic therapy.

Criteria for approval to move to Phase II included correct identification of appropriate cases for treatment 75 percent of the time, less than five percent inappropriate cases or with contraindications for thrombolytic therapy, and no more than one percent of the selected cases found to have initial life-threatening hemorrhagic conditions.

Beginning in November 1988, Phase II compared initiation of thrombolysis with tissue plasminogen activator (TPA) in the field to initiation in the hospital. During Phase II, which lasted for 24 months, one half of the patients meeting the case definition received paramedic administration of intravenous thrombolytic therapy and one half received the same pre-hospital diagnostic steps but treatment administered after transport to the hospital. The 360 patients with symptoms for six hours or less, no risk factors for serious bleeding, and ST-segment elevation, were selected by paramedics and a remote physician for inclusion in the trial. They were allocated to aspirin and alteplase treatment initiated before or after hospital arrival. Intravenous sodium heparin was administered to both groups in the hospital. Nineteen hospitals in the Seattle and King County areas participated. The primary endpoint was a ranked composite score (combining death, stroke, serious bleeding, and infarct size). The relation between time to treatment and outcome (composite score, infarct size, ejection fraction, and mortality) was also assessed.

In Phase II, a myocardial infarction registry was established to include all patients hospitalized in the area with a discharge diagnosis of acute myocardial infarction. The registry placed the trial findings in a community-based perspective and aided in the interpretation of trial findings by assessing the proportion of cases seeking paramedic care and determining the characteristics of these patients in contrast to those arriving at the hospital by other means. The records of all patients admitted directly to coronary care units in Seattle and King County were surveyed. Study personnel examined differences, if any, between paramedic and other transport groups regarding clinical history, treatment, complications, and hospital discharge rates. Of the cases in the registry, 300 who arrived at the hospital by means other than paramedic transport were compared to the paramedic-transported cases with respect to the incidence of new Q-wave infarction on the first and last electrocardiogram, an interview to determine onset, severity, and duration of symptoms, and reasons for choosing one type of care over the other during the emergency situation.