Mortality rates range from 6 to 19 percent during the first year after an acute myocardial infarction (MI), and about 3 or 4 percent per year after that. Younger patients can be successfully classified as high, intermediate or low risk by using noninvasive procedures such as a dipyridamole thallium scan, but this technique, although safe in the elderly, has not been used for post-MI risk stratification. Jain and colleagues conducted a prospective study to determine if dipyridamole thallium imaging could predict major cardiac events in elderly patients after acute MI.
Patients who were admitted with an acute MI were included in the study if they were older than 65 years of age. Contraindications to dipyridamole thallium testing were the main exclusion criteria; these included persistent symptomatic bradycardia, ventricular arrhythmias, unstable angina, decompensated heart failure and severe hyper- or hypotension. The most common reason for exclusion, though, was physician or patient refusal (82 percent). Intravenous dipyridamole (0.56 mg per kg, up to 60 mg) was given to study participants, followed by one minute of exercise (leg swinging or hand squeezing). Thallium (3 mCi) was then injected. Patients exercised for two more minutes, at which time single photon emission computed tomographic thallium (SPECT) images were taken. Three to four hours later, a second injection of thallium (1 mCi) took place. If a patient had a fixed defect on the reinjection scan, further images were obtained on day 2. Symptoms, vital signs and electrocardiograms were monitored during the study. All patients underwent echocardiography to assess left ventricular function. Follow-up data were collected at six weeks, six months, 12 months, 18 months and at the end of the study. The end points were nonfatal reinfarction and all-cause mortality.
None of the 73 patients in the thallium group had adverse reactions to the dipyridamole thallium testing. However, 14.2 percent of patients required sublingual nitroglycerin or intravenous aminophylline to relieve angina during the test. More than one half (60.3 percent) of the patients had reversible perfusion defects on thallium scan. In this group of 44 patients, 13 died during the follow-up period, and six had nonfatal reinfarctions. Among the patients without reversible defects on thallium imaging, there were five deaths and no reinfarctions. Recurrent events were predicted by the following risk factors: (1) left ventricular dysfunction (ejection fraction of less than 50 percent); (2) no use of aspirin; (3) no use of beta blockers, and (4) reversible perfusion defect on thallium scan. When these risk factors were examined in patients who died or had a reinfarction, it was seen that patients who had no more than one risk factor had a 6 percent risk of death or reinfarction. Those who had two risk factors had a 41 percent incidence of one of the major outcomes during follow-up, and those who had three or all four risk factors had an 83 percent incidence of death or reinfarction during the two years after hospital discharge. Patients with reversible defects on dipyridamole thallium scanning who underwent revascularization procedures had a zero percent mortality on follow-up; patients who were not revascularized had a 37 percent incidence of mortality at follow-up.
The authors conclude that both echocardiography and dipyridamole thallium testing provide important information that can allow noninvasive stratification of elderly patients with acute MI. Coronary revascularization, even in elderly patients who are found to be at high risk, may improve outcomes.
editor's note: This study also tangentially underscores what many other studies have found: namely, the benefits of aspirin and beta blocker use in elderly patients who have had an acute myocardial infarction. It appears that prescribing these two medications, even for a patient with left ventricular dysfunction and a reversible perfusion defect on dipyridamole thallium scan, could reduce the incidence of death or reinfarction two years after discharge from 83 percent to 41 percent.—g.b.h.