Unmasking Potential Myocardial Ischemia with Dipyridamole Thallium Imaging in Patients with Normal Submaximal Exercise Thallium Tests

Abstract

We evaluated the ability of dipyridamole thallium imaging to unmask evidence of ischemia in patients who previously had a normal submaximal exercise thallium test. In a prospective analysis of 385 consecutive exercise thallium tests, 72 (19%) were judged to be normal submaximal tests after meeting the following two criteria: (1) peak heart rate achieved was less than 85% predicted for age and/or treadmill time was shortened, and (2) there was no angina, no diagnostic ST segment depression and no thallium redistribution. Twenty-one of these patients consented to return for dipyridamole thallium imaging within a mean period of 18 days, 6 (29%) of whom demonstrated thallium redistribution during dipyridamole imaging that was not present during exercise thallium imaging. There were no differences in age, sex, peak heart rate achieved, treadmill time, history of previous myocardial infarction and cardiac medications in patients with and without redistribution. One of the 6 patients developed severe unstable angina within 6 months. In patients with normal submaximal exercise thallium tests, inferences about the maximal potential for ischemia should be made with caution.