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.
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