Effect of Drugs on Dipyridamole Thallium Results
ABSTRACT & COMMENTARY
Synopsis: The continuance of anti-anginal therapy during dipyridamole stress SPECT thallium results in underestimation of coronary artery disease severity.
Source: Sharir T, et al. J Am Coll Cardiol 1998;31: 1540-1546.
Dipyridamole stress thallium-201 single photon emission computed tomography (SPECT) imaging is a popular test for the non-invasive diagnosis of coronary artery disease (CAD) because it can be applied to patients who cannot exercise adequately, yet gives similar results to exercise thallium-201. It is well known that anti-anginal drugs can impair the diagnostic sensitivity of exercise stress tests, but little is known about their effects on pharmacologic stress. Thus, Sharir et al evaluated 26 patients who had undergone dipyridamole SPECT off anti-anginal drugs and had reversible defects. They were restarted on anti-anginal therapy 11 ± 6 days later. Anti-anginal therapy consisted of calcium blockers in 81%, nitrates in 73%, and beta blockers in 31%. SPECT images were visually scored by an experienced but blinded reader and quantitatively assessed by a polar map display using commercial software. Visual and quantitative perfusion defects were larger on the non-drug study. Since all patients had at least one significant coronary artery lesion, sensitivity and specificity were determined for the three major vessel territories independently. The off-drug sensitivity for left anterior descending, circumflex, and right coronary artery lesions was 93%, 79%, and 100%, respectively. The on-drug values dropped to 64%, 50%, and 70%, respectively (P < 0.01). Specificity was not significantly different between the two tests. Sharir et al conclude that the continuance of anti-anginal therapy during dipyridamole stress SPECT thallium results in underestimation of CAD severity.
COMMENT BY MICHAEL H. CRAWFORD, MD
This study has shown similar results as studies of other stress testing modalities, despite the fact that dipyridamole has different hemodynamic effects as compared to exercise and dobutamine. This suggests that reduced myocardial oxygen demand is not the mechanism for blunting the sensitivity of the test. In fact, heart rate and blood pressure at rest and during dipyridamole were not altered by anti-anginal medications. Also of interest is the fact that the ECG response to exercise was no different between the two tests. This suggests that reduced ischemia may not be the mechanism of less sensitivity on anti-anginal drugs. It has been established that only one-third of patients with CAD given dipyridamole get myocardial ischemia manifested by angina and ECG changes. Thus, an alteration in ischemia induced by medications is not likely to explain the results. Nitroglycerin can reduce left ventricular volume and reduce myocardial oxygen demand, and three- fourths of the patients were on nitrates. Thus, a reduction in myocardial oxygen demand cannot be completely excluded. More likely, the calcium blocker and nitrates the patients were on increased coronary blood flow to ischemic areas and blunted the vasodilator effect of dipyridamole-reducing the difference in perfusion between normal and ischemic areas. Since nitroglycerin and calcium blockers affect different arterial segments, the exact mechanism cannot be elucidated from this study.
Problems with this study include the small study group and the retrospective design. Patients were not prospectively studied, but patients who happened to stop their anti-anginal medications were identified and studied after restarting their drugs. Whether this group is a representative sample of their angina population is unclear. It is interesting that specificity was not significantly effected, which suggests that anti-anginal drugs do not cause false-positive tests. In fact, there was a trend in this study toward higher specificity on drugs, so perhaps drugs reduce false-positives. Also, true sensitivity and specificity for the detection of CAD was not tested; instead, individual vessel territory sensitivity and specificity were tested. Whether similar results would be obtained in a prospective diagnostic study is unclear. Nevertheless, this study strongly supports the concept that all diagnostic stress tests should be done off anti-anginal drugs to maximize sensitivity for the detection of CAD.
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