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Predictive Value of Dipyridamole Thallium Imaging in a Patient with Myocardial Bridging but Without Fixed Obstructive Coronary Artery Disease

Department of Radiology, Massachusetts General Hospital

Correspondence: For reprints contact: Tsunehiro Yasuda, MD, Department of Radiology, Division of Nuclear Medicine, Tilton 2, Massachusetts General Hospital, Boston, MA 02114.

ABSTRACT

Both dipyridamole and exercise-²⁰¹Tl imaging are sensitive, specific and of prognostic value in patients with suspected coronary artery disease, following myocardial infarction, and undergoing major noncardiac surgery. Though reported sensitivities and specificities vary widely from 60% to 100%, the consensus is that both are between 80% and 90% for both dipyridamole and exercise studies (23). Moreover, when compared directly in the same study populations, the two have equal sensitivities and specificities (9,11,13,23). Transient thallium perfusion abnormalities are the most consistent predictors of adverse cardiac events and have more predictive power than clinical and angiographic parameters. Thallium reversibility may be a better predictor of adverse cardiac events than angiography since it represents more of a physiologic rather than a purely anatomic evaluation of the heart. It is difficult to make an exact comparison of some of the studies in the literature because they use different patient populations, sometimes define coronary stenosis in different ways, may have different cardiac endpoints and may not differentiate between reversible and fixed thallium perfusion defects. Exercise has the advantage of a graded examination and more experience historically and is of importance in a detailed study of cardiopulmonary hemodynamics, as in cardiac transplantation. Dipyridamole is more useful in patients who cannot achieve symptom-limited, submaximal exercise. It may also be more useful for patients who are bedridden or have peripheral vascular disease. Angina occurs less frequently with dipyridamole. Dipyridamole has superior sensitivity, specificity and predictive value, when compared with exercise testing, in the patient who cannot achieve the 85% maximal predicted heart rate or is limited by cardiac symptoms.

Once a test for making the diagnosis of coronary artery disease has been selected and performed, the result must be interpreted with respect to epidemiologic principles. In this patient, the normal dipyridamole-thallium study diminishes the post-test probability of disease. The post-test probability is not zero, as neither the specificity nor negative predictive value is 100% for thallium testing. However, the patient's prognosis is excellent regardless of angiography results, because several studies have demonstrated a 1% or less risk of infarction or death per year in patients with normal dipyridamole or exercise-thallium studies (14,15,24,28,29). It would have been reasonable and perhaps advisable not to proceed to angiography in view of the thallium result.

The conflicting reports of the clinical implications of myocardial bridging, unlike the consensus from thallium imaging, do not enable accurate prediction of an outcome based on the angiographic findings alone for the patient presented here. Also, the literature contains no controlled follow-up studies of patients with bridging. Thus, based on the angiographic findings alone, a recommendation for prognosis or therapy cannot be made for this patient. However, the studies of dipyridamole-thallium imaging performed in other populations suggest that even in the presence of an abnormal coronary arteriogram, a normal thallium study confers a good prognosis. The patient presented here most likely has a good prognosis, for there is only moderate bridging of the left anterior descending artery and the dipyridamole-thallium study was normal.