The article by Christian and colleagues [1] underestimates the diagnostic value of myocardial perfusion scintigraphy, as done in most medical centers. The authors used a rigorous cutoff for disease to assess the incremental value of thallium-201 imaging in diagnosing coronary artery disease. Although previous analyses [2] have suggested that scintigraphy is most useful when reserved for patients with an intermediate probability of ischemia, the Mayo Clinic patients had a 40% incidence of typical anginal pain, and presumably most could exercise satisfactorily. In many nuclear cardiology laboratories, an explosive growth has occurred in pharmacologic testing [3]; this modality serves a group in whom exercise-based noninvasive testing is inconclusive. The Mayo Clinic study ignores those patients who cannot exercise, as well as those with abnormal baseline electrocardiograms.

We examined the usefulness of scintigraphy in a different referral context, in which most of our patients could not exercise to an appropriate end point. For patients who could do some dynamic exercise, we performed stress testing with a combination of intravenous dipyridamole and symptom-limited ergometry [4]. To simplify the determination of incremental usefulness and to avoid the potential pitfalls of multivariate analysis [5], we calculated the number of patients in whom ischemia was correctly identified during stress redistribution thallium procedures using the electrocardiographic findings (ST-segment depression) and the concurrent scintigraphic findings (reversible defects or increased lung uptake of thallium); we considered abnormal baseline electrocardiograms and fixed thallium defects to be indeterminate.

As determined from Table 1, the value of reversible defects was greatest when scintigraphy was targeted at identifying a focal area of stress-induced hypoperfusion corresponding to angiographic criterion A or B. In comparison to ST-segment depression, reversible defects showed an incremental diagnostic value in 170 of the 503 patients (34%) with criterion A, in 134 (27%) with criterion B, but in only 47 (9%) with criterion C. In contrast, lung uptake showed its most striking incremental value in 137 patients (27%) with criterion C, an angiographic target similar to that used in the study of Christian and colleagues. Moreover, both scintigraphic markers appeared more useful when stress testing was done with dipyridamole-based tests than when exercise alone was used. These inferences from Table 1 were corroborated by stepwise discriminant analyses.

Scintigraphy fills a small but useful niche in the overall assessment of patients with cardiac disorders and is certainly not confined to the discernment of severe disease. It provides a reasonably accurate way to indicate the most hemodynamically important stenosis. In patients who can exercise and have typical angina, the exertion of daily activities probably represents the most "cost-effective" test; in those with atypical symptoms and an interpretable electrocardiogram, the treadmill test becomes useful. In other groups, scintigraphy may well be justified.