Optimal Use of Stress Thallium
abstract & commentary
Synopsis: The finding of only one abnormal territory on thallium perfusion imaging does not necessarily establish a benign prognosis but, considering clinical and other exercise variables, can increase the accuracy of the prognostic analysis.
Source: Kwok JM, et al. J Am Coll Cardiol 2000; 35:335-344.
Thallium stress testing is of proven prognostic value in patients with known or suspected coronary artery disease (CAD). However, the distinction between single-vessel and severe multivessel disease is imperfect. Thus, Kwok and colleagues from the Mayo Clinic sought to identify clinical and exercise variables that would detect patients with three-vessel or left main disease (3VLMD) from among patients with single rest or exercise perfusion defects on thallium scintigraphy. First, they studied a training population of 264 patients with one thallium defect who had undergone coronary angiography. The prevalence of 3VLMD was 26% and four clinical and exercise variables were independent predictors of 3VLMD by multivariate analysis; magnitude of ST depression (x2 =10); lower rate-pressure product (x2 =8); diabetes (4); and hypertension (3). No thallium variables added to the prediction of severe CAD. They then studied this analysis in a validation cohort of 474 patients with one thallium defect treated medically and followed for a median of seven years. These patients were divided into three risk groups based upon the multivariate model: low probability of 3VLMD (< 15%); intermediate (15-35%) and high (> 35%). In the training population, the prevalence of 3VLMD in the low-risk group was 15%, intermediate 22%, and high 51%. So half the patients with one thallium defect in the high-risk group actually had 3VLMD. Survival in the validation population at eight years was 89% in the low-risk group, 73% intermediate, and 75% high (P < 0.001). Kwok et al conclude that the finding of only one abnormal territory on thallium perfusion imaging does not necessarily establish a benign prognosis, but considering clinical and other exercise variables can increase the accuracy of the prognostic analysis accuracy.
Comment by Michael H. Crawford, MD
The accurate noninvasive identification of patients with 3VLMD is important because revascularization has been shown to reduce mortality in such patients. In this group of patients with one arterial territory defects by stress thallium scintigraphy, more than one-quarter had 3VLMD. Previous studies of patients with known 3VLMD have shown that only 29% of such patients had three distinct territories positive on stress thallium imaging. Other studies have shown that the addition of exercise variables increases the detection of 3VLMD to 39%. This study extends these observations and identifies which clinical and exercise variables are useful for detecting 3VLMD and predicting outcome: magnitude of ST depression; low rate pressure product; diabetes; and hypertension. The results of this study suggest that those at high or intermediate risk based upon these clinical and exercise variables plus a one-territory thallium defect should be considered for coronary angiography.
Why do some patients with 3VLMD only exhibit one-territory involvement on stress thallium? There are several possible reasons. The exercise test may have been stopped early for other reasons before ischemia could develop in more than one region. Reduced sensitivity for detecting circumflex CAD may have resulted in a one-territory defect in LMD. Also, thallium scintigraphy measures relative flow. If one vessel is more critically narrowed than the other two and they are equally narrowed, only one territory may be relatively lacking in flow. Other studies have shown that only 70% of individual vessel lesions are detected by thallium scans in patients with 3VD, whereas 83% of single-vessel patients are detected. Finally, the overall prognosis in the test cohort was relatively good. The mortality was 1-3% per year across all three risk groups. In fact, cardiac death and myocardial infarction (MI) rates were no different for the three risk groups. These data suggest that this may have been a cohort with limited (one-territory) ischemia despite 3VLMD.
This study also suffers from the usual problems with such studies: conducted in a tertiary referral center; referral bias of patients undergoing catheterization; comparing perfusion defects to an anatomical gold standard; and the lack of gated wall motion studies, which would help detect 3VLMD. However, the data suggest that not all patients with presumed single-vessel disease based upon thallium scintigraphy results should be managed conservatively. Diabetics, hypertensives, and those with low double products and marked ST depression with exercise should be considered for coronary angiography and possible revascularization. Of interest, 50-75% of the patients in the training and validation groups had typical angina or prior MI. Many would catheterize most of such patients, which would leave the smaller group of patients with no or atypical symptoms as the main focus of this noninvasive first approach.
A single myocardial area of ischemia on exercise thallium perfusion scanning means:
a. single-vessel disease.
b. a 25% chance of three-vessel or left main disease.
c. a poor long-term prognosis.
d. other exercise variables are not predictive.
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