Clinical Brief

Exercise Thallium Accuracy in Pacemaker Patients

Source: Lakkis NM, et al. J Am Coll Cardiol 1997; 29:1221-1225.

It is well known that left bundle branch block on the electrocardiogram (ECG) can result in false-positive exercise thallium scintigraphy results. Patients with right ventricular apical pacemakers have similar ECG patterns during pacing, so Lakkis and colleagues from Baylor College of Medicine in Houston, TX, studied the accuracy of exercise perfusion scintigraphy in 105 pacemaker patients. They had exercise thallium studies and coronary angiography within seven days of each other. Patients with prior myocardial infarction were excluded. The ECG showed continuous pacing in 39 patients before exercise and in 28 patients during exercise. Patients were subgrouped, based upon the agreement between the stress testing and catheterization results. Only two (8%) of the 24 patients with normal cath and normal exercise study were continuously paced, whereas 11 (78%) of 14 patients with normal cath and abnormal thallium were continuously paced. In patients with false-positive perfusion defects, the defects were small, mild, reversible, and the majority were inferior (71%). Overall sensitivity/specificity was 98%/84% in non-paced rhythm and 86%/15% in paced rhythm (P < 0.01). Corresponding accuracy was 91% nonpaced and 54% paced. Lakkis et al conclude that patients with right ventricular apex pacemakers, who are continuously paced during exercise, have a high incidence of false-positive thallium defects. This finding is consistent with other studies showing false-positive perfusion defects in patients with Wolff-Parkinson-White and left bundle branch block. In contrast to left bundle branch block, which causes septal defects, pacing more often causes inferior defects. Presumably, the abnormal electrical activation is exaggerated as heart rate increases with rate responsive pacemakers—leading to perfusion defects with exercise. In patients with left bundle branch block, dipyridamole stress obviates the problem since heart rate is not accelerated much. It is not known whether pharmacologic testing would be better in pacemaker-dependent patients. —mhc