The paradox of negative exercise stress ECG/positive thallium scintigram. Ischemic ST-segment counterpoise as the underlying mechanism.

A 62-year-old woman with exertional angina underwent an exercise thallium-201 stress test. She exercised for 4 minutes on the treadmill using a modified Bruce protocol, reaching 94% of her predicted maximal heart rate, and stopped because of chest pain and fatigue. No ST-segment depression was detected at peak exercise or in the recovery period. In contrast, the thallium-201 myocardial single-photon emission computed tomography images in the short-axis, vertical long-axis, and horizontal long-axis views revealed severe myocardial ischemia involving the anterior, septal, posteroinferior, and posterolateral planes of the heart. Coronary arteriography showed severe stenosis of the left anterior descending and right coronary arteries. The information from the exercise electrocardiogram (ECG), thallium-201 myocardial scintigraphy, and a coronary angiogram suggested that the false negative ECG response was due to ischemic ST-segment counterpoise (i.e., cancellation of ischemic ST-segment vectors, generated by equally extensive and severe ischemia involving myocardial planes opposite each other.