False negative ECG response to exercise testing due to "ischemic ST-segment counterpoise"

Purpose: The negative exercise ECGs (ECG-) are often puzzling. We hypothesized that ST-segment vectors generated by myocardial ischemia (MI) from regions opposite each other (MRO) might lead to ECG-[("ischemic ST-segment counterpoise" (ISTC)]. Methods: We studied 77 patients (pts) with MI on T1-201 perfusion scintigraphy (T1): 42 pts had a ECG- at ≥85% of the maximal predicted heart rate, and 35 pts had a positive ECG (ECG+). Stress SPECT T1 "bullseye" maps were divided into [anterior (A), posterior (P), septal (S), and lateral (L)] 90° sectors, and, the mean relative count rates (CR) from these sectors were used to calculate the A/P or P/A and the S/L or L/S ratios, with the lower CR entered as a numerator; the smaller of the two ratios was employed as an index of the greatest difference in the perfusion of MRO. Results: This variable was 0.81±0.07 (x±SD) for the patients with ECG-and 0.73±0.1 for the patients with ECG+ (P=0.0005) Conclusions: ISTC in pts with MI appears to be a determinant of the exercise ECG response. Clinical Implications: An exercise-ECG-only test may not suffice for diagnosis of MI involving multiple heart territories.