Electrocardiographic diagnosis of remote posterior wall myocardial infarction using unipolar posterior lead V₉

The accuracy of four electrocardiographic criteria for diagnosing remote posterior myocardial infarction was assessed prospectively in 369 patients undergoing exercise treadmill testing with thallium scintigraphy. Criteria included the following: 1) R-wave width ≥ 0.04 s and R-wave ≥ S-wave in V₁; 2) R-wave ≥ S-wave in V₂; 3) T-wave voltage in V₂ minus V₆ ≥ 0.38 mV (T-wave index); 4) Q-wave ≥ 0.04 s in left paraspinal lead V₉. Twenty-seven patients (7.3 percent)met thallium criteria for posterior myocardial infarction, defined as a persistent perfusion defect in the posterobase of the left ventricle. Sensitivities for the four criteria ranged from 4 to 56 percent, and specificities ranged from 64 to 99 percent. Posterior paraspinal lead V₉ provided the best overall predictive accuracy (94 percent), positive predictive value (58 percent), and ability to differentiate patients with and without posterior myocardial infarction of any single criterion (p < .0001). Combining the T-wave index with lead V₉ further enhanced the diagnostic yield: the sensitivity for detecting posterior infarction by at least one of these criteria was 78 percent, and when both criteria were positive, specificity was 98.5 percent. It is concluded that a single, unipolar posterior lead in the V₉ position is superior to standard 12-lead electrocardiographic criteria in diagnosing remote posterior myocardial infarction, and that combining V₉ with the T-wave index maximizes the diagnostic yield.