An Important Concern for the Analysis of Culprit Artery

The ECG is considered the most important initial clinical test for diagnosing myocardial ischemia or MI. In the ST-segment elevation MI (STEMI) emergency setting, the presence of RBBB may confound the electrocardiographic diagnosis of acute MI. New RBBB associated with acute extensive anterior wall MI often suggests a proximal LAD lesion.² Lead V₁ reflects electrical activity of the right ventricle and left ventricular septum, and its ST-segment elevation may be associated with more extensive basal anterior and septal involvement.⁵ In cases of proximal LAD occlusion above the first septal and first diagonal branch, V₁ also shows ST-segment elevation.³ In the Figure, A, however, ST-segment elevation is absent in V₁. There may be 3 plausible explanations: acute total LMT occlusion without collateral circulation, multiple blood supply in lead V₁, and secondary ST-T wave changes.^3,4,6,7 (1) In 2012, Fiol et al² first reported the STEMI pattern with RBBB and left anterior fascicular block of proximal LAD without ST-segment elevation in lead V₁, suggesting acute total occlusion of LMT without collateral circulation. In these patients, the absence of ST-segment elevation in lead V₁ may be related to acute occlusion of the left circumflex artery that results in ST-segment depression in right precordial leads, counteracting ST-segment elevation in lead V₁ caused by occlusion of LAD.³ In the present case, application of supplemental ECG leads using posterior leads revealed no ST-segment elevation. Coronary angiography also did not reveal LMT occlusion. Therefore, reciprocal ST-segment depression in lead V₁ caused by left circumflex branch was excluded. (2) The basal portion of the interventricular septum may be supplied with blood by the septal branches and the conus branch of LAD and the conus branch of the right coronary artery.^6,7 Accordingly, in patients with proximal LAD occlusion, the presence of the large conus branches may protect the interventricular septum, which manifests with absence of ST-segment elevation in lead V₁. However, in the present patient, there is a Q wave in lead V₁, which is evidence of septal involvement, and coronary angiography revealed the conus branch of the right coronary artery had not reached the interventricular septum without the conus branch of LAD. (3) Secondary ST-T changes may mask acute myocardial ischemia. Zhong-qun et al⁸ reported on a patient after acute LAD occlusion proximal to the first septal branch with concomitant RBBB, where the ECG showed no ST-segment elevation in lead V₁. The underlying mechanism may be that RBBB results in secondary repolarization changes, depressing the ST-segment elevation in lead V₁,⁴ as in the present case.