Precordial ST Segment Depression nvestigated in Relation to Coronary Supply in Acute Inferoposterior Myocardial Infarction

In inferoposterior myocardial infarction (MI), unexplained ST changes often appear on the precordial leads of electrocardiograms (ECG) obtained soon after onset of the infarction but not on leads recorded a few hours later. Although the reason behind the temporary abnormality has not been clarified, speculations have been proposed that the transient electrocardiographic changes denote mirror images, reciprocal change or ischemic changes occurring in the anterior wall of the left ventricle. By general consensus, ischemia or infarction in the anterior ventricular area may cause ST segment depression. We studied the precordial leads obtained from 63 consecutive patients of acute inferoposterior MI (51 men, 12 women, 58.9 years old) to evaluate precordial ST segment depression for possible use in assessing infarct site. Standard 12-lead ECGs were recorded within six hours of onset and evaluated in relation to results of coronary angiography and myocardial contrast echocardiography (MCE) performed during the acute phase. Deviations shown in the precordial ST segment were scrutinized for correlations with the artery responsible for the infarct and with the dominant coronary artery, viz. either right coronary artery (RCA) or left circumflex artery (LCX). In the posterior wall of left ventricle the boundary between the region supplied by RCA and that supplied by LCX was estimated by MCE demarcation. With significant frequency (p<0.01), leads V1-6 showed upslope type ST segment depression in cases of right coronary artery obstruction, whereas, in left circumflex artery obstruction (LCX) the shape was characteristically horizontal or downslope in leads V1-4, with ST segment elevation in V5, 6 (p<0.01). Curiously, however, precordial ST segment depression was horizontal or downslope in leads obtained from patients with right coronary obstruction at dominant segment 4PL. As a whole, the left ventricular surface was supplied by both RCA and LCX generally contributing the main supply. The dominant coronary artery was defined as the main artery supplying the posterior wall of the left ventricle as visualized by angiography. The LCX was the dominant artery in 44 patients (70%), the RCA in 9 (14.2%) and balance type was found in 10 (15.8%). In patients with left coronary dominance, enhanced area in MCE included the area between the posterolateral wall and the posteroseptum. In patients with right coronary dominance, the enhanced area covered a large extent of the posterior wall. The most definitive electrocardiographic changes were those elicited by infarctions related to left coronary dominance, i. e., the LCX. These results suggest that ST segment shifts appearing on the precordial leads obtained during acute inferoposterior myocardial infarction correspond to a mirror image of adverse changes caused in the posterior wall of the left ventricle. Furthermore, the changes were shown to be mediated largely by the dominant coronary artery responsible for the main supply of blood to the left ventricular posterior wall.