The left main coronary artery arises from the left coronary cusp and bifurcates into the left anterior descending (LAD) and the left circumflex coronary arteries. Since the left main supplies a very large amount of myocardium, occlusion is often fatal resulting in cardiogenic shock. Frequently, a left bundle branch block is seen on the ECG during left main occlusion, however changes of an anterior myocardial infarction may be seen as well.
Anomalies of the left main coronary artery can occur including arising from the right coronary cusp or the pulmonary artery. Congenital absence of the left main is common and a benign finding (the LAD and circumflex arise separately from the left coronary cusp).
Intravascular ultrasound (IVUS) is frequently used to diagnose left main coronary stenosis since angiographically this can be difficult in certain situations.
The below ECG tracing is from a patient with an acutely occluded left main coronary artery. This ECG shows dramatic ST depression that is downsloping consistent with ischemia. The changes are quite pronounced. Classically it has been taught that ST segment elevation in lead aVR greater than the ST elevation in lead V1 along with ST depression in the precordial leads is consistent with left main occlusion.