This chapter will focus on the recognition of myocardial ischemia and infarction, one of the most important aspects of electrocardiography. It’s absolutely vital that you master the ECG manifestations of acute and chronic coronary syndromes because you are going to use these skills to make urgent clinical decisions. Let’s start our discussion with a review of anatomy.
We learned in Chapter 2 that the heart muscle derives its supply of oxygen from the coronary arteries. The two coronary openings (ostia) give rise to the right coronary (RCA) and left main (LM) coronary arteries (Figure 10-1).
The RCA circles around the right (anterior) surface of the heart, coursing in the plane of the AV groove that divides the atria and the ventricles. This vessel normally supplies the right atrium and the right ventricular free wall. In 85% of individuals, the RCA gives rise to the posterior descending artery (PDA) that supplies the inferior surface of the left ventricle; a pattern termed a right dominant circulation. Septal branches from the PDA supply the inferior portion of the interventricular septum.
The LM coronary artery divides into the left anterior descending (LAD) and left circumflex (LCx) coronary arteries. The LAD courses in the plane of the interventricular septum and supplies the anterior wall of the left ventricle. Septal branches supply the majority of the interventricular septum and diagonal branches supply portions of the anterolateral wall. The LCx circles leftward in the plane of the AV groove, typically supplying the posterior and lateral portions of the left ventricle. Anterolateral and posterolateral obtuse marginal branches supply their corresponding segments of the left ventricle. In 15% of individuals, the PDA arises from the LCx and is called a left dominant circulation.
It is important to note that the coronary anatomy depicted in Figure 10-1 is only a generality because there is wide variability from person to person. In some individuals, the left main trifurcates into three branches, the LAD, LCx, and a diagonal-type vessel termed the ramus intermedius. In most individuals the first branch of the LAD is the first diagonal artery, but in others it is the first septal artery. The size and distribution of the LCx and marginal vessels also vary widely. All these variations impact the ECG patterns of myocardial infarction.
Circulation to the Cardiac Conduction System
The coronary arteries provide circulation to the specialized conduction system. Accordingly, cardiac conduction abnormalities may be present in acute coronary syndromes. The SA node is supplied by the RCA in 60% of individuals and the LCx in 40%. The circulation to the AV node follows the coronary dominance as determined by the vessel that gives rise to the PDA; ...