Fig. 11.9 Dependenceofmyocardialinfarctiononthelocationandnatureofthediminishedperfusion.Left, Patternsoftransmuralinfarctionresultingfrommajorcoronaryarteryocclusion.Therightventriclemaybeinvolvedwithocclusionoftherightmaincoronaryartery(not depicted). Right, Patternsofinfarc-tionresultingfrompartialortransientocclusion(top), globalhypotensionsuperimposedonfixedthree-vesseldisease(middle), orocclusionofsmallintra-myocardialvessels(bottom). Posterior Restoration of flow (reperfusion) LVRV Permanent occlusion of left anterior descending branch Transient/partial obstruction regionalsubendocardial infarct Permanent occlusion of left circumflex branch Global hypotension circumferentialsubendocardial infarct Permanent occlusion of the posterior descending branch of the right coronary artery Small intramural vessel occlusions microinfarcts Anterior typically results in infarction of the anterior wall of the left ventricle, the anterior two thirds of the ventricular septum, and most of the heart apex; more distal occlusion of the same vessel may affect only the apex. Similarly, acute occlusion of the proximal left circumflex (LCX) artery (seen in 15% to 20% of MIs) causes necrosis of the lateral left ventricle, and proximal right coronary artery (RCA) occlusion (30% to 40% of MIs) affects much of the right ventricle. Coronary occlusion in the proximal left anterior descending artery has been dubbed the “widow maker” because so much myocardial territory is perfused by that vessel, and acute obstructions are often fatal.
The posterior third of the septum and the posterior left ventricle are perfused by the posterior descending artery. The posterior descending artery can arise from either the RCA (in 90% of individuals) or the LCX. By convention, the coronary artery—either RCA or LCX—that gives rise to the posterior descending artery and thereby perfuses portions of the inferior/posterior left ventricle and the posterior third of the septum is considered the dominant vessel. Thus, in a right dominant heart, occlusion of the RCA can lead to posterior septal and posterior wall ischemic injury. In comparison, in a left dominant heart, where the posterior descending artery arises from the circumflex artery, occlusion of the LCX generally affects the left lateral wall as well as the posterior third of the septum, and the inferior and posterior wall of the left ventricle.
Occlusions also can occur within secondary branches, such as the diagonal branches of the LAD artery or marginal branches of the LCX artery. Interestingly, atherosclerosis is primarily a disease of epicardial vessels; significant atherosclerosis or thrombosis of penetrating intramyocardial branches of coronary arteries is rare—although these can be affected by vasculitis or vasospasm, and can be occluded by embolization.
Even though the three major coronary arteries are end arteries, these epicardial vessels are interconnected by numerous intercoronary anastomoses (collateral circulation). Although these channels are normally closed, gradual narrowing of one artery allows blood to flow from highto low-pressure areas through the collateral channels. In this manner, gradual collateral dilation can provide adequate perfusion to areas of the myocardium despite occlusion of an epicardial vessel. Based on the size of the involved vessel and the degree of collateral circulation, myocardial infarcts may take one of the following patterns:
Transmural infarctions involve the full thickness of the ventricle and are caused by epicardial vessel occlusion through a combination of chronic atherosclerosis and acute thrombosis; such transmural MIs typically yield ST segment elevations on the electrocardiogram (ECG) and can have negative Q waves with loss of R wave amplitude. These infarcts are also called ST-segment elevated MIs (STEMIs).