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In 1912, James Herrick reported the first description of left main coronary artery (LMCA) occlusion when he described the case of a 55-year-old man who died of cardiogenic shock and was subsequently found on autopsy to have extensive necrosis of the left ventricle associated with thrombotic occlusion of the LMCA superimposed on an area of atherosclerotic narrowing.1 It is now estimated that 4% to 6% of patients undergoing coronary angiography have significant LMCA disease defined as ≥50% stenosis, with more than two thirds noted to have concurrent multivessel disease.2,3 Insofar that interventional cardiologists are usually the first in diagnosing and often determining treatment strategies for LMCA stenoses, it is important to have a thorough understanding of the disease process in order to make informed decisions for this complex disease process.

On angiography, the LMCA is designated as either protected or unprotected. The “protected” LMCA refers to the presence of patent coronary artery bypass graft (CABG) to either the left anterior descending or left circumflex artery or the presence of collateral vessels from the right coronary artery to an occluded major branch of the left coronary artery. This distinction is of particular importance as it directly influences the risk and, therefore, the approach to treatment. When faced with unprotected left main disease, the decision is complex and often is between percutaneous coronary intervention (PCI) or CABG. In this chapter, we will review data and topics concerning LMCA anatomy, assessment and significance of LMCA stenosis, prognosis, treatment, and factors that play a crucial role in successful unprotected left main PCI.


The LMCA originates from the left coronary sinus of Valsalva and gives origin to the left anterior descending coronary artery (LAD), the left circumflex coronary artery (LCx), and in 12% of cases, the ramus intermedius. The LMCA itself is a relatively short arterial segment arising from the aortic root in the area of the aortic valve leaflets and extending leftward for a short distance to its point of bifurcation.4 In adults, it ranges from 1 to 25 mm in length and between 2.0 and 5.5 mm in diameter.5 In the absence of coronary anomalies, the LMCA lies in a relatively fixed anatomic position in relation to the left ventricle and great arteries, bordered anteriorly by the right ventricular outflow tract, inferiorly by the superior margin of the left ventricle, superiorly by the descending portion of the pulmonary artery, and posteriorly by the left atrium.5 Although initially believed to have more of a funnel-shaped structure, more recent sophisticated imaging studies using multidetector computed tomography have shown 3 possible additional shapes: (1) biconcave morphology; (2) tapering morphology; or (3) a combined morphology involving a cone-shaped ostium and a tubular-shaped shaft (Figure 16-1).6

Figure 16-1

Four of the most common shapes of the left main artery: ...

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