The left main coronary artery (left main stem) arises from the left aortic sinus of Valsalva as a short trunk of 5-15 mm before bifurcating into the left anterior descending and left circumflex arteries. It travels between the pulmonary trunk and the left atrial appendage and may trifurcate in 15-20% of individuals, giving rise to an intermediate (ramus intermedius) artery supplying the lateral wall. The left main supplies approximately 75% of the left ventricular myocardium.
Left main coronary artery disease carries the highest mortality risk of any coronary lesion pattern, as stenosis threatens the entire territory of both the LAD and LCx. Unprotected left main disease traditionally required surgical bypass (CABG), but percutaneous coronary intervention with drug-eluting stents is now acceptable in selected patients per the EXCEL and NOBLE trials. Left main dissection during PCI is a catastrophic complication requiring emergency surgery. The left main is also at risk during aortic root surgery, aortic valve replacement, and cardiac transplantation.
Critical stenosis of the left main stem threatens the anterior and lateral left ventricular walls simultaneously, producing severe angina and high sudden cardiac death risk; managed by CABG with LIMA-to-LAD anastomosis or PCI with intravascular imaging guidance in selected anatomically favourable cases.
Type A aortic dissection extending into the left main coronary ostium causes acute anterior ST elevation MI and cardiogenic shock as a catastrophic complication requiring emergency aortic repair with coronary revascularisation.
