Left Coronary Artery

The left coronary artery (LCA) is one of the two main arteries supplying blood to the myocardium. It originates from the left aortic sinus of the ascending aorta and quickly bifurcates into major branches that supply the majority of the left side of the heart, including the left atrium, most of the left ventricle, and the anterior two-thirds of the interventricular septum. It plays a critical role in systemic circulation and myocardial function.

Origin and Course

The LCA arises from the left posterior aortic sinus (also called the left coronary sinus), just above the left cusp of the aortic valve. It passes between the left auricle and pulmonary trunk before entering the left atrioventricular (coronary) sulcus, where it typically divides into its two principal branches.

Main Trunk (Left Main Coronary Artery)

The initial segment of the LCA is referred to as the left main coronary artery. It is usually short—about 1 to 2 cm in length—before it bifurcates. In some individuals, a trifurcation occurs, producing an additional branch (ramus intermedius).

Major Branches of the LCA

1. Left Anterior Descending Artery (LAD)

• Also known as: Anterior interventricular artery
• Course: Descends in the anterior interventricular sulcus toward the apex of the heart

Supplies:

• Anterior wall of the left ventricle
• Anterior two-thirds of the interventricular septum
• Part of the right ventricle (adjacent to septum)
• Apex of the heart

Important branches:

• Diagonal branches: Supply anterior wall of the left ventricle
• Septal perforators: Penetrate the interventricular septum

2. Circumflex Artery (LCx)

• Course: Follows the left atrioventricular (coronary) sulcus around the left border of the heart toward the posterior aspect

Supplies:

• Left atrium
• Lateral and posterior walls of the left ventricle
• In some individuals, the posterior interventricular septum (in left-dominant circulation)

Important branches:

• Left marginal artery (obtuse marginal): Runs along the left margin of the heart, supplying the lateral wall of the left ventricle
• Posterior interventricular artery (in left dominance): Supplies the posterior septum and diaphragmatic surface of ventricles

Variation in Coronary Dominance

Coronary dominance is based on which artery gives rise to the posterior interventricular artery (PDA):

• Right dominant (≈70%): PDA arises from RCA
• Left dominant (≈10–15%): PDA arises from LCx (a branch of the LCA)
• Co-dominant (≈15–20%): PDA receives contributions from both RCA and LCA

In left-dominant hearts, the LCA supplies a larger portion of the myocardium, increasing its functional and clinical significance.

Areas Supplied by LCA

• Most of the left ventricle (anterior, lateral, posterior walls)
• Left atrium
• Anterior two-thirds of the interventricular septum
• Apex of the heart
• Part of the right ventricle (adjacent to septum)

Topographic Relationships

• Anterior: Left auricle and pulmonary trunk
• Posterior: Left atrioventricular groove (circumflex course)
• Right: Interventricular septum (via LAD)
• Left: Lateral wall of the left ventricle

Clinical Notes (Minimal)

The LAD is often referred to as the "widowmaker" because occlusion here can cause a large anterior myocardial infarction. The LCA supplies critical cardiac structures, so its obstruction can have severe consequences. Coronary angiography is commonly used to assess LCA patency.

Topographic Summary

• Origin: Left aortic sinus
• Main branches: Left anterior descending (LAD), Circumflex (LCx)
• Additional branches: Diagonal, septal, obtuse marginal
• Supplies: Left ventricle, left atrium, anterior septum, part of right ventricle, apex
• Dominance: Left-dominant if PDA arises from LCx