5.1 Coronary Anatomy & Dominance

Every coronary angiogram you assist in the cath lab begins with the same mental map: which vessel feeds which territory, and which artery defines dominance. For the RCIS exam—and for safe daily practice—you must instantly recognize the left main (LM) bifurcation, trace the LAD and circumflex (LCx), follow the right coronary artery (RCA) around the AV groove, and identify where the posterior descending artery (PDA) originates. Dominance is not an academic label; it tells you which vessel supplies the inferior wall, the posterior septum, and—during right-dominant circulation—the AV node.

Left Main and Proximal Coronary Tree

The left main coronary artery (LM) arises from the left sinus of Valsalva and typically measures 1–2 cm before dividing. In most patients it bifurcates into:

• Left anterior descending artery (LAD) — courses in the anterior interventricular (AV) groove toward the apex
• Left circumflex artery (LCx) — wraps in the left AV groove toward the crux

Approximately 30–35% of patients have a ramus intermedius branch arising from the LM between the LAD and LCx; it supplies a portion of the lateral wall similar to a large obtuse marginal.

Left main stenosis ≥50% is considered hemodynamically significant. Because the LM feeds roughly two-thirds of left ventricular mass, significant LM disease carries high-risk implications for both medical management and revascularization planning.

Major Vessel Territories

Left Anterior Descending (LAD)

The LAD is the largest coronary artery in most hearts. It supplies:

• Anterior left ventricular free wall
• Anterior two-thirds of the interventricular septum (via septal perforators)
• Apex in most patients
• Anterolateral wall through diagonal branches (D1, D2, etc.)

Proximal LAD occlusion produces anterior STEMI patterns and can threaten large myocardial territory. Septal perforators arising from the proximal LAD are targets during septal ablation for hypertrophic cardiomyopathy.

Left Circumflex (LCx)

The LCx travels in the left AV groove and gives rise to obtuse marginal (OM) branches that supply the lateral wall. In left-dominant hearts, the LCx continues to the crux and gives rise to the PDA and posterolateral branches—supplying the inferior wall and posterior septum.

Right Coronary Artery (RCA)

The RCA arises from the right sinus of Valsalva, travels in the right AV groove, and supplies:

• Right ventricle (via acute marginal branches)
• Inferior LV (through PDA and posterolateral branches in right-dominant hearts)
• SA node in approximately 60% of patients (right-dominant)
• AV node in approximately 90% of patients when circulation is right-dominant

The conus branch is often the first RCA branch; it may arise separately from the right sinus (anatomic variant relevant during aortic root procedures).

Coronary Dominance

Dominance is defined by the origin of the posterior descending artery (PDA), which runs in the posterior interventricular groove.

In right-dominant circulation—the most common pattern—the RCA crosses the crux, gives off the PDA, and continues as posterolateral branches. Inferior STEMI from proximal RCA occlusion reflects this anatomy.

In left-dominant circulation, the LCx is large, wraps to the crux, and gives rise to the PDA. The RCA may appear small and terminate before the crux. Inferior ischemia may localize to LCx/PDA territory rather than RCA.

In codominant circulation, both the RCA and LCx contribute to posterior circulation. Angiographers must carefully define which vessel supplies the dominant PDA segment before PCI.

Clinical and Exam Relevance

Dominance affects:

• ECG localization — inferior leads (II, III, aVF) reflect RCA territory in right-dominant hearts but may reflect LCx/PDA territory in left-dominant hearts
• Conduction complications — AV nodal ischemia is more likely with RCA disease in right-dominant hearts
• PCI planning — guide catheter selection and backup support depend on vessel size and takeoff angles
• Graft planning — CABG targets depend on which vessel supplies the PDA and posterolaterals

Anatomic Variants Worth Knowing

RCIS Lab Application

Before contrast injection, confirm:

1. LM patency and bifurcation anatomy
2. LAD size, diagonal branches, and septal supply
3. LCx size and OM branches
4. RCA course, PDA origin, and dominance classification
5. Collateral filling when chronic occlusion is present

Document dominance in the procedure report—surgeons, EP specialists, and interventionalists all rely on this descriptor.

Conduction System Blood Supply (High-Yield)

The RCIS exam frequently links coronary anatomy to arrhythmia complications during ischemia:

In left-dominant hearts, the LCx may contribute more posterior supply, but the SA node still arises from the RCA in most patients. Always correlate ECG rhythm changes during RCA injection or inferior ischemia with this anatomic framework.