Coronary Arteries, Dominance & Venous Drainage

The coronary circulation is one of the most heavily examined Anatomy–Cardiology overlap areas in NEET PG. Mastery means being able to map an ECG lead group to a coronary artery, name the territory of septum/conduction tissue supplied, and recall venous tributaries draining into the coronary sinus. These notes integrate gross anatomy, dominance, conduction-tissue supply, infarct territories, and bypass-graft anatomy.

Orientation: the two ostia

The heart is supplied by two coronary arteries that arise from the ascending aorta, just above the aortic valve cusps, within the aortic sinuses (sinuses of Valsalva).

• Right coronary artery (RCA) → arises from the anterior (right) aortic sinus.
• Left coronary artery (LCA) → arises from the left posterior aortic sinus.
• The posterior (non-coronary) sinus gives off no coronary artery.

High-yield: Coronary arteries fill during diastole (during systole the cusps and myocardial compression occlude flow). Hence tachycardia (which shortens diastole) compromises coronary perfusion — a recurring physiology link.

The coronary arteries are functional end arteries — anastomoses exist but are usually insufficient to prevent infarction if sudden occlusion occurs.

Left coronary artery (LCA)

The LCA is short (the left main / LMCA, ~1 cm), passing between the pulmonary trunk and left auricle, then dividing into:

1. Anterior interventricular artery (Left Anterior Descending, LAD)

Runs in the anterior interventricular groove to the apex, often winding around it.

Supplies:

• Anterior wall of left ventricle
• Anterior two-thirds of the interventricular septum (IVS)
• Apex of the heart
• Gives diagonal branches (to LV anterolateral wall) and septal perforators.

High-yield: LAD = "widow's maker." Occlusion → anteroseptal/anterior wall MI (leads V1–V4). It supplies the apex and anterior 2/3 of IVS.

2. Circumflex artery (LCx)

Runs in the left atrioventricular (coronary) groove, around the left border to the posterior surface.

Supplies:

• Left atrium
• Posterior + lateral wall of left ventricle
• Gives the left marginal artery (to obtuse/left margin of LV).
• In left-dominant hearts (~15%), it gives the posterior interventricular artery (PDA).

High-yield: LCx occlusion → lateral wall MI (leads I, aVL, V5, V6).

Right coronary artery (RCA)

Arises from the anterior aortic sinus, runs in the right atrioventricular groove between the right atrium and right ventricle, curving to the inferior (diaphragmatic) surface.

Branches and supply:

• SA nodal artery (in ~60% from RCA, ~40% from LCx) — the first branch in most.
• Right marginal artery (along acute/inferior margin of RV).
• AV nodal artery — typically from RCA at the crux of the heart.
• Posterior interventricular artery (Posterior Descending Artery, PDA) in right-dominant hearts (~85%).

Supplies:

• Right atrium and right ventricle
• SA node and AV node (in most people)
• Inferior (diaphragmatic) wall of LV and posterior 1/3 of IVS (via PDA, in right dominance).

High-yield: RCA occlusion → inferior wall MI (leads II, III, aVF) ± bradyarrhythmia/heart block (because RCA supplies SA and AV nodes in most). Inferior MI may extend to RV infarct (clear lung fields + raised JVP + hypotension; ST elevation in V4R).

Coronary dominance

Dominance is defined by which artery gives the posterior interventricular (descending) artery, PDA, and therefore reaches the crux (junction of AV grooves and posterior interventricular groove).

Dominance type	Frequency	PDA arises from	Notes
Right dominant	~85%	RCA	Most common; RCA supplies AV node, posterior IVS
Left dominant	~8–15%	LCx	Entire septum + posterior LV via left system
Co-dominant (balanced)	~5–7%	Both RCA & LCx	PDA from RCA, posterolateral from LCx

High-yield: Dominance is decided by the origin of the PDA, NOT by which artery supplies more myocardium. (Even in "right dominance," the LCA supplies the larger mass of LV myocardium.)

Arterial supply of the conduction system

This is a classic single-best-answer favourite.

Structure	Usual blood supply
SA node	SA nodal artery — RCA in ~60%, LCx in ~40%
AV node	AV nodal artery — RCA in ~80–90% (right dominance), branch at the crux
Bundle of His	AV nodal artery + septal branches of LAD (dual supply)
Right bundle branch & anterior fascicle of LBB	LAD (septal perforators)
Posterior fascicle of LBB	Dual: LAD + PDA (relatively protected)

High-yield: Because both SA and AV nodes are most often RCA-supplied, inferior MI (RCA) is classically associated with sinus bradycardia and AV block. Anteroseptal MI (LAD) affecting the His-bundle/bundle branches tends to cause more sinister infrahisian (wide-complex) blocks with poorer prognosis.

ECG localisation → culprit artery (integrated MCQ core)

Stepwise reading approach:

Identify lead group with ST elevation → assign wall → map to artery → predict complication.

Region / Wall	ECG leads	Likely artery
Septal	V1–V2	LAD (septal branches)
Anterior	V3–V4	LAD
Anteroseptal	V1–V4	LAD
Lateral	I, aVL, V5, V6	LCx (or diagonal of LAD)
Inferior	II, III, aVF	RCA (or LCx if left-dominant)
Posterior	Tall R + ST depression V1–V2 (mirror); confirm V7–V9	RCA / LCx
Right ventricle	V4R ST elevation	Proximal RCA

Flow: ST-elevation leads → wall → artery → conduction risk.

• Inferior (II, III, aVF) → RCA → bradycardia/AV block, possible RV infarct → check V4R; avoid nitrates/morphine (preload-dependent).
• Anteroseptal (V1–V4) → LAD → pump failure, VSD, bundle branch block, LV aneurysm at apex.

High-yield: In inferior MI with ST elevation greater in III than II + ST depression in lead I, the lesion is in the RCA; if ST elevation is greater in II with no reciprocal depression in I, suspect a left-dominant LCx.

Coronary anastomoses & collaterals

Anastomoses (functionally limited) occur between:

• LAD and PDA (around the apex / in the IVS).
• LCx and RCA in the posterior AV groove.
• Branches of LAD and LCx on the anterior LV.

Gradual (chronic) stenosis allows collaterals to enlarge and become protective; sudden occlusion gives no time for collaterals → infarction. Thebesian (venae cordis minimae) veins open directly into the chambers and contribute a small anatomical right-to-left shunt.

Venous drainage of the heart

Three routes:

1. Coronary sinus (largest, ~60% of venous return) → drains into right atrium between the IVC opening and the AV orifice, guarded by the valve of the coronary sinus (Thebesian valve).
2. Anterior cardiac veins → drain RV directly into right atrium (bypass coronary sinus).
3. Venae cordis minimae (Thebesian veins) → drain directly into all four chambers (mainly right).

Coronary sinus & its tributaries

The coronary sinus lies in the posterior atrioventricular (coronary) groove and is the persistent remnant of the left horn of the sinus venosus / left common cardinal vein (duct of Cuvier).

Tributary	Accompanies / runs with	Drains
Great cardiac vein	Anterior interventricular artery (LAD), then LCx	Anterior heart, becomes coronary sinus
Middle cardiac vein	Posterior interventricular artery (PDA)	Posterior/inferior wall, apex
Small cardiac vein	Right marginal artery / runs in right AV groove	Right border
Posterior vein of LV	—	Posterolateral LV
Oblique vein of left atrium (vein of Marshall)	—	Marks the proximal end of the coronary sinus; remnant of left SVC

High-yield: The great cardiac vein begins at the apex with the LAD; the middle cardiac vein accompanies the PDA. The point where the oblique vein of Marshall joins marks the beginning of the coronary sinus (and is the embryological boundary with the great cardiac vein).

Mnemonic — coronary sinus tributaries: "Great Middle Small Posterior Oblique" → "Good Medical Students Pass Out." (Great, Middle, Small cardiac veins, Posterior vein of LV, Oblique vein of Marshall.)

High-yield: A persistent left superior vena cava drains into the right atrium via a dilated coronary sinus — explaining a "dilated coronary sinus" finding on echo. Marshall's vein is its vestige.

Coronary artery bypass graft (CABG) anatomy

Frequently asked as integrated surgery–anatomy.

• Left internal thoracic (mammary) artery, LIMA → grafted to LAD. It is the graft of choice because of the highest long-term patency (>90% at 10 years) and need not be detached proximally (pedicled from subclavian).
• Right internal thoracic artery (RIMA) and radial artery → additional arterial conduits.
• Great saphenous vein → reversed to respect valves; used for RCA/LCx territories; lower long-term patency than arterial grafts.

High-yield: LIMA-to-LAD is the single most important CABG fact — best patency, mortality benefit. Reversed great saphenous vein is the commonest venous conduit.

Clinical correlation snapshot

• Angina pectoris: referred pain via T1–T4 sympathetic afferents → left chest, medial arm, jaw; classic visceral referred pain.
• Coronary steal: dipyridamole/adenosine dilate normal vessels, diverting flow from stenosed territory (used in stress imaging).
• LV aneurysm: complication of large anterior (LAD) MI involving the apex; persistent ST elevation.
• Ventricular septal rupture: more common with LAD (anterior IVS) or PDA (posterior IVS) infarcts.
• Papillary muscle rupture: posteromedial papillary muscle has single blood supply (PDA) → more vulnerable in inferior MI (RCA), causing acute mitral regurgitation.

High-yield: The posteromedial papillary muscle is supplied only by the PDA (single supply) → ruptures in inferior MI. The anterolateral papillary muscle has dual supply (LAD + LCx) → more protected.

Key differentials / look-alikes in MCQs

If the stem says…	Think…
Anterior 2/3 IVS + apex	LAD
Posterior 1/3 IVS + AV node	PDA (RCA in right dominance)
Dilated coronary sinus on echo	Persistent left SVC
Inferior MI + hypotension + clear lungs	RV infarct (proximal RCA), check V4R
Bradycardia/heart block post-MI	RCA territory (SA/AV node)
Best CABG graft	LIMA to LAD
Vein running with PDA	Middle cardiac vein

Recently asked / exam angle

• "Anterior two-thirds of the interventricular septum is supplied by?" → LAD (anterior interventricular artery).
• "Coronary dominance is determined by the artery giving off the posterior interventricular artery" — true/false style.
• "SA node is supplied by which artery in majority?" → RCA (~60%).
• ECG with ST elevation in II, III, aVF → culprit RCA; next step right-sided leads (V4R).
• "Middle cardiac vein accompanies which artery?" → posterior interventricular (PDA).
• "Dilated coronary sinus" → persistent left superior vena cava.
• "Graft with best long-term patency in CABG" → LIMA (left internal mammary).
• Embryological origin of coronary sinus → left horn of sinus venosus.
• Papillary muscle most likely to rupture in inferior MI → posteromedial (single PDA supply).
• The Thebesian valve guards the opening of the coronary sinus into the right atrium.

Rapid revision

1. Two coronary arteries arise from right and left aortic sinuses; the non-coronary (posterior) sinus gives none.
2. Coronaries fill in diastole; tachycardia hurts perfusion.
3. LAD = anterior wall + anterior 2/3 IVS + apex; leads V1–V4; "widow's maker."
4. LCx = lateral/posterior LV; leads I, aVL, V5, V6.
5. RCA = RA, RV, inferior LV, SA & AV nodes, posterior 1/3 IVS (right dominance); leads II, III, aVF.
6. Dominance = origin of the PDA; right-dominant in ~85%.
7. SA node: RCA ~60% / LCx ~40%; AV node: RCA ~80–90%.
8. Inferior MI → bradycardia/AV block ± RV infarct (check V4R, avoid nitrates).
9. Coronary sinus drains ~60% of cardiac venous blood into RA; guarded by Thebesian valve; embryologically from left horn of sinus venosus.
10. Tributaries: Great (with LAD), Middle (with PDA), Small cardiac veins, posterior vein of LV, oblique vein of Marshall (= persistent left SVC remnant; marks CS origin).
11. Dilated coronary sinus → persistent left SVC.
12. LIMA-to-LAD = best CABG graft patency; posteromedial papillary muscle (single PDA supply) ruptures in inferior MI.