Coronary Circulation and Cardiac Conduction

Both coronary arteries arise from the aortic sinuses (of Valsalva), immediately distal to the aortic valve, filling predominantly during diastole, when the aortic cusps are closed and not obstructing the ostia, and when myocardial relaxation allows unimpeded flow into the compressed coronary vessels of systole.

Right Coronary Artery (RCA)

• Origin: Right aortic sinus.
• Course: Runs in the right atrioventricular (coronary) groove, between the right atrium and right ventricle, passing around the inferior/acute margin of the heart toward the posterior surface.
• Major branches:
• SA nodal artery: in the majority of individuals (arises from the RCA in ~60% of people, from the circumflex in the remainder).
• Right marginal artery: runs along the inferior (acute) margin of the heart, supplying the right ventricle.
• Posterior interventricular artery (posterior descending artery, PDA): in a right-dominant circulation (the majority, ~70-85% of people), arises from the RCA, running in the posterior interventricular groove.
• AV nodal artery: typically arises near the crux of the heart (where the coronary and posterior interventricular grooves intersect), usually from the RCA in a right-dominant system.
• Territory supplied: Right atrium, most of the right ventricle, the inferior/diaphragmatic surface of the left ventricle (via the PDA, in right-dominant circulations), the SA node (usually) and AV node (usually), and the posterior third of the interventricular septum.

Left Coronary Artery (LCA)

• Origin: Left aortic sinus.
• Course: Short main stem, passing behind the pulmonary trunk before dividing into two major branches.
• Anterior interventricular artery (left anterior descending, LAD):
• Runs in the anterior interventricular groove, toward the apex.
• Supplies the anterior two-thirds of the interventricular septum, most of the anterior wall of the left ventricle, and part of the right ventricle's anterior surface.
• The single most commonly occluded coronary vessel, given the large myocardial territory it supplies, occlusion here is often termed the "widow-maker" for the extensive infarction it can cause.
• Circumflex artery:
• Runs in the left atrioventricular (coronary) groove, around the left (obtuse) margin of the heart toward the posterior surface.
• Gives off the left marginal artery (obtuse marginal), supplying the lateral wall of the left ventricle.
• In a left-dominant circulation (~10-15% of people), continues around to give the posterior interventricular artery instead of the RCA.

Marginal Branches

• Right (acute) marginal artery: from the RCA, along the inferior heart margin.
• Left (obtuse) marginal artery: from the circumflex artery, along the left heart margin.

Posterior Interventricular Artery (PDA)

• Runs in the posterior interventricular groove, supplying the posterior third of the interventricular septum and part of the posterior/inferior walls of both ventricles.
• Its origin (RCA vs circumflex) defines coronary dominance.

• Defined by which artery gives rise to the posterior interventricular (posterior descending) artery and, typically, the AV nodal artery.
• Right-dominant (~70-85% of individuals), PDA arises from the RCA.
• Left-dominant (~10-15%), PDA arises from the circumflex artery.
• Co-dominant (~5-10%), both RCA and circumflex contribute branches to the posterior interventricular territory.
• Clinically relevant to predicting the territory affected by occlusion of a given vessel, and to surgical/interventional planning.

• Coronary arteries are functionally described as end-arteries for practical clinical purposes, meaning that sudden occlusion typically produces infarction of the supplied territory, since the naturally occurring anastomotic connections between branches are generally too small to provide adequate immediate collateral flow.
• However, small anastomotic channels do exist (particularly subendocardially and between branches of the same or different coronary arteries), and can enlarge over time in the setting of chronic, gradual coronary stenosis, providing some protective collateral circulation that is absent in acute, sudden occlusion.

Venous drainage largely parallels the arterial supply, converging predominantly into the coronary sinus.

• Great cardiac vein: Begins near the apex, ascends in the anterior interventricular groove (alongside the LAD), then continues in the left atrioventricular groove to reach the coronary sinus, draining the territory of the LAD and circumflex arteries.
• Middle cardiac vein: Ascends in the posterior interventricular groove (alongside the PDA), draining into the coronary sinus.
• Small cardiac vein: Runs alongside the RCA in the right atrioventricular groove, draining into the coronary sinus (or sometimes directly into the right atrium).
• Coronary sinus: The main venous channel, lying in the posterior part of the left atrioventricular groove, receiving the great, middle, and small cardiac veins (among smaller tributaries); drains directly into the right atrium, via its own ostium (guarded by the thebesian valve).
• Anterior cardiac veins: Small veins draining the anterior right ventricle directly into the right atrium, bypassing the coronary sinus.
• Venae cordis minimae (thebesian veins): Very small veins draining directly into the cardiac chambers themselves (all four chambers, though most numerous in the right atrium/ventricle), represent a minor component of the normal physiological right-to-left "shunt," since some of this blood bypasses the pulmonary circulation entirely.

• SA (sinoatrial) node: Located at the junction of the superior vena cava and right atrium, near the upper end of the crista terminalis, the normal primary pacemaker of the heart, generating the intrinsic rhythm that spreads across the atrial myocardium.
• AV (atrioventricular) node: Located within the triangle of Koch (bounded by the tricuspid valve annulus, the coronary sinus ostium, and the tendon of Todaro), near the base of the interatrial septum, receives the depolarisation wave from the atria, imposing a brief physiological delay (allowing atrial contraction to complete ventricular filling before ventricular contraction begins) before passing the impulse onward.
• AV bundle (bundle of His): Continuation of the AV node, passes through the fibrous skeleton of the heart (the only normal electrical connection between atria and ventricles) at the membranous part of the interventricular septum, then divides into right and left bundle branches.
• Right bundle branch: Descends along the right side of the interventricular septum, continues through the septomarginal trabeculation (moderator band) to reach the anterior papillary muscle of the right ventricle, then distributes as Purkinje fibres.
• Left bundle branch: Descends along the left side of the interventricular septum, typically dividing into anterior and posterior fascicles, distributing more diffusely across the larger left ventricular myocardial mass.
• Purkinje fibres: Fine terminal branches of the conduction system, distributing throughout the ventricular subendocardium, ensuring rapid, near-simultaneous depolarisation of the ventricular myocardium (essential for coordinated, efficient contraction).

• This dual/overlapping blood supply to the AV bundle and bundle branches provides some relative protection compared to the SA and AV nodes, which are more dependent on a single dominant vessel, relevant to predicting which coronary occlusions are more likely to produce significant conduction block.

Coronary artery occlusion:

• Sudden thrombotic occlusion (typically atherosclerotic plaque rupture) of a coronary artery produces ischaemia and, if prolonged, infarction of the myocardial territory it supplies, since coronary arteries function as end-arteries in the acute setting.
• Symptoms and ECG changes correspond anatomically to the territory affected, forming the basis of infarction localisation.

Myocardial infarction territories:

• RCA occlusion proximal to the SA/AV nodal branches can also cause bradyarrhythmias or heart block, given the shared blood supply to the conduction system, a clinically important overlap between "vascular" and "electrical" complications of the same infarct.

Conduction block anatomy:

• AV nodal blocks (1st, 2nd, 3rd degree) most often relate to RCA territory ischaemia/infarction, given the typical AV nodal artery origin from the RCA.
• Bundle branch blocks more often relate to LAD territory disease (septal perforator involvement), given the bundle branches' predominant LAD-derived blood supply.
• Anatomical understanding of this differential blood supply helps predict and interpret the pattern of conduction abnormality associated with a given coronary territory's ischaemia.