Coronary arteries

Origins and flow

• Right coronary artery (RCA) arises from the anterior (right) aortic sinus; left coronary artery from the left posterior aortic sinus. The short left main stem runs behind the pulmonary trunk and divides into the left anterior descending (LAD, anterior interventricular) and the circumflex arteries.
• Left ventricular flow occurs mainly in diastole, because systolic contraction squeezes the intramural vessels shut. Tachycardia shortens diastole and reduces flow; the subendocardium is most vulnerable.
• Regulation: coronary flow is matched to oxygen demand by local metabolic vasodilatation. During exercise, rising myocardial metabolism releases adenosine, nitric oxide, CO2 and H+, and falling oxygen tension dilates the arterioles; this, not sympathetic nerves, is the main reason flow increases several-fold. Oxygen extraction at rest is already near maximal, so extra oxygen can only come from extra flow.

Territories

• Branches of the RCA: the conus artery (usually its first branch, to the infundibulum of the right ventricle; it can join an LAD branch to form a collateral ring, and sometimes arises directly from the aorta, so it may be missed on selective RCA angiography), SA nodal artery, right marginal, AV nodal and, in right-dominant hearts, the posterior interventricular (posterior descending) artery.
• Branches of the left system: diagonals and septal perforators from the LAD; obtuse marginals from the circumflex.
• Dominance is defined by the artery that gives the posterior interventricular artery: right in about 70%, left (circumflex) in about 10%, co-dominant in about 20%. Size is irrelevant.
• Papillary muscles: the posteromedial muscle usually has a single supply (posterior descending artery), so it ruptures more often after inferior MI, causing acute mitral regurgitation; the anterolateral has a dual supply (LAD and circumflex).

Collaterals and occlusion

• Coronary arteries are functional end arteries: anastomoses exist but are tiny. A sudden occlusion therefore causes infarction.
• With slow, progressive narrowing, collateral vessels enlarge over months, so a completely occluded artery may be found with normal wall movement and no infarct. Collaterals rarely carry enough flow for exercise, so angina often persists.
• Most acute coronary occlusions result from rupture or erosion of an atherosclerotic plaque with superimposed thrombosis; diabetes accelerates atherosclerosis.

Cardiac veins

• Bleeding from a vein beside the left coronary artery or circumflex comes from the great cardiac vein.
• The coronary sinus lies in the posterior part of the left atrioventricular groove and opens into the right atrium between the IVC opening and the tricuspid orifice, guarded by the Thebesian valve. It develops from the left horn of the sinus venosus, which the oblique vein joins.
• The coronary sinus returns most of the venous blood of the left ventricle, about 75%; the rest comes back through anterior cardiac and Thebesian veins. Thebesian veins emptying into the left heart add slightly to the physiological shunt.
• Leads for left ventricular pacing (cardiac resynchronisation) are passed through the coronary sinus into a lateral cardiac vein.

Remember The great cardiac vein runs with the LAD, the middle cardiac vein with the posterior descending artery, and anterior cardiac veins bypass the sinus.

Bypass grafts

• Left internal thoracic artery to LAD has the best long-term patency; the radial artery and great saphenous vein are also used.
• A saphenous vein graft is reversed so that its valves do not block arterial flow from the aorta. Reversal places the narrow distal end at the high-flow aortic inflow, creating a size mismatch.
• Before radial harvest, the Allen test confirms ulnar supply to the hand.

Great veins of the thorax

Each brachiocephalic vein is formed behind the sternoclavicular joint by the internal jugular and subclavian veins; the two join behind the right first costal cartilage to form the SVC, which enters the right atrium at the level of the third costal cartilage. The left brachiocephalic vein crosses above the aortic arch and is at risk in tracheostomy in children.

Clinical points

• Angina is carried by sympathetic afferents (T1–T5) and referred to the chest, inner left arm and jaw.
• Inferior MI with right ventricular infarction: hypotension, clear lungs and a raised JVP; avoid nitrates and give fluids.
• Left main stem disease threatens the LAD and circumflex together and is treated by bypass surgery.

Exam trap The AV node is usually supplied by the RCA, so inferior MI causes heart block.

Exam trap Anterior cardiac veins drain directly into the right atrium, not the coronary sinus.

How FCPS asks about this

• Commonest artery occluded in MI: LAD; anterior leads V1–V4.
• Circumflex occlusion: lateral wall, leads I, aVL, V5–V6; the great cardiac vein accompanies it in the AV groove.
• Vein accompanying the anterior interventricular artery: great cardiac vein; middle cardiac vein accompanies the posterior descending.
• Proportion of LV venous blood returned by the coronary sinus: about 75%; the sinus develops from the left horn of the sinus venosus.
• Origin of the conus artery: right coronary artery (sometimes directly from the aorta).
• Complete RCA occlusion with normal inferior wall movement: collaterals formed during slow progression.
• Why a saphenous vein graft is reversed: so its valves do not block arterial flow. Coronary flow rises in exercise through metabolic vasodilatation (adenosine).