The heart and its coronary arteries

1. Four chambers, and which one faces you

Seen from the front, as on a chest X-ray, most of the heart is right ventricle, lying right behind the sternum. The right atrium forms the right border, the left ventricle the apex and the left border. The left atrium is the most posterior chamber, lying directly on the oesophagus.

Surgeon’s corner: A stab wound beside the sternum hits the right ventricle first. An enlarged left atrium (mitral valve disease, atrial fibrillation) can be seen with a probe in the oesophagus: that is why transoesophageal echo shows it so well.

2. The great vessels and the aortic valve

The pulmonary trunk leaves the right ventricle in front; the aorta leaves the left ventricle behind it and to its right, and the two twist around each other. The aortic valve has three cusps, named after the coronary arteries that arise from the sinuses behind them: right coronary, left coronary and non-coronary.

Surgeon’s corner: Before a TAVI the height of the coronary ostia is measured on CT: a low ostium can be covered by the pushed-aside leaflet. In an aortic dissection the flap can reach the right coronary ostium: inferior ST elevation in a patient with tearing chest pain — never give thrombolysis without thinking of it.

3. The left coronary artery: LAD and circumflex

The left main stem, usually about 1 cm long (0.5–2 cm), runs behind the pulmonary trunk and divides into the left anterior descending (LAD) and the circumflex. The LAD runs in the anterior interventricular groove to the apex: its diagonal branches feed the anterior wall, its septal branches the front two thirds of the septum. The circumflex runs in the left atrioventricular groove and gives obtuse marginal branches to the lateral wall.

Surgeon’s corner: A left main occlusion puts most of the left ventricle at risk: cardiogenic shock or arrest. A critical (subtotal) left main stenosis typically shows ST elevation in aVR with widespread ST depression. The septal branches feed the bundle branches: a new bundle branch block in an anterior infarction suggests a large, proximal LAD occlusion.

4. The right coronary artery and dominance

The right coronary artery leaves the right sinus and runs in the right atrioventricular groove, round the acute margin, to the crux on the back of the heart. On the way it gives the sinus node artery (in about 60%) and acute marginal branches to the right ventricle; at the crux come the posterior descending artery (in the posterior interventricular groove) and the AV node artery, then posterolateral branches. The artery that gives the posterior descending artery is called dominant: the right in about 70–80%, the circumflex in about 10%, both in the rest.

Surgeon’s corner: Inferior infarction with bradycardia or AV block: the RCA feeds the sinus and AV nodes. Usually transient (ischaemia plus a vagal reflex), but keep atropine and pacing ready.

5. Who supplies which wall

Orange, the LAD: anterior wall, front two thirds of the septum and the apex. Green, the circumflex: lateral wall (and the inferior wall too in a left-dominant heart). Blue, the right coronary artery: right ventricle, inferior wall, back third of the septum and the AV node. Turn the heart to see each wall.

Surgeon’s corner: This map is the key to the ECG: each group of leads faces one of these walls, so the leads with ST elevation point to the culprit artery — that is the next lesson.

6. The cardiac veins and the coronary sinus

The veins run back with the arteries: the great cardiac vein up with the LAD and round with the circumflex, the middle cardiac vein with the posterior descending artery. They empty into the coronary sinus, in the posterior atrioventricular groove, which opens into the right atrium next to the tricuspid valve.

Surgeon’s corner: Cardiac resynchronisation therapy: the left ventricular lead is passed from the right atrium into the coronary sinus and out into a lateral or posterolateral vein, to pace the lateral wall. In heart surgery, cardioplegia can be given backwards through the coronary sinus.

7. Inside: valves and papillary muscles

The mitral leaflets are held by chordae on two papillary muscles. The anterolateral one has a double blood supply (LAD diagonals and circumflex); the posteromedial one usually only one, the posterior descending artery. The tricuspid valve has three leaflets; its septal leaflet lies on the septum, close to the AV node and the bundle of His.

Surgeon’s corner: Two to seven days after an inferior infarction, sudden pulmonary oedema, often with a new systolic murmur (which can be soft or absent): think of a ruptured posteromedial papillary muscle (acute mitral regurgitation). Bedside echo, then emergency surgery.