Heart Anatomy
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Direct answer
Each heart valve is best heard downstream of its opening, not over its anatomical seat: the mitral valve at the apex beat in the left fifth intercostal space at the midclavicular line, the tricuspid at the lower left sternal edge, the pulmonary in the left second intercostal space and the aortic in the right second intercostal space, because sound travels with blood flow. Two-thirds of the heart lies left of the midline in the middle mediastinum. Around this scaffold FMGE arranges the coronary territories (left anterior interventricular artery to the anterior wall and septum, right coronary artery to the inferior wall and nodes), the pericardial sinuses and the anatomy of pericardiocentesis.
What you must remember
- The apex beat is normally felt in the left fifth intercostal space about 9 cm from the midline, just medial to the midclavicular line; left ventricular hypertrophy displaces it down and out.
- Left ventricular wall thickness is 8 to 12 mm, roughly three times the right ventricle's 3 to 5 mm.
- The right coronary artery usually gives the posterior interventricular branch — right dominance, present in about 80 to 90 per cent — and supplies the right ventricle, the inferior wall, the sinoatrial node in about 60 per cent and the atrioventricular node in about 80 per cent.
- The left anterior interventricular (anterior descending) artery supplies the anterior wall, apex and anterior two-thirds of the septum, and is the artery most often occluded in infarction; the circumflex supplies the lateral wall.
- The conduction system: the sinoatrial node at the upper end of the crista terminalis; the atrioventricular node in the triangle of Koch, bounded by the tendon of Todaro, the septal cusp of the tricuspid valve and the coronary sinus opening.
- The transverse sinus of the serous pericardium lies between the arteries in front and the veins behind — a finger passed here controls the great vessels in cardiac surgery; the oblique sinus is the recess behind the left atrium.
- The mitral valve has two cusps and the largest orifice; the aortic, pulmonary and tricuspid have three.
- Clotted haemopericardium causing tamponade is drained through the left xiphocostal angle, needle directed up and back toward the left shoulder.
A typical case: inferior infarction with a slow pulse
An elderly man develops crushing retrosternal pain; his electrocardiogram shows ST elevation in the inferior leads with a rate of 40 and barely recordable blood pressure. Follow the anatomy to the artery. Inferior leads face the diaphragmatic surface, supplied by the right coronary artery; the bradycardia and dropped beats point to ischaemia of the atrioventricular node, which that artery feeds in about 80 per cent — artery and node agree. Had the stem shown anterior ST elevation with a new right bundle branch block, the septal territory of the left anterior interventricular artery would be implicated instead, with pump failure rather than block as the expected complication. Auscultation follows the same geography: a new pansystolic murmur loudest at the apex radiating to the axilla after infarction suggests mitral regurgitation from papillary muscle dysfunction. Translate every lead, murmur and rhythm disturbance into the coronary artery or valve that owns that surface.
Where students slip
The recurring error is auscultating "over the valve": the pulmonary valve is heard in the left second intercostal space though it sits behind the left third, and the aortic area is the right second space though the valve lies midline — flow carries sound, and aortic regurgitation is often loudest at the left sternal edge. The second error is attributing nodal bradycardia always to the right coronary artery: about 20 per cent of atrioventricular nodes are circumflex supplied, and stems reward the majority figure. Candidates also confuse the transverse and oblique sinuses — the transverse sinus is the one a finger passes through behind the aorta, the basis of aortic clamping in surgery.
Frequently asked questions
Where is the mitral valve best auscultated?
At the apex beat — left fifth intercostal space in the midclavicular line — because sound is carried downstream into the ventricle.
Which artery supplies the atrioventricular node in most people?
The right coronary artery, in about 80 per cent; hence inferior infarction is complicated by heart block.
What is the artery most commonly occluded in acute myocardial infarction?
The left anterior interventricular (anterior descending) artery, supplying the anterior wall and septum.
What are the boundaries of the triangle of Koch?
The tendon of Todaro, the septal cusp of the tricuspid valve and the coronary sinus opening, enclosing the atrioventricular node.
Which pericardial sinus lies between the great arteries and veins?
The transverse sinus, between aorta and pulmonary trunk in front and the pulmonary veins and superior vena cava behind.
How is pericardiocentesis performed anatomically?
Through the left xiphocostal angle, needle directed upward and backward toward the left shoulder, entering the pericardial sac in front of the heart.