Describe the chambers of the heart, its conducting system and its blood supply. Add a note on the surface marking of the valves.

General Features

• About the size of a closed fist; weighs 250–300 g
• Apex — formed by the left ventricle, in the fifth left intercostal space 9 cm from the midline
• Base — formed mainly by the left atrium, directed backward

Interior of the Chambers

• The moderator band carries the right branch of the AV bundle to the anterior papillary muscle, ensuring it contracts before the rest of the ventricle

Valves and Their Surface Marking

Conducting System

• Triangle of Koch — bounded by the tendon of Todaro, the septal cusp of the tricuspid valve, and the opening of the coronary sinus; the surgical landmark for the AV node
• The AV node delays conduction by about 0.1 second, allowing the atria to empty before the ventricles contract
• Nerve supply — sympathetic (T1–T5) accelerates; vagus slows. The nodes are richly supplied

Blood Supply

• The LAD is the most commonly occluded artery — the "widow-maker"
• Dominance is decided by which artery gives the posterior interventricular branch: right dominant in about 70%
• Venous drainage — the coronary sinus (great, middle and small cardiac veins) opens into the right atrium; also anterior cardiac veins and venae cordis minimae
• Coronary filling occurs in diastole, since systolic compression of the intramural vessels obstructs flow

Fibrous Skeleton and Nerve Supply

• The fibrous skeleton consists of four rings around the valve orifices, joined by the right and left fibrous trigones
• Functions — provides attachment for the valve cusps and the myocardium, keeps the orifices patent, and electrically insulates the atria from the ventricles so that the AV bundle is the only conducting pathway
• Sympathetic supply (T1–T5) — increases rate, force and coronary flow; fibres from the cervical and upper thoracic ganglia
• Parasympathetic (vagus) — decreases rate and force; acts chiefly on the nodes and atria
• They form the superficial and deep cardiac plexuses at the level of the sternal angle
• Pain fibres run with the sympathetic nerves to T1–T5, which is exactly why cardiac pain is referred to the arm and jaw

Applied Aspects

• Myocardial infarction — the coronaries are functional end arteries, so occlusion infarcts the territory. LAD occlusion affects the anterior wall and septum; right coronary occlusion the inferior wall, often with bradycardia because the AV node is involved
• Referred cardiac pain — to the left arm, jaw and epigastrium, because cardiac afferents enter the cord at T1–T5 alongside somatic afferents from those regions
• Atrial septal defect — usually a patent foramen ovale or ostium secundum defect at the fossa ovalis
• Heart block follows damage to the conducting system; the AV node is vulnerable in inferior infarction and in septal surgery
• Pericardiocentesis is performed in the left xiphocostal angle, directed toward the left shoulder, to avoid the pleura and the internal thoracic artery
• Valvular disease — the mitral valve is most often affected by rheumatic fever, still common in India; mitral stenosis gives a mid-diastolic murmur at the apex with the patient in the left lateral position
• Right ventricular hypertrophy follows pulmonary hypertension; left ventricular hypertrophy follows systemic hypertension and aortic stenosis, displacing the apex beat outward and downward
• Cardiac catheterisation reaches the right side through the femoral vein and the left side retrogradely through the femoral or radial artery
• Infective endocarditis affects the mitral and aortic valves most often, and the tricuspid in intravenous drug users
• The apex beat is the lowest and outermost point of definite cardiac pulsation, and its displacement is the simplest bedside evidence of cardiac enlargement