Heart and pericardium

Position, surfaces and borders

The heart lies in the middle mediastinum inside the pericardium, two-thirds to the left of the midline, rotated so that the right side faces forward and the left atrium faces backward.

• Anterior (sternocostal) surface: mostly the right ventricle, with part of the right atrium and a strip of left ventricle. A stab wound just left of the sternum usually enters the right ventricle first.
• Inferior (diaphragmatic) surface: mainly the left ventricle, with part of the right ventricle; lies on the central tendon.
• Base (posterior surface): mostly the left atrium, which lies directly in front of the oesophagus. An enlarged left atrium (mitral stenosis) indents the barium-filled oesophagus, can cause dysphagia and may stretch the left recurrent laryngeal nerve (hoarseness, Ortner syndrome).
• Apex: left ventricle, normally in the left 5th intercostal space, midclavicular line.

Chambers

• Right atrium: the crista terminalis separates the rough musculi pectinati (in front, from the primitive atrium) from the smooth sinus venarum (behind, from the sinus venosus). The SA node sits at the upper end of the crista near the SVC opening. Openings: SVC, IVC, and the coronary sinus between the IVC opening and the tricuspid valve.
• Fossa ovalis: the floor is septum primum; the rim (limbus) is septum secundum. A probe-patent foramen ovale allows paradoxical embolism, and the fossa is the site of the transseptal puncture used to reach the left atrium.
• AV node: in the triangle of Koch, bounded by the coronary sinus opening, the tendon of Todaro and the septal cusp of the tricuspid valve.
• Right ventricle: coarse trabeculae carneae, three papillary muscles and the septomarginal trabecula (moderator band), which carries the right bundle branch to the anterior papillary muscle. Its smooth outflow is the infundibulum (conus arteriosus).
• Left atrium: smooth walled except its auricle, where thrombi form in atrial fibrillation; receives four pulmonary veins. Atrial muscle extends a short way into the pulmonary veins as myocardial sleeves; ectopic firing from these sleeves triggers most paroxysmal AF, so ablation isolates the pulmonary veins, reached by a transseptal route.
• Left ventricle: wall about three times thicker than the right; two papillary muscles (anterolateral, posteromedial); smooth aortic vestibule below the aortic valve.

Valves and papillary muscles

• Atrioventricular valves: tricuspid (three cusps) and mitral (two: anterior and posterior). Cusps are tethered by chordae tendineae to papillary muscles, which contract during systole to stop the cusps everting into the atria. They do not open or close the valves; pressure differences do that.
• Papillary muscle rupture after MI, usually 3–5 days later, causes sudden acute mitral regurgitation with pulmonary oedema. The posteromedial muscle is the usual victim because it has a single supply (posterior interventricular artery); the anterolateral has a dual supply (LAD and circumflex).
• Semilunar valves: aortic and pulmonary, three cusps each. The aortic sinuses above the cusps give the right and left coronary arteries.
• A congenital bicuspid aortic valve (about 1–2% of people) gives an ejection click and an ejection systolic murmur in the right 2nd space, calcifies early (aortic stenosis in middle age), can leak, and is associated with coarctation and aortic dilatation or dissection.
• The fibrous skeleton anchors the valves and insulates atria from ventricles, so the AV bundle is the only normal conducting path.

Surface markings and auscultation

Sounds are heard where blood flow carries them, not over the valve itself.

• The bell, applied lightly, picks up low-pitched sounds: the mid-diastolic rumble of mitral stenosis at the apex with the patient turned onto the left side, and S3 or S4. Pressing the bell firmly stretches the skin into a diaphragm and loses them.
• The diaphragm picks up high-pitched sounds: aortic regurgitation at the left sternal edge with the patient sitting forward in expiration, and most systolic murmurs.
• Secundum ASD (excessive resorption of septum primum or a short septum secundum, at the fossa ovalis) gives wide fixed splitting of S2 and a soft ejection murmur in the pulmonary area from increased pulmonary flow, not from the low-pressure shunt itself.

Conduction system and its blood supply

Impulses travel from the SA node through atrial muscle to the AV node, then the bundle of His (which pierces the fibrous skeleton), the right and left bundle branches (the left splits into anterior and posterior fascicles), and the Purkinje fibres.

So inferior MI (RCA) often causes sinus bradycardia and AV block that is usually temporary, while heart block in anterior MI (LAD) means extensive septal necrosis and a worse outlook.

Coronary arteries and veins

• Dominance is defined by which artery gives the posterior interventricular artery: right in about 70–80%, left (circumflex) in about 10%, co-dominant in the rest. Size of the artery is irrelevant.
• The posterior third of the septum is supplied by the posterior interventricular artery.
• Left ventricular coronary flow occurs mainly in diastole, so tachycardia shortens perfusion time.
• ECG territories: anterior (V1–V4) LAD; lateral (I, aVL, V5–V6) circumflex; inferior (II, III, aVF) RCA.
• CABG: the left internal thoracic artery is grafted to the LAD (best long-term patency); great saphenous vein and radial artery are used for other grafts.
• Veins: most blood drains through the coronary sinus (posterior AV groove) into the right atrium. Tributaries: great cardiac vein (with the LAD, then the circumflex), middle cardiac vein (with the posterior interventricular artery), small cardiac vein. Anterior cardiac veins open directly into the right atrium; the smallest (Thebesian) veins open into every chamber.

Pericardium

• Fibrous pericardium: tough, fused with the central tendon of the diaphragm and the adventitia of the great vessels; it cannot stretch acutely, so a rapid 150–200 mL effusion can cause tamponade (Beck triad: hypotension, raised JVP, muffled heart sounds).
• Serous pericardium: parietal layer lining the fibrous sac and visceral layer (epicardium) on the heart, with 15–50 mL of fluid between.
• Transverse sinus: behind the aorta and pulmonary trunk, in front of the SVC and left atrium; a finger or clamp passed through it controls the great arteries in cardiac surgery.
• Oblique sinus: a blind recess behind the left atrium, bounded by the pulmonary veins and IVC.
• Nerve supply: fibrous and parietal layers by the phrenic nerves (C3–C5), so pericarditic pain is sharp and referred to the shoulder tip; the visceral layer is insensitive. The phrenic nerves run on the pericardium in front of the lung roots.
• Constrictive pericarditis: a thick, often calcified pericardium (in Pakistan most often after tuberculosis) stops diastolic filling. Signs: raised JVP that rises on inspiration (Kussmaul sign), early diastolic pericardial knock, soft heart sounds, ascites out of proportion to ankle oedema.
• Pericardiocentesis: subxiphoid approach, needle at the left xiphocostal angle aimed at the left shoulder, ideally under ultrasound.

Nerve supply of the heart

Sympathetic fibres (T1–T4/5) increase rate and force; the vagus slows the rate. Pain afferents run with sympathetic nerves to T1–T4, so cardiac pain is felt centrally and down the inner left arm (T1–T2 dermatomes).

Remember RCA runs the nodes and the inferior wall; LAD runs the front of the septum and the apex.

Exam trap Dominance depends on the origin of the posterior interventricular artery, not on which artery is bigger.

Exam trap The right atrium forms the right border on X-ray, but the right ventricle forms most of the anterior surface.

How FCPS asks about this

• Chamber entered by a left parasternal stab wound: right ventricle, which forms most of the anterior surface.
• Chamber related to the oesophagus: left atrium; enlargement in mitral stenosis indents the barium swallow.
• Artery supplying the SA and AV nodes: usually the RCA, so inferior MI causes bradycardia and heart block.
• Function of papillary muscles: prevent cusp eversion in systole; posteromedial rupture after MI gives acute mitral regurgitation.
• Structure carrying the right bundle branch: the moderator (septomarginal) band.
• Space used to clamp the aorta and pulmonary trunk: transverse pericardial sinus; oblique sinus lies behind the left atrium.
• Raised JVP rising on inspiration with calcified pericardium after TB: constrictive pericarditis.