Cardiac Anatomy and Physiology: Structure, Coronary Supply, and Hemodynamics
The Cardiac Anatomy and Physiology domain tests the structural and electrical basis for what you see on the monitor: chamber anatomy, valve and blood flow, coronary supply, the conduction system, electrophysiology, and hemodynamic relationships. Questions often ask why a particular occlusion produces a particular rhythm or lead pattern.
Blood flow and valves
Deoxygenated blood enters the right atrium from the superior and inferior venae cavae, passes the tricuspid valve to the right ventricle, and leaves through the pulmonic valve into the pulmonary arteries. Oxygenated blood returns via the pulmonary veins to the left atrium, crosses the mitral valve, and is ejected from the left ventricle through the aortic valve. AV valves (tricuspid, mitral) have chordae tendineae and papillary muscles; semilunar valves (pulmonic, aortic) do not.
Conduction system and rates
The SA node fires at 60–100 bpm, the AV junction at 40–60, and Purkinje fibers at 20–40. Impulses travel from the SA node through internodal pathways and Bachmann's bundle to the atria, then to the AV node, which delays conduction about 0.1 s, then to the bundle of His, bundle branches, and Purkinje network. Sympathetic beta-1 stimulation raises rate and contractility; vagal stimulation slows SA and AV nodal conduction.
Coronary supply
The left main coronary artery divides into the LAD (anterior wall, anterior septum, apex) and circumflex (lateral wall). The RCA supplies the right ventricle, inferior wall, and in most people the AV node (about 85–90%) and SA node (about 60%). In left dominance the circumflex supplies the posterior descending artery.
Cardiac cycle and hemodynamics
Systole ejects blood; diastole fills the ventricles, with atrial contraction contributing a late "kick." Normal EDV is about 120 mL, ESV about 50 mL, stroke volume about 70 mL, and ejection fraction 55–70%. Cardiac output = heart rate × stroke volume, normally 4–8 L/min. Frank-Starling: increased preload stretches fibers and increases force; afterload is the resistance the ventricle must overcome.
Worked example
An inferior myocardial infarction is accompanied by sinus bradycardia and Mobitz I block. The RCA supplies the inferior wall and, in most patients, the AV node and SA node. Ischemia of those nodes explains both the slow sinus rate and the AV conduction delay, and the expected ECG changes are in II, III, and aVF.
Traps
- Assuming the RCA always supplies the SA node; the circumflex does in about 40% of people.
- Confusing pulmonary arteries (deoxygenated) with pulmonary veins (oxygenated).
- Mixing up the tricuspid and mitral valves.
- Reversing preload and afterload.
- Treating the AV node as the heart's primary pacemaker.