Cardiac Anatomy and Physiology
Anatomy that drives the questions
The left main coronary artery divides into the left anterior descending (LAD) and left circumflex (LCx). The LAD supplies the anterior wall, the anterior two-thirds of the septum through septal perforators, and the apex through diagonals. The LCx supplies the lateral wall via obtuse marginals. The right coronary artery (RCA) supplies the right atrium and ventricle, the inferior wall via the posterior descending artery (PDA), and usually the SA node (about 60%) and AV node (about 85-90%). Dominance is defined by which artery gives off the PDA, not by vessel caliber: right dominance is about 85%, left about 8%, codominant about 7%.
Physiology and hemodynamics
Know normal pressures: RA 2-6 mmHg, RV 25/4, PA 25/10 (mean 15), PCWP 8-12, LV 120/8, aorta 120/80. Cardiac output is 4-8 L/min; index 2.5-4.0 L/min/m2. Left ventricular perfusion occurs mainly in diastole, which is why tachycardia and aortic regurgitation worsen ischemia. Valve areas: normal aortic 3-4 cm2 with severe stenosis below 1.0 cm2; normal mitral 4-6 cm2 with severe stenosis below 1.0 cm2.
Traps
- Confusing PA systolic pressure with mean PA pressure.
- Assuming PCWP always equals LVEDP; in mitral stenosis or a stiff ventricle, PCWP underestimates LVEDP.
- Mixing up pulmonary vascular resistance (PVR) with systemic vascular resistance (SVR).
Worked rule
Transpulmonary gradient (TPG) = mean PA - PCWP; PVR in Wood units = TPG / CO. If mean PA is 19, PCWP is 12, and CO is 5 L/min: TPG = 7, PVR = 1.4 WU (about 112 dyn.s.cm-5), which is normal (under 3 WU).