Anatomy, Pathophysiology, and Natural History of Congenital Lesions
Segmental analysis
Describe every heart in sequence: atrial situs, ventricular loop, atrioventricular and ventriculoarterial connections, then associated lesions. This discipline prevents mislabeling complex anatomy (for example, congenitally corrected transposition is atrioventricular and ventriculoarterial discordance, with the morphologic right ventricle supporting the systemic circulation and progressive systemic AV valve regurgitation plus heart block).
Shunt physiology
Shunt direction and magnitude depend on relative resistances and compliance, not defect size alone. Qp/Qs estimates the ratio of pulmonary to systemic flow. Chronic left-to-right shunting raises pulmonary vascular resistance; when resistance exceeds systemic, the shunt reverses (Eisenmenger physiology) with cyanosis and a poor prognosis. Small VSDs often close spontaneously but carry lifelong endocarditis risk; they do not require prophylaxis.
Obstructive and cyanotic lesions
Pressure-loaded ventricles (aortic stenosis, coarctation) hypertrophy and develop diastolic dysfunction, ischemia, and arrhythmia. In tetralogy of Fallot, the dynamic subpulmonary obstruction makes physiology afterload-dependent - falling systemic vascular resistance worsens right-to-left shunting and deepens cyanosis. Transposition of the great arteries creates parallel circulations requiring mixing at the atrial, ventricular, or ductal level.
The Fontan circulation
Passive pulmonary blood flow depends on low pulmonary vascular resistance, unobstructed pathways, and adequate ventricular function. Elevated resistance manifests as protein-losing enteropathy, plastic bronchitis, hepatic fibrosis, and arrhythmia.
Traps
Questions reverse shunt direction by ignoring resistance, or offer pregnancy as safe in Eisenmenger syndrome. Do not describe pulmonary vascular disease as reversible once Eisenmenger physiology is established.
Rule of thumb
Ask two questions of any shunt: what are the relative resistances now, and what will they be in 20 years? The answer predicts direction and complications.