Congenital Heart Disease Pathophysiology and Lesion Classification
Classification framework
Congenital heart lesions are usually grouped by physiology: left-to-right shunt lesions (atrial septal defect, ventricular septal defect, patent ductus arteriosus), obstructive lesions (coarctation, aortic stenosis, pulmonary stenosis), cyanotic lesions with decreased pulmonary blood flow (tetralogy of Fallot, pulmonary atresia, tricuspid atresia), cyanotic lesions with increased pulmonary blood flow or mixing (transposition of the great arteries, total anomalous pulmonary venous connection, truncus arteriosus, single ventricle), and complex lesions with ductal-dependent circulation. Shunt magnitude depends on defect size and the relative resistance of the systemic and pulmonary beds. After birth, falling pulmonary vascular resistance increases left-to-right shunting, so a large VSD may cause heart failure at about 4–8 weeks rather than immediately. Long-standing large shunts can raise pulmonary vascular resistance and reverse flow (Eisenmenger physiology).
Ductal dependence
Ductal-dependent systemic circulation includes hypoplastic left heart syndrome, critical aortic stenosis, and severe coarctation; ductal-dependent pulmonary circulation includes pulmonary atresia, critical pulmonary stenosis, and some forms of tetralogy of Fallot. Prostaglandin E1 maintains ductal patency.
Traps
Do not equate cyanosis with a single mechanism. Tetralogy of Fallot causes cyanosis with decreased pulmonary vascularity, while transposition causes cyanosis with increased pulmonary vascularity. A left-to-right shunt can later produce cyanosis if Eisenmenger physiology develops.
Rule of thumb
Cyanosis plus decreased pulmonary vascular markings suggests obstruction to pulmonary blood flow; cyanosis plus increased markings suggests mixing or transposition.
Worked example
A cyanotic newborn with a single loud second heart sound, a normal-sized heart, and increased pulmonary vascular markings most likely has transposition of the great arteries until proven otherwise.