Congenital Heart Disease: Anatomy, Physiology, and Management
Physiology first
Congenital lesions are best organized by shunt direction, pulmonary blood flow, and ductal dependence. Left-to-right shunts (VSD, ASD, PDA) raise pulmonary blood flow and produce a murmur from turbulence, not from the size of the defect. A small restrictive VSD has a loud, harsh murmur because the pressure gradient is high; a large unrestrictive VSD may have a softer murmur but causes heart failure, poor growth, and pulmonary hypertension. Right-to-left lesions (TOF, Eisenmenger physiology) cause cyanosis; squatting raises systemic vascular resistance and reduces the right-to-left shunt in TOF. Ductal-dependent lesions include critical coarctation, interrupted aortic arch, critical aortic stenosis, pulmonary atresia, and hypoplastic left heart; prostaglandin E1 keeps the ductus open.
Traps
Do not equate murmur loudness with shunt severity. A fixed, widely split S2 suggests ASD, whereas a hyperdynamic precordium and failure to thrive suggest a large VSD. The hyperoxia test can support a cardiac cause of cyanosis but is not a substitute for echocardiography. In Eisenmenger syndrome, closure of the defect is contraindicated; oxygen may improve saturations but does not reverse fixed pulmonary vascular disease.
Worked example
A 6-week-old has tachypnea, poor feeding, hepatomegaly, and a harsh holosystolic murmur. The murmur is loud, but the infant is in heart failure, so the defect is likely large, not small. Start anticongestive therapy, monitor growth, and obtain echocardiography. Rule of thumb: murmur intensity tracks the pressure gradient; clinical compromise tracks shunt volume and pulmonary vascular resistance.