Clinical Judgment: Cardiovascular
Transition, shunts, and the ductus
Fetal circulation depends on right-to-left flow through the foramen ovale and ductus arteriosus and on high pulmonary vascular resistance (PVR). At birth, lung inflation and rising oxygen drop PVR, and the ductus constricts over hours to days. Anything that re-raises PVR or relaxes the ductus - hypoxemia, acidosis, hypercarbia, cold stress, sepsis - can send the infant back toward fetal physiology.
Two ductal-dependent patterns
Right-sided obstructive lesions (pulmonary atresia, critical pulmonary stenosis, tetralogy with pulmonary atresia) need a patent ductus to get blood to the lungs: deep cyanosis that does not improve with oxygen. Left-sided obstructive lesions (hypoplastic left heart, critical aortic stenosis, coarctation, interrupted aortic arch) need the ductus for systemic output: a 2- to 10-day-old who turns gray, acidotic, and oliguric with weak or absent femoral pulses as the ductus closes.
Bedside tools
Four-extremity blood pressures; pre- and post-ductal saturation; hyperoxia test (100% oxygen for 10 minutes - a PaO2 above roughly 150 mmHg favors lung disease, below 100 mmHg favors cyanotic heart disease); echocardiogram. Prostaglandin E1 keeps the ductus open but causes apnea, hypotension, and fever. Indomethacin, ibuprofen, or acetaminophen close a hemodynamically significant PDA; all are avoided with renal failure, necrotizing enterocolitis, bleeding, or thrombocytopenia.
Traps
Assuming cyanosis is always pulmonary; trusting a single normal saturation in coarctation; forgetting that a pre/post-ductal difference of about 3% or more suggests ductal-level right-to-left shunting; and labeling ductal-closure shock as sepsis.
Rule of thumb
Blue and unresponsive to oxygen means heart. Shock plus weak femoral pulses in a week-old means left-sided obstructive lesion - start PGE1 and get an echo.