Patent ductus arteriosus — formative SAQs

SAQ 1 (10 marks)

Question. A 4-year-old child is referred with a continuous "machinery" murmur at the upper left sternal edge, bounding pulses and a widened pulse pressure. Outline (a) the underlying anatomy and pathophysiology, (b) the key bedside and echocardiographic features that define the haemodynamic burden, and (c) the differential diagnosis. [1]

(a) Anatomy and pathophysiology. The patent ductus arteriosus is persistent postnatal patency of the fetal vessel connecting the main pulmonary artery to the descending aorta. Normally the duct closes functionally within 12 to 24 hours of birth as oxygen tension rises (closing oxygen-sensitive potassium channels, causing calcium influx and smooth-muscle contraction) and prostaglandin E2 falls, then anatomically over two to three weeks by intimal proliferation to become the ligamentum arteriosum. The congenital PDA represents failure of this closure in a structurally mature duct, and the persistent channel produces a left-to-right shunt once pulmonary vascular resistance falls below systemic resistance, the direction and magnitude of which are set by the PVR:SVR balance and the duct diameter. [1]

(b) Bedside and echocardiographic burden. The bedside signature is the continuous machinery murmur at the upper left sternal edge (it does not respect the cardiac cycle because the aorta-to-pulmonary-artery gradient persists throughout), with bounding collapsing pulses and a widened pulse pressure and low diastolic pressure reflecting diastolic run-off into the pulmonary bed. Echocardiography defines the burden by integrating the duct size and flow pattern, the peak systolic flow velocity (high velocity = restrictive duct with normal pulmonary pressure; low velocity = elevated pulmonary pressure), the left atrial to aortic root ratio (above 1.5 suggesting left-heart volume overload), and left ventricular size and pulmonary pressure estimation. [1] [6]

(c) Differential diagnosis. The continuous-murmur differentials are the coronary arteriovenous fistula, a systemic arteriovenous malformation, an aortopulmonary window and the innocent venous hum (infraclavicular, abolished by head turning or neck-vein pressure). The bounding-pulse differentials are aortic regurgitation and coarctation (bounding upper-limb pulses with weak femorals, which may coexist with a PDA and must be excluded). In the failing infant, a ventricular or atrioventricular septal defect mimics the pulmonary over-circulation and failure to thrive, and the echocardiogram resolves the differential. [1]