machinery murmur

**Definition:** A continuous murmur (begins in systole, peaks near S2, continues through diastole) with a harsh, churning, "machinery-like" quality. Also called a Gibson murmur.

**Classic cause: Patent ductus arteriosus (PDA)** - The ductus arteriosus connects the proximal descending aorta to the left pulmonary artery. It normally closes functionally within hours to days after birth (increased O2, decreased prostaglandin E2) and becomes the ligamentum arteriosum. - When it persists, blood shunts left-to-right (aorta to pulmonary artery) because aortic pressure exceeds pulmonary pressure in both systole and diastole. This produces a continuous murmur. - Best heard at the left infraclavicular region or left upper sternal border.

**Associated findings in PDA:** - Wide pulse pressure with bounding pulses (diastolic runoff into the pulmonary circulation) - Left atrial and left ventricular volume overload and enlargement, which can progress to heart failure - Pulmonary overcirculation, which can lead to pulmonary hypertension - Eisenmenger syndrome: pulmonary hypertension reverses the shunt (right-to-left), causing differential cyanosis (the lower extremities are cyanotic, the upper extremities are spared). The murmur disappears as the pressure gradient equalizes.

**Risk factors:** - Congenital rubella (classic) - Prematurity (the ductus is less responsive to O2 and more sensitive to prostaglandins) - Genetic syndromes, such as Down syndrome - High altitude

**Management:** - Indomethacin or ibuprofen (NSAIDs) inhibit prostaglandin synthesis and close a PDA. Acetaminophen is an alternative. - Prostaglandin E1 (alprostadil) keeps the ductus open. Use it in ductal-dependent lesions such as transposition of the great arteries, pulmonary atresia, and coarctation. - Surgical or transcatheter closure if medical therapy fails or the PDA is large.

**Other causes of continuous murmurs (differential):** - Arteriovenous fistula (including dialysis access) - Ruptured sinus of Valsalva aneurysm - Coronary artery fistula - Venous hum (benign, in children; disappears when supine or with jugular venous compression) - Aortopulmonary window - Mammary souffle (pregnancy and lactation)

**High-yield pearls:** - Do not confuse the continuous murmur with a to-and-fro murmur. Continuous murmurs span S2 without a break. A to-and-fro murmur (such as aortic stenosis with regurgitation) has separate systolic and diastolic components divided by S2. - Maneuvers: the murmur is not markedly changed by respiration, and a venous hum is the benign mimic. - Boards association: "machinery murmur" = PDA, until proven otherwise.