Overview

The ductus arteriosus develops from the left 6th pharyngeal arch artery. It runs from the start of the left pulmonary artery to the underside of the aortic arch.

Patency in the fetus. Its wall is rich in smooth muscle. It stays open because fetal PO₂ is low and because prostaglandin E2 (from the placenta and the ductus itself) and prostacyclin relax it.

Closure after birth. Arterial PO₂ rises and prostaglandins fall, because the placenta is gone and the lungs now break prostaglandins down. The smooth muscle constricts, closing the duct functionally within about 12–24 hours in term babies. The inner lining then thickens and the duct scars down (anatomical closure), usually within 2–3 weeks. The fibrous remnant is the ligamentum arteriosum. The left recurrent laryngeal nerve hooks under the arch of the aorta just beside it.

Function

In the fetus, shunts right ventricular output from the pulmonary trunk into the descending aorta, bypassing the lungs.

Clinical significance

Patent ductus arteriosus (PDA)

A PDA is common in premature babies and is associated with congenital rubella and birth at high altitude. Blood flows from the aorta to the pulmonary artery (left to right) throughout the cardiac cycle. This produces a continuous "machine-like" murmur below the left clavicle, bounding pulses and a wide pulse pressure. A large shunt causes heart failure and, eventually, pulmonary hypertension.

If the shunt reverses (Eisenmenger syndrome), deoxygenated blood enters the aorta below the arm arteries. The toes then become blue and clubbed while the fingers stay pink (differential cyanosis).

Pharmacological control

In premature babies the ductus can be closed with drugs that block prostaglandin synthesis (indomethacin or ibuprofen) or with paracetamol. Otherwise it can be closed with a catheter device or by surgical ligation, during which the recurrent laryngeal nerve is at risk. To keep the ductus open in duct-dependent heart lesions, prostaglandin E1 (alprostadil) is infused.

Mnemonics

Open or shut

"Indomethacin closes, PGE keeps the duct open."

Test yourself

In the fetus, the ductus arteriosus connects:

The pulmonary trunk (left pulmonary artery) to the aortic arch
The umbilical vein to the inferior vena cava
The right atrium to the left atrium
The ascending aorta to the superior vena cava

Answer: The pulmonary trunk (left pulmonary artery) to the aortic arch

The ductus arteriosus runs from the start of the left pulmonary artery to the underside of the aortic arch, beyond the left subclavian artery. It carries most right ventricular output past the high-resistance fetal lungs. After birth it becomes the ligamentum arteriosum.