Introduction
The arterial duct originates from the sixth aortic arch and in fetal life allows the majority of fetal right ventricular output to be diverted away from the lungs to the lower body and placenta. The arterial duct is structurally composed of a thicker intima than adjacent vessels, prominent media with spirally arranged smooth muscle and an adventitia. During normal fetal development, the arterial ductal smooth muscle becomes increasingly sensitive to arterial oxygen tension such that the abrupt increase at birth causes constriction of the ductal smooth muscle. This effect is blunted by circulating prostaglandins, predominantly derived from the placenta. Following delivery, the level of placentally derived prostaglandin and prostacyclin falls precipitously, and metabolism by the lung increases. This renders the ductal smooth muscle more sensitive to oxygen, initiating a vasoconstriction response. Subsequent necrosis of the inner wall and fibrosis ultimately lead to complete obliteration of the duct. Ductal closure is complete within two weeks of birth in two-thirds of term patients and nearly all by one year of age. Persistent patency of the duct in a full-term infant is defined as patency beyond three months of age. This anomaly makes up 12 to 15 per cent of congenital heart defects and up to 30 per cent of defects found in premature infants. Eighty per cent of premature infants weighing less than 1,200 g will present with this condition.
Clinical Presentation
With increasing prematurity and lower birth weight, the immaturity of the ductal vasoconstrictor response results in an inability of the arterial duct to occlude post-natally. As the pulmonary vascular resistance falls, increasing left-to-right shunt will occur. The magnitude of the shunt will depend predominantly on the size of the patent ductus arteriosus (PDA), and the resultant runoff causes a low diastolic pressure and even reversal of flow in the descending aorta during diastole. This can cause reduced end-organ perfusion and an increased incidence of necrotizing enterocolitis, broncho-pulmonary dysplasia and cerebral bleeding complications. In older infants, the increased pulmonary blood flow can be entirely asymptomatic and is diagnosed after hearing a murmur or when it causes failure to thrive and recurrent chest infections. Large PDAs place a significant over-circulation and result in congestive cardiac failure and pulmonary hypertension if untreated.
In duct-dependent congenital cardiac lesions, part (or all) of the systemic circulation depends on the patency of the arterial duct for supply. Examples include aortic interruption and infantile coarctation of the aorta and various degrees of left heart hypoplasia. Similarly, any condition with pulmonary atresia or critical pulmonary stenosis depends on ductal patency to provide adequate pulmonary blood flow. In these conditions, maintaining or re-establishing ductal patency through infusion of synthetic prostaglandin E2 is lifesaving and allows for resuscitation and diagnostic workup before urgent repair is undertaken. In cyanotic heart conditions, prostaglandin infusion can also increase mixing at the ductal level to allow adequate saturations to be maintained until surgical repair is undertaken.
Clinical Findings and Management
Large PDAs will cause signs of congestive heart failure in neonates and infants with lung congestion, tachypnoea, poor feeding and enlarged liver edge. There is a ‘machinery murmur’ in both systole and diastole, heard loudest in the second intercostal space and radiating over the back. Pulses are usually bounding with wide pulse pressure. CXR shows enlarged heart with plethoric lungs, and the ECG may show some left ventricular hyperplasia (LVH) in older infants and children. Diagnosis is confirmed on echocardiography (Figure 5.1).
Despite the frequent occurrence of patent arterial duct and the seemingly simple nature of the defect, considerable controversy still exists regarding its management. Prior to widespread antibiotic therapy and surgical closure, infective endarteritis, ductal aneurysms and pulmonary vascular disease due to long-term left-to-right shunt were significant problems but are now seen rarely. No adequate randomized, placebo-controlled trial has been undertaken, and therefore, the timing and indications for closure of a PDA remain under debate. As safe options for trans-catheter occlusion of PDAs have become available, the vast majority of infants and children over 4 kg in weight will be treated by the percutaneous route, and surgical ligation in this group has become rare.
The majority of patients referred for PDA ligation in current practice are therefore very small, premature neonates, typically smaller than 1.5 kg. A trial of expectant management is indicated in all premature neonates without lung disease of prematurity (usually >1.5 kg in weight), as the majority of ducts will close spontaneously. If the duct is large and persistent and the neonate symptomatic (failing attempts to wean from ventilation) with signs of left heart volume overload on echocardiogram, most centres would advocate a course of medical management in the first instance. In borderline cases, the traditional echocardiographic criterion of a significant PDA was that the diameter of the left atrium on a long-axis parasternal view was more than 1.5 times the diameter of the ascending aorta – indicating a large volume load on the left heart. However, the indication is usually more subjective, guided by a history of a premature neonate failing to wean from ventilation with clear evidence of a moderate or large PDA and pulmonary congestion on CXR.
Traditionally, this has comprised three doses of indomethacin, but recent randomized trials have demonstrated ibuprofen to have a lower incidence of intra-cerebral haemorrhage and renal dysfunction with similar efficacy. Some more recent randomized trials have found similar efficacy with paracetamol but with a lower incidence of renal impairment. If medical management has failed or is contra-indicated (such as the presence of an intra-cerebral bleed or necrotizing enterocolitis) and a haemodynamically significant PDA with left heart volume overload persists, PDA closure is indicated. Percutaneous device occlusion has been successful in children down to 1.2 kg in weight and can be undertaken through a 4F sheath inserted via the femoral artery in the majority of ductal anatomies (Figure 5.2). The most common complications reported are arterial injury and device embolization. In the presence of a very large duct, where device embolization poses a significant risk, and in very small babies, surgical duct ligation remains the therapeutic option of choice.
