Patent ductus arteriosus, aortopulmonary window, sinus of valsalva fistula, aortoventricular tunnel
Patent ductus arteriosus, aortopulmonary window and sinus of Valsalva fistula share common pathophysiology in that each of these lesions usually results in a continuous steal of blood from the systemic circulation during both systole and diastole. Since coronary blood flow occurs during diastole there is a risk that coronary blood flow will be importantly compromised. If the communication is large it can result in a severe degree of congestive heart failure. Aortoventricular tunnel is a closely related anomaly that shares some of the pathophysiologic features of these anomalies.
PATENT DUCTUS ARTERIOSUS
A patent ductus arteriosus results from a failure of normal transition from the fetal to the postnatal circulation. Usually
ductal closure occurs initially by constriction of smooth muscle within the wall of the ductus.1 This results in contact between the opposing intimal cushions which leads to thrombosis. There is subsequently fibrosis over several weeks and months and the ductus evolves to become the ligamen-tum arteriosum.
During fetal life the patency of the ductus is maintained by both local and circulating prostaglandin. After birth increased pulmonary blood flow metabolizes prostaglandin and absence of the placenta removes an important source of prostaglandin. Subsequently there is a marked decrease in the circulating level of prostaglandin. It is also thought that there is an increased level of circulating vasoconstric-tive substances.2,3 In addition to a fall in the level of prostaglandin the increased partial pressure of oxygen in the blood passing through the ductus is another stimulus to ductal constriction. In term infants ductal closure usually occurs within the first 24 hours after birth. In preterm neonates the immature ductal tissue is much less reactive to oxygen and persistent patency of the ductus is therefore much more likely.
Relationship of the ductus to the recurrent laryngeal nerve
Embryologically the ductus represents persistence of the dis¬tal component of the left sixth aortic arch.4 The embryology of the aortic arch is reviewed in detail in Chapter 29. It is important to recall that the left sixth aortic arch originates in the neck (branchial = gill). Therefore the left recurrent laryn¬geal nerve is carried down into the thoracic cavity as the heart and proximal great vessels migrate from a more cervical to a thoracic position. On the right side there is usually resorp-tion of the right sixth aortic arch as well as the right fifth aor¬tic arch. Thus the right recurrent laryngeal nerve comes to pass around the remnant of the fourth aortic arch which per¬sists as the right subclavian artery.
Atypical ductal anatomy
When there is a right sided aortic arch because of persistence of the right sided embryological arches rather than left sided arches the ductus usually arises from the proximal descend¬ing aorta in conjunction with the left subclavian artery.5 The dilated structure which represents the origin of both the left subclavian artery as well as the ductus is often referred to as the diverticulum of Kommerell. This diverticulum lies posterior to the esophagus. The ductus subsequently passes anteriorly to join the origin of the left pulmonary artery thereby completing a vascular ring. The ductus does not always arise from the diverticulum of Kommerell in cases of right sided aortic arch. When there is mirror image branching, i.e.