Aortopulmonary Transposition in the Fetus: Speculation on Pathophysiology and Therapy

Abstract

ABSTRACT:Fetuses with transposition and abnormalities of the foramen ovale and/or ductus arteriosus detected by ultrasound may develop severe hypoxemia postnatally. Higher than normal oxygen content in the pulmonary artery has been considered to be responsible. Patterns of blood flow in the normal fetus and the fetus with aortopulmonary transposition were reviewed. Well-oxygenated ductus venosus is preferentially directed through the foramen ovale into the left atrium. Normally this produces a higher oxygen content in the ascending aorta. In the fetus with transposition, pulmonary arterial oxygen content is higher. Pulmonary vascular resistance is decreased and the ductus arteriosus constricted. Increased pulmonary venous return to the left atrium tends to close the foramen ovale. Changes are more likely in the last trimester because sensitivity of the pulmonary circulation and ductus arteriosus increases. Severe ductus arteriosus constriction could result in pulmonary arterial hypertension and increased pulmonary arteriolar smooth muscle development. Variability of responses could be related to the proportion of umbilical venous blood passing through the ductus venosus. It is proposed that, in fetuses with evidence of abnormalities of the ductus arteriosus andør the foramen ovale, methods to occlude the ductus venosus be developed to avoid progressive changes. I n 1974, I proposed that, as a result of the patterns of blood flow in the fetus, the pulmonary circulation would be exposed to a higher oxygen saturation in the fetus with aortopulmonary transposition than in the normal fetus. This would induce pulmonary vasodilatation and increase pulmonary blood flow. As a result, the ductus arteriosus may be narrowed, because a greater proportion of the blood ejected by the right ventricle would be diverted into the pulmonary circulation away from the ductus (1). At that time it was not possible to examine the fetus in utero. However, with the advent of ultrasonography, two reports have described the presence of abnormalities of the foramen ovale and/or the ductus arteriosus in some fetuses with aortopulmonary transposition (2,3). The possible role of increased pulmonary arterial oxygen saturation in causing these changes was reiterated (2). Proposed mechanisms responsible for these changes and a possible approach to management are presented. COURSE OF THE CIRCULATIONIn the adult, blood circulates in series. It is ejected by the left ventricle and passes through the systemic circulation to the right atrium. It is then ejected by the right ventricle to the lungs where it is oxygenated and then passes into the left atrium and ventricle. Thus, oxygenated arterial and poorly oxygenated venous blood are well separated on the two sides of the heart.Normal fetus. The pattern of blood flow in the normal fetus is shown Figure 1 (1,4). In the fetus, blood is oxygenated in the placenta and returns to the body through the umbilical veins. Umbilical venous blood may pass through the portal venous system into the hepatic venous sy...

Impaired cerebral development in fetuses with congenital cardiovascular malformations: Is it the result of inadequate glucose supply?

“…Poorly oxygenated venous blood from the superior and inferior vena cava passes preferentially through the tricuspid valve to the right ventricle and is ejected into the pulmonary trunk to be distributed to the lung and through the ductus arteriosus to the descending aorta. In the fetal lamb, the SaO2 is 60-65 percent in carotid arterial and 50-55 percent in descending aortic, blood (21).…”

Lung Circulation

“…As breathing commences, pulmonary blood flow increases, reaching total CO in the human infant (519). During the transition from fetal to neonatal life P PA gradually decreases, and within hours approaches 50% of systemic pressure (158,540). As the systemic vascular resistance and pressure become greater than P PA , the foramen ovale closes.…”

“…The ductus arteriosus begins to close within the first few hours after birth (436), closing completely by 2 days after birth (280). In the normal infant, P PA reaches adult levels within the first 2 weeks of life (158,540). During maturation, the myogenic response is also lost.…”

Prenatal Echocardiographic Assessment of Foramen Ovale Appearance in Fetuses with <smlcap>D</smlcap>-Transposition of the Great Arteries and Impact on Neonatal Outcome

“…We also reviewed the Doppler flow pattern of the pulmonary veins when available, assessing the possible presence and extent of the reversed A-wave [10,19] .…”