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Learn more on d-Transposition of Great Arteries in the post-natal TGA section.
Case of a transposition of great arteries (TGA) with intact ventricular septum. The aortic orifice is anterior right with respect to the pulmonary orifice with parallel course of the great arteries
Fetal Life / Embryology:
In TGA, the initial distribution of combined foetal blood flow generally remains normal, meaning foetal perfusion is preserved, and there's typically no issue with coarctation. However, the critical difference lies in oxygen saturation. In the normal fetus, there is streaming of oxygenated flow from the placenta, the ductus venosus ("Arantius" duct), right atrium towards the left atrium, via the foramen ovale. Oxygenated blood is streamed from the right atrium (RA) to the left atrium (LA) due to a directional flow mechanism within the right atrium, facilitated by its valve system. This system ensures that blood returning from the placenta, which is more oxygenated, is preferentially directed towards the left atrium and subsequently to the left ventricle, the ascending aorta, coronary arteries, and the brain. This selective streaming prevents the complete mixing of oxygenated blood from the placenta with the deoxygenated blood returning from the rest of the fetal body. Conversely, the blood directed towards the right ventricle is less oxygenated. The RA valve system include:
Eustachian Valve: A remnant of fetal circulation located at the junction of the inferior vena cava (IVC) and RA. In fetal life, it helps direct oxygenated blood from the IVC toward the foramen ovale. In some neonates and preterm infants, a prominent Eustachian valve can persist and continue to direct flow toward the PFO.
Thebesian Valve: Located at the coronary sinus ostium. Less involved in streaming toward the PFO but can be part of the RA valve complex.
Chiari Network: A fenestrated, web-like structure in the RA, often near the Eustachian valve. Can influence intracardiac flow patterns and sometimes direct blood toward the PFO. Patent
In transposition, oxygenated blood enters the left atrium from the right atrium via the foramen ovale. The left atrium feeds oxygenated blood into the left ventricle, which is connected to the pulmonary artery. This high oxygen content in the pulmonary arteries leads to some degree of pulmonary vasodilation. Because the PVR are relatively lower than a normal fetus because of this phenomenon, there is a decrease in the net amount of right to left transductal shunt, which can limit the growth of the ductus arteriosus, as well as limit the contribution of flow at the isthmus level to the descending aorta, which may promote a narrowing of the aorta and a coarctation. Blood in the aorta (originating from the right ventricle) is less oxygenated since it is filled preferentially by the flow coming from the SVC and IVC via the tricuspid valve to the RV. This altered oxygenation leads to consequences:
Cerebral impact: Less oxygenated blood reaching the brain can lead to a decrease in brain size and oxygen consumption, and potentially an increase in cerebral blood flow with vasodilation of the middle cerebral artery, risking white matter lesions and neurodevelopmental issues.
Foramen Ovale (FO): Due to increased pulmonary blood flow (due to vasodilation), more relative flow returns via the pulmonary veins to the left atrium, reducing the need for flow through the foramen ovale. This drop in the net amount of flow from the RA to the LA due to increased LA filling can cause the foramen ovale to be smaller than normal (more restrictive).
Ductus Arteriosus (DA): The increased oxygen content in the pulmonary artery can lead to a constriction or restriction of the ductus arteriosus antenatally. Ductal constriction increases blood flow to the pulmonary arteries, raising the pulmonary venous return and left atrial pressure. This elevated left atrial pressure pushes the flap of the foramen ovale in utero, creating a restrictive atrial septum even before birth. The increased pulmonary blood flow in fetal life may lead to pulmonary vascular remodelling. This is also believed to be why these neonates have a higher risk of pulmonary hypertension in the post-natal setting. Those with persistent PH due to arterial remodelling can be quite severe and be associated with a grim prognosis when persistent chronically.
Neonatal Implications: A small foramen ovale and a restricted ductus arteriosus in TGA are major problems at birth. The FO is a crucial shunt for mixing blood between the systemic and pulmonary circulations in TGA, and its restriction can lead to severe cyanosis and mortality.
Conceptually and physiologically, a closure of the ductus venosus antenatally would promote blood mixing before it reaches the atria, thereby equalizing oxygen saturation between the ventricles and preventing FO/DA restriction. However, this is not something to be done clinical practice as it can lead to many other problems (hydrops, fetal ascities, fetal demise, etc.).
Fetal Echo Findings (Checklist) - See Fetal TGA section.
A comprehensive checklist is required to form a full assessment:
Great Arteries: Assess the relationship and relative size of the great arteries. In TGA, the arteries appear parallel rather than crossing. The right ventricle gives rise to the aorta, which is usually positioned more anteriorly and rightward compared with the pulmonary artery.
Outflow Tracts: Assess the health of the semilunar valves, especially the pulmonary valve, as it will become the neo-aortic valve after surgery. Evaluate for left ventricular outflow tract obstruction, which is more common than right ventricular outflow tract obstruction.
Ventricular Septal Defect (VSD): Look for a VSD, present in about 50% of cases. An outlet (membranous) VSD is most common. Malalignment of the conal septum may cause:
Posterior malalignment → subpulmonic obstruction
Anterior malalignment → obstruction beneath the aortic valve, potentially associated with coarctation of the aorta
Ductus Arteriosus: Look for a small ductus arteriosus (less than 3 mm after 36 weeks) or bidirectional flow. This can be associated with restriction fo the foramen ovale or post-natal pulmonary hypertension.
Direction through the ductus: In d-transposition of the great arteries, the pulmonary artery receives more oxygenated blood than in the normal fetus. Indeed, the blood returning from the placenta via the ductus venosus and streaming towards the inter-atrial shunt will enter the left atrium and fills the left ventricle which is connected to the pulmonary artery. This higher oxygen content within the pulmonary artery and ductus promotes increased pulmonary blood flow and smooth muscle constriction at the ductal level. This can lead to premature ductal narrowing. As the ductus becomes more restrictive, pulmonary arterial pressure rises, and there is increased pulmonary blood flow with elevated pulmonary venous return. The elevated pulmonary venous return relative to what a normal fetus would see will raise the pulmonary venous velocities and eventually will increase the relative left atrial pressure. This can decrease the magnitude of the right to left inter atrial shunt (which promotes a less significant growth of the hole as the pregnancy advances) and will also push the foramen ovale flap toward the right atrium, making inter-atrial mixing more restrictive. With pregnancy evolution, the reduction of inter-atrial right to left shunting magnitude relative to fetus body habitus will reduce the left atrial preload and left ventricular filling. This can lead to eventually a reversal of the shunt at the ductal level because there is decreased flow into the pulmonary artery. As such, a bidirectional ductus or left to right ductus is a concern for low pulmonary arterial filling and LV preload - raising the concern for post-natal restriction at the level of the inter-atrial shunt.
IVC and Atrial Septum: Confirm that the IVC connects to the right atrium — this access is required for balloon atrial septostomy (BAS) and important to outline for your interventional cardiologist in the prenatal evaluation (or immediate post-natal assessment to determine access if BAS is necessary). Those with intererupted IVC may not be eligible for an access through the typical routes and strategies using neck vessels will be considered. Assess the size and mobility of the atrial septum.
Predicting Restrictive Atrial Septum (High specificity, low sensitivity):
Identifying a restrictive atrial septum is essential for delivery planning. Even if markers are absent, clinicians must be prepared for balloon septostomy in all cases. Risk factors include: • Atrial Septum Hypermobile flap swinging into both atria Foramen ovale <1/3 of total atrial septal length Bidirectional atrial flow Fixed flap with limited motion (angle <30°) • Ductus Arteriosus Small size (<3 mm after 36 weeks) Bidirectional or restrictive flow (pulsatility index <1.8) • Pulmonary Veins Peak systolic velocity >41 cm/s
Predicting Restrictive Atrial Septum (High specificity, low sensitivity) - Unfortunately there is no great marker to predict whether the patient will require a BAS and teams should always be ready to act urgently. Even if markers are absent, clinicians must be prepared for balloon septostomy in all cases.
Risk factors include:
Atrial Septum Hypermobile flap swinging into both atria Foramen ovale <1/3 of total atrial septal length
Bidirectional atrial flow
Fixed flap with limited motion (angle <30°)
Ductus Arteriosus Small size (<3 mm after 36 weeks)
Bidirectional or restrictive flow (pulsatility index <1.8)
Pulmonary Veins Peak systolic velocity >41 cm/s
Fetal Echocardiography Images
Fetal Echocardiography Images
Sweep showing the RV and LV, as well as the MPA coming off the LV and dividing into the RPA and the LPA.
In this view, one may appreciate the pulmonary valve which is posterior to the aortic valve. The aorta is coming off the anterior right ventricle.
The aorta and the pulmonary artery are parallel.
Ventricular septum should be interrogated in multiple views by colour using a low velocity filter ("Nyquist") to evaluate for the presence of a ventricular septal defect. Here, there is no evidence of a septal defect. A VSD should never fully reassure regarding the need for post-natal septostomy to ensure adequate mixing.
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