Case description

The case below describes a term newborn admitted to the NICU following prenatal concern for a possible left-sided obstructive lesion. Antenatal imaging had demonstrated right ventricular predominance associated with a relatively small transverse aortic arch, raising concern for possible coarctation of the aorta or another left-sided obstructive lesion. Despite these prenatal findings, the infant was born vigorous and remained clinically well, with normal oxygen saturations, palpable femoral pulses, preserved systemic perfusion, and no evidence of cardiovascular compromise. Because of the antenatal concerns, an early postnatal echocardiogram was obtained. The initial study demonstrated markedly elevated right ventricular pressures, estimated to be suprasystemic, together with a large patent ductus arteriosus exhibiting bidirectional to predominantly right-to-left shunting, occasionally becoming entirely right-to-left. The right ventricle was mildly dominant, while the left ventricle appeared relatively underfilled but structurally normal. The aortic arch appeared mildly hypoplastic (or underfilled), although antegrade flow was maintained throughout and there was no evidence of discrete coarctation or ductal-dependent systemic circulation. The configuration outlined a "bovine arch" - a common human anatomical variant with a common origin of the brachiocephalic trunk and left common carotid artery; a misnomer as bovines do not have this type of arch configuration (See Reference: "Although the term “bovine arch” evokes resemblance with the aortic arch branching configuration found in cattle, the “true bovine arch” found in cattle bears no resemblance to any of the common aortic arch variations found in humans. Paradoxically, the bovine arch is considered a norm in rodents and beasts of prey, and it was its angiographic appearance from which its name was derived"). 

Despite the striking echocardiographic findings, the infant remained asymptomatic with stable vital signs, normal lactate concentrations, reassuring end-organ perfusion, and no metabolic acidosis. The discrepancy between the echocardiographic severity and the reassuring clinical presentation prompted a conservative management strategy with close surveillance. From a hemodynamic perspective, the findings were interpreted as representing an exaggerated form of transitional physiology rather than structural cardiovascular disease. In fetal life, relative left ventricular stiffness or impaired left ventricular filling (possibly by a "restrictive" foramen ovale) may reduce left atrial filling and left ventricular preload, which may diminish antegrade flow through the aortic arch. This can result in an apparently small transverse arch and right ventricular predominance (by preferential right ventricular preloading) despite the absence of true left-sided obstruction. Indeed, a relatively reduced magnitude of right-to-left shunting across the foramen ovale may contribute to the exaggerated right ventricular dominance observed relative to left ventricular dimensions. Reduced interatrial shunting results in greater blood retention within the right heart, increasing right ventricular preload and promoting greater antegrade pulmonary blood flow. This physiology may have several downstream consequences. Increased fetal pulmonary blood flow exposes the pulmonary circulation to higher shear stress and potentially to a modest increase in oxygen content relative to normal fetal physiology. Although speculative, these changes could contribute to subtle pulmonary vascular remodeling or delayed pulmonary vascular relaxation after birth. Concurrently, the reduction in left ventricular preload decreases left-sided output across the aortic arch, contributing to relative hypoplasia (or appearance of due to underfilling) of the transverse arch without fixed obstruction. Thus, the prenatal findings of right ventricular predominance and a small aortic arch may not necessarily reflect structural disease but rather altered fetal and transitional flow dynamics.

After birth, this physiology can manifest as elevated pulmonary vascular resistance, high right ventricular pressure, and right-to-left ductal shunting despite preserved clinical status. Importantly, this pattern may closely mimic more concerning conditions such as persistent pulmonary hypertension of the newborn or ductal-dependent systemic circulation. Careful integration of the echocardiographic findings with the clinical examination is therefore essential. In this case, the absence of systemic hypoperfusion, the presence of palpable femoral pulses, good urine output and the lack of metabolic compromise supported a strategy of watchful waiting. Management was therefore focused on supporting the transition rather than aggressively lowering pulmonary vascular resistance. The infant was monitored closely with serial targeted neonatal echocardiography, pre- and post-ductal oxygen saturations, blood pressure trends, and clinical markers of systemic perfusion. Low-flow supplemental oxygen (0.25 L/min) was initiated to provide gentle pulmonary vasodilation (since oxygen is a pulmonary vasodilator) and facilitate gradual pulmonary vascular relaxation while minimizing perturbation of the systemic circulation. No pulmonary vasodilators, mechanical ventilation, or invasive interventions were initiated.

Over the ensuing days, serial echocardiograms demonstrated progressive improvement in the transitional physiology. Right ventricular pressures steadily decreased, ductal shunting evolved from predominantly right-to-left to bidirectional and eventually left-to-right, and right ventricular size normalized. The infant remained clinically stable throughout this period, further supporting the interpretation that the initial findings represented delayed cardiopulmonary transition rather than fixed pathology. Interestingly, intermediate echocardiographic studies suggested persistent abnormalities of left ventricular diastolic properties. Features including mild left atrial enlargement, elevated left ventricular filling indices, pulmonary venous Doppler patterns suggestive of increased left atrial pressure, and moderate mitral regurgitation were observed. These findings support the possibility that relative left ventricular stiffness or impaired left ventricular compliance may represent the upstream driver of the fetal flow alterations, explaining both the right ventricular predominance and the reduced antegrade aortic arch flow observed prenatally.

Physiology hypothesis

This case raises the intriguing possibility that a subset of infants with prenatal right ventricular predominance and suspected aortic arch hypoplasia may share a common physiologic substrate characterized by altered fetal atrial shunting and relative heightened left ventricular stiffness. One proposed mechanism involves a relatively small or less permissive foramen ovale, or alternatively a relatively stiff left ventricle with elevated filling pressures, which limits right-to-left interatrial shunting during fetal life. Reduced atrial shunting would increase right ventricular preload and augment pulmonary blood flow while simultaneously reducing left ventricular preload and left-sided output. The resulting flow imbalance could explain the combination of fetal right ventricular predominance and mild aortic arch hypoplasia (either by limited growth or underfilling) observed antenatally. Increased pulmonary blood flow during fetal life may also expose the pulmonary vasculature to greater shear stress and relatively higher oxygen concentrations, potentially promoting subtle pulmonary vascular remodeling or delayed pulmonary vascular relaxation after birth. Such infants may therefore present during the neonatal transition with elevated pulmonary vascular resistance, higher than expected right ventricular pressures, and even right-to-left ductal shunting despite otherwise reassuring clinical status. A right-to-left ductal shunt should always prompt careful evaluation of whether this shunting pattern is necessary to support systemic blood flow. Indeed, obstruction of the aortic arch or aortic isthmus proximal to the ductus arteriosus may also present with right-to-left ductal shunting, as the duct becomes an essential source of perfusion to the descending aorta and lower body. Distinguishing between these scenarios is therefore critical and requires integration of echocardiographic findings by pediatric cardiology expertise with the clinical examination, including assessment of femoral pulses, upper and lower extremity blood pressures, markers of systemic perfusion, and serial imaging of the aortic arch as the ductus constricts. Importantly, it is not uncommon for infants referred prenatally with suspected coarctation of the aorta or transverse arch hypoplasia to demonstrate relatively normal arch anatomy after birth, without evidence of fixed obstruction. Rather than structural disease, some of these infants exhibit delayed cardiopulmonary transition characterized by persistently elevated pulmonary vascular resistance or even frank persistent pulmonary hypertension of the newborn (PPHN), resulting in elevated right ventricular pressures and right-to-left ductal shunting. In such cases, the ductal shunt does not serve to maintain systemic perfusion but instead reflects the elevated afterload faced by the right ventricle during the transitional period. The challenge lies in recognizing that both pathophysiologic states may initially produce remarkably similar echocardiographic findings. Serial assessments are therefore often necessary, as the natural evolution of the physiology can be highly informative. Progressive ductal constriction accompanied by normalization of right ventricular pressures, conversion to left-to-right ductal shunting, and demonstration of unobstructed antegrade flow through the aortic arch strongly support a diagnosis of exaggerated or delayed transitional physiology rather than ductal-dependent left-sided obstructive disease.

An important implication of this physiology is that therapies aimed at lowering pulmonary vascular resistance should not be initiated reflexively in the presence of right-to-left ductal shunting. A right-to-left PDA is not synonymous with pulmonary vascular disease; it may instead represent a necessary pathway for systemic perfusion in the setting of aortic arch obstruction or other ductal-dependent left-sided lesions. In such cases, the ductus arteriosus serves as the primary source of blood flow to the descending aorta (and sometimes of the ascending aorta and coronaries if retrograde flow from the duct to the ascending arch) and lower body. Introducing supplemental oxygen, inhaled nitric oxide, or other pulmonary vasodilators without first excluding a ductal-dependent systemic circulation may be detrimental. By lowering pulmonary vascular resistance, these interventions may preferentially increase pulmonary blood flow and reduce the pressure gradient driving right-to-left ductal shunting, thereby decreasing systemic blood flow to the lower body and potentially precipitating systemic hypoperfusion or cardiovascular collapse. For this reason, the interpretation of right-to-left ductal shunting requires careful assessment of the entire cardiovascular physiology. The direction of ductal flow must always be interpreted in conjunction with a detailed evaluation of aortic arch anatomy, ventricular size and function, upper and lower extremity blood pressures and oxygen saturations, femoral pulses, and markers of systemic perfusion. Serial echocardiographic examinations are often necessary, particularly during the first days of life when the pulmonary circulation and ductus arteriosus are rapidly changing. In the present case, the reassuring clinical examination, preserved systemic perfusion, absence of arch obstruction on serial studies, and eventual spontaneous normalization of right ventricular pressures and ductal flow direction all supported the diagnosis of exaggerated transitional physiology rather than ductal-dependent systemic circulation. Nevertheless, the case highlights the importance of multidisciplinary management and close collaboration between neonatology, targeted neonatal echocardiography specialists, and pediatric cardiology. The distinction between delayed transition with elevated pulmonary vascular resistance and a ductal-dependent left-sided obstructive lesion can be subtle, yet it carries fundamentally different therapeutic implications. Expertise in neonatal and pediatric cardiovascular physiology is therefore essential to avoid both undertreatment of true obstructive lesions and overtreatment of transitional physiology. Recognition of an exagerated transitional physiology is important because it argues for cautious interpretation of early echocardiographic findings and supports a conservative management strategy in clinically stable infants. Future studies examining fetal atrial shunting patterns, pulmonary blood flow, left ventricular diastolic properties, and postnatal pulmonary vascular adaptation may help determine whether this represents a distinct transitional phenotype among infants referred prenatally for suspected coarctation or left-sided obstructive lesions.

• EDA Ratio: The RV/LV end-diastolic area (EDA) ratio has been studied as a predictor of fetal CoA, with two key studies offering somewhat different conclusions depending on how the ratio is applied. 

  • Amar et al. (2024) — EDA ratio as the most sensitive overall predictor In this retrospective case-control study of 75 neonates with prenatal suspicion of CoA (16 confirmed, 59 false-positive), the RV/LV EDA ratio was the most sensitive predictor in the overall cohort, identifying all cases of true CoA (100% sensitivity) - see attached article. Key findings: An RV/LV EDA ratio cutoff > 1.24 had 100% sensitivity and 69.5% specificity (AUC = 0.88). Before 30 weeks' gestational age, the EDA ratio performed exceptionally well (AUC = 0.92), alongside the RV/LV end-diastolic width ratio (AUC = 0.96). After 30 weeks, the RV/LV end-diastolic width ratio (AUC = 0.95), ascending aorta Z-score (AUC = 0.93), and LV end-diastolic width Z-score (AUC = 0.91) outperformed the EDA ratio. The study used the first available fetal echocardiogram rather than the last exam before delivery, which is an important methodological distinction.

  • DeVore et al. (2021) — In contrast, DeVore et al. studied 108 fetuses (54 true CoA, 54 false-positive) using speckle-tracking analysis of the last examination prior to delivery. They found that RV/LV area disproportion > 90th centile was present in 80% of true CoA fetuses but also in 76% of false-positive fetuses — a non-significant difference. This suggests that the EDA ratio alone cannot reliably distinguish true CoA from false-positive cases when both groups have already been selected based on ventricular disproportion. However, when the EDA ratio was incorporated into a multivariate speckle-tracking model combining 28 variables (epicardial size, ventricular shape, sphericity indices, and contractility parameters), the model achieved an AUC of 0.98 with 96% sensitivity and 96% specificity. Reconciling the findings: The discrepancy likely reflects differences in study design. Amar et al. used the first fetal echo (often earlier in gestation) and compared confirmed CoA to all false-positives, finding the EDA ratio valuable as a screening tool with high sensitivity. DeVore et al. used the last echo before delivery in a cohort already pre-selected for ventricular disproportion, where the EDA ratio alone had limited discriminatory value — but adding shape and functional parameters via speckle tracking dramatically improved accuracy.

• Doppler-Based Markers

  • Aortic isthmus flow disturbance — An important Doppler marker beyond simple measurements of arch size. Abnormal isthmal flow patterns, including increased diastolic forward flow and delayed retrograde flow peaks, are strongly associated with true coarctation compared with isolated arch hypoplasia. The diastolic-to-systolic velocity-time integral (VTI) ratio at the aortic isthmus has emerged as a powerful discriminator, outperforming traditional metrics such as the isthmus Z-score and pulmonary artery-to-aorta ratio. More recently, the VTId:VTIs ratio has demonstrated excellent diagnostic performance, achieving very high specificity in distinguishing fetuses ultimately requiring surgical intervention from those with benign arch hypoplasia.

  • Bidirectional flow at the foramen ovale — Bidirectional shunting at the foramen ovale has been independently associated with coarctation of the aorta. Interestingly, the presence of a redundant foramen ovale flap appears to be inversely associated with coarctation, suggesting that this finding may actually be reassuring. This distinction is important and differs from the presence of a thickened or restrictive foramen ovale valve, which may contribute to ventricular and arch disproportion.

• AV Valve Disproportion

  • Tricuspid valve/mitral valve (TV/MV) diameter ratio — This marker is distinct from ventricular disproportion and has emerged as an independent predictor of coarctation. Elevated TV/MV ratios are associated with increased risk and appear particularly useful in the second trimester, when they add diagnostic value beyond measurements of the aortic isthmus and ductal arch.

  • Aortic valve Z-score — A smaller aortic valve is consistently associated with coarctation. Abnormal aortic valve morphology, including bicuspid aortic valve, has also been identified as an independent predictor and may reflect more generalized left-sided hypoplasia.

• Aortic Isthmus/Duct Ratio: Aortic isthmus/arterial duct diameter ratio < 0.7 — The isthmus-to-ductal ratio is among the strongest predictors of coarctation and consistently outperforms isolated measurements of isthmus size. This parameter captures the relative discrepancy between the systemic and pulmonary outflow pathways and is therefore related to, but distinct from, aortic isthmus hypoplasia alone. Gestational age-specific thresholds may further improve its predictive performance.

• Ascending Aorta Z-Score: Although early studies reported conflicting results, more recent investigations have identified a reduced ascending aortic diameter as an important independent predictor of coarctation. In some series, the ascending aorta Z-score has demonstrated among the highest sensitivities and specificities of all prenatal measurements, suggesting that diffuse left-sided hypoplasia may be an important component of the disease spectrum.

• Novel Morphometric and Angular Measures:

  • Transverse aortic arch–descending aorta (TAo-DAo) angle — Novel angular measurements of the aortic arch have shown remarkable diagnostic performance. A narrower TAo-DAo angle is strongly associated with coarctation, particularly when combined with transverse arch diameter and isthmus Z-score. Similarly, the ascending aorta–descending aorta (AAo-DAo) angle has demonstrated near-perfect discrimination in some cohorts.

  • Carotid-subclavian artery index (CSAI) — The CSAI is defined as the distance between the left common carotid and left subclavian arteries normalized to the overall arch length. Lower values reflect increased spacing of the head and neck vessels, a feature commonly observed in fetuses with tubular arch hypoplasia, and have demonstrated excellent diagnostic performance.

  • Distal aortic arch (DA) index — The DA index combines the ductus arteriosus-to-isthmus diameter ratio with additional arch measurements and has emerged as another promising composite marker with high sensitivity and specificity for predicting postnatal coarctation.

• Left Ventricular Longitudinal Strain: Functional assessment of the fetal myocardium may provide incremental value beyond conventional morphologic markers. Reduced LV longitudinal strain has been shown to distinguish fetuses requiring surgical intervention from those with benign arch hypoplasia and, in some studies, has outperformed traditional high-risk criteria such as flow reversal at the foramen ovale or aortic arch.

• Fetal Cardiac MRI: Fetal cardiac MRI is emerging as an important adjunct when echocardiographic findings are equivocal. MRI-derived measurements of aortic flow and isthmal displacement may improve prediction of postnatal intervention and offer superior visualization of the aortic arch, particularly in cases limited by fetal position or acoustic shadowing from the spine.

• Serial Assessment: The value of serial echocardiographic assessment deserves emphasis as a marker in its own right. Aortic isthmus Z-scores that improve over time are generally associated with normal postnatal outcomes, whereas persistently low Z-scores are more likely to predict true coarctation requiring intervention. Gestational age-specific algorithms may further refine prediction, as the optimal discriminating parameters appear to differ between the second and third trimesters.

The physiological rationale for these fetal markers stems from the redistribution of blood flow in utero. Coarctation is frequently associated with intracardiac anomalies that reduce flow through the left side of the heart, such as left ventricular outflow tract obstruction or specific types of ventricular septal defects. When flow to the ascending aorta is curtailed, the right ventricle must compensate by handling a larger proportion of the combined cardiac output, which it ejects through the pulmonary trunk and ductus arteriosus to the descending aorta. This increased volume load results in the characteristic dilation of the right heart chambers and the pulmonary artery. Simultaneously, because the aortic isthmus normally develops in response to the volume of blood it conducts, the reduced flow from the left ventricle leads to its underdevelopment and tubular hypoplasia. The foramen ovale valve may appear restricted or rigid because a decrease in left heart filling or compliance can elevate left atrial pressure, pushing the septum primum toward the right and limiting the normal right-to-left fetal shunt.

Despite these signs, many cases of coarctation are not detected prenatally. The physiological rationale for missed cases often involves a heart that appears balanced if the coarctation is isolated and localized without significant flow redistribution in utero. Additionally, a degree of right-sided preponderance is considered physiological as the fetus reaches the third trimester, making it difficult to distinguish from a pathological state without comparative data from the second trimester. In cases where the ductal ampulla remains wide and only gradually occludes postnatally, the aortic obstruction may not become hemodynamically manifest or echocardiographically visible until well after birth.

Left sided SVC: The relationship between persistent left SVC (PLSVC) and fetal coarctation of the aorta (CoA) is nuanced and depends on whether the PLSVC is bilateral (with right SVC present) versus a single/isolated left SVC (absent right SVC), and whether other cardiac anomalies coexist. Importantly, a major systematic review and meta-analysis in Circulation found that PLSVC alone did not carry a statistically significant increased risk for CoA (P=0.85) when used as a prenatal ultrasound predictor (Reference). Among fetuses with bilateral SVC, coarctation of the aorta is the most frequently associated left ventricular outflow tract abnormality, reported in approximately 20–21% of cases. [Reference] Conversely, more than one in five newborns diagnosed with CoA present with bilateral SVC. A systematic review and meta-analysis of PLSVC found CoA in 21.3% (95% CI, 13.6–30.3%) of cases, leading to recommendations for serial follow-up throughout pregnancy to rule out evolving coarctation. However, this high co-occurrence rate reflects the fact that PLSVC is frequently found alongside other structural heart defects — the association may be driven by the presence of additional cardiac anomalies rather than the PLSVC itself. When PLSVC is truly isolated (no associated cardiac, extracardiac, or vascular anomalies), the risk of CoA appears minimal.

1st Echocardiography 

Mechanism of Paradoxical Septal Motion on M-Mode

Normally on M-mode, the interventricular septum (IVS) moves posteriorly (toward the LV center) during systole and anteriorly during diastole, in concert with the LV posterior wall. Paradoxical septal motion (PSM) refers to the septum moving anteriorly (toward the RV) during systole — opposite to its expected direction. The underlying mechanism depends on the etiology but fundamentally reflects an alteration in the transseptal pressure gradient, ventricular loading conditions, or electrical activation sequence. Three core pathophysiological mechanisms produce PSM:

1. Altered transseptal pressure gradient (pressure overload) — When RV systolic pressure approaches or exceeds LV pressure, the septum is pushed leftward during systole. On parasternal short-axis, this produces a D-shaped LV at end-systole. In the neonatal period, this is most commonly seen with pulmonary hypertension. 

2. RV volume overload — Diastolic RV distension shifts the septum leftward during diastole, flattening or reversing its normal curvature. During systole, the septum returns to its normal circular configuration, producing net rightward (paradoxical) motion on M-mode. The key distinction is that the D-shaped LV occurs at end-diastole rather than end-systole. The LV eccentricity index is significantly elevated at end-diastole but normal at end-systole in pure volume overload, versus (often) elevated at both time points in pressure overload. 

3. Abnormal electrical activation — In LBBB or RV pacing, early RV free wall contraction pulls the septum leftward before the LV free wall contracts, producing a characteristic "septal flash" followed by paradoxical rightward rebound stretch when the LV lateral wall activates late. 

4. Pericardial effusion / tamponade — Exaggerated respiratory variation in septal position with inspiratory leftward septal bulge ("septal bounce") due to enhanced ventricular interdependence.

5. Post-cardiac surgery (POPS) — Paradoxical septal motion occurs in a high proportion of patients after cardiac surgery, likely due to loss of pericardial restraint and altered cardiac mobility rather than ischemia. The septum thickens normally despite paradoxical excursion, and the phenomenon gradually resolves over weeks to months as postoperative adhesions form. 

6. Normal neonatal transition — This is a critical consideration. In healthy term newborns, septal flattening and abnormal wall motion are present during the first 3–5 days of life due to the physiologic transition from fetal right-dominant circulation, with elevated RV systolic pressure persisting until pulmonary vascular resistance falls. In preterm infants, this distortion persists longer — up to several weeks — and causes M-mode shortening fraction to underestimate true LV function (2D area shortening is more reliable during this period). Septal curvature normalizes progressively over the first 14 days as the transseptal pressure gradient increases. 

Echocardiography 2 - on Low Flow Oxygen to decrease PVR

Echocardiography 3 - Off Low Flow Oxygen 1/4 LPM - 60/40 (49)

Follow-up at 3 months

At outpatient pediatric cardiology follow-up, echocardiography demonstrated near-complete normalization of the cardiovascular findings. A very small restrictive patent ductus arteriosus remained present with exclusively left-to-right shunting and a peak gradient of 54 mmHg, indicating pulmonary artery pressures substantially below systemic pressure. The aortic arch was reassessed and considered normal, with no evidence of coarctation or residual obstruction. The previously noted mild transverse arch hypoplasia was interpreted as a developmental variant related to the branching arrangement of the head and neck vessels rather than true pathology. The infant was found to have a left aortic arch with a common origin of the brachiocephalic artery and left common carotid artery (a so-called "bovine arch" variant), an anatomic configuration that can give the appearance of mild narrowing of the proximal transverse arch and likely contributed to the prenatal and early postnatal concerns. Importantly, there was no Doppler evidence of obstruction across the arch. A trivial mitral regurgitation and a patent foramen ovale with two small left-to-right shunting orifices remained present, both of minimal hemodynamic significance. Tricuspid regurgitation was physiologic, with a right ventricular-to-right atrial gradient of only 18 mmHg, consistent with normalization of pulmonary artery pressures. Right ventricular dimensions and systolic function were normal, including a normal TAPSE and normal fractional area change. Left ventricular systolic function was likewise normal. The complete clinical and echocardiographic normalization strongly supports the initial hypothesis that the neonatal presentation reflected an exaggerated but ultimately benign transitional physiology rather than structural heart disease. The evolution of the ductal shunting pattern, normalization of right ventricular pressures, and absence of residual arch obstruction all suggest that the apparent prenatal left-sided abnormalities were primarily flow related. This case therefore illustrates how fetal flow dynamics and transitional physiology can produce echocardiographic findings that mimic serious congenital heart disease, while ultimately resolving spontaneously with time and supportive care.