EXPLANATION

Hybrid palliation consists of bilateral PAB placement, PDA stenting, and, when indicated, balloon atrial septostomy or atrial stent placement. This strategy provides initial stabilization for patients with HLHS variants while avoiding a complex cardiopulmonary bypass operation in the neonatal period. Although some institutions employ hybrid palliation for all patients, most reserve it for those with high-risk features such as prematurity, low birth weight, genetic conditions, ventricular dysfunction or significant non-cardiac comorbidities.¹

PDA stenting in neonates with ductal-dependent pulmonary or systemic blood flow was first reported in the early 1990s. Although early outcomes were discouraging, subsequent technical advances and increased operator experience have made PDA stenting the preferred initial intervention for ductal-dependent pulmonary blood flow at many centers, offering lower mortality and shorter hospital stays compared with a surgical systemic-to-pulmonary shunt.² A high rate of reintervention remains a reality of PDA stenting. In a recent retrospective study of 214 infants across 24 centers who underwent hybrid palliation, 21% required PDA stent re-intervention.¹ Indications for re-intervention include shunt thrombosis, inadequate stent coverage at either ductal end resulting in obstruction, somatic growth leading to inadequate stent caliber, pulmonary artery distortion, and retrograde aortic arch obstruction (RAAO).²

RAAO, also known as reverse coarctation, is a recognized complication of the hybrid procedure, with a reported incidence of 10-29%.³ It occurs when the PDA stent protrudes into the aortic arch or becomes stenotic, effectively obstructing retrograde flow to the proximal arch vessels and coronary arteries.⁴ RAAO increases interstage morbidity and mortality, due to coronary artery hypoperfusion, and therefore requires prompt recognition and intervention. Diagnosis relies on a combination of clinical and echocardiographic findings, as no standardized definition exists. Clinically, RAAO may present as ST-segment changes, a blood pressure gradient characterized by higher pressures in the lower extremities, and poor weight gain. Echocardiographic features include increased velocity from the PDA into the transverse aorta, worsening systemic ventricular function, and progressive atrioventricular valve regurgitation. Patients with a small aortic root (z-score < -3) are at particularly high risk for RAAO and should undergo close surveillance with serial echocardiograms. Management options include catheter-based stent placement in the retrograde aortic arch or surgical conversion to Norwood palliation or a comprehensive stage II procedure.^2,3

This presentation is most consistent with RAAO. Although stent thrombosis in a patient with HLHS can cause markedly decreased systemic blood flow with ST changes along with right ventricular dysfunction, it would not create a blood pressure gradient. Partial left pulmonary artery obstruction can be difficult to diagnose acutely and typically does not result in immediate clinical changes; however, over time, it can lead to differential pulmonary blood flow and impaired growth of the left pulmonary artery.

REFERENCES

1. Argo MB, Barron DJ, Eghtesady P, et al. Outcomes after hybrid palliation for infants with critical left heart obstruction. J Am Coll Cardiol. 2023 Oct 3;82(14):1427-41.

2. Bentham JR, Kang SL. Re-interventions following patent ductus arteriosus stenting during the inter-stage period. Pediatr Cardiol. 2025 Oct 4. doi: 10.1007/s00246-025-04029-7.

3. Egan MJ, Hill SL, Boettner BL, et al. Predictors of retrograde aortic arch obstruction after hybrid palliation of hypoplastic left heart syndrome. Pediatr Cardiol. 2011 Jan;32(1):67-75.

4. Hibino N, Cismowski MJ, Lilly B, et al. Potential Molecular Mechanism of Retrograde Aortic Arch Stenosis in the Hybrid Approach to Hypoplastic Left Heart Syndrome. Ann Thorac Surg. 2015;100(3):1013-1020. doi:10.1016/j.athoracsur.2015.04.125