Circular Shunt Physiology

Circular Shunt Physiology in the Neonatal Period

A circular shunt is a critical pathophysiological condition first described by Shone et al. in 1962. It involves abnormal blood recirculation through complete intracardiac or intra- and extracardiac communications, where blood bypasses the capillary beds and returns to its originating chamber. This differs from typical left-to-right or right-to-left shunts because the absence of capillary beds allows blood to flow more freely, resulting in a larger shunt magnitude. The hemodynamic impact of a circular shunt is profound, as it diverts significant volumes of blood from systemic or pulmonary circulation without contributing to effective systemic perfusion. This can lead to cardiovascular instability, systemic hypoperfusion, metabolic acidosis, and organ failure. If left untreated, it can progress to multiple organ failure and death.

Specific Lesions and Mechanisms Leading to Circular Shunt: Circular shunts typically arise from a combination of congenital cardiac defects that permit blood flow between cardiac chambers or between cardiac chambers and great arteries, bypassing capillary beds. Notable examples include:

• Ebstein's Malformation/Anomaly (EA) with Tricuspid Valve Dysplasia (TVD):

  • This is the most commonly associated condition with circular shunt physiology.

  • Mechanism: It is characterized by unrestricted ductal flow via a patent ductus arteriosus (PDA) and significant pulmonary and tricuspid regurgitation. The right ventricle (RV) produces insufficient pressure for adequate antegrade pulmonary blood flow, causing pulmonary circulation to be maintained exclusively by the aorta via the PDA. A substantial volume of blood entering the duct does not proceed to the lungs but instead leaks back through the incompetent pulmonary valve into the dilated right heart, then returns via the patent foramen ovale (PFO) to the left heart. The left cardiac output fills the aorta and there is diastolic steal filling the underfilled pulmonary artery. This creates an ineffective recirculation loop between both ventricles that does not contribute to systemic perfusion, leading to systemic hypotension, multiple-organ failure, and potentially death.

• Pulmonary Atresia with Intact Ventricular Septum (PA/IVS) and Tricuspid Regurgitation (TR):

  • A circular shunt can develop if blood flows from the aorta into the coronary arteries, then retrogradely into the RV through sinusoids, and from there back into the right atrium (RA) via tricuspid regurgitation. This creates a loop that bypasses both systemic and pulmonary circulations, providing no effective perfusion. If coronary perfusion becomes dependent on right ventricular pressure, this is hemodynamically dangerous and may lead to myocardial ischemia. Right ventricular-dependent coronary circulation (RVDCC) is considered a contraindication to biventricular repair in PA/IVS.

  • A similar circular shunt physiology can also occur after balloon dilation of the pulmonary valve, particularly if significant secondary pulmonary insufficiency develops, especially when combined with severe tricuspid regurgitation, a large PDA or BT-shunt, and an inter-atrial shunt.

• Gerbode Defect:

  • This rare septal defect is located in the atrioventricular septum, connecting the right atrium and the left ventricle.

  • Mechanism: Left ventricular blood is shunted directly into the right atrium, increasing right atrial pressure. The right atrium then decompresses into the left atrium, and subsequently re-enters the left ventricle. This limits effective forward flow into the aortic outflow tract, causing the blood to recirculate ineffectively between chambers.

• Original Description by Shone et al. (1962):

  • The initial description of a circular shunt was associated with a ventricular septal defect (VSD), pulmonary valvular stenosis (PS), congenital tricuspid insufficiency (TR), and a patent oval foramen (PFO). In this specific combination, shunted blood returns to the originating cardiac chamber without transversing a capillary bed. LV to the RV via the VSD, RV to RA due to significant TR. RA to LA due to inter-atrial shunt. LA to LV, which then circles again. 

• Post-Bidirectional Glenn Procedure:

  • A circular shunt has been reported to occur following a bidirectional Glenn procedure in an infant with tricuspid atresia, ventricular septal defect, pulmonary stenosis, and a persistent left superior caval vein draining into the right atrium via the coronary sinus.

  • Mechanism: In this scenario, in the presence of antegrade blood flow into the pulmonary artery, the cavo-pulmonary anastomosis decompressed the pulmonary artery to the right atrium through the innominate vein, left superior caval vein, and coronary sinus, creating the substrate for the circular shunt.

Clinical Presentation:

• General: The impact on hemodynamics is profound, leading to a significant diversion of blood without effective systemic perfusion. This results in cardiovascular instability, systemic hypoperfusion, metabolic acidosis, and organ failure, potentially leading to multiple organ failure and death if not treated.

• Newborns: May exhibit pronounced tachypnea and acidemia or hypoxemia. These babies are often critically ill and typically require care in intensive care units. They can present with profound acidosis, hypoxia, and low cardiac output. 

• In Utero/Fetuses: Severe cases, particularly when associated with Ebstein's anomaly, can lead to fetal heart failure, hydrops fetalis, or intrauterine death if not managed appropriately. Umbilical artery end-diastolic flow may be reduced or reversed due to vascular steal.

Management Strategies:

Interventions should be implemented immediately once a circular shunt is diagnosed. Management strategies primarily focus on interrupting or regulating the shunt. Cardiac anesthesiologists must possess a high level of awareness and expertise in managing anesthetic techniques related to these conditions, as they significantly impact hemodynamics during interventional or cardiac surgical procedures.

• For Ebstein's Anomaly and Circular Shunt:

  • Prenatal Management: Nonsteroidal anti-inflammatory drugs (NSAIDs) can be administered transplacentally to induce constriction of the patent arterial duct, thereby reducing the magnitude of the shunt. This is an established method for managing fetuses with severe Ebstein's anomaly and circular shunt. Studies have shown that NSAID therapy can lead to ductal constriction in a high percentage of fetuses (82%) and reduce the likelihood of fetal demise (6% compared to 50% in non-responders). Furthermore, it has been observed to resolve hydrops in 50% of affected fetuses. Early initiation of this treatment is advised to prevent potential brain injury resulting from hypoperfusion.

    1. Side Effects: Oligohydramnios (reduced amniotic fluid) is a common side effect, observed in 59–67% of fetuses on NSAID therapy, sometimes prompting delivery. Transient postnatal renal failure requiring dialysis has been reported in some neonates. Irreversible ductal closure is a rare but significant risk. Despite these risks, the benefits may outweigh them given the high mortality associated with untreated circular shunts.

  • Neonatal Management: Immediate postnatal stabilization may necessitate extracorporeal membrane oxygenation (ECMO). For severe cases in neonates with very low estimated right ventricular systolic pressure (RVSP) (less than 30–40 mmHg), surgical interventions like the Starnes procedure or main pulmonary artery (PA) ligation, while maintaining prostaglandin E (PGE) infusion, are indicated. If neonates have a higher estimated RVSP (greater than 40 mmHg) and are hemodynamically stable, initial medical management for 48–72 hours to allow pulmonary vascular resistance (PVR) to decrease is pursued (interventions to decrease PVR such as providing iNO and oxygen to promote forward flow towards LPA and RPA) with avoidance of PGE or even ligation of the duct to interrupt the circular shunt and to assess the RV's ability to maintain oxygen saturations above 75% without the PDA. Fetuses successfully treated prenatally with NSAIDs often undergo neonatal surgery immediately after birth to eliminate the circular shunt and have improved survival outcomes. Optimizing outcomes requires a comprehensive approach that includes prenatal diagnosis and management, alongside a detailed understanding of the associated pathophysiology.

References:

1. 1. Maddali, M. M., Al-Maskari, S. N., & Al Kindi, H. N. (2025). Circular Shunt: A Loop Not to Be Ignored. Journal of Cardiothoracic and Vascular Anesthesia, 39, 813–817.

   2. Peña, F. L., Emanuelson, T. W., Todman, S. H., Jones, R. C., & Mahajan, S. (2024). Fetal circular shunt in Ebstein’s anomaly and non-steroidal anti-inflammatory treatment. Journal of Neonatal-Perinatal Medicine, 17, 63–69.

   3. Freud, L. R., Wilkins-Haug, L. E., Beroukhim, R. S., Lafranchi, T., Phoon, C. K., Buzzard, C. J., ... & Tworetzky, W. (2018). Abstract 14979: Prenatal NSAID Therapy to Mitigate Circular Shunt Physiology in Fetuses With Severe Ebstein Anomaly. Circulation, 138(Suppl_1).

   4. Mustafa, H. J., Aghajani, F., Bairmani, Z. A., & Khalil, A. (2024). Transplacental non-steroidal anti-inflammatory drugs versus expectant management in fetal Ebstein anomaly with circular shunt: systematic review and meta-analysis. Prenatal Diagnosis, 44(6-7), 773–782.

   5. Torigoe, T., Mawad, W., Seed, M., Ryan, G., Marini, D., Golding, F., ... & Jaeggi, E. (2019). Treatment of fetal circular shunt with non-steroidal anti-inflammatory drugs. Ultrasound in Obstetrics & Gynecology, 53(6), 841–846.