Circular Shunt Physiology

Circular Shunt Physiology in the Neonatal Period

Diagrams by CHD Doodles (Yaeji Kim)

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 fulguration and dilation of the pulmonary valve, when 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.

• Absent Pulmonary Valve Syndrome (APVS) with Patent Ductus Arteriosus: 

  • Neonates with an APVS are essentially almost never born with a duct. 

  • A rare variation of APVS involves a persistently patent arterial duct coexisting with a VSD and severe pulmonary regurgitation. This can result in a "circular shunt" where blood ejected from the left ventricle across the aortic valve eventually returns to the right ventricle through the patent arterial duct and the regurgitant pulmonary valve. Having a ductus arteriosus in utero is described as fatal in this condition due to the likelihood of a circular shunt

• Single Ventricle Anatomy: 

  • Necessary Conditions: For a circular shunt to form in a single ventricle patient, two main conditions must be met: 

    1. There must be an outflow from the heart to the lungs. This means that circular shunts do not occur in cases of pulmonary atresia. Often, this involves pulmonary stenosis. 

    2. In addition to this pulmonary outflow tract, there must be another source of pulmonary blood flow originating from the systemic circulation. A common example given is a Blalock-Taussig-Thomas (BTT) shunt, which might be present because the patient has pulmonary stenosis and insufficient blood flow to the lungs from the heart.

    3. A circular shunt arises when there is significant pulmonary insufficiency (PI) or regurgitation, even if the pulmonary valve is stenotic. If this valve fails to close properly, blood in the main pulmonary artery can backflow from the pulmonary arteries (PAs) into the right ventricle (RV). The ineffective circuit then develops: mixed blood exits the heart through the aortic valve, crosses the BTT shunt into the PAs, then flows back down the main PA into the right ventricle due to PI. This blood then continues to mix in the heart, exits through the aorta, and the cycle repeats, effectively meaning the blood goes "around and around," never reaching the body or the lungs effectively.

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. 

• Ebstein’s Anomaly and Circular Shunt – Prenatal and Postnatal Management: 

  • Prenatal Management: In cases of severe Ebstein’s anomaly with circular shunt physiology (as classified by the GOSE system), prenatal intervention can be considered. Transplacental administration of nonsteroidal anti-inflammatory drugs (NSAIDs) may induce constriction of the ductus arteriosus, thereby reducing the volume of the circular shunt. This strategy is an established approach in managing fetuses with severe Ebstein’s anomaly. Studies have demonstrated that NSAID therapy results in ductal constriction in approximately 82% of cases and is associated with a significant reduction in fetal demise (6% vs. 50% in non-responders). Additionally, hydrops fetalis resolved in about 50% of treated fetuses. Early initiation is advised to minimize the risk of brain injury due to hypoperfusion. Adverse Effects: Oligohydramnios is a frequent complication, reported in 59–67% of cases, and may lead to the need for early delivery. Transient postnatal renal failure requiring dialysis has also been documented. While irreversible ductal closure is rare, it remains a serious concern. Despite these risks, the potential benefits of therapy may outweigh the harms, given the high mortality associated with untreated circular shunt physiology. 

  • Postnatal Management in Two-Ventricle Circulation (e.g., Ebstein’s Anomaly with Pulmonary Insufficiency): Management focuses on disrupting the components perpetuating the circular shunt—most commonly severe pulmonary insufficiency and a patent ductus arteriosus (PDA). Counterintuitively, closure or restriction of the PDA—either pharmacologically or surgically—can be beneficial in this context, as it interrupts the recirculation loop. This is in contrast to the usual practice of maintaining ductal patency to support pulmonary blood flow. Supportive strategies include intubation and mechanical ventilation, pharmacologic support of cardiac output, and pulmonary vasodilation using oxygen and inhaled nitric oxide (iNO). iNO, in particular, can promote pulmonary arterial vasodilation and enhance antegrade flow from the main pulmonary artery to the right and left pulmonary arteries, as blood preferentially follows the path of least resistance. In severe cases, extracorporeal membrane oxygenation (ECMO) may be required during the immediate postnatal period. 

• Circular Shunt in Single-Ventricle Physiology (e.g., Pulmonary Stenosis with Systemic-to-Pulmonary Shunt): In single-ventricle patients with a systemic-to-pulmonary shunt (such as a modified Blalock-Taussig-Thomas shunt) and significant pulmonary regurgitation, a circular shunt can develop. In this setting, blood exits the heart via the aorta, traverses the shunt to the pulmonary artery, and regurgitates through the pulmonary valve into the right ventricle, creating a hemodynamically ineffective loop. This phenomenon impairs systemic and pulmonary perfusion. Definitive treatment typically requires surgical intervention—either ligation of the main pulmonary artery or closure of the pulmonary valve—to disrupt the circuit and restore effective circulation.

References:

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