7.4 Single Ventricle Physiology & Heterotaxy Syndromes

Single Ventricle Physiology

"Single ventricle" or "univentricular heart" is an umbrella term for a diverse group of complex congenital heart defects where only one ventricular chamber is sufficiently developed to support the circulation. In these conditions, complete surgical biventricular repair is impossible. Instead, patients undergo a series of palliative surgeries (the Fontan pathway) to route systemic venous return directly to the pulmonary arteries, bypassing the heart entirely and leaving the single ventricle to pump oxygenated blood to the body.

Common Single Ventricle Lesions

Aside from Hypoplastic Left Heart Syndrome (HLHS), which is a specific form of single ventricle physiology, several other distinct morphologies exist:

• Tricuspid Atresia: Characterized by complete absence (imperforation) of the tricuspid valve. There is no direct communication between the right atrium and the right ventricle. Blood must flow through an atrial septal defect to the left heart. The left ventricle is the dominant, functional chamber, while the right ventricle is severely hypoplastic and typically receives blood only via a ventricular septal defect (VSD).
• Double Inlet Left Ventricle (DILV): In this malformation, both the mitral and tricuspid valves (or a common atrioventricular valve) open directly into a single, dominant left ventricular chamber. A rudimentary, hypoplastic right ventricular outflow chamber is often present and connected via a bulboventricular foramen (a VSD).
• Hypoplastic Right Heart Syndrome (HRHS): A spectrum of lesions, including severe pulmonary atresia with intact ventricular septum, where the right ventricle is profoundly underdeveloped and unable to function as an effective pulmonary pump.

Prenatal diagnosis of these lesions hinges on careful four-chamber and outflow tract evaluations. Identifying the dominant ventricle's morphology (trabecular pattern, moderator band) and defining the atrioventricular and ventriculoarterial connections are essential for planning postnatal care.

Heterotaxy Syndromes (Situs Ambiguus)

Heterotaxy, or Situs Ambiguus, refers to an abnormal arrangement of internal organs and vessels across the left-right axis of the body. It is a failure of normal left-right asymmetry programming during embryogenesis. The presentation falls somewhere between normal arrangement (situs solitus) and complete mirror-image reversal (situs inversus). Heterotaxy is highly associated with complex congenital heart disease.

Heterotaxy is broadly categorized based on the morphology of the atrial appendages into Right Atrial Isomerism and Left Atrial Isomerism.

Right Atrial Isomerism (Asplenia Syndrome)

Right Atrial Isomerism is characterized by bilateral right-sidedness. The fetus develops organs with right-sided morphological features on both sides of the body.

Key anatomical findings include:

• Spleen: Absent (Asplenia). This leads to significant immunological compromise postnatally.
• Lungs: Bilateral right lungs (each having three lobes and a short, eparterial bronchus).
• Atria: Bilateral right atrial appendages (broad, blunt shape).
• Vascular Anomalies: The inferior vena cava (IVC) and the descending aorta run parallel on the same side of the spine, a classic ultrasound marker for right isomerism.
• Cardiac Defects: Cardiac malformations in right isomerism are typically severe and complex. They frequently include a complete Atrioventricular Septal Defect (AVSD), single ventricle physiology, Pulmonary Atresia or severe stenosis, and Total Anomalous Pulmonary Venous Return (TAPVR). The combination of a single ventricle and TAPVR presents a formidable surgical challenge.

Left Atrial Isomerism (Polysplenia Syndrome)

Left Atrial Isomerism is characterized by bilateral left-sidedness. The fetus develops organs with left-sided morphological features on both sides.

Key anatomical findings include:

• Spleen: Multiple small spleens (Polysplenia) are typically present, though function is usually normal.
• Lungs: Bilateral left lungs (each having two lobes and a long, hyparterial bronchus).
• Atria: Bilateral left atrial appendages (narrow, finger-like shape).
• Vascular Anomalies: The hallmark finding is Interruption of the Inferior Vena Cava (IVC). The intrahepatic segment of the IVC is missing. Systemic venous return from the lower body bypasses the liver and continues superiorly via a dilated azygos or hemiazygos vein, which empties into the superior vena cava. This "azygos continuation of the IVC" is a highly reliable prenatal marker.
• Cardiac Defects: Cardiac defects are often less structurally severe than in right isomerism. They frequently involve partial or complete AVSDs and left-sided obstructive lesions (like coarctation).
• Rhythm Disturbances: A critical complication of left atrial isomerism is the abnormal development of the conduction system. Fetuses are at a very high risk for developing complete heart block (CHB). The combination of an AVSD and complete heart block is highly suggestive of left isomerism and carries a poor prognosis, potentially leading to hydrops fetalis.