Three Stages of Single Ventricle Palliation

Surgical palliation for single ventricle physiology is accomplished through a staged approach, gradually redirecting systemic and pulmonary circulations to create a stable physiology in the absence of two functional ventricles. Each stage has distinct objectives and introduces unique hemodynamic consequences.

Stage 1: Initial Palliation (e.g., Norwood Procedure)

The first stage focuses on establishing reliable systemic output and controlled pulmonary blood flow. Key steps include enlargement of the atrial septal defect (ASD) to ensure adequate mixing of systemic and pulmonary venous return, reconstruction of the aortic arch to secure systemic circulation, and creation of a source of pulmonary blood flow using either a modified Blalock–Taussig shunt or a right ventricle–to–pulmonary artery conduit.

A major limitation of this stage is that pulmonary blood flow is effectively “stolen” from systemic output, producing a volume-loaded single ventricle in which total cardiac output is the sum of both systemic and pulmonary flows (Qp + Qs). Excessive pulmonary flow can therefore reduce systemic perfusion and impose a heavy volume burden on the ventricle.

Stage 2: The Glenn Procedure

The second stage, the bidirectional Glenn, separates the pulmonary and systemic circuits by connecting the superior vena cava (SVC) to the pulmonary arteries. Pulmonary blood flow becomes passive, driven by SVC return, and thus Glenn flow approximates Qsvc (upper body venous return). This eliminates the systemic-to-pulmonary shunt and reduces ventricular volume load.

However, systemic oxygen saturation remains incomplete, since inferior vena caval (IVC) return bypasses the pulmonary circulation at this stage. In addition, pulmonary blood flow remains highly dependent on pulmonary vascular resistance (PVR), such that elevations in PVR directly reduce pulmonary flow and oxygenation.

Stage 3: Fontan Completion

The final stage of palliation is the Fontan procedure, in which all systemic venous return—including IVC blood—is routed passively into the pulmonary arteries. This arrangement results in a marked improvement in systemic oxygen saturation. For the Fontan to function effectively, pulmonary artery pressure must remain lower than central venous pressure (CVP), with low PVR and adequate venous driving pressure.

Despite improved oxygenation, Fontan physiology is inherently a pressure-loaded circulation. The single ventricle faces the combined afterload of both systemic and pulmonary vascular beds, while persistent systemic venous hypertension predisposes patients to complications such as pleural effusions, protein-losing enteropathy, and congestive hepatopathy.

Summary

The staged palliation of single ventricle physiology transitions from the Norwood (Stage 1), through Glenn (Stage 2), to Fontan completion (Stage 3). Each stage addresses the limitations of the previous circulation: Stage 1 secures systemic output at the cost of volume overload, Stage 2 unloads the ventricle but leaves residual desaturation, and Stage 3 achieves complete separation with improved oxygenation but at the expense of a pressure-loaded circuit. This sequence underscores the delicate balance of physiology that defines single ventricle palliation.