Stage 2 Palliation: The Glenn Circulation

Stage 2 palliation for single ventricle physiology is typically achieved through the bidirectional Glenn procedure, which connects the superior vena cava (SVC) directly to the pulmonary arteries. This represents a major hemodynamic shift from the Stage 1 circulation, reducing ventricular workload and improving systemic efficiency.

Hemodynamic Changes from Stage 1

Following Stage 1 palliation, pulmonary blood flow (PBF) is supplied through a systemic-to-pulmonary shunt, which diverts cardiac output away from systemic circulation and contributes to volume overload of the single ventricle. The Glenn anastomosis eliminates this “steal,” so that cardiac output (COsv) is directed entirely to systemic flow (Qs). This markedly decreases ventricular volume load and improves long-term ventricular preservation.

Mechanism of the Glenn Circulation

With the SVC connected to the pulmonary arteries, the lungs are now interposed between the SVC and the systemic venous return. Pulmonary blood flow occurs passively, driven by venous pressure gradients rather than ventricular ejection. This passive nature makes Glenn flow highly sensitive to changes in venous return and pulmonary vascular resistance (PVR).

Determinants of Glenn Flow

The amount of pulmonary blood flow in Glenn physiology is determined primarily by SVC flow (Qsvc). This, in turn, depends on the distribution of systemic venous return between the upper and lower body.

• When upper body flow increases, Glenn flow rises and oxygenation improves.
• When lower body (IVC) flow predominates, desaturation occurs, since IVC return bypasses the pulmonary circulation at this stage.

Additionally, Glenn flow is strongly influenced by pulmonary vascular resistance. Elevated PVR reduces pulmonary blood flow, leading to worsening desaturation.

Clinical Perspective

The Glenn circulation is generally well tolerated in early childhood, when a relatively large fraction of systemic blood flow returns via the SVC. However, its reliance on passive pulmonary flow and sensitivity to PVR mean that desaturation, venovenous collaterals, and pulmonary arteriovenous malformations may develop over time. Ultimately, the Glenn is an intermediate stage, intended to unload the ventricle and prepare the circulation for completion of the Fontan procedure.

Summary

Stage 2 palliation with the bidirectional Glenn shunt eliminates the volume burden of Stage 1 by removing the systemic-to-pulmonary shunt and directing all ventricular output to systemic circulation. Pulmonary blood flow is supplied passively from the SVC, making it dependent on the ratio of upper- to lower-body venous return and on pulmonary vascular resistance. While effective in the short to medium term, these physiologic constraints underscore why the Glenn is a transitional step toward the Fontan circulation.