Single Ventricle Physiology

Single ventricle physiology refers to a circulation in which only one functional ventricular chamber supports both systemic and pulmonary circulations. This arrangement creates a unique hemodynamic state, in which oxygen saturation, blood flow distribution, and cardiac output are fundamentally different from those in a biventricular circulation.

Oxygen Saturation

In single ventricle physiology, arterial oxygen saturation is essentially identical in the pulmonary artery and aorta. This is because systemic venous and pulmonary venous return mix completely (or nearly completely) before being ejected by the common ventricular chamber. Thus, systemic desaturation is an inherent characteristic of this physiology, with the degree of hypoxemia determined by the balance between systemic and pulmonary blood flows.

Balance of Pulmonary and Systemic Blood Flow

The ratio of pulmonary to systemic flow (Qp/Qs) is determined primarily by the relative resistances of the two circuits:

• Pulmonary vascular resistance (PVR) regulates forward flow into the lungs.
• Systemic vascular resistance (SVR) governs outflow to the systemic circulation.

A low PVR favors pulmonary blood flow at the expense of systemic perfusion, while elevated PVR restricts pulmonary flow and worsens systemic desaturation. Thus, management strategies must often aim to fine-tune PVR and SVR to achieve a physiologic balance.

Cardiac Output

In this setting, total systemic venous return is ejected into a single outflow chamber. Cardiac output is therefore expressed as:

COsv = Qp + Qs

This reflects the fact that the single ventricle must provide the combined output for both pulmonary and systemic circulations. Consequently, excessive pulmonary flow not only steals blood from systemic perfusion but also imposes a volume load on the ventricle, contributing to heart failure. Conversely, inadequate pulmonary flow results in profound systemic desaturation.

Clinical Implications

The physiology of a single ventricle requires continuous attention to the delicate balance of systemic and pulmonary circulations:

• Monitoring oxygen saturation provides insight into flow distribution.
• Ventilatory strategies (e.g., adjusting FiO₂, PEEP, or PaCO₂) can influence PVR.
• Pharmacologic manipulation of SVR (e.g., vasodilators or vasoconstrictors) shifts the Qp/Qs ratio.
• Surgical palliation (staged Norwood–Glenn–Fontan pathway) progressively separates the two circulations to mitigate volume load and improve systemic oxygenation.

Summary

Single ventricle physiology is characterized by complete intracardiac mixing, equal arterial oxygen saturation in the aorta and pulmonary artery, and a total cardiac output equal to the sum of systemic and pulmonary flows. The balance between PVR and SVR dictates oxygenation and perfusion, making careful regulation of vascular resistances the cornerstone of management.