SvO₂: Balance of Oxygen Delivery and Demand

Mixed venous oxygen saturation (SvO₂) reflects the equilibrium between systemic oxygen delivery and oxygen consumption. It is a global indicator of tissue oxygenation and is widely used in critical care and congenital heart disease management.

Oxygen delivery (DO₂) is determined by cardiac output (CO) and arterial oxygen content (CaO₂). The latter depends on hemoglobin concentration, arterial oxygen saturation (SaO₂), and the small dissolved fraction of oxygen in plasma. Oxygen consumption (VO₂) represents the uptake of oxygen by the tissues and is calculated from the arteriovenous oxygen content difference. The relationship can be summarized as:

• DO₂ = CO × CaO₂
• CaO₂ = (Hb × SaO₂ × 1.34) + (0.003 × PaO₂)
• VO₂ = DO₂ × (SaO₂ – SvO₂)
• SvO₂ = SaO₂ – VO₂/DO₂

In normal biventricular physiology, SvO₂ typically ranges from 65% to 75%, corresponding to an oxygen extraction ratio of approximately 25–35%. Values below this range generally indicate inadequate delivery relative to demand, while abnormally high SvO₂ may signify impaired tissue oxygen utilization.

Causes of low SvO₂ include reduced cardiac output, anemia, hypoxemia, or increased oxygen demand (fever, agitation, shivering). Causes of high SvO₂ include sepsis with impaired oxygen extraction, mitochondrial dysfunction, excessive sedation, or left-to-right intracardiac shunts.

Thus, SvO₂ is best interpreted as a dynamic marker of the interaction between cardiac function, oxygen transport, and metabolic demand. Continuous monitoring provides valuable guidance for therapeutic adjustments and is particularly important in managing critically ill patients and those recovering from congenital heart surgery.