Shunt Equation

The Shunt Fraction calculator computes the fraction that venous blood bypasses oxygenation in the capillaries of the lung. 

INPUTS: Choose units and enter the following:

• (CcO₂) End-pulmonary-capillary Oxygen Content
• (CaO₂) Arterial Oxygen Content
• (CvO₂) Mixed Venous Oxygen Content

Shunt Fraction (SF): The calculator returns the factor as a real number and percentage.

The Math / Science

Shunt refers to blood that enters the arterial system without going through ventilated areas of the lung causing the PO2 of arterial blood to be less than that of alveolar PO2.The pulmonary shunt fraction (Qs/Qt) is frequently calculated in critically ill patients to monitor the effectiveness of pulmonary oxygenation. The breathing of pure oxygen often results in higher calculated Qs/Qt values that have been attributed to the development of atelectasis, ventilation-perfusion imbalance, or both. To interpret properly the changes in calculated Qs/Qt that occur when the inspired oxygen fraction is altered, the changes produced in all the variables affecting Qs/Qt must be known.

The oxygen content of mixed arterial blood (CaO₂) is determined by the content of oxygen in the blood that reached ventilated alveoli (CcO2), the content of oxygen in blood that bypassed ventilated alveoli (CvO₂), and the proportion of the two. Thus the shunt formula is:

SF = (CcO₂ - CaO₂) / (CcO₂ - CvO₂)

where:

• SF is the shunt factor.
• CcO₂ is the volume of blood in the ventilated alveoli (End-pulmonary-capillary Oxygen Content).
• CaO₂ is the volume of blood in the arterial blood flow (Arterial oxygen content).
• CvO₂ is the volume of blood that bypasses the ventilated alveoli (Mixed Venous Oxygen Content).