Anatomic shunt: Calculation

A shunt refers to blood that bypasses oxygenation in the lungs. There exists a spectrum of shunt from absolute shunt in which the blood receives no oxygen (V/Q=0) to areas that are perfused in excess of their ventilation (0 < V/Q < 1). Examples of absolute shunt include normal anatomic shunting due to bronchial, pleural, and thebesian veins. This normal anatomic shunting is up to 5% of cardiac output and accounts for the small A-a gradient that exists in all patients. Other examples of absolute shunt include right to left intracardiac shunt, vascular tumors, consolidated pneumonia, ARDS, lobar atelectasis, and pulmonary edema.

Areas of well oxygenated blood (V/Q = 1), absolute shunt (V/Q = 0), and areas with shunt effect (0 < V/Q < 1) combine to create venous admixture. Venous admixture is defined as the mixing of reoxygenated blood with shunted (non-reoxygenated) blood distal to the alveoli. The amount of total shunting can be quantified utilizing the shunt equation. Though shunting typically occurs along a continuum, the equation has been developed from a two-compartment model. One compartment has perfect gas exchange therefore all blood is oxygenated. In the second compartment all blood is shunted and receives no oxygen.

See equation 1

Where Q_s = shunt, Q_t = cardiac output, CcO₂ = oxygen content of pulmonary end capillary blood, CaO₂ = oxygen content of arterial blood, CvO₂ = oxygen content of mixed venous blood

This equation can be simplified because pulmonary end capillary blood is assumed to be completely saturated and because hemoglobin concentration is the same throughout the entire vasculature.

See equation 2

Where S_aO₂ = arterial saturation and S_vO₂ = venous saturation