• Right-to-left intrapulmonary shunting (also referred to as physiologic shunting, wasted blood flow, and venous admixture) is the pathologic phenomenon whereby venous blood is shunted past the alveoli without taking up oxygen. This blood then returns to the left side of the heart as venous blood with a low oxygen tension.1,2

• Right-to-left intrapulmonary shunting is expressed as a fraction or percentage of shunted blood flow (Qs) to total blood flow (Qt) as expressed by the equation (Qs/Qt). The normal physiologic shunt is less than 5% and is caused by venous blood from the bronchial and coronary veins returning to the left side of the heart as desaturated blood.1,2

• Shunting of blood past the alveoli means that a certain percentage of the blood flows through an area of lung that receives no ventilation. Examples of conditions in which shunt is present include acute respiratory distress syndrome, acute lung injury, atelectasis, pneumonia, and pulmonary edema with fluid-filled alveoli.

• As the percentage of the shunted cardiac output increases, the mixture of venous shunted blood with arterial blood increases with a concomitant decrease in the arterial oxygen tension. The extent of the hypoxemia depends on the amount of the lung parenchyma that is not ventilated.

• The hallmark of right-to-left intrapulmonary shunting is persistent hypoxemia despite high concentrations of inspired oxygen (called refractory hypoxemia).1,2

• For evaluation of shunt, heparinized* arterial and mixed venous blood samples are analyzed with a cooximeter to determine saturation. Use of the calculated saturation obtained in conjunction with blood gas analysis or with pulse oximetry is not as accurate.^1,2

• Calculator

• Qs/Qt equation

• Mixed venous and arterial blood gas and saturation measurements

• Pulmonary artery catheter, for drawing mixed venous blood samples, or venous oxygen saturation (SVO₂) catheter. If an SVO₂ catheter is used, the mixed venous saturation recorded on the monitor can be used for the calculation (as long as in vitro and in vivo calibrations have been done according to manufacturer’s recommendations)