What Is Shunting in Respiration?

In a normal human, blood exits the right ventricle of the heart and is oxygenated by the lung. The oxygenated blood then returns to the left side of the heart where it is pumped out to the rest of the body. However, under certain conditions, oxygenation does not occur despite normal breathing and heartbeat. This is called respiratory shunting and it can be caused by both pulmonary and cardiac conditions.

Perfusion Without Ventilation

• When blood perfuses areas of the lung that are not being ventilated, there is no oxygen for them to pick up. This can happen when the lung is collapsed or filled with fluid. Because the lung is not capable of inflating or holding more oxygen, this type of shunt does not improve when oxygen is administered to the patient.

Ventilation Without Perfusion

• In this form of shunting, the lung is inflating and functioning normally, but there is no blood to be oxygenated. This can occur when cardiac output is too low to properly perfuse all areas of the lung.

Diffusion Abnormailty

• When gases cannot diffuse properly across the blood-lung interface, carbon dioxide cannot move out of the blood and oxygen cannot move in. Therefore, despite normal perfusion and ventilation, blood is still not oxygenated. This can occur in patients with unusually thick membranes surrounding the air sacs in their lungs, caused by fibrosis or Acute Respiratory Distress Syndrome.