A person with a ventilation/perfusion (V/Q) defect has hypoxemia and is treated with supplemental O2. The supplemental O2 will be most helpful if the person’s predominant V/Q defect is
Supplemental O2 (breathing inspired air with a high Po2) is most
helpful in treating hypoxemia associated with a ventilation/perfusion (V/Q) defect if the
predominant defect is low V/Q. Regions of low V/Q have the highest blood flow. Thus, breathing high Po2 air will raise the Po2 of a large volume of blood and have the greatest influence on the total blood flow leaving the lungs (which becomes systemic arterial blood). Dead space (i.e., V/Q = ∞) has no blood flow, so supplemental O2 has no effect on these regions. Shunt (i.e., V/Q = 0) has no ventilation, so supplemental O2 has no effect.
Regions of high V/Q have little blood flow, thus raising the Po2 of a small volume of blood will have little overall effect on systemic arterial blood.