Figure 49-27 Schematic representation of two-lung ventilation versus one-lung ventilation. Typical values for fractional blood flow to the nondependent and dependent lungs are shown, as well as arterial oxygen tension (PaO₂ ) and shunt (QS/QT) for the two conditions. QS/QT during two-lung ventilation is assumed to be distributed equally between the two lungs (5% to each lung). The essential difference between two-lung and one-lung ventilation is that during one-lung ventilation, the nonventilated lung has some blood flow and therefore has an obligatory shunt, which is not present during two-lung ventilation. The 35% of total flow perfusing the nondependent lung, which was not shunt flow, was assumed to be able to reduce its blood flow by 50% by hypoxic pulmonary vasoconstriction. The increase in QS/QT from two-lung to one-lung ventilation is assumed to be solely due to the increase in shunt through the nonventilated, nondependent lung during one-lung ventilation. (From Benumof JL: Anesthesia for Thoracic Surgery. Philadelphia, WB Saunders, 1987.)