DISCUSSION: Nearly 80 cases of WLL have been performed for treatment of PAP in our institution. According to our previous observations, the oxygen saturation was not dramatically reduced in single-lung ventilation compared with that in the ventilation of both lungs; moreover, the oxygen saturation may show some increase after a certain duration of lung lavage with constant setting of the ventilator. We found that ventilation/perfusion could change at any time despite no change in diffusion of the ventilated lung. When the lung on one side is lavaged and the other ventilated, pulmonary vasoconstriction due to a chemoreceptor reflex in the lavaged lung, reduces perfusion of the lavaged lung and improves the ventilation/perfusion, which can counteract the reduction in oxygen saturation due to decreased diffusion area with single-lung ventilation. Additionally, intrapulmonic pressure increases when lavage fluid is introduced into the bronchus and alveoli, leading to pulmonary capillary contraction and shunting of pulmonary blood flow through the ventilated lung[5-6], which also counteracts the lowering oxygen saturation. This conform wtih previous studies[7-8].