One lung ventilation has become accepted procedure where lung isolation and sepration is required in thoracic surgeries . During one lung ventilation (OLV) diseased lung  is  collapsed which is being operated while ventilation of the other lung is required to facilitate better  surgical access and visibility in  lung surgeries. We use double-lumen endotracheal tubes (DLT) or bronchial blocker for OLV.  This technique helps to prevent contamination of normal lung and improve surgical ability.

During one-lung ventilation (OLV)  the nonventilated lung remains perfused but not ventilated, leading to an increase in shunt fraction and a decrease in oxygenation. This obligatory intrapulmonary shunt may produce hypoxemia because of collapse of the nondependent lung and increased atelectatic areas in the dependent lung. It leads to ventilation -perfusion mismatch.

Hypoxic Pulmonary Vasoconstriction shunts divert blood away from poorly oxygenated areas to better ventilated lung segments, thereby optimizing ventilation-perfusion matching, reducing shunt fraction and optimizing systemic O2 delivery.

Oxygenation during OLV can be improved by increasing blood flow to the ventilated lung or decreasing blood flow to the nonventilated lung.

Carbon dioxide is a potent vasodilator in cerebral and systemic circulation. However the effect of CO2 on pulmonary circulation is unclear.

Permissive hypercapnia refers to the ventilator strategy that allows the Pco2 to rise in a controlled fashion in an effort to reduce lung injury and the burden of gas exchange. Although a Pco2 value of 40 mm Hg is considered normal in healthy humans, higher values may be adaptive in disease. Permissive hypercapnia can be achieved by reductions in respiratory rate or tidal volume, or both. 

In this study, we tested the hypothesis that   hypercarbia within permissible limits may have  effect in improving arterial oxygenation compared to normocarbia during OLV in patients undergoing lung surgeries.