The sampling apparatus and its interpretation play important roles in ventilation of neurosurgical cases. We use capnography - side stream or mainstream- for all cases. Main stream capnography shows instant changes in ventilation where as side stream takes longer time- few breaths before the changes are seen, especially using longer tubing. Side stream capnography can also show lower values in case of leak in the system due to loose connection etc. Since end-tidal CO2 is surrogate marker of PaCO2, it is important to measure arterial CO2  where cerebral hemodynamics need to be maintained very stringently using CO2 control. Hence, we send arterial gases at least once or twice for all major cases. This also helps us in determining PaCO2-ETCO2 gradient. This gradient can also change intraoperatively depending upon the position, FiO2, type of ventilation etc. besides the venous air embolism, making multiple assessments of arterial blood gases. Lastly, when, blood gas machine is not in the OR complex, significant lag time in processing can show false low values, which we have observed especially in night time.