 Mechanical ventilation interferes with cerebral perfusion via the changes in intrathoracic pressure andor as a consequence of hypocapnia. This latter occurs frequently following traditional ventilation strategies with relatively high tidal volume and respiratory rate. New trends in anesthesia intend to promote protective lung ventilation by keeping a normocapnic or even mildly hypercapnic state. However, cerebral vascular vasotonicity is carbon dioxide CO2-dependent with hypocapnia potentially leading to vasoconstriction and subsequent decrease in cerebral blood flow. Changes in cerebral vasoreactivity can be assessed by the near infrared spectroscopy NIRS device. This monitoring evaluates the changes in various parameters deoxygenated hemoglobin, oxygenated hemoglobin, the tissue oxygenation index TOI and the tissue hemoglobin index THI that act as surrogate for cerebral vasoconstriction. 