During the study PaCO2 will be maintained at the value before induction of anaesthesia by ventilator adjustments. Internal carotid artery CO2 reactivity before induction of anaesthesia will be determined by evaluations of blood flow and PaCO2 during normoventilation and during hyperventilation to reduce PaCO2 by 1.5 kPa as guided by end-tidal CO2 tension (PetCO2) and measurements will be conducted when PetCO2 has been stable for 5 min. The CO2 reactivity during anaesthesia will be determined by evaluations of internal carotid artery blood flow and PaCO2 at a PaCO2 at the value before induction of anaesthesia and 1.5 kPa above and below that value as guided by PetCO2 and measurements will be conducted when PetCO2 has been stable for 5 min. The CO2 reactivity before and during anaesthesia is calculated as the percentage change in internal carotid artery blood flow per kPa change in PaCO2. Analysis of internal carotid blood flow will be after correction for CO2 reactivity.