BACKGROUND	To determine the effects of nitrous oxide on cerebrovascular reactivity to carbon dioxide ( CCO ( 2 ) R ) during sevoflurane anaesthesia in children , middle cerebral artery blood flow velocity ( V ( mca ) ) was measured over a range of end-tidal carbon dioxide concentrations ( E ' ( CO ( 2 ) ) ) , using transcranial Doppler ( TCD ) ultrasonography .\