A comparison between reaction time measurement and critical flicker fusion frequency under rising nitrous oxide inhalation in healthy subjects.

Ten subjects volunteered to inhale a test gas containing 0, 10, 20 and 30% nitrous oxide. After equilibration at each nitrous oxide concentration, the reaction time was measured using both the single-hand and the double-hand methods, and then the critical flicker fusion frequency was measured. Using the flicker funsion frequency test, significant changes from the normal range were found at 20 and 30% nitrous oxide in the test gas, whereas no significant changes were found with the fusion flicker frequency test. No significant prolongation was found with either double-hand or single-hand reaction time measurements until 30% nitrous oxide in the test gas was reached. It is concluded that critical flicker fusion frequency measurement is a simpler, more time-saving and more sensitive method for the estimation of the changes which nitrous oxide produces in healthy subjects than reaction time measurement, and that critical flicker fusion frequency measurement will presumably be a useful supplement to clinical examination as an objective method for the estimation of postanaesthetic recovery in patients.