Nitrous oxide influences pattern EP estimate of visual acuity.

The effect of nitrous oxide (N2O) inhalation on the pattern EP with the onset and disappearance of checkerboard patterns was studied in four healthy volunteers. Checkerboards with checks ranging from 2 to 55 min of arc were presented to subjects sedated at various levels. Under the highest level introduced, 60% N2O, fixation of the pattern was still adequate. The major consistent changes in the pattern onset EP under N2O inhalation are (a) preferential attenuation of the response to small checks and (b) an increase of the response amplitude to large checks. The change in the amplitude versus check-size curve under N2O inhalation restricts the assessment of visual acuity on the basis of the pattern EP. In patients sedated with N2O, EP methods can be used only as a gross indicant of visual acuity.