Postural control reduced by subanesthetic nitrous oxide narcosis.

The effects of subanesthetic (21%) nitrous oxide (N2O) narcosis on postural control, as measured by posturography were studied in 12 subjects. Vibration induced body sway, with open and closed eyes, and body sway induced by a visual stimulus were evaluated. Adaptation was measured as the quotient of body sway variance between the second and first halves of each trial. Change in postural strategy was evaluated as change in the variance of shear forces relative to change in body sway. Subjective evaluation of narcosis was recorded. Body sway variance increased significantly during exposure to N2O, and equally for all body sway inducing stimuli. With the eyes open, adaptation to vibratory perturbation was significantly reduced by N2O. Postural strategy was unaffected by N2O, but differed significantly between stimuli, with relatively less involvement of shear forces during vibratory perturbation in the eyes open condition than during the other body sway inducing stimuli. Subjective evaluation of narcosis correlated with body sway variance during the visual stimulus. The present findings indicate that subanesthetic N2O narcosis reduces postural control. Adaptation to a balance disturbing stimulus, with undisturbed vision, is decreased by N2O. Visual clues are shown to be crucial to the postural strategy adopted to maintain balance. The effects of N2O are assumed to be due to impairment of sensorimotor integration in the CNS.