Electroretinographic responses to the addition of nitrous oxide to halothane in rats

The electroretinograms of 20 anesthetised but spontaneously breathing rats (Wistar) were recorded over a period of 60 minutes. Two anesthetical procedures were applied to each of these rats respectively: (I) halothane/nitrous oxide/oxygen (2% halothane in 0.81 N2O: 0.41 O2/min) and (II) halothane/nitrogen/oxygen (2% halothane in 0.81 N2: 0.41 O2/min). From the 10th up to the 60th min of anesthesia the ERG amplitudes decreased gradually (I: a-wave 17% and b-wave 14%; II: a-wave 15% and b-wave 12%). Body temperature was experimentally maintained at nearly 37.0 degrees C. Under considerable diminution in both procedures the frequency of respiration was significantly higher under procedure I in the first 20 minutes only. Under procedure I the b-wave amplitudes were significantly higher in contrast to procedure II. This occurred at all applied stimulus intensities (6; 24; 100; 400 and 1600 Lux at the cornea). The a-wave amplitudes and the peak times of the a- and b-waves did not show any significant differences between the two experimental conditions. These ERG responses to nitrous oxide in addition to halothane are discussed on the basis of the respiratory, circulatory and cerebral effects of these anesthetics.