Changes in the somatosensory evoked potentials and spontaneous electroencephalogram of broiler chickens during exposure to gas mixtures

1. Six week-old broiler chickens implanted with electroencephalogram (EEG) recording and somatosensory stimulating electrodes were exposed to either 90% argon in air, a mixture of 30% carbon dioxide and 60% argon in air or a mixture of 30% oxygen and 40% carbon dioxide (balance nitrogen) for 2 min, to determine the times to onset of changes in spontaneous EEG and the loss of somatosensory evoked potentials (SEPs) and thus unequivocal loss of consciousness. 2. In addition, after a 2 min exposure to the carbon dioxide-oxygen mixture, some broilers were allowed to recover in air and their EEGs and SEPs were continuously recorded until the return of normal EEG and SEPs. During this period, the time to return of response to comb pinching was also determined in 10 broilers. 3. All broilers exposed to either argon or the carbon dioxide-argon mixture died within 2 min, whereas, only 3 out of 17 broilers died during the 2 min exposure to the carbon dioxide-oxygen mixture. 4. During exposure to argon, unlike the other 2 gas mixtures, the majority of broilers showed high amplitude, low frequency electrical activity in the EEG on average at 10 s. The mean times to onset of EEG suppression were 17, 19 and 40 s after exposure to argon, the carbon dioxide-argon mixture and the carbon dioxide-oxygen mixture, respectively. An isoelectric EEG occurred on average at 58 and 41 s after exposure to argon and the carbon dioxide-argon mixture, respectively. An isoelectric EEG did not occur in broilers which were exposed to the carbon dioxide-oxygen mixture. 5. The SEPs were abolished in broilers on average 32 and 24 s after exposure to argon and the carbon dioxide-argon mixture, respectively. During exposure of broilers to the carbon dioxide-oxygen mixture the SEPs were abolished in the majority of birds on average at 47 s, however, 2 out of 14 birds retained their SEPs for the entire period of 2 min exposure to this gas mixture. 6. During the recovery after exposure to the carbon dioxide oxygen mixture, response to comb pinching and SEPs returned either at the time of, or soon after, the onset of high frequency electrical activity in the suppressed EEG of broilers. The mean times to return of response to comb pinching and SEPs were 52 and 43 s, respectively. 7. Based on the time to onset of EEG suppression or loss of SEPs, exposure of broilers to either 90% argon in air, or a mixture of 30% carbon dioxide and 60% argon in air, resulted in quicker loss of consciousness than during exposure to a mixture of 40% carbon dioxide, 30% oxygen and 30% nitrogen. The time to return of consciousness after a 2 min exposure to the carbon dioxide-oxygen mixture was also found to be rapid.