Transient neocortical, hippocampal and amygdaloid EEG silence induced by one minute inhalation of high concentration CO2 in swine.

The study is part of a series of investigations performed with the ultimate goal of obtaining an objective evaluation of the ethical aspects and the narcotic efficiency of CO2 inhalation used as pre-slaughter anaesthesia for swine. Six Yorkshire swine were exposed twice to 80% CO2 for 1 min during simultaneous recording of the EEGs from the frontal neocortex, the dorsal hippocampus, and the amygdaloid region via permanently implanted electrodes. In five of the animals myoclonic jerks started at 28 +/- 1 s of CO2 exposure and lasted for 6 +/- 2 s. Neocortical slow wave (delta) activity and increased amplitude of the hippocampal theta (5-7 Hz) waves (i.e. EEG changes seen during the second stage of barbiturate anaesthesia) had developed before the brief period of myoclonic jerks. After this period the EEG activity gradually declined, resulting in neocortical EEG silence at the end of the exposure. This apparent isoelectricity lasted for on average 1 min. The return of the neocortical EEG activity exhibited a pattern reverse to its disappearance, but was much prolonged in comparison to the EEG extinction. Pre-exposure neocortical EEG pattern was not regained until 3-5 min post-exposure. In eight out of 11 experiments the CO2 inhalation also induced hippocampal EEG silence lasting for on average 30 s. EEG flattening was further obtained when recording from the amygdaloid nuclear complex and the adjacent pyriform cortex. The observed changes in the neocortical and hippocampal EEGs suggest that the present swine were unconscious already when they exhibited motor reactions. This does not exclude the possibility that CO2-independent stress/arousal factors present in a slaughterhouse environment may facilitate the development of motor phenomena similar to seizures, with the result that such reactions become manifest before the neocortical EEG exhibits an anaesthesia pattern. The duration of the observed EEG silence implies that, from the ethical point of view, exsanguination might safely be performed within 1 min after the moment when the animal is removed from the high concentration CO2. However, the slow return to a pre-exposure neocortical EEG pattern suggests that the swine remains unconscious for at least another minute.