Assessment of unconsciousness during carbon dioxide stunning in pigs
The aim of this study was to assess unconsciousness in pigs during exposure to CO2 through changes in the middle latency auditory evoke potentials (MLAEP) of the central nervous system (CNS), blood parameters (pH, carbon dioxide partial pressure [pCO2], oxygen partial pressure [pO2], oxygen saturation [SatO2] and bicarbonate [HCO− 3]), behaviour and the corneal reflex. The MLEAP did not decrease significantly until after 60 s exposure to CO2. The blood parameters (decreased pH, pO2 and SatO2 and increased pCO2 and HCO− 3) changed 53 s after the onset of immersion. The burst suppression index (BS%) and the A-line ARX index (AAI) from the MLEAP recovered basal levels at 136 and 249 s, respectively. The first blood parameter to return to basal levels was HCO− 3 at 76 s of exposure, followed by SatO2 at 180 s, pH and pO2 at 210 s and pCO2 at 240 s. During exposure to the gas, pigs exhibited lateral head movements and sneezing (10.3 s), gasping (23.5 s) and vocalisation (26.1 s). Furthermore, all pigs demonstrated muscular excitation after between 19 and 39 s exposure, when the AAI and BS% values were not significantly different from basal values. It was suggested, therefore, that these excitatory movements represent conscious movement, indicative of aversion to the gas. According to our results, loss of consciousness began, on average, after 60 s inhalation of 90% CO2. During exposure to the gas, decreased brain activity was seen, immediately following the changes in blood parameters. Following exposure, the restoration of blood parameters to basal levels allows a return to normal brain activity.
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