CO₂ Stunning in Pigs: Physiological Deviations at Onset of Excitatory Behaviour

Simple Summary Stunning by carbon dioxide inhalation is commonly used in pigs before slaughter. However, the method is controversial because seemingly aversive behaviours are observed before the pigs appear to lose consciousness, i.e., fall over. It is unknown whether the aversive behaviours occur while pigs are conscious. The aim of the current study was to characterise behaviours observed during carbon dioxide stunning and relate these to physiological variables recorded when each behaviour began. We recorded physiological variables we considered to be essential in maintaining consciousness, and at a high sampling frequency. In total, 13 behaviours were characterised, 7 of which were examined more closely. The results show that some aversive behaviours were observed while physiological variables were both compatible and incompatible with consciousness. However, the physiological variables recorded at the onset of the behaviours "continuous neck dorsiflexion" and "agonal gasping" were sufficiently deranged in all pigs to justify the conclusion that these animals were unconscious when the behaviours began. These findings are important when considering animal welfare in relation to current stunning methods. Stunning by carbon dioxide (CO2) inhalation is controversial because it is associated with vigorous movements and behaviours which may or may not be conscious reactions. Furthermore, it is unknown whether some behaviours might indicate the transition into unconsciousness. Our study objective was to investigate the loss of consciousness during CO2 stunning by linking physiological variables (in particular pH, PaO2 and PaCO2) to the onset of observed behaviours. A total of 11 cross-bred pigs were studied. A tracheostomy tube, venous and arterial cannulae were placed under sevoflurane anaesthesia. After recovery from this, and a "wash out" period of at least 30 min, arterial blood samples were taken (and baseline values established) before 90-95% CO2 in medical air was administered through the tracheostomy tube. Subsequent behaviours were video-recorded and key physiological variables were evaluated using an anaesthetic monitor and the frequent sampling of arterial blood (albeit with inconsistent inter-sample intervals). After the study, behaviours were classified in an ethogram. At the onset of behaviours categorised as "vigorous movement extremities", "opisthotonos" and "agonal gasping" pH values (range) were: 6.74-7.34; 6.66-6.96 and 6.65-6.87, while PaCO2 (kPa) was 4.6-42.2, 24.4-51.4 and 29.1-47.6. Based upon these values, we conclude that the pigs were probably unconscious at the onset of "opisthotonos" and "agonal gasping", but some were probably conscious at the onset of "vigorous movements".