Co and N2 Are No Humane Alternatives to Co2 Euthanasia in Rats.

Carbon dioxide (CO ₂ ) is commonly used for euthanasia of laboratory animals but has potential negative side effects. There is an urgent need to identify alternatives causing minimal distress while providing a fast and reliable loss of consciousness (LOC). We evaluated different CO ₂ concentrations (30% and 70%) and explored carbon monoxide (CO) and nitrogen (N ₂ ) as alternatives for a more humane euthanasia in rats. Isoflurane anesthesia was included as a reference. We assessed relevant physiological parameters in rats using a multifaceted approach combining EEG, ECG, telemetry, EMG, respiration, behavior, biochemical stress parameters, and lung histopathology. 70% CO ₂ resulted in rapid LOC and a faster death compared to 30% CO ₂ thus minimizing the time of consciousness in which distress and eventually pain can be experienced. Nevertheless, a higher amount of distress (e.g. gasping, arrhythmia or seizures) experienced until LOC might outweigh the benefits of a faster LOC. CO and N ₂ resulted in longer times to LOC, induced seizures before LOC, increased stress levels and caused higher lung damage. Therefore, CO and N ₂ are no humane alternatives to CO ₂ and should not be used for euthanasia of rodents. However, combining CO ₂ with anesthetic gases might be suitable to induce a more humane euthanasia.