A COMPARISON OF EUTHANASIA METHODS IN RATS, USING CARBON-DIOXIDE IN PREFILLED AND FIXED FLOW-RATE FILLED CHAMBERS

The two methods (prefilled and fixed flow rate filled chambers) recomm ended in the 1993 AVMA Euthanasia Panel report for using carbon dioxid e to euthanatize rats were evaluated in terms of their effect on behav ior and selected blood gas values. Responses were videotaped during ex posure to greater than or equal to 90% carbon dioxide in a prefilled c hamber or a gradually filled chamber, using a fixed flow rate of 20% c hamber volume/min. Arterial blood samples were taken to determine part ial pressure of oxygen, partial pressure of carbon dioxide, pH, and ox ygen saturation prior to entering the chamber and at time points deter mined by rats' responses to carbon dioxide. Rats showed similar reacti ons when exposed to carbon dioxide by either method. Significant diffe rences in mean time for each response to occur were seen between eutha nasia methods. Maintaining a near atmospheric oxygen chamber concentra tion by using a 75% CO2: 20% O-2: 5% N-2 gas mixture to gradually fill the chamber did not change rats' reactions upon exposure. Significant differences were found between pre-exposure values and values from sa mples obtained when rats became immobile after entering the prefilled chamber. Partial pressure of carbon dioxide significantly increased, a nd pH and percent oxygen saturation significantly decreased from pre-e xposure values in all samples obtained after rats entered the graduall y filled chamber. Partial pressure of oxygen in these rats was greater than or equal to pre-exposure levels in all samples. Rats appeared se dated because of the anesthestic effects of carbon dioxide when immobi lity was observed. Distress was not observed in the rats when either m ethod of euthanasia was used.