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) recommended in the 1993 AVMA Euthanasia Panel report for using carbon dioxide to euthanatize rats were evaluated in terms of their effect on behavior and selected blood gas values. Responses were videotaped during exposure to > or = 90% carbon dioxide in a prefilled chamber or a gradually filled chamber, using a fixed flow rate of 20% chamber volume/min. Arterial blood samples were taken to determine partial pressure of oxygen, partial pressure of carbon dioxide, pH, and oxygen saturation prior to entering the chamber and at time points determined by rats' responses to carbon dioxide. Rats showed similar reactions when exposed to carbon dioxide by either method. Significant differences in mean time for each response to occur were seen between euthanasia methods. Maintaining a near atmospheric oxygen chamber concentration by using a 75% CO2: 20% O2: 5% N2 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 samples obtained when rats became immobile after entering the prefilled chamber. Partial pressure of carbon dioxide significantly increased, and pH and percent oxygen saturation significantly decreased from pre-exposure values in all samples obtained after rats entered the gradually filled chamber. Partial pressure of oxygen in these rats was greater than or equal to pre-exposure levels in all samples. Rats appeared sedated because of the anesthetic effects of carbon dioxide when immobility was observed. Distress was not observed in the rats when either method of euthanasia was used.