HUMANE AND PRACTICAL IMPLICATIONS OF USING CARBON-DIOXIDE MIXED WITH OXYGEN FOR ANESTHESIA OR EUTHANASIA OF RATS

A series of studies was undertaken to determine whether CO2, can be us ed as a humane as well as practical agent for euthanasia or anesthesia of rats. Human volunteers rated the degree of discomfort associated w ith breathing 50 to 100% CO2, mixed with oxygen. Increasing concentrat ions of CO2 were judged as progressively more noxious, from ''highly u npleasant'' for 50% CO2, to ''painful'' for 100% CO2. The practical as pects of anesthesia and euthanasia with 50 to 100% CO2, were studied, using male Sprague Dawley rats. Time to anesthesia and death were inve rsely related to CO2, concentration, as were the frequency and severit y of adverse reactions, including seizures and hemorrhaging from the n ose. The severity of edema and hemorrhage, which were observed on hist ologic examination of the lungs of all rats euthanized with CO2, were greatest in the animals exposed to the lowest concentrations. There we re no significant. effects of CO2, concentration on time to recumbency or recovery, and there were no significant effects of precharging ver sus not precharging the chamber on any of the parameters studied. It w as concluded that, although CO2, can be used in a humane manner, the c oncentrations that are least likely to cause pain and distress are ass ociated with the longest times to anesthesia and death, highest incide nce of unwanted side effects, and most severe histologic changes in th e lungs. Acceptably humane and reasonably practical euthanasia or anes thesia can be achieved using a nonprecharged chamber and a low gas flo w rate so that conscious animals are never exposed to CO2 concentratio ns >70%.