The replacement of air with nonanesthetic gases produces a deficiency of available oxygen, and animals develop hypoxia. Without sufficient oxygen, the brain becomes depressed, and the animals lapse into unconsciousness. These gases are not recommended for euthanasia of newborn animals, because newborn animals have been accustomed to low oxygen concentration in the uterus and are more resistant to hypoxic conditions (Rowsell, 1981).
Although some of the agents have been widely used, most have sufficient disadvantages that preclude their common use for euthanasia of laboratory animals. Two disadvantages in some instances are that excitement and struggling are associated with the initial action on the CNS and initiating vapors can cause anxiety during induction.
Most of the agents are hazardous to attending personnel and other animals (AVMA, 1986). Halothane, methoxyflurane, enflurane, and isoflurane are expensive and thus impractical for routine use. Ether is explosive and therefore not recommended. Carcasses of animals killed by ether require special storage, handling, and disposal, because ether fumes are retained in the bodies. Chloroform should not be used, because it is extremely hepatotoxic and potentially carcinogenic for humans. Nitrous oxide alone is not potent enough to be useful. Hydrogen cyanide and CO are dangerous to personnel and are not recommended.
CO2 is a well-accepted, commonly used gas for euthanasia of laboratory animals other than newborns. Inhalation of at least 40% CO2 has a rapid anesthetic effect that proceeds to respiratory arrest and death if exposure is prolonged. It is effective for use in small laboratory animals, including rodents, rabbits, cats, poultry, small dogs, and swine. It is rapid, painless, humane, easy to use, relatively inexpensive, nonflammable, nonexplosive, nearly odorless, and heavier than air. If used in well-ventilated areas, it is much safer than most of the other inhalational agents. Waste gas poses no substantial environmental hazard. CO2 is available in cylinders as a compressed gas or in a solid state as ''dry ice." The compressed-gas form is preferable. Animals should be kept out of direct contact with dry ice, to prevent chilling or freezing. CO2 is heavier than air, so chambers used for euthanasia should be filled from the bottom. Anxiety and struggling are minimized if the chamber is precharged. Relatively slow introduction of the gas into the chamber tends to minimize animal anxiety; high-speed flow causes turbulence and noise. Animals typically become unconscious within 45–60 seconds of the beginning of CO2 exposure. With continued depression of vital centers, hypoxia and death occur soon. Animals should remain in the chamber for at least 5 or 6 minutes and then examined closely to determine that all vital signs have ceased before they are removed from the chamber and disposed of.