persons and other nearby persons and animals. The selection of a procedure also depends on the skill of the personnel performing euthanasia and the number of animals to be killed.

The criteria that should be used in selecting an agent or method for euthanasia include:

• Lack of pain perception by the animal.

• Relative rapidity and irreversibility.

• Minimal stress, apprehension, and fear in the animal or in nearby animals.

• Minimal interference with experimental protocol.

• Simultaneous interruption of consciousness and reflex mechanisms.

• Safety of personnel.

• Reliability.

• Efficiency and ease of procedure.

• Cost and availability.

• Minimal psychologic stress for attending personnel and observers.

• Minimal adverse environmental impact.

• Minimal potential for human drug abuse.

Each means of euthanasia has advantages and disadvantages. It is unlikely that any agent or method will meet all the above criteria in a given situation.

An acceptable means of euthanasia must have an initial depressant action on the CNS that causes relatively rapid unconsciousness and insensitivity to pain. Therefore, an overdose of a chemical anesthetic is a desirable means of euthanasia. Once the animal is anesthetized, the method used to kill it is less important. In some research protocols, the use of chemical agents is contraindicated, and physical methods must be used.

When the animal is unconscious, it is desirable to stop its heart action as soon as possible, to reduce the flow of blood to its brain. A technique need not be considered inhumane merely because the heartbeat persists for a longer period, as long as the animal is unconscious (CCAC, 1980). There are three basic causes of death: hypoxia, either direct or indirect; direct depression of neurons vital for life functions; and damage to brain tissue (Sawyer, 1988).

Attitudes, sensitivities, and scientific knowledge regarding euthanasia are changing. A number of agents or methods formerly used are now considered undesirable, because they are inhumane, dangerous to personnel, or unaesthetic. Use of strychnine, nicotine, magnesium sulfate, potassium hydrochloride, hydrocyanic acid, curare, or succinylcholine is not considered acceptable (Lumb and Moreland, 1982). Others have sufficient disadvantages to exclude them from a list of recommended agents and methods.

Agents and methods recommended for euthanasia (Table 7-1) can be grouped into three categories: inhalational agents, physical methods, and noninhalational pharmacologic agents. The selection of the appropriate technique depends on an evaluation of the factors and criteria previously discussed.