Anesthesia

Pain: Recognition of pain depends upon intact pathways from pain receptors to the thalamus and cerebral cortex, as well as functional cerebral cortex and subcortical structures. Thus any means that renders the cerebral cortex nonfunctional, such as hypoxia or drug depression, prevents pain. When this happens, stimuli that evoke motor nerve reflexes that may be painful to the conscious animal are not painful in the unconscious animal. Equally painful stimuli administered to animals chemically paralyzed by curare or succinylcholine will not evoke a motor reflex simply because of paralysis, but will cause pain because of the conscious state. Hence, it is possible that unconscious animals may feel no pain but respond to certain stimuli, and paralyzed animals may feel pain but cannot respond. Thus, movement is not a reliable indicator of pain, and paralyzing agents (i.e., succinylcholine and curare) are strictly prohibited as euthanatizing agents. The methods used for prevention or relief of pain and distress in scientific experimentation with living animals will be dependent upon the kind of procedures used on the animals. Selection of an appropriate anesthetic, analgesic, or tranquilizer require the assistance of an experienced professional.

Anesthesia:

The local or general loss of sensation. General anesthesia is achieved by depressing the brain receptors of pain, thus producing a general anesthetic effect, although not necessarily blocking local responses such as spinal cord reflex arcs. Therefore, it is possible to have good levels of general anesthesia but still have motor reflexes such as pinch-pad and corneal reflexes present. These should not be mistaken for purposeful responses to pain. They can however, be abolished by deepening the level of anesthesia. Great care must be exerted when general anesthesia is made too deep since not only are pain receptors depressed, but also the vital centers of the brain and brain stem including respiratory, cardiac, hypothalamic, etc. When depressed for too long, heart and respiratory function cease and death ensues unless heroic measures are taken--if they are available.

Regardless of the species involved, some principles of general anesthesia are universal and worth keeping in mind. They include:

1. Maintain patent airway. This is essential if trouble arises and the subject is to survive. Nothing must block the ability to breathe freely and easily. With small rodents that are obligate nose breathers, a patent airway is easily maintained if the nostrils are not blocked.

2. Avoid hypothermia.Core body temperature can fall alarmingly, particularly in small animals, during the course of prolonged general anesthesia. Hypothermia added to other factors can produce an irreversible sequence of events leading to death. Thermostatically controlled heating pads should always be used in animal surgery.

3. Administer anesthetic to effect.Technically, because of wide variation within and between species, there is no such thing as predetermined anesthetic dose of a drug. General anesthesia must be given to effect, as measured by physiological parameters and response to stimuli. Most anesthetic deaths can be attributed to not following this principle. This is especially true for parenterally administered drugs such as barbiturates. Once they are injected, there is little the anesthetist can do to control the outcome; therefore, great care is necessary when administering these drugs. Most methods of general anesthesia as listed in Table 1 are generally safe and reliable if properly administered and monitored.