SURGERY, PAIN PERCEPTION AND ANESTHESIA: To carry out surgical procedures on animals, pain perception must be completely suppressed. This can be achieved either by general anesthesia, which produces loss of consciousness, or by local or regional anesthesia. While different agents can appear to provide similar levels of hypnosis (sleep), the degree of intraoperative analgesia produced can vary with the agent. It is important to ensure that an anesthetic regimen is selected that provides an appropriate degree of intra-operative analgesia. If an anesthesia is being induced simply to provide humane restraint while non-painful procedures are carried out, then only light anesthesia, with little pain suppression, will be required. Conversely, if potentially painful surgical procedures are to be undertaken, then deep anesthesia, with complete suppression of pain perception, is necessary.

GENERAL ANESTHESIA: General anesthesia involves the required features of loss of consciousness, analgesia, suppression of reflex activity and muscle relaxation. Often a single drug can produce all aspects of general anesthesia or it can be achieved by a combination of agents, each making a contribution to the overall effect. The latter approach has the advantage that undesirable side effects can often be minimized. The side effects of anesthetics are usually dose dependant. Giving several drugs in combination, at relatively low doses, often results in less effect on major body systems than that following anesthesia with single anesthetic agents. The best anesthetic regimens, including those involving inhalant anesthetics, usually involve an initial tranquilization and/or muscle relaxation followed by general anesthesia using an anesthetic drug. Such multiple agents are not generally or sequentially used for rodent anesthesia where general anesthesia using inhalant anesthetics may routinely be induced using only the anesthetic agent. There are distinct advantages and disadvantages of this type of regimen as noted below.

ADMINISTRATION BY INHALATION: Inhalant anesthetics may be supplied to the patient either by anesthetic machines using face masks or endotracheal tubes or by using enclosed chambers containing anesthetic vapors. Anesthesia machines are designed to supply oxygen, anesthetic gases and volatile anesthetic agents to the animal. The gases pass through a vaporizer which is designed for use with a particular volatile anesthetic agent. Flowmeters and calibrated vaporizers on the machines control anesthetic vapor and oxygen levels. Breathing circuits on machines may be open or closed. A widely used breathing circuit is an open facemask, which is a simple and convenient way of delivering anesthetic gases to an animal. Expired gases pass around the edges of the masks and, depending upon the gas flow rate, rebreathing of exhaled gas can be minimized and dilution of the anesthetic gases by breathing room air can be avoided. Such an apparatus should always be used in a scavenging hood. The most serious disadvantage in using a simple open circuit (besides the safety issue for the surgeon) is that it is not possible to assist ventilation artificially in case of emergency. Both semi-closed and T-piece breathing circuits have been developed. The T-piece circuit was developed and intended to provide a circuit of low resistance and little dead space for use on infants. It has been used widely on small animals such as rabbits and cats where it is connected directly to an endotracheal tube or close-fitting mask. Occluding the “exhaust” end, which is fitted with a reservoir, controls ventilation. Closed breathing circuits are systems in which expired carbon dioxide is absorbed in a soda lime canister. Use of such circuits should be limited to experienced anesthetists. This system has routinely been used for larger animals but has now been adapted for use in anesthetizing small animals and rodents where an inline, sealable chamber is utilized to place the animal.

When controlled breathing, anesthetic machines are not available, the “drop method” using a bell jar technique may be used. Equipment ranges from the classic glass bell or desiccation jar with tight fitting lid, to plastic chambers with clear lids or aquaria. All chambers should have bottom screens, which separate the anesthetic soaked media (cotton balls or gauze) from the animal in order to prevent contact with the irritating anesthetic agent. Adequate scavenging (chemical fume hood, charcoal filtered safety hood, hood connected to house exhaust) must be used to prevent unintentional exposure of humans and animals to waste anesthetic gases. Inhalant anesthetic agents are poured onto the media and the chamber is closed to allow the anesthetic to vaporize inside the chamber (~5 minutes). An animal is then place into the chamber where it becomes anesthetized as it inhales the volatized anesthetic. After removal of the animal from the chamber, one may administer additional anesthetic during a procedure by placing an anesthetic soaked gauze or cotton containing nose cone over the face of the animal. For rats and mice, a 35-60 cc syringe barrel may be used for the nose cone. Unless adequate scavenging is available, this method of prolonging inhalant anesthesia is not recommended.

INHALANT ANESTHETICS: Many inhalant anesthetics are available for use in animals. One of the most widely used for rodent anesthesia is methoxyflurane (Metofane). However, this agent is no longer manufactured and halothane and isoflurane are being used as replacements. Methoxyflurane has been the preferred agent when using the bell jar technique because room temperature and pressures permit a 3% concentration to easily develop. This concentration of methoxyflurane is safe, whereas, lethal concentrations of halothane and isoflurane develop under identical conditions. The potency of each anesthetic agent is indicated by its, minimum alveolar concentration (MAC) value. This value, most commonly referred to simply as MAC, is the alveolar concentration of an anesthetic required to block the response to a specified stimulus in the animals. The lower the MAC value, the lower the concentration required, so the more potent the anesthetic. Information concerning the range of concentrations of the different inhalation anesthetics which are required for induction and maintenance of anesthesia are given below.