Anesthesia and Analgesia Guidelines

• Neuromuscular blocking agents do not provide relief from pain and should not be used as the sole agent for animals having surgery or involved in potentially painful or distressful procedures.
• Individual, strain, stock or breed variation in responses to anesthetics and analgesics may be encountered.
• Procedures that may potentially cause more than momentary of slight pain or distress to the animals must be performed with appropriate sedation, analgesia, or anesthesia, unless the procedure is justified for scientific reasons on the animal use protocol and approved by the IACUC. Note, animals anesthetized during potentially painful procedures but not provided post-procedural analgesics may still experience pain or distress.

Inhalation Anesthesia - delivery of volatile anesthetic agents via the respiratory tract

Open drop ("bell jar")

• Anesthetic concentration within the jar cannot be controlled and lethal concentrations can rapidly accumulate.
• Liquid anesthetic is volatilized by placing anesthetic moistened material (i.e. cotton balls, gauze) into the bottom of the jar.
• Anesthetic moistened material should be covered with mesh or some other suitable material to prevent direct animal contact with the liquid phase anesthetic.
• This method had historically been most safely used with methoxyflurane because it reaches a maximum concentration of ~ 3%, but this anesthetic is no longer commercially available. Other commercially available anesthetic agents reach concentrations far exceeding the level needed for safe anesthesia.
• This method contains no provisions for scavenging excess gases and should be used within a fume hood to minimize personnel exposure.

Anesthetic Chamber

• May be used with species capable of comfortably fitting into the anesthetic chamber.
• The animal is placed into the chamber for induction, then removed from the chamber with anesthesia maintained by delivery through a face/head mask.
• Chamber and mask delivery incorporate addition of a calibrated vaporizer into the circuit for precise control of the concentration of anesthetic gas

Nonrebreathing (open) system

• Used with a precision vaporizer and therefore permitting control over anesthetic concentrations.
• Patient continuously exposed to "fresh" gas mixture.
• May be used with face/head mask or intubation.

Rebreathing (closed) system

• Used with a precision vaporizer and therefore permitting control over anesthetic concentrations.
• Exhaled gases flow through soda lime which removes all the carbon dioxide.
• Conserves anesthetic and oxygen.
• May be used with face/head mask or intubation.
• Due to rodents' small tidal volumes (the amount of gas entering the respiratory tract during a single breath) this method is rarely used in these species.

Tracheal intubation may not be practical in smaller species. If tracheal intubation is attempted, care should be taken to avoid traumatizing the oropharygeal structures and pulmonary tissue. Advantages of inhalant anesthetic techniques include: 1) increased control over anesthetic concentrations and depth of anesthesia (excluding bell jar technique), 2) decreased time of recovery from anesthesia, and 3) ability to choose agents (e.g. isoflurane) that require minimal metabolism, biotransformation for excretion and thus contribute minimal variability to overall goals of the research project.

Chronic exposure to trace amounts of inhalant anesthetics is associated with adverse human health effects. Inhalant anesthetics should only be used where waste gases can be safely removed from the work environment. Low fresh gas flows will minimize the amount of gases requiring scavenging. Anesthetic chambers or delivery systems such as masks that are not easily sealed should be used inside fume hoods. Only fume hoods that do not recirculate exhausted gases into other areas are acceptable. Commercially available canisters may be used for absorption of halogenated anesthetics but are not effective in absorbing nitrous oxide. The canisters have a limited useable life and must be used and changed as per the manufacturer's recommendations.

B. Parenteral Anesthesia - administration of anesthetic agents usually by injection (ex. IM, SQ, IP, IV).

It is advisable to monitor anesthetized animals so that therapeutic intervention can occur or anesthesia can be terminated prior to a fatal outcome.

Parameters which can be used to monitor the physiological status and depth of anesthesia include:

• recumbency and loss of purposeful movements
• loss of reflexes (e.g. corneal, pedal, pinnae reflexes)
• muscle relaxation
• lack of vocalization
• response to aversive stimulation (ex. pinching the toes or tail)
• respiratory rate and pattern
• pulse rate
• arterial blood pressure

In most instances, respiratory assessments are limited to observations of chest wall movement and cardiovascular assessment to palpation of the heart pulse through the chest wall.

Most anesthetics cause a depression of the thermoregulatory mechanism, predisposing animals to hypothermia. Smaller species are particularly prone to hypothermia due to their large surface area to body mass ratio allowing a greater loss of body heat. Heat loss is further compounded by hair removal, wetting with antiseptics, cold surfaces, opening of body cavities, and cold surgical solutions. Hypothermia may lead to a decline in metabolism and excretion of the anesthetic agent.

Methods to minimize heat loss in rodents during anesthesia and recovery include:

1. Provide supplemental heat

• increase surgery room temperature
• recirculating warm water blanket
• heat lamps and electric heating pads (Inadvertent thermal injuries may occur when using these products. If used, the temperature nearest the animal should be monitored and not permitted to exceed 42^oC.)
• administration of warmed subcutaneous or intraperitoneal fluids (saline, Lactated Ringers, etc.) intra and/or post-operatively
• supplemental heat may be required for the support of body temperature during the procedure and recovery from anesthesia

2. Minimize heat loss

• insulate the patient from the surgical table
• avoid wetting excessive areas with antiseptic or other solutions
• minimization of organ exposure from body cavities during surgery

Presurgical administration of analgesics or local anesthetics, referred to as "preemptive analgesia", may delay the onset and severity of post-procedural pain. The beneficial effects of preemptive analgesia may not completely eliminate the need for post-operative analgesics but will likely minimize the dosage needed to maintain animal comfort.

In humans, the assessment of pain and suffering is usually communicated verbally. However, since animals cannot communicate their discomfort to humans verbally, their clinical appearance must be interpreted to determine when they are in pain or discomfort. Anesthetics may provide relief from pain during the anesthetic period but offer little pain relief after recovery from anesthesia. When in doubt as to the degree of pain experienced by animals, procedures that are considered to be painful to humans should also be considered to be painful to animals.

The following clinical signs may be associated with pain/distress in animals:

• lack of responsiveness or depression
• immobility or reluctance to move
• lack of appetite or thirst leading to weight loss and/or dehydration
• abnormal vocalization or vocalization during handling
• abnormal posture (i.e. hunched)
• changes in respiratory pattern
• apprehensive or aggressive behavior
• poorly groomed or soiled hair coat
• self-mutilation (i.e. biting, scratching)

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