Anesthesia & Analgesia

Pain Relief in Animals

Analgesia is defined as the absence of sensibility to pain. Analgesia is present during appropriately administered general anesthesia. However, after an animal recovers from general anesthesia no analgesic effect remains. Experience in human and veterinary surgery has shown that pain and discomfort are common sequelae to surgical procedures and the healing period that follows. The anatomical structures and physiological processes involved in pain perception are similar in both humans and other mammals. Therefore, it is assumed that other mammalian species experience pain or discomfort after surgery.

• Public Health Service Policy and The Guide for the Care and Use of Laboratory Animals describe the need to avoid pain and distress as imperative.
  
• The U.S. Government Principals for the Care and Use of Laboratory Animals Used in Testing, Research, and Training directs investigators to consider that procedures that cause pain or distress in human beings may cause pain or distress in other animals.
  
• There is evidence in the literature suggesting that pain and it's associated stress increase variability in experimental results.
  
• Animals that are likely to experience more than momentary or slight pain should receive post procedural analgesia unless analgesic use will interfere with scientific objectives. Withholding of analgesics for scientific reasons must be justified in writing by the principal investigator.

Recognizing Pain

Assessment of the degree of pain or discomfort experienced by an animal can be challenging. While the physiologic mechanisms of pain perception are similar in all mammals the ability to tolerate and cope with pain may be vastly different in different species. For example, prey species such as rodents have adapted to hide overt signs of pain to avoid signaling to a predator that they are ill and would be an easy meal. A rodent experiencing mild to moderate pain may display only subtle behavioral signs associated with its discomfort.

Moderate to severe pain in rodents leads to more obvious changes in normal physiology and behavior. Accurate recognition of these changes requires that research personnel have some knowledge of normal behavior and physiology for the species they are using.

Clinical signs associated with pain in rodents

• Decreased activity or a reluctance to move
• Hunched or other abnormal posture or gait (e.g., lameness)
• Rough, greasy-looking coat (due to lack of normal grooming)
• Dark, red material around the eyes and nose in rats
• Excessive licking or chewing of a body part or area
• Decreased appetite and/or weight loss
• Unusual aggressiveness when handled
• Rats - following abdominal surgery:

• Stretching and back arching
• Abdominal pressing onto the cage floor
• Frequent sudden short movements

Physiologic changes that may be observed in rodents experiencing pain include:

• Fluctuations in body temperature, heart and breathing rates and blood pressure
• Weight loss
• Dehydration

Managing Pain

The recommended approach to managing postoperative pain is based on several considerations.

• Animals that are experiencing mild to moderate pain may display only subtle behavioral signs of pain.
• Since it is often difficult to assess how much pain an animal is experiencing, federal regulations direct us to consider that a procedure that causes pain in humans would also be expected to cause pain in animals.
• Pain is more easily controlled by administration of analgesics before pain starts than after the animal is experiencing pain.

Recommendations:

• Animals that undergo a painful procedure such as surgery and are likely to experience post procedural pain, should receive preemptive analgesic medication (before the animals recover from anesthesia).
• Additional dosing of analgesic medication may be predetermined or based on a subjective assessment of the degree of pain experienced by the animal.
• A predetermined analgesic regimen should be incorporated into the experimental design based on a prediction of how much pain will occur after an experimental procedure. All animals in the experiment are treated with the same amount and type of drug, with administration usually beginning immediately after surgery before the animal awakens. Treatments may be repeated for a prescribed period of time. This type of regimen insures that all animals will receive some type of analgesic treatment, but it does not relieve research personnel of their responsibility to observe and evaluate each animal after surgery. Animals that continue to show signs of pain or have other complicating problems after surgery need to be assessed individually and treated appropriately.

Analgesic Drugs and Delivery Methods

There are many drugs and routes of administration available for alleviation of pain in rodents. Some drugs, such as the opiates (example: buprenorphine or morphine), provide pain relief through their action on the central nervous system. Non-steroidal anti-inflammatory drugs (NSAIDS) relieve pain by decreasing inflammation and, in some cases, direct analgesic action. Long acting local anesthetics such as bupivacaine, provide postoperative analgesia by blocking nerve conduction.

In most cases analgesics should be administered by injection. Analgesic drugs may be placed in drinking water or food for oral delivery, but these methods run the risk of inaccurate dosing, lack of consumption due to palatability and degradation of the drug over time as it sits in the water bottle or food product.