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Peri-anesthetic pain management strategies (Proceedings)

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Nov 01, 2009

There are a variety of options for providing analgesia to veterinary patients in the peri-anesthetic period. In the past, pain relief has not always been a priority in the veterinary field as some practitioners have not always believed that animals were capable of feeling pain. Today, pain management strategies in veterinary patients have improved drastically and the chance of decreasing patient suffering is greatly improved by providing appropriate analgesia.

Pre-emptive analgesia

Pre-emptive analgesia is the provision of an analgesic drug prior to the induction of a painful stimulus. Generally, if analgesia is provided before pain occurs it is easier to control. This occurs because administration of an analgesic before pain occurs prevents the occurrence of central sensitization, also called "wind-up". In surgical patients it is possible to administer an analgesic ahead of surgery to help achieve this, but it is not possible in every situation. Patients that come in after being hit by car, with intervertebral disk disease, fractures, or osteoarthritis cannot receive true pre-emptive analgesia. However, if these patients receive appropriate analgesics in a timely manner, the occurrence of central sensitization can be minimized as can the development of chronic pain states.

Multi-modal analgesia

When different classes of drugs are combined, they can have a synergistic effect. This means that the drugs when given together are more effective than any one drug administered alone. This technique reduces the total amount of each drug that is needed and therefore reduces the incidence of side effects from administering larger amounts of individual drugs. Combining different drug classes to provide analgesia also offers the advantage of being able to inhibit pain sensation at different points along the pain pathway.
     • NSAIDS- prevent pain and inflammation by interfering with prostaglandin synthesis (meloxicam, carprofen)
     • Opioids- bind to opioid receptors in the brain and dorsal horn of the spinal cord to produce sedation and analgesia (butorphanol, morphine, oxymorphone, hydromorphone, fentanyl, buprenorphine)
     • Tramadol- Centrally acting analgesic for treatment of moderate to severe pain. Acts as a weak agonist at mu opioid receptor. Acts to increase serotonin and norepinephrine.
     • Methadone- binds to the mu opioid receptor but also acts as a weak NMDA receptor antagonist
     • Gabapentin- helpful for management of intractable pain→ chronic pain, pain from nerve injury or extensive trauma.
     • Alpha 2 Agonists- provide sedation, muscle relaxation as well as analgesia by binding to receptors in the CNS. They also bind to other alpha 2 receptors throughout the body to exert their other effects (xylazine, medetomidine, dexmedetomidine).
     • Ketamine- should not be used alone for analgesia but excellent as an adjunctive analgesic at sub-anesthetic doses. Acts as an NMDA receptor antagonist.
     •Local Anesthetics- cause a reversible depression of nerve impulses by blocking voltage gated sodium channels→ loss of sensation as well as motor control (lidocaine, bupivicaine).

Epidural analgesia

Epidural analgesia (or anesthesia) is provided by injecting an analgesic drug into the space around the spinal cord. The drug will then diffuse out to reach the spinal nerves and produce analgesia. Epidurals are most commonly performed for surgeries that involve the rear limbs such as cranial cruciate ligament repair, rear limb amputations, and total hip replacements. Epidurals can also be used for abdominal surgeries such as cesarean sections and splenectomies. Epidural administration should be avoided in patients that are coagulopathic, have generalized sepsis, have a skin infection or pathology at the level of the L/S junction, or patient's with neurologic dysfunction. The main advantage of providing epidural analgesia is that the amount of inhalant and other analgesics needed to maintain a surgical plane of anesthesia can generally be greatly reduced, and post-operative analgesia is achieved with minimal sedation and other systemic effects.

Brachial plexus block

A brachial plexus block can be performed for surgeries distal to the elbow joint. Bupivicaine is most commonly used for this block due to its length of blockade (up to 6 hours), compared to lidocaine (1-2 hours). Bupivicaine has an onset time of about15-30 minutes. Drug doses should always be carefully calculated and the dose of bupivicaine should not exceed 1.5-2.0 mg/kg. If the volume is very small, the drug can be diluted with saline to provide an adequate volume for administration. The brachial plexus block is not difficult to perform but care must be taken to avoid IV administration of drug as well as penetration of the thoracic wall.