Aarnes Turi Kenna - Handbook of Small Animal Regional Anesthesia and Analgesia Techniques

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Phillip Lerche

The use of regional anesthesia as a component of perioperative pain management has gained acceptance and popularity in small animal practice over the past few decades. Reasons for this include the fact that many of the regional blocks are straightforward to perform, requiring moderate technical skill given familiarity with patient anatomy; they can be conducted relatively safely given an understanding of local anesthetic drug pharmacology, complications and side effects; and they contribute to the two major tenets of treating pain: pre-emptive and multimodal analgesia.

Providing pre-emptive analgesia by performing regional anesthesia prior to surgery leads to a drastic reduction in intraoperative nociceptive (pain) stimulation. This results in a decrease in anesthetic maintenance drug as well as intra- and postoperative analgesic requirements, thereby decreasing the incidence of drug side effects during surgery, and improving postoperative patient comfort as well as duration of pain relief. Some techniques can be continued postoperatively to assist in managing pain after particularly painful surgeries once the patient has recovered from anesthesia, e.g. instilling local anesthetic into a chest tube after thoracotomy, or injecting local anesthetic into an epidural or spinal catheter after pelvic limb or abdominal surgery.

The experience of pain, a sensory process involving the nociceptive pathway, is complex, and involves several steps. Noxious stimuli involving mechanical, chemical or thermal injury to tissue are first transduced into electrical stimuli by peripheral nociceptors (pain receptors). These electrical impulses are then transmitted to the spinal cord, where they are modulated by neurons in the dorsal horn of the gray matter of the spinal cord. Here, impulse intensity can be increased (amplified) or decreased (suppressed). Finally, the nociceptive signals are projected via lateral nerve fibers to the brain where they are perceived.

Whereas most analgesic drugs either decrease the amount of excitatory neurotransmitters, or increase the level of inhibitory neurotransmitters released in the nociceptive pathway, drugs used to provide regional anesthesia block sodium channels in neurons. This completely prevents sensory neurons from transmitting noxious stimuli from the periphery to the brain and spinal cord, or from the spinal cord to the brain in the case of epidural or spinal analgesia, thus providing effective pain relief for the duration of the block. Using regional anesthetic techniques in conjunction with other analgesic drugs that act in different ways on the nociceptive fibers (e.g. with opioids, alpha-2 agonists, ketamine) results in multimodal analgesia, contributing to an overall decrease in excitatory neurotransmission within the pain pathway both during and after surgery. This approach allows for the lowest effective dose of each drug to be used, which decreases side effects and enhances patient safety.