Reasons to provide regional anesthesia
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.
History of regional anesthesia/analgesia
The use of a local anesthetic drug was first demonstrated in 1884 when cocaine was used to desensitize the eye prior to surgery. Due to cocaine being habit forming and having a low safety margin, as well as the emergence of techniques allowing artificial synthesis of chemical compounds in the 1900s, non-toxic, non-addictive local anesthetics were sought, discovered, and manufactured. Initially, amino ester-type compounds were produced, until in 1943 lidocaine, an amino amide drug, was developed. Amide-type local anesthetic drugs are preferred for their longer duration of action, and several compounds in this group were discovered in the latter half of the 20th century, including mepivacaine, bupivacaine, and ropivacaine.
Principles of the major techniques
Topical application
Local anesthetic drops, e.g. proparacaine, can be directly applied to the eye for immediate relief of keratoconjunctival pain, although prolonged use delays corneal healing and is not recommended. Local anesthetic can also be directly applied to exposed tissue. Drug is directly deposited into the surgical field by dripping from a syringe, or soaking surgical sponges in local anesthetic and applying them to the tissue. Local anesthetic can also be instilled into the chest via a thoracostomy tube to desensitize the pleura following chest surgery, and into the abdominal cavity to treat pain following incision of the peritoneum. Local anesthetic cream is available as a mixture of lidocaine and prilocaine, which is used to desensitize skin for intravenous catheter placement. Lidocaine is also available as a transdermal patch.
Regional infiltration
Continuous regional analgesia is accomplished by placing fenestrated “soaker” catheters in areas that are not amenable to peripheral or regional analgesic techniques. The catheter is then attached to an infusion pump or an elastomeric bulb which delivers a set rate of local anesthetic over a specified period of time.
Intravenous regional analgesia
Analgesia can be provided to a distal limb by placing an esmarch bandage and injecting local anesthetic (lidocaine only) into a vein.
Intra-articular injection
Local anesthetics injected into joints have a long duration of action due to slow systemic uptake. There is in vitro evidence that local anesthetics may be detrimental to chondrocyte health, with preservative-free formulations being preferred. However, in vivo, this has not been shown to be definitively the case (Chu et al, 2008).
Peripheral nerve blockade
Individual or groups of sensory nerves supplying a specific region are located by palpation, electrophysiology, ultrasound or varying combinations of two or three of these techniques. Local anesthetic is then deposited adjacent to, but not into, the nerves. Nerves are typically blocked at sites proximal and distant to t...
