The pain stops here: local anesthetic analgesics (Proceedings)

Currently, the only analgesics available to veterinarians that prevent pain transmission are local anesthetics, which facilitate a variety of veterinary procedures. In addition to short-term analgesia, incorporation of local blocks can reduce the long-term consequences of pain. These tools are also readily available and are not controlled. This lecture will review local anesthetic benefits, techniques and case examples where incorporation of local anesthetics is simple and effective.

Benefits of local anesthetics

Although there are over 50 different local anesthetics available, the most commonly used local anesthetics in small companion animal medicine include lidocaine and bupivacaine. Local anesthetics are antinociceptive because they block neuronal conduction—thus preventing transmission of noxious stimuli to the spinal cord and brain. Neuronal conduction is blockaded via the sodium channel, which stops depolarization of the desensitized neurons. While this simplified explanation summarizes the overall effects of local anesthetics, individual variation at different nerves can occur. For example, it makes sense that blockade of smaller nerve fibers would occur first (secondary to a greater concentration of drug to nerve size), but small C-fibers actually require more drug for blockade. This probably relates to the type of sodium channels C-fibers express, but highlights the need for individualizing a proposed local block based on which nerves are targeted. Blockade also appears more rapid if the nerve is stimulated at the time of blockade, warranting the use of a nerve stimulator, if available.

In addition to preventing transmission of noxious stimuli, there are other benefits to using local anesthetics. There is some evidence that local anesthetics may have dose dependent anti-inflammatory properties. Although there are a variety of mechanisms by which these anti-inflammatory actions occur, it is likely a combination of physical changes (such as a preservation of the endothelial barrier with use of local anesthetics) as well as direct effects on mediators of inflammation. Local anesthetics appear to inhibit both pro-inflammatory mediator release, as well alter pro-inflammatory mediator activity. Additionally, local anesthetics also appear to alter all stages of leukocyte migration to inflamed tissue, and they may also help reduce free radical formation. This information would suggest that sites where a local block is incorporated may be less likely to have local infection, which is an added benefit to amenable sites for blockade.

Other important benefits of these drugs include their relative inexpensiveness, ready availability (non-controlled) and ease of use.

Local blocks important in daily practice

Dental blocks are incorporated to an anesthetic plan to help manage noxious stimuli associated with surgery of the jaw, surgery of the face, and dental procedures. The nerves targeted include the mental, inferior alveolar (mandibular), and infraorbital nerves. The mental nerve foramen is easily palpable rostrally between the canine and the first premolar tooth, on the buccal side of the mandible. Palpation and blockade of the inferior alveolar nerve is more complicated; the foramen is either externally or internally performed, at the caudal aspect of the mandible (lingual, ventral and rostral to the angular process). The infraorbital canal is readily palpable; the foramen is ventral to the eye where the zygomatic arch meets the maxilla, along the buccal mucosa. This foramen is very shallow in the feline so needle insertion should not be deep. Remember, as with any local block, to aspirate before depositing local anesthetic at any of these foramens, and remain within the toxic dose limits of local anesthetic. In smaller species, combine the local anesthetic with saline to provide more volume.

In addition to blocks of specific foramen, local anesthetics can facilitate procedures like minor mass removal and biopsies by infiltrating the subcutaneous tissue and skin and thus desensitizing the proposed surgical area. To ensure the local anesthetic is not given systemically, aspirate with the syringe prior to administration. If blood is aspirated, the needle and syringe should be removed and re-directed. Total local anesthetic dose must remain below the toxic dose. Success of this type of block is related to the concentration of the local anesthetic at the site of innervation.

In certain blocks, specific nerves are targeted, such as the distal paw block. There is no conclusive evidence that either infiltration or specific nerve blocks improve post-operative analgesia, but with the ease of performing such blocks and the sound theory behind the analgesic benefits, the effort is warranted. The distal paw block, occasionally referred to as the four-point block, provides desensitization to the superficial branches of the radial, the palmar and dorsal cutaneous ulnar, and the median nerve. Desensitization of the radial nerves is performed proximal to the carpal joint, and on the dorsomedial aspect of the paw. The ulnar nerve is desensitized at two points: proximal and lateral to the accessory carpal bone. The medial carpal pad is the landmark for desensitizing the median nerve, which is blocked proximal to this site.

Inclusion of preservative free local anesthetic for epidural drug deposition unquestionably provides desensitization of the hind limbs. While it is possible to only provide sensory blockade without loss of motor innervation (differential blockade), the possibility of motor blockade must be considered, especially in a large patient where staff is physically not capable of assisting the patient in moving. However, sensory nerves may be more sensitive to the effects of local anesthetics than motor nerves. The effects of local anesthetic epidurally administered are relatively short-lived compared to those of morphine; in general, the epidural nerve regions regain sensation within four to six hours after an epidural is performed.

Other routes of administration

Topical administration of local anesthetic can facilitate minimally invasive procedures such as small local biopsies or catheter placements, as well as desensitize large wounds post surgery. Eutectic mixture of local anesthetics (EMLA) cream, a combination of lidocaine and prilocaine, is now generically available. When applied topically, it can desensitize skin within 10-20 minutes of application. Use of this cream significantly increased the success rate of jugular catheterization in feline patients (Wagner et al. 2006). It is important for practitioners to wear gloves when applying this product to a patient's skin. Five percent lidocaine patches successfully desensitize incision sites they are placed over, resulting in greater patient comfort. Both products have little systemic uptake, making them safe for use in even small patients.

Intravenous regional anesthesia, or Bier block, is a useful procedure for compromised patients requiring an invasive procedure of the distal limb. The patient is sedated to the point of compliance, and the limb is clipped and prepped to the level of the elbow. A suitably sized catheter is placed as distal (and directed distally) as possible in the cephalic vein and secured. An esmarch bandage, beginning distally, is used to exsanguinate the limb, and a tourniquet is applied above the elbow. Lidocaine is then infused through the catheter and the limb is given 5-10 minutes for the block to take effect. Although the block may provide desensitization for up to 90 minutes with the tourniquet in place, it is important that the tourniquet remain in place for NO LESS than 15-20 minutes, to prevent a large bolus of lidocaine from rapidly diffusing into the systemic circulation. For this same reason, bupivacaine should NEVER be used for intravenous regional analgesia techniques.

Local anesthetics may also provide analgesia when administered systemically, in addition to their use at local sites. Mechanisms behind this are unclear, but may relate to damaged tissues expressing more or different sodium channels. Lidocaine has been used systemically as an anti-arrhythmic for many years, and continues in this role currently. However, its incorporation to a balanced analgesic plan both alone and with other drugs post-operatively is relatively easy, inexpensive and effective. Additional case examples will be discussed during the lecture.

Local anesthetics are versatile drugs with a multitude of applications resulting in greater patient comfort, and the only option that prevents conduction of noxious input. Once local anesthetic administration is routinely incorporated into veterinary care, these options become simple and quick adjunct techniques to improve patient comfort. Improved patient comfort has both short-term and long-term benefits for patients, with little additional expense and record keeping for veterinarians. Therefore, for veterinarians looking to expand their analgesic options, local anesthetics are an excellent choice.

Reference

Wagner, K. A., K. J. Gibbon, T. L. Strom, J. R. Kurian & L. A. Trepanier (2006) Adverse effects of EMLA (lidocaine/prilocaine) cream and efficacy for the placement of jugular catheters in hospitalized cats. J Feline Med Surg, 8, 141-4.