Introduction

Recently local and regional anesthetic techniques have gained widespread acceptance in small animal practice. The mechanism of action of local anaesthetics (lidocaine, mepivacaine, ropivacaine and bupivacaine) is to interrupt nervous conduction by occupying a specific receptor site located in axonal Na+ channels, thus restricting passage of this ion through the channel. Usually, the effect is restricted to the site of application and the action of these agents is quickly reversed due to its short half-life and subsequent decrease in concentration. Local anesthetics are organic bases of the amide group, with a pKa ranged between 7.6 and 8.1 with a high affinity for tissue proteins. These characteristics, as well its low systemic toxicity make the use of local anesthetics a valuable option in regional anesthetic techniques. These agents are often used with opioids, alfa2-receptor agonists, dissociatives and anti-inflammatory drugs as part of multimodal strategy to manage pain in small animals.

Table 1. Local anesthetics properties.

Differential Sensory/Motor Blockade

An important clinical consideration is the ability of local anesthetic agents to cause a differential blockade of sensory and motor fibers. Small nerve fibers tend to be more susceptible to the action of local anesthetics than large nerves fibers. Some local anesthetics (bupivacaine, ropivacaine) selectively block sensory rather than motor function. The required time for tissue desensitization (onset-time) is higher in ropivacaine than lidocaine, but similar to bupivacaine. Ropivacaine in an equal analgesic dose produces a shorter duration of the motor blockade than bupivacaine. This may be an advantage when the proposed blockade involves muscle structures taking part in respiratory movements or translation. However there are reports indicating that the difference between both drugs is not significant for lower concentrations (0.125%) mostly used in pain treatment.

Techniques

Local and regional anesthetic techniques are easier to perform in small animals that have been sedated or anesthetized. Some indications are obvious, but the local anesthetic techniques have a broader application than just surgical procedure. To perform it, practitioners should review the sensory innervation to the zone to block. Percutaneous desensitization, after induction of anesthesia, is also suggested; however the veterinarian must be familiar with landmarks.

Nerves Frequently Blocked

A. Dental Nerve Blocks

There are a number of conditions for which we can use dental nerve blocks. The obvious indication is for preemptive and postoperative analgesia for dental extractions; however dental nerve blocks provide excellent analgesia for many procedures in the oral cavity and head.

1.  Infraorbital nerve block

2.  Mandibular alveolar nerve block

3.  Maxillary or caudal infraorbital nerve block

4.  Mental nerve block

Most nerves can be blocked with 0.5 to 1.0 mL of 2% lidocaine, 0.25-0.5% bupivacaine or 0.2-0.5% ropivacaine.

B. Cervical and Thoracic Nerves Blocks

The regional blockade of cervical and thoracic nerves provides analgesia for surgical procedures and pain relieves involving forelimb and thorax. During forelimb amputation, desensitization of the brachial plexus may be carried out intraoperatively by infiltration of 0.25% bupivacaine 15 minutes prior to transection.

1.  Brachial plexus

a.  Axillary approach: useful for pain treatment located in structures distal elbow. Dose: 0.8 mL/kg 1.5% lidocaine with epinephrine, 0.25% bupivacaine or 0.2% ropivacaine.

b.  Paravertebral approach: Nerves of brachial plexus (C6; C7; C8 and T1) are blocked as they exit intervertebral foramina. Dose: 0.5 to 1.0 mL of 2% lidocaine, 0.25-0.5% bupivacaine or 0.2-0.5% ropivacaine. Useful to improve analgesia and muscular relaxation of the shoulder and elbow.

2.  Intercostals nerves. Intercostal nerve block during thoracotomy is performed by infiltration of 0.25% bupivacaine on the posterior aspect of the rib close to the spinal column while avoiding the intercostal artery and spanning at least 2 intercostal spaces each side of the incision.

C. Thoracic and Pelvic Limbs Nerves

Selective block of radial, ulnar, median and musculocutaneous nerves in forelimb or lumbar and sacral nerves in hindlimb and perineum, may provide analgesia as well as facilitate anesthetic management during surgical procedures. Most nerves can be blocked with 1.2 to 2.0 mL of 2% lidocaine, 0.25-0.5% bupivacaine or 0.2-0.5% ropivacaine. During hindlimb amputation, desensitization of femoral and sciatic nerves may be carried out intraoperatively by infiltration of 0.25% bupivacaine 15 minutes prior to transection. Sciatic nerve is a very big branch and, as well as with the brachial plexus, is mandatory the infusion of large amount of the local anesthetic to produce a solid block (0.8 mL/kg 1.5% lidocaine with epinephrine, 0.25% bupivacaine or 0.2% ropivacaine).

In cats, prior to onychectomy of the fore paw, injection of 0.2 mL of 1% lidocaine or 0.25% bupivacaine subcutaneously at each site of the dorsomedial aspect of the carpus, just proximal to the joint (block of the superficial branches of the radial nerve), and medial and lateral to the carpal pad (block of median nerve and palmar and dorsal cutaneous branches of the ulnar nerve) will confer analgesia for approximately 30 minutes to four hours depending on the solution used. For the hind paw, subcutaneous injection on the dorsomedial aspect of the tarsus just distal to the joint (selective block of distal branches of the common peroneal and tibial nerves) and on the ventromedial aspect of the tarsus just distal to the joint (superficial branches of the tibial nerve are blocked) will give approximately 30 minutes to four hours of analgesia.

References

1.  Lemke KA, Dawson SD. Local and regional anesthesia. Vet Clin N Amer:Sm Anim Pract. 2000; 30(4): 839-857.

2.  Otero E. Pablo. Dolor. Evaluación y tratamiento en pequeños animales. Editorial Inter-médica. Argentina; 2004.

3.  Skarda RT. Local and regional anesthetic and analgesic techniques: In: Thurmon JC. Tranquilli WJ, Benson GJ, eds. "Lumb and Jones" Veterinary Anesthesia, Baltimore, MD, Williams and Wilkins, 1996, pp 426-447.

4.  Tranquilli WJ.; Grimm KG.; Lamomt LA. Pain management for small animal practitioner. Jackson, Wy: Teton New Media; 2000.