Local and Regional Anesthesia

MANAGEMENT OF PAIN 0195-5616/00 $15.00 + .OO

LOCAL AND REGIONAL ANESTHESIA

Kip A. Lemke, DVM, MS, and Susan D. Dawson, PhD

Until recently, local and regional anesthetic techniques had not gained widespread acceptance in small animal practice. Lack of instruction at North American veterinary colleges, limited availability of publications describing techniques, and failure to approve new local anesthetics for veterinary use have contributed to the confusion surrounding the use of local and regional anesthetic techruques in dogs and cats. These techniques can be used safely and effectively to manage or pre-empt pain in a variety of clinical settings. Local anesthetic solutions and eutectic creams can be used topically for dermal and mucosal pain. Somatic tissues can be infiltrated with local anesthetic solutions to relieve pain associated with trauma and inflammation. Intravenous regional, peripheral neural, and central neural techniques can be used to manage pain in nonsurgical patients and to manage as well as pre-empt pain in surgical patients. Local anesthetics have the unique ability to produce complete blockade of sensory nerve fibers and prevent or pre-empt the development of secondary (central) sensitization to pain. For this reason, local and regional anesthetic techniques are often used with opioids, a,-receptor agonists, dissociatives, and anti-inflammatory drugs as part of a multimodal strategy to manage pain in small animals.

CLINICAL PHARMACOLOGY

General Considerations

Chemical Structure Local anesthetics are tertiary amines connected to an aromatic ring by an

ester or amide linkage, and they are classified as aminoesters (e.g., procaine) or

From the Departments of Companion Animals (KAL) d d Anatomy and Physiology (SDD), Atlantic Veterinary College, University of Prince Edward Island, Charlottetown, Prince Edward Island, Canada

VETERINARY CLINICS OF NORTH AMERICA: SMALL ANIMAL PRACTICE

VOLUME 30 NUMBER 4 - JULY 2000 839

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aminoamides (eg., lidocaine, bupivacaine), respectively. The aromatic ring is lipophilic, and substitution of alkyl groups on the aromatic ring or tertiary amine increases lipid solubility and potency. The tertiary amine readily accepts protons and is hydrophilic. At physiologic pH (7.4), local anesthetics exist in equilibrium as an uncharged base and a charged cation. The pH at which the concentration of the base is equal to that of the cation is defined as the dissocia- tion constant (pK,). Local anesthetics have pK, values greater than physiologic pH, and most of the drug exists in the hydrophlic protonated form. The charged cation does not penetrate lipid membranes as readily as the uncharged base, and drugs with pK, values closer to physiologic pH tend to have a more rapid onset of action.

Mechanism of Action

Local anesthetics block the generation and conduction of nerve impulses by inhibiting voltage-gated sodium channels. The resting membrane potential of nerve cells is near the potassium equilibrium potential (-70 mV). During generation of the action potential, voltage-gated sodium channels open briefly, allowing sodium ions to flow into the cell and depolarize the cell membrane. Local anesthetics bind to a hydrophilic site within the sodium channel on the inner surface of the cell membrane, block activation of the channel, and prevent depolarization of the cell membrane.7, 30 The drug must first pass through the cell membrane as an uncharged base to reach the intracellular site. Once inside the cell, the uncharged base is protonated and the charged cation binds to the receptor. Repetitive stimulation of nerve fibers increases binding affinity of the receptor site for local anesthetics and facilitates development of neural blockade (frequency-dependent blockade).

Differential Blockade

Small nerve fibers tend to be more susceptible to the action of local anesthetics than large nerve fibers. Myelinated fibers also tend to be blocked more readily than unmyelinated fibers of the same diameter. Generally, autonomic fibers (small unmyelinated C fibers and myelinated B fibers) and pain fibers (small unmyelinated C fibers and myelinated A6 fibers) are blocked before other sensory and motor fibers (large myelinated Ay, AP, and Aci fibers). Sensory fibers are also more sensitive to blockade by local anesthetics because they have longer action potentials and discharge at higher frequencies than other types of fibers (i.e., frequency-dependent blockade). Some local anesthetics (e.g., bupivacaine, ropivacaine) selectively block sensory rather than motor function.15, 16* 39

Absorption and Metabolism

Systemic absorption of local anesthetics is determined primarily by dose and route of administration. Uptake from mucosal, pleural, and peritoneal surfaces is rapid and complete. Local anesthetics are also absorbed quickly after epidural administration, but absorption is much slower after subcutaneous administration. Most local anesthetics cause peripheral vasodilation, which ac- celerates systemic absorption. In the past, vasoconstrictors (e.g., epinephrine) were added to local anesthetic solutions to reduce the rate of systemic absorption and prolong the duration of action. Given the availability of long-acting drugs (e.g., bupivacaine, ropivacaine) with high margins of safety, the addition of vasoconstrictors to local anesthetic solutions is no longer necessary. After sys-

LOCAL AND REGIONAL ANESTHESIA 841

temic absorption, uptake by the lungs plays an important role in the distribution of most local anesthetics. Degree of protein binding influences the duration of action of these drugs. Aminoesters (e.g., procaine) tend to be more hydrophlic, less protein-bound, and have a shorter duration of action. Conversely, aminoamides (e.g., lidocaine, bupivacaine) tend to be more lipophilic, highly protein- bound, and have a longer duration of action. The principal metabolic pathway for most local anesthetics is enzymatic hydrolysis. Aminoesters are hydrolyzed by plasma cholinesterase, and metabolites are excreted in the urine. Plasma cholinesterase is produced by the liver, and hydrolysis of aminoesters is some- what dependent on hepatic function. Most aminoamides undergo N-dealkylation and subsequent hydrolysis in the hepatic endoplasmic reticulum, and metabolites are excreted in the urine.’

Adverse Effects

Local anesthetics are relatively safe if they are used correctly. Administration of an excessive dose and accidental intravenous administration are probably the most common causes of systemic toxicity in small animals. Doses of local anesthetics, especially those for cats and small dogs, should always be calculated carefully. Local anesthetics can also cause direct damage to tissues, allergic reactions, and methemoglobinemia. Aminoamides are highly protein-bound, and displacement by other drugs that are also highly protein-bound (eg., nonsteroi- dal anti-inflammatory drugs) may precipitate the development of systemic toxicity. Additionally, hypoproteinemic animals may be more susceptible to local anesthetic toxicity, and doses should be reduced accordingly.

The toxicity of most local anesthetics increases with potency. In dogs, the relative central nervous system toxicity of lidocaine, etidocaine, and bupivacaine is 1:3:5.24 Systemic reactions to local anesthetics involve primarily the central nervous system and the cardiovascular system. Acute central nervous system toxicity occurs at much lower doses than those required to produce acute cardiovascular system toxicity. In dogs, the dose of lidocaine, bupivacaine, or etidocaine required to produce irreversible cardiovascular collapse is four to six times greater than that required to produce conv~lsions.~~ Muscle twitching and convulsions are usually the first signs of local anesthetic toxicity observed in small animals. If large doses are given, convulsions are followed by unconscious- ness, coma, and respiratory arrest.”, 27 Acid-base status also affects the central nervous system toxicity of local anesthetics. Elevated arterial Pco2 (68-81 mm Hg) decreases the convulsive dose of procaine, lidocaine, and bupivacaine by approximately 50% in cats.14

At higher doses, local anesthetics affect the heart and peripheral vasculature. Blockade of sodium channels in Purkinje fibers and the myocardium depresses electrophysiologic and mechanical function of the heart.9, 26, 29 At extremely high doses, local anesthetics cause sinus bradycardia and sinus arrest. Toxic doses of local anesthetics also cause peripheral vasodilation and profound hypotension. In dogs anesthetized with pentobarbital, intravenous administration of lidocaine (16 mg / kg) induces bradycardia and depressed hemodynamic function, intravenous administration of mepivacaine (12 mg/ kg) induces minimal changes in hemodynamic and electrophysiologic function, and intravenous administration of etidocaine (8 mg/ kg) or bupivacaine (4 mg/kg) depresses hemodynamic and electrophysiologic f~nct ion .~ Administration of toxic doses of bupivacaine can cause ventricular arrhythmias and cardiovascular collapse in dogs and cats8, 21

Further, bupivacaine is more cardiotoxic than ropivacaine when equivalent doses are administered to conscious dogs.I7 Acute systemic toxicity induced by intrave-

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nous administration of local anesthetics can be successfully treated by support- ing ventilation (i.e., endotracheal intubation, oxygen supplementation, positive- pressure ventilation) and controlling convulsions by administering small doses of thobarbiturates.l8

In addition to systemic toxicity, local anesthetics can cause localized nerve and muscle damage. Some concentrated local anesthetic solutions as well as those containing the preservative sodium bisulfite can be neurotoxic. Skeletal muscle changes have been observed after intramuscular administration of lidocaine, mepivacaine, bupivacaine, or etidocaine.2, 3, 38 Concurrent use of vasoconstrictors (e.g., epinephrine) increases tissue oxygen demand and decreases local oxygen supply and may delay wound healing or cause tissue necrosis. Addition- ally, local anesthetic solutions containing vasoconstrictors should not be used to produce neural blockade of distal extremities that have limited collateral circula- tion (e.g., paws).

Aminoester local anesthetics (eg , procaine) can also induce local and systemic anaphylactic reactions. This class of drugs is derived from p-amino benzoic acid, which is a common allergen. Although allergic reactions to aminoamide local anesthetics are extremely rare, solutions containing methylparaben, a preservative whose structure is similar to that of p-amino benzoic acid, can also induce allergic reactions.

Selected Drugs

Procaine

Procaine is an aminoester with an intermediate onset (10-15 minutes) and a short duration of action (30-60 minutes). The drug has a pK, of 8.9, is 6% protein-bound, and has a relative potency of one. The drug was once used in small animals for local infiltration anesthesia and peripheral nerve blocks. Be- cause procaine has a short duration of action, and because it can cause allergic reactions, the drug is used infrequently in small animals.

Lidocaine

Lidocaine is an aminoamide with an intermediate onset (10-15 minutes) and an intermediate duration of action (60-120 minutes). The drug has a pK, of 7.7, is 65% protein-bound, and has a relative potency of two. Lidocaine is the most versatile and widely used local anesthetic in small animal practice. The drug is effective topically and can be used for local infiltration and intravenous regional anesthesia as well as for peripheral and central (epidural, intrathecal) nerve blocks. Several formulations are available, including a 2% jelly, 10% spray, and 2% solution (20 mg/mL). In dogs, intravenous administration of lidocaine at a dose of 22 k 6.7 mg/kg (mean k SD) induces convulsions, and signs of central nervous system toxicity (salivation, muscle tremors) become apparent after administration of 36% of the convulsant dose." In cats, intravenous administration of lidocaine at a dose of 11.7 k 4.6 mg/kg (mean 2 SD) induces

Based on these results, healthy dogs and cats should not be given doses of lidocaine that exceed 12 and 6 mg/kg, respectively. For example, a healthy 3-kg cat should not be given more than 18 mg of lidocaine (0.9 mL of a 2% solution).


Mepivacaine

Mepivacaine is an aminoamide with a rapid onset (5-10 minutes) and a duration of action (90-180 minutes) comparable to that of lidocaine. The drug has a pK, of 7.6, is 75% protein-bound, and has a relative potency of two. Mepivacaine causes less tissue irritation and has a higher therapeutic index than lidocaine. The drug is not effective topically but can be used for local infiltration anesthesia as well as for peripheral and central (epidural) nerve blocks. Several formulations are available, including a 2% solution (20 mg/mL) that is labeled for veterinary use.

Bupivacaine

Bupivacaine is an aminoamide with a slow onset (20-30 minutes) and a long duration of action (240-360 minutes). The drug is structurally related to mepivacaine, has a pK, of 8.1, is 96% protein-bound, and has a relative potency of eight. Bupivacaine is not effective topically but can be used for local infiltration anesthesia as well as for peripheral and central (epidural, intrathecal) nerve blocks. Several formulations are available, including a 0.5% solution (5 mg/mL) and a preservative-free 0.5% solution labeled for epidural and intrathecal use. Bupivacaine produces sensory blockade with limited motor blockade. Because of the drug‘s long duration of action and selective sensory blockade, it is usually the best choice for management of surgical and nonsurgical pain in small animals. Bupivacaine is selectively cardiotoxic, however, and its use should be avoided in small animals with ventricular arrhythmias (ventricular premature contractions, ventricular tachycardia). In dogs and cats, intravenous administration of bupivacaine at doses of 5.0 + 2.2 and 3.8 -t 1.0 mg/kg (mean +- SD), respectively, induces convulsions.8, 24 Based on these results, healthy dogs and cats should not be given doses of bupivacaine that exceed 2 mg/kg. For example, a healthy 3-kg cat should not be given more than 6 mg of bupivacaine (1.1 mL of a 0.5% solution).

Etidocaine

Etidocaine is an aminoamide with a rapid onset (5-10 minutes) and a duration of action (180-300 minutes) comparable to that of bupivacaine. The drug has a pK, of 7.7, is 94% protein-bound, and has a relative potency of six. Etidocaine is not effective topically but can be used for local infiltration anesthesia as well as for peripheral and central (epidural) nerve blocks. Several formulations are available, including a 0.75% solution (7.5 mg/mL). Etidocaine produces preferential motor blockade, and the drug’s cardiotoxicity is comparable to that of bupivacaine.

Ropivacaine

Ropivacaine is an aminoamide with a rapid onset (5-10 minutes) and a duration of action (180-300 minutes) comparable to that of bupivacaine. The drug is structurally related to mepivacaine and bupivacaine, has a pK, of 8.1, is 94% protein-bound, and has a relative potency of eight. Ropivacaine can be used for local infiltration anesthesia as well as for peripheral and central (epidural, intrathecal) nerve blocks. Several formulations are available, including a 0.5% solution (5 mg / mL). Ropivacaine produces selective sensory blockade compara-

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ble to that of bupivacaine, and the drug's cardiotoxicity is approximately half that of bupivacaine.**

GENERAL TECHNIQUES

Local and regional anesthetic techniques are easier to perform in animals that have been sedated or anesthetized. Healthy dogs can be sedated with acepromazine or medetomidine, or a combination of acepromazine or medetomidine with an opioid (e.g., butorphanol, oxymorphone). Healthy cats can be sedated with acepromazine, or a combination of acepromazine with an opioid (e.g., butorphanol, oxymorphone). In fractious cats, low doses of ketamine (5-10 mg / kg administered intramuscularly) can be used for immobilization. Local anesthetics can also be used to reduce anesthetic requirements and enhance analgesia and muscle relaxation during general anesthesia. Small animal veterinarians should appreciate the fact that most injectable (e.g., thiopental, propofol) and lnhalation (e.g., halothane, isoflurane) anesthetics simply produce uncon- sciousness and are not analgesics per se. Preoperative administration of local anesthetics can pre-empt the development of pain and improve patient comfort postoperatively." 41

Topical Application

Topical application of local anesthetics is used primarily for minor diagnostic and surgical procedures. Lidocaine spray (lo%, 100 mg/mL) can be used to desensitize oral, nasal, and pharyngeal mucous membranes during invasive diagnostic procedures (e.g., foreign body removal, endoscopy) and placement of nasal catheters for tube feeding or administration of oxygen. The dose of topical lidocaine is similar to that administered intravenously and should not exceed 18 mg (0.18 mL) in a 3-kg cat. Sterile lidocaine jelly (2%, 20 mg/mL) can be used to facilitate placement of urethral catheters in male and female dogs and cats. A sterile 2% formulation in a cartridge is extremely useful for depositing in the vaginal vault of female dogs and cats. The amount of lidocaine jelly to use is similar to that for intravenous administration, as this is well absorbed across mucous membranes and can be equally as toxic should this dose be exceeded, especially in cats. Most local anesthetics are ineffective when applied topically to intact skin, but a eutectic mixture of lidocaine and prilocaine (EMLA cream) penetrates the stratum comeum and provides superficial analgesia. Eutectic creams can be used to facilitate placement of venous or arterial catheters in dogs and cats.I9 The cream must be applied under an occlusive dressing and requires at least 20 minutes to desensitize the skin. Benzocaine, a potential alternative, should not be used as it has been associated with methemoglobinemia in cats.40

Local Infiltration

Local infiltration is used primarily for removal of dermal or subcutaneous tumors and for repair of superficial lacerations. This technique can also be used to facilitate invasive diagnostic procedures (e.g., cutaneous or percutaneous biopsy). After aseptic preparation of the skin, 2% lidocaine solution is injected using a 25-gauge (0.5 mm), 0.6-in (16 mm) or 22-gauge (0.7 mm), 1-in (25 mm) needle in cats and small dogs or larger dogs, respectively. A small subcutaneous


bleb (0.5-2.0 mL) is usually sufficient to desensitize the skin for removal of small dermal tumors and for cutaneous or percutaneous biopsy in small to large patients, respectively. Larger tumors and superficial lacerations require a wider area of desensitization and larger volumes of 1% (less irritation than 2%) local anesthetic solution. Line and inverted V-blocks can be used to facilitate surgical removal or repair these lesions. Subcutaneous tumors require a deeper area of desensitization, and placement of lidocaine solution in a triangular or rectangu- lar pattern around these masses can facilitate surgical removal. The size of the lesion is the limiting factor for the use of local anesthetics in cats, as the potential for toxicity is higher than in dogs, and doses should be calculated carefully based on the above recommendations for intravenous dosing.

CANINE TECHNIQUES

Intravenous Regional Anesthesia

Intravenous regional anesthesia can be used to produce short-term (<2 hours) anesthesia of distal extremities (i.e., paws) in dogs?* An Esmarch bandage is applied to desanguinate the extremity, and a tight rubber tourniquet is placed proximal to the bandage. The bandage is removed, and lidocaine solution (0.25%4.5%) is injected intravenously at a dose of 2.5 to 5.0 mg/ kg using an accessible vein (i.e., cephalic or saphenous) distal to the tourniquet. The paw is desensitized within 10 minutes, and the tourniquet can be left in place for up to 90 minutes. Once the tourniquet is removed, sensation returns within 15 minutes and analgesia persists for up to 30 minutes.

Peripheral Neural Blockade

Cranial Nerves Selective blockade of distal branches of cranial nerves can provide short-

term analgesia as well as facilitate anesthetic management of dogs admitted for dental and surgical procedures (Fig. 1). Most nerves can be blocked with 0.5 to

Zygomatic n.

V Mental nn. Inferior alveolar n.

Figure 1. Canine cranial nerves-a lateral view of the canine skull with the distribution of the relevant branches of the trigerninal (cranial nerve V) nerve.

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1.0 mL of 2% lidocaine or 0.5% bupivacaine solution using a 22-gauge (0.7 mm) 1.0- to 1.5-in (25-40 mm) needle. As always, aseptic technique should be used, doses should be calculated carefully, and syringes should be aspirated before injection of local anesthetic solution to avoid inadvertent intravascular administration.

The infraorbital nerve provides sensory innervation to the upper dental arcade, soft and hard palates, and muzzle. The nerve can be blocked proximally as it enters the maxillary foramen, or it can be blocked distally as it exits the infraorbital foramen. The site of injection for the proximal block is located dorsal and caudal to the last molar and ventral to the junction of the zygomatic arch and maxilla. The site of injection for the distal block is located at the infraorbital foramen, which is dorsal to the rostral edge of the upper fourth premolar. Infraorbital nerve blocks are easy to perform and improve the anesthetic and pain management of dogs that require extraction or repair of teeth in the upper dental arcade.

The inferior alveolar nerve provides sensory innervation to the lower dental arcade and chin. The nerve can be blocked proximally as it enters the mandibular foramen, or it can be blocked distally as it exits the middle mental foramen. The site of injection for the proximal block is located on the ventromedial aspect of the ramus of the mandible dorsal and rostral to the angular process. The site of injection for the distal block is located at the middle mental foramen, which is ventral to the lower first and second premolars. Inferior alveolar nerve blocks are easy to perform and improve the anesthetic and pain management of dogs that require extraction or repair of teeth in the lower dental arcade.

Chronic otitis externa and otitis media in dogs are extremely painful condi- tions that often require surgical intervention. Preoperative blockade of the auriculotemporal nerve (cranial nerve V) and the great auricular nerve (cervical nerve 11) may reduce isoflurane requirements during surgery and improve recovery from anesthesia in dogs that undergo unilateral or bilateral total ear canal ablation and bulla osteotomy6 The site of injection for blockade of the auriculotemporal nerve is caudal and dorsal to the masseter and rostral to the vertical ear canal. The site of injection for blockade of the great auricular nerve is ventral to the wing of the atlas and caudal to the vertical ear canal.

Cervical and Thoracic Nerves

Selective and regional blockade of cervical and thoracic nerves can provide short-term analgesia as well as facilitate anesthetic management of dogs admitted for surgical procedures involving the front leg and thorax (Figs. 2 and 3). Most nerves can be blocked with 1 to 2 mL of 2% lidocaine or 0.5% bupivacaine solution using a 22-gauge (0.7 mm) 1.0- to 1.5-in (2540 mm) needle. As always, aseptic technique should be used, doses should be calculated carefully, and syringes should be aspirated before injection of local anesthetic solution to avoid inadvertent intravascular administration.

Blockade of the brachial plexus can be used to manage pain before and after surgical correction of front limb fracture^.'^ Traditionally, the block has been performed by injection of local anesthetic solution into the axillary space at the level of the s h o ~ l d e r . ~ ~ , ~ ~ Although this techmque is easy to perform, large amounts of anesthetic solution are required, onset is slow (20-30 minutes), structures proximal to the elbow are not anesthetized, and incomplete blockade of the plexus is relatively common. An alternate technique, whch is best described as a paravertebral brachial plexus block, has been used successfully at the Atlantic Veterinary College for several years. The paravertebral block is a


Figure 2. Lateral view of the canine skeleton, showing landmarks for the paravertebral brachial plexus nerve block.

modification of a technique taught to anesthesiology residents at the University of Illinois. Nerves of the brachial plexus (C6, C7, C8, T1) are blocked as they exit intervertebral foramina. To perform h s block, the scapula is moved caudally and the large transverse process of the sixth cervical vertebra and the head of the first rib are palpated. Next, C6 and C7 are blocked by injecting local

Figure 3. Medial view of the canine right thoracic limb showing the distribution of the nerves of the brachial plexus.

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anesthetic solution dorsal to the cranial and caudal margins of the transverse process of the sixth cervical vertebra. Finally, C8 and T1 are blocked by injecting local anesthetic solution dorsal to the cranial and caudal margins of the head of the first rib. When performing the paravertebral brachial plexus block, needles should be directed caudally to avoid epidural or intrathecal injection. Addition- ally the vertebral artery and branches of the costocervical artery run in close proximity to the nerves as they exit intervertebral foramina, and syringes should always be aspirated before injection. Unilateral blockade of the phrenic nerve may also occur but does not seem to compromise pulmonary function in conscious or anesthetized dogs. Given the potential for blockade of the phrenic nerve and paralysis of the diaphragm, this block should not be performed bilaterally. Unlike the traditional technique, the paravertebral braclual plexus block has distinct landmarks that are easy to palpate, requires small amounts of local anesthetic solution, has a fast onset, and produces anesthesia of the thoracic limb distal to the scapula. Both techniques are unreliable in obese or heavily muscled animals, because landmarks are obscured. The paravertebral brachial plexus block improves analgesia and muscle relaxation for surgery of the shoulder and elbow, and it can be used preoperatively and postoperatively to manage pain in dogs with humeral fractures.

Anesthesia of the front leg distal to the elbow joint can be achieved by selective blockade of the median, ulnar, musculocutaneous, and radial nerves.25 The median, ulnar, and musculocutaneous nerves are blocked by injecting local anesthetic solution proximal to the medial epicondyle of the humerus and between the biceps and triceps. The brachial artery and vein are adjacent to these nerves, and syringes should be aspirated before injection. Pulsation of the braclual artery can be used to locate these three nerves. The radial nerve is blocked by injecting local anesthetic solution proximal to the lateral epicondyle and between the brachialis and triceps. In most dogs, these nerves can be palpated digitally, and direct pressure elicits an avoidance response in conscious animals. The block can be used preoperatively and postoperatively to manage pain in dogs with radial, ulnar, and metacarpal fractures.

Selective intercostal nerve block is used to control pain in dogs with rib fractures or discrete pleural lesions as well as before and after intercostal thoracotomy.2". 33, 36 In contrast to parenteral administration of morphine or oxymorphone, selective intercostal nerve block with bupivacaine has minimal effects on minute ventilation and blood gas values postoperatively in dogs after intercostal thorac~tomy.~ With the exception of obese animals, intercostal nerve blocks are relatively easy to perform in dogs. Intercostal nerves and vessels lie adjacent to the caudal border of each rib, and two or three nerves on either side of the incision site or lesion should be blocked. Local anesthetic solution is injected distal to the angle of the rib near the insertions of the epaxial (longissimus, iliocostalis) muscles. Needles are directed dorsally and medially and "walked off" the caudal border of each rib. Syringes should be aspirated before injection of local anesthetic solution. Pneumothorax may occur in a small number of animals, and dogs should be observed closely for 20 to 30 minutes after performance of the block. Animals with severe pulmonary disease, which rely on their intercostal muscles to maintain ventilation and oxygen saturation, should be watched continuously.

Intrapleural regional anesthesia may not control pain as effectively as selective intercostal nerve blockade, but the technique is easier to use in conscious dogs with thoracostomy tubes. Uptake of local anesthetics is rapid after intrapleural administration, and systemic toxicity can occur if doses are not calculated carefully. Intrapleural administration of bupivacaine can be used postoper-


atively to manage pain in dogs undergoing intercostal or sternal thoracotomy.’O, 35, 36 After surgical placement of a thoracostomy tube, 0.5% bupivacaine is

administered at a dose of 1.5 mg/kg and the tube is flushed with saline solution. Next, the dog is placed with the incision side down for 10 to 20 minutes to allow local anesthetic solution to pool near the incision site. Bupivacaine then diffuses across the pleura and blocks intercostal nerves adjacent to the site. Onset of analgesia is rapid and persists for 4 to 6 hours. If a thoracostomy tube was not placed during surgery, or if the tube has been removed, bupivacaine can be administered by placing a 16-gauge (1.7 mm) 3.25-in (83 mm) over-the- needle catheter into the pleural space using aseptic technique. In the awake patient, the placement of bupivacaine into the pleural space can be quite painful. At the Ontario Veterinary College, 0.2 mL/kg of 0.5% bupivacaine solution with the addition of 0.02 mEq/kg of sodium bicarbonate and normal saline to a final solution of 10 to 20 mL (for small to large dogs) raises the pH to -7.4. When administered slowly, this mixture is well tolerated by the patient (Dr K. Mathews, personal communication). Pneumothorax is a potential complication of intrapleural regional anesthesia, but this complication can be managed easily if a thoracostomy tube is in place. Strict aseptic techmque is required. At a dose of 1.5 mg/kg, intrapleural administration of bupivacaine has minimal effects on cardiovascular and pulmonary functioning in dogs.l2, 23, 35 Intrapleural regional anesthesia is unreliable in animals with extensive pleural adhesions.

Peritoneal Space

Pain caused by pancreatitis can be difficult to manage with opioids alone. The technique described previously for intrapleural bupivacaine can also be useful as an adjunct to opioids in dogs with pancreatitis. The bupivacaine plus sodium bicarbonate mixture, with saline added to a final volume of at least 10 to 20 mL, is placed into the abdomen via abdominocentesis at the level of the umbilicus, performed with an 18- or 20-gauge over-the-needle catheter. Strict adherence to dosing is important, as the drug is readily absorbed across the inflamed peritoneum. The patient is placed in dorsal recumbency for 10 minutes and then in ventral recumbency with the hindquarters slightly elevated for a total of 20 minutes for both positions. Strict aseptic technique is required (K. Mathews, personal communication).

Lumbar and Sacral Nerves

Selective blockade of lumbar and sacral nerves can provide short-term analgesia as well as facilitate anesthetic management of dogs admitted for surgical procedures involving the hind leg and perineum (Fig. 4). Most nerves can be blocked with 1 to 2 mL of 2% lidocaine or 0.5% bupivacaine solution using a 22-gauge (0.7 mm) 1.0- to 1.5-in (2540 mm) needle. As always, aseptic technique should be used, doses should be calculated carefully, and syringes should be aspirated before injection of local anesthetic solution to avoid inadvertent intravascular administration.

Anesthesia of the hind leg distal to the hip can be acheved by selective blockade of the saphenous, common peroneal, and tibia1 nerves.” The saphenous nerve is a branch of the femoral nerve and provides sensory innervation to the medial surface of the thgh, stifle, and lower leg. The nerve runs through the femoral triangle on the medial surface of the thigh and lies cranial to the femoral artery and vein. Pulsation of the femoral artery can be used to locate the saphenous nerve. The nerve is blocked by injecting local anesthetic solution

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Figure 4. Lateral view of the canine vertebral column and left pelvic limb, showing the level of termination of the spinal cord and the nerves of the lumbosacral plexus.

cranial to the femoral artery, and syringes should be aspirated before injection. The common peroneal nerve is a branch of the sciatic nerve and provides sensory innervation to the dorsal aspect of the lower leg. The nerve runs laterally over the gastrocnemius and around the fibula and is blocked by injecting local anesthetic solution dorsal and posterior to the head of the fibula. The tibial nerve is also a branch of the sciatic nerve and provides innervation to the caudal aspect of the lower leg. The nerve can be blocked above the stifle by injection of local anesthetic solution between the medial and lateral heads of the gastrocnemius. The tibial nerve can also be blocked below the stifle by injecting local anesthetic solution between the superficial flexor tendons and the long digital extensor tendon distal to the lateral saphenous vein. Selective blockade of the saphenous, peroneal, and tibial nerves can be used preoperatively and postoperatively to manage pain in dogs with tibial, fibular, and metatarsal fractures.

Intra-articular Analgesia

Intra-articular bupivacaine has been assessed for postoperative analgesia in dogs after stifle surgery. Details on administration of intra-articular analgesics are provided elsewhere in this issue.


FELINE TECHNIQUES

Peripheral Neural Blockade

Cranial Nerves

Selective blockade of distal branches of cranial nerves can provide short- term analgesia as well as facilitate anesthetic management of cats admitted for dental and surgical procedures (Fig. 5). Most nerves can be blocked with 0.1 to 0.3 ml of 2% lidocaine or 0.5% bupivacaine solution using a 25-gauge (0.5 mm) 0.6-in (16 mm) needle. As always, aseptic technique should be used, doses should be calculated carefully, and syringes should be aspirated before injection of local anesthetic solution to avoid inadvertent intravascular administration.

The infraorbital nerve provides sensory innervation to the upper dental arcade, soft and hard palates, and muzzle. The nerve is blocked as it exits the infraorbital foramen. The foramen is located ventral to the eye at the junction of the zygomatic arch and maxilla. The infraorbital nerve block is easy to perform and improves anesthetic and pain management of cats that require extraction or repair of teeth in the upper dental arcade.

The inferior alveolar nerve provides sensory innervation to the lower dental arcade and chin. The nerve can be blocked proximally as it enters the mandibular foramen, or it can be blocked distally as it exits the mental foramina. The site of injection for the proximal block is located on the ventromedial aspect of the ramus of the mandible dorsal and rostra1 to the angular process. The site of injection for the distal block is located between the mental foramina, caudal and ventral to the lower canine. Inferior alveolar nerve blocks are easy to perform and improve the anesthetic and pain management of cats that require extraction or repair of teeth in the lower dental arcade.

Cervical and Thoracic Nerves

Selective and regional blockade of cervical and thoracic nerves can provide short-term analgesia as well as facilitate anesthetic management of cats admitted

Zygomaticofacial n. gomaticotemporal n.

Infiatrochlear n

Auriculotemporal n.

Zygomaticofacial n. Frontaln. / 7-r-nn7aticotemporal n.

Auriculotemporal n.

Y \

Mental M. Inferior alveolar n.

Figure 5. Feline cranial nerves-a lateral view of the feline skull with the distribution of the relevant branches of the trigeminal (cranial nerve V) nerve.

852 LEMKE & DAWSON

for surgical procedures involving the front leg and thorax (Figs. 6 and 7). Most nerves can be blocked with 0.1 to 0.3 mL of 2% lidocaine or 0.5% bupivacaine solution using a 25-gauge (0.5 mm), 0.6-in (16 mm) needle. As always, aseptic technique should be used, doses should be calculated carefully, and syringes should be aspirated before injection of local anesthetic solution to avoid inadvertent intravascular administration.

Blockade of the brachial plexus can be used to manage pain before and after surgical correction of front limb fractures. Nerves of the brachial plexus (C6, C7, C8, T1) are blocked as they enter the axillary space. The brachial plexus block is performed by injecting 1 mL of 2% lidocaine (20 mg) or 0.5% bupivacaine (5 mg) solution between the shoulder and thoracic wall. A 22-gauge (0.7 mm), 1-in (25 mm) needle is placed into the axillary space just above the shoulder, and local anesthetic solution is injected cranial to the first rib and caudal to the cranial border of the scapula. Although the technique is easy to perform, large amounts of local anesthetic solution are required and structures proximal to the elbow are not anesthetized. Additionally, the axillary artery and vein run in close proximity to the brachial plexus, and syringes should be aspirated before injection. Unilateral blockade of the phrenic nerve may also occur but does not seem to compromise pulmonary function in conscious or anesthetized cats. Given the potential for blockade of the phrenic nerve and paralysis of the diaphragm, this block should not be performed bilaterally. The brachial plexus block improves analgesia and muscle relaxation for surgery of the elbow and can be used preoperatively and postoperatively to manage pain in cats with fractures of the radius and ulna.

Selective blockade of distal branches of the radial, median, and ulnar nerves can facilitate anesthetic and pain management of cats admitted for onychec- t ~ m y . ~ * Superficial branches of the radial nerve are blocked by injecting local

Figure 6. Medial view of the feline right thoracic limb showing the distribution of the nerves of the brachial plexus.


Radial n

Dorsal Palmar

Figure 7. Dorsal and palmar views of the feline front paw, showing distribution of nerves.

anesthetic solution subcutaneously on the dorsomedial aspect of the carpus just proximal to the joint. The median nerve and the palmar and dorsal cutaneous branches of the ulnar nerve are blocked by injecting local anesthetic solution subcutaneously medial and lateral to the carpal pad. This technique is easy to perform and seems to provide exceptional short-term analgesia postoperatively.

Selective intercostal nerve blockade is used to control pain in cats with rib fractures or discrete pleural lesions as well as before and after intercostal thoracotomy. With the exception of obese animals, selective intercostal nerve block is relatively easy to perform in cats. Intercostal nerves and vessels lie adjacent to the caudal border of each rib, and two or three nerves on either side of the incision site or lesion should be blocked. Local anesthetic solution is injected distal to the angle of the rib near the insertions of the epaxial muscles. Needles should be directed dorsally and medially and walked off the caudal border of each rib. Syringes should be aspirated before injection of local anesthetic solution. Pneumothorax may occur in a small number of animals, and cats should be observed closely for 20 to 30 minutes after performance of the block. Animals with severe pulmonary disease, which rely on their intercostal muscles to maintain ventilation and oxygen saturation, should be watched continuously.

Lumbar and Sacral Nerves

Selective and regional blockade of lumbar and sacral nerves can provide short-term analgesia as well as facilitate anesthetic management of cats admitted for surgical procedures involving the hind leg and perineum (Figs. 8 and 9). Most nerves can be blocked with 0.1 to 0.3 mL of 2% lidocaine or 0.5% bupivacaine solution using a 25-gauge (0.5 mm), 0.6-in (16 mm) needle. As always, aseptic techtuque should be used, doses should be calculated carefully, and syringes should be aspirated before injection of local anesthetic solution to avoid inadvertent intravascular administration.

Anesthesia of the hind leg distal to the hip can be achieved by selective blockade of the saphenous, common peroneal, and tibia1 nerves. The saphenous

854 LEMKE & DAWSON

Figure 8. Lateral view of the feline vertebral column and left pelvic limb, showing the level of termination of the spinal cord and the nerves of the lumbosacral plexus.

Dorsal

/

Tibia1 n.

Lateral

o u Plantar

Figure 9. Dorsal and plantar views of the feline hind paw, showing distribution of nerves.


nerve is a branch of the femoral nerve and provides sensory innervation to the medial surface of the thigh, stifle, and lower leg. The nerve runs through the femoral triangle on the medial surface of the thigh and lies cranial to the femoral artery and vein. Pulsation of the femoral artery can be used to locate the saphenous nerve. The nerve is blocked by injecting local anesthetic solution cranial to the femoral artery, and syringes should be aspirated before injection. The common peroneal nerve is a branch of the sciatic nerve and provides sensory innervation to the dorsal aspect of the lower leg. The nerve runs laterally over the gastrocnemius and around the fibula and is blocked by injecting local anesthetic solution posterior to the head of the fibula. The tibial nerve is also a branch of the sciatic nerve and provides innervation to the caudal aspect of the lower leg. The tibial nerve can be blocked by injecting local anesthetic solution between the superficial flexor tendons and the long digital extensor tendon. Selective blockade of the saphenous, peroneal, and tibial nerves can be used preoperatively and postoperatively to manage pain in cats with tibial, fibular, and metatarsal fractures.

Selective blockade of distal branches of the common peroneal and tibial nerves can facilitate anesthetic and pain management of cats admitted for ony- chectomy. Superficial branches of the common peroneal nerve are blocked by injecting local anesthetic solution subcutaneously on the dorsomedial aspect of the tarsus just distal to the joint. Superficial branches of the tibial nerve are blocked by injecting local anesthetic solution subcutaneously on the ventromedial aspect of the tarsus just distal to the joint. m s technique is easy to perform and seems to provide exceptional short-term analgesia postoperatively.

SUMMARY

Local anesthetics have the unique ability to produce complete blockade of sensory nerve fibers and prevent or pre-empt the development of secondary (central) sensitization to pain.4l For this reason, local and regional anesthetic techniques are often used with opioids, a,-receptor agonists, dissociatives, and anti-inflammatory drugs as part of a multimodal strategy to manage pain." Lidocaine and bupivacaine are the local anesthetics used most commonly in dogs and cats. Lidocaine has a fast onset (10-15 min) and an intermediate duration of action (60-120 min), and is used for short diagnostic and surgical procedures. Bupivacaine has a slow onset (20-30 min) and a long duration of action (240-360 min), and is used to control pain both preoperatively and postoperatively. Local anesthetics are relatively safe if they are administered correctly. Administration of an excessive dose and accidental intravenous administration are probably the most common causes of systemic toxicity in small animals. Doses of local anesthetics, especially those for cats and small dogs, should always be calculated carefully. In many animals, the most simple and elegant way to control pain perioperatively is to perform a local or regional anesthetic block. Veterinarians should not hesitate to incorporate local and regional anesthetic techniques into their pain management strategies for dogs and cats.

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Kip A. Lemke, DVM, MS Atlantic Veterinary College

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Local and Regional Anesthesia