For each technique, perform a sterile scrub, wear sterile gloves, and use only sterile needles, syringes, and catheters. Strict adherence to aseptic technique will reduce complications from infection. An overview of each technique is presented in Table 1. The onset of the blockade and duration of analgesia depend on the local anesthetics chosen and the use of adjunctive agents.
INTRA-ARTICULAR STIFLE BLOCK
Lidocaine or bupivacaine are commonly used; morphine or buprenorphine can also be used alone or, more commonly, in combination with a local anesthetic.4,5 Clonidine, an alpha2 agonist, is effective in people as an adjunct to local anesthetics when administered intra-articularly,6 and the alpha2 agonist fadolmidine was shown to suppress peripheral arthritic pain when administered into the stifle joints of rats.7 As outlined in the previous article, adding alpha2 agonists, opioids, or both to a local anesthetic may enhance efficacy and extend duration of regional anesthetic techniques. Bupivacaine, with or without an opioid or alpha2 agonist, is most commonly used in intra-articular blocks, with onset in about 30 minutes and a duration of action, when given alone, of four to six hours.
INTERCOSTAL NERVE BLOCKS
Local anesthetics can be administered into the pleural cavity (between the visceral and parietal pleura) to provide analgesia after a lateral or sternal thoracotomy, in patients with thoracic trauma, and in patients with cranial abdominal pain, such as that associated with acute pancreatitis.1,8-10 The mechanism of analgesia has been suggested to be due to diffusion of local anesthetic through the parietal pleura, causing intercostal nerve block, blockade of the thoracic sympathetic chain and splanchnic nerves, and diffusion of the anesthetic into the ipsilateral brachial plexus, resulting in blockade of the parietal peritoneum.8-11 Because cranial abdominal nerves enter the spinal cord at the level of the thorax, intrapleural administration of a local anesthetic blocks the cranial abdominal nerves, and this technique may be useful for acute pancreatitis or cranial abdominal surgical procedures.9
Local anesthetics and adjunctive agents can be administered through a chest tube, if present. Follow the anesthetic drugs with 3 to 5 ml of sterile saline solution to flush the tube. If a chest tube is not in place, a hypodermic needle, butterfly catheter, or intravenous catheter can be used. Systemic analgesics and sedatives should be considered for conscious patients. After sterile preparation and infiltration of the site of interest with 0.2 to 0.5 ml of 2% lidocaine, penetrate the chest wall between the ribs with the stylet and catheter. Once the pleural space has been reached, advance the catheter off the end of the stylet to avoid lung laceration. Attach a three-way stopcock and syringe to the catheter as soon as the stylet is removed to avoid inducing a significant pneumothorax, and make sure the catheter is never left open to the environment. A hypodermic needle or butterfly catheter should have the stopcock attached, closed to the needle, before penetration of the chest wall.
Local anesthetic administered intrapleurally is rapidly and extensively absorbed systemically, so be sure to avoid using toxic doses. Inflammation and the resulting tissue acidity may reduce the efficacy of local anesthetics and result in increased systemic absorption. If possible, place the patient with the incision site down, and maintain the patient in that position during needle placement and drug injection and for at least 10 minutes afterward, as this may facilitate local anesthetic pooling over the incision and blocking of the adjacent intercostal nerves.9 This position may be uncomfortable for the patient, so it is best performed after systemic administration of an analgesic, such as an opioid, to improve patient tolerance.
FORELIMB NERVE BLOCKS
Intraoperative brachial plexus block
Blocking the nerves of the brachial plexus can be performed intraoperatively by depositing lidocaine onto the visualized nerve or by injecting directly into the perineurium, the connective tissue sheath that surrounds a bundle of nerve fibers, three to five minutes before transection. This technique can provide excellent postoperative analgesia after forelimb amputation. The duration can be extended by adding bupivacaine and other adjunctive drugs such as opioids and alpha2 agonists. A similar blockade of the lumbar plexus nerves can be performed for hindlimb amputation.
Paravertebral and traditional brachial plexus blocks
For regional forelimb blockade when the nerves will not be visualized during the surgical procedure, a paravertebral technique or a traditional brachial plexus block technique is used. Local anesthetics can be used alone or in combination with opioids, alpha2 agonists, or both. Either brachial plexus block technique could result in blockade of the phrenic nerve (C5), and performance of bilateral brachial plexus blocks may inadvertently paralyze the diaphragm, although this may not cause clinically significant respiratory depression.11,12 Pneumothorax can also occur with both types of blocks. In addition, inadvertent injection into the thoracic dural sheath could occur with the paravertebral technique, which could result in high sympathetic blockade, systemic hypotension, and severe respiratory depression.
With the patient in lateral recumbency and using sterile technique, move the scapula caudally to expose the large transverse process of the sixth cervical vertebra and the first rib. Block the ventral branches of C6 and C7 as they cross the dorsal surface of the transverse process of C6 by inserting a needle dorsal to the process and directing it toward the cranial and then caudal margins of the process. Block the ventral branches of C8 and T1 on the lateral surface of the first rib by directing the needle to the cranial and caudal border of the dorsal part of the first rib, close to the articulation with the vertebra. A modified technique has also been described.11 With this method, C6 and C7 are blocked as described above; however, C8 and T1 are approached by identification of the axillary artery and costochondral junction of the first rib. Local anesthetic is deposited along the cranial margin of the first rib, 1 to 2 cm dorsal to the costochondral junction. Paravertebral techniques are technically more difficult than the traditional brachial plexus block, particularly in obese animals.
Place the patient in lateral recumbency, and after preparing the skin using aseptic technique, insert a spinal needle into the axillary region, medial to and at the level of the shoulder joint, directed toward the costochondral junction and parallel to the vertebral column. The needle's distal end should lie just caudal to the spine of the scapula. Aspirate the syringe to avoid intravascular administration, and then inject two-thirds of the dose. Inject the remaining one-third as you slowly withdraw the needle. Increasing the volume of local anesthetic used by diluting it with sterile saline solution up to 50% can improve the degree of blockade by increasing the volume injected.
Radial, median, ulnar, and musculocutaneous nerve blocks
For procedures of the elbow and antebrachium, the radial, median, ulnar, and musculocutaneous nerves may be blocked proximal to the humeral epicondyles. These nerves can often be palpated, making this technique relatively straightforward to perform.
After sterile preparation, approach the radial nerve on the lateral aspect of the distal humerus. Palpable just proximal to the lateral epicondyle, the radial nerve is located between the brachialis muscle and the lateral head of the triceps.11 The median, ulnar, and musculocutaneous nerves are located close to one another on the medial aspect of the forelimb, proximal to the medial epicondyle. The brachial artery is situated among these nerves. The musculocutaneous nerve is cranial to the artery while the median and ulnar nerves are located caudally. Identify the artery with palpation, and aspirate the syringe before depositing local anesthetic and adjunct drugs near these nerves, avoiding intra-arterial, intravenous, and intraneural injection.
The use of nerve stimulators to locate peripheral nerves is routine in human surgical procedures, and the body of literature is extensive. Several meta-analyses of the human literature indicate that peripheral nerve blockade supplies equivalent postoperative analgesia and an improved side effect profile compared with epidural analgesia.17,18
A recent study in dogs demonstrated the efficacy of a nerve stimulation-guided blockade of the brachial plexus for analgesia distal to the shoulder.16 Another report described the successful use of a nerve stimulator to perform a brachial plexus block for carpal arthrodesis in a dog.19 Subsequently, the use of a nerve stimulator to identify the brachial plexus of 20 dogs undergoing distal thoracic limb procedures was described.20 Ten dogs received a mixture of lidocaine and bupivacaine, and 10 dogs received an equal volume of saline solution. Intraoperative and postoperative opioid requirements were significantly lower in dogs that received the local anesthetics. The use of a nerve stimulator to assist in the placement of an indwelling perineural catheter near the brachial plexus of a dog with severe trauma of the distal thoracic limb has also been reported.15
A recent abstract compared the use of a nerve locator to achieve lumbar plexus blockade with bupivacaine vs. systemic administration of methadone to dogs undergoing hindlimb orthopedic procedures.21 Intraoperative isoflurane requirements were lower in the locoregional group, and cardiovascular parameters were improved.21 The use of a nerve locator to facilitate mandibular nerve blocks in crocodilians has been described.22 A recent study described dissection of the brachial plexus, sciatic and femoral nerves, identification of relevant anatomical landmarks, and the use of a nerve stimulator to locate these nerves in four anesthetized dogs.14 Disappointingly, the use of a nerve locator did not result in increased staining of the brachial plexus with new methylene blue when compared with a blind technique in one study.23
The volumes of local anesthetic for use in brachial plexus, lumbar plexus, and sciatic nerve blocks, based on distribution of an injected lidocaine and methylene blue solution, have been investigated.24 Local anesthetic volumes of 0.3 ml/kg were found to adequately stain the nerves of the brachial plexus, whereas 0.05 ml/kg was judged appropriate for sciatic nerve block. The lumbar plexus appeared to require a volume of 0.4 ml/kg, but statistical significance was not reached.
For a traditional brachial plexus block, when the musculocutaneous nerve is stimulated, the biceps muscle will contract, causing flexion of the elbow.25 Injecting local anesthetic should cause the muscle movement to immediately cease.9,14-16,19,25 Refer to Table 1 for drug choices and dosages.
The use of a nerve locator for the paravertebral block has been described11 ; however, the exact muscle movements that should be elicited to achieve a successful block are still being defined. Various motor movements may be elicited from stimulation of individual nerves within the brachial plexus and range from dorsal displacement of the scapula to extension of the triceps.11 An in-depth review of the paravertebral blockade is provided elsewhere.11 If a hiccupping movement of the diaphragm is elicited, the phrenic nerve has been located, and the needle should be repositioned before injection of local anesthetic.
By adopting the above techniques, you can greatly reduce your patients' pain after surgery or trauma. These techniques do not require expensive equipment or drugs, yet can provide effective analgesia of prolonged duration, reducing the requirements for systemic analgesics to control pain. Look for upcoming articles on maxillary and mandibular nerve blocks and epidural anesthesia and analgesia.
The authors wish to thank Gregory Hirshoren, Instructional Resources, College of Veterinary Medicine, The University of Tennessee, for the photos that accompany this article.
Christine Egger, DVM, MVSc, DACVA
Lydia Love, DVM
Department of Small Animal Clinical Sciences
College of Veterinary Medicine
The University of Tennessee
Knoxville, TN 37996
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