Food-animal practitioners commonly perform local-anesthetic techniques due to the dangers associated with general anesthesia. Local anesthetic techniques usually are simple, cheap and have relatively few side effects. Unless otherwise stated, "local anesthetic agent" refers to lidocaine or carbocaine. It is preferable to clip the hair and perform a surgical prep of the site before anesthetic agents are injected, particularly those injected deep into tissues.
The most common reason to perform surgery on the eye or eyelids is for squamous-cell carcinoma. For anesthesia prior to hyperthermia or freezing of lid lesions, or H-plasty, a simple line block with a local anesthetic agent will work. For lesions on the globe, it is helpful to proptose the globe for better exposure and immobilization. I try to proptose the globe without relaxation from a Peterson or retrobulbar block so there is less chance of abrasions and drying of the cornea following the procedure, especially if animals are traveling in an open trailer following the procedure. A topical local anesthetic labeled for the eye should be applied.
For enucleation of the eye, I prefer the Peterson eye block. A small amount of local anesthetic is placed subcutaneously at the notch formed by the supraorbital process and the zygomatic arch. A 14-gauge, 1-inch needle is placed through the skin, then an 18-gauge, 6-inch needle slightly curved is placed through the first needle with the tip aimed slightly caudal and ventral until I encounter bone. Although explanations of this technique by others describes repositioning the needle until it passes medial to the coronoid process of the mandible, I have rarely been successful at this. However, by depositing 20 mls of local anesthetic (after aspiration to make sure a vessel has not been entered), I am usually successful in obtaining anesthesia and resultant relaxation and protrusion of/to the globe. This technique does not block sensation to the lids. Blocking the auriculopalpebral branch of the facial nerve is described, but is not always successful and only blocks the lower lid. So, I prefer to block both upper and lower lids with a line of local anesthetic about 3 centimeters from the lid margins.
There is much debate among practitioners over the use of a Peterson eye block vs. the retrobulbar block. Both can be effective if performed correctly, and both can have deadly side effects. The Peterson eye block is purportedly safer and more effective if done correctly, but is more difficult to perform.
Blocking the cornual nerve desensitizes the horn for dehorning. Five mls to 10 mls of a local anesthetic agent is deposited subcutaneously and relatively superficially midway between the lateral canthus of the eye and the base of the horn along the zygomatic process. Complete anesthesia may take 10 minutes to occur. In older animals with larger horns, a partial ring block, especially posterior to the horn, may be needed.
For knife castration, a line of local anesthetic agent is placed in the scrotum at the proposed incision site. Injection of a local anesthetic agent into each testicle is then performed. The volume of the agent depends on the size of the testes. I inject until I get backpressure. It is important to remove the testes within a few minutes of injection into them to prevent any toxic side effects from lidocaine or carbocaine going systemically.
For teat lacerations, an inverted V block immediately proximal to the laceration is usually sufficient. For severe, extensive lacerations, a complete ring block at the base of the teat may be necessary.
Surgery of the foot
In general, intravenous regional anesthesia is preferred for surgery of the foot. A tourniquet is placed proximal to the fetlock immediately prior to injection (vein will be maximally distended immediately after the tourniquet is placed). Three sites of injection are available. One vein runs down the center of the dorsal aspect of the pastern. Another vein runs approximately 2 cm dorsal to the dewclaw, on both the lateral and medial sides of the foot. A 20-gauge needle or butterfly catheter is inserted into one of these veins and 15 cc to 20 cc of local anesthetic agent is administered. Alternatively, a 20-gauge, 1.5-inch needle is inserted into the dorsal aspect of the pastern, in the groove between the proximal phalanges, just distal to the fetlock. Many times a vein is entered in the interdigital space and can be used to inject the anesthetic agent. It is only necessary to administer an anesthetic into one of these veins to provide anesthesia to the entire area distal to the tourniquet. The tourniquet can be safely left on for up to an hour to provide hemostasis during surgical procedures.
In feet with severe cellulitis, local intravenous anesthesia can be difficult. In these cases, a four-point nerve block or a simple ring block will also work. The four-point nerve block anesthetizes the area from the pastern distally. To perform the procedure, a 20-gauge, 1-inch needle is inserted into the dorsal aspect of the pastern, in the groove between the proximal phalanges, just distal to the fetlock. Five mls of lidocaine is administered deep, and another 5 mls superficially. This injection is repeated on the palmar/plantar aspect of the pastern, just distal to the dewclaws. Next, palpate the nerve over the lateral aspect of the fetlock, approximately 2 cm dorsal and proximal to the dewclaw. Administer 5 mls of lidocaine over the nerve and repeat on the medial side. The two interdigital injections performed in the four-point block can be used for removal of an interdigital fibroma.
Most bovine practitioners are familiar with the use of lidocaine and/or carbocaine for caudal epidural anesthesia. But other pharmacologic agents can be used with this technique. Epidural administration of xylazine (0.05 mg/kg) or xylazine/lidocaine combination offers similar anesthesia to lidocaine but the duration is longer (~4 hours) and systemic effects (sedation, salivation, ataxia) can occur. Because of the systemic effects, I do not use xylazine caudal epidurals for obstetric work. However, I have used it successfully in cattle that chronically strain due to rectal or vaginal prolapse, vaginal irritation, etc. Although the duration of anesthesia is still relatively short, it's my opinion that the systemic sedation effects are helpful in decreasing straining. I have not experienced problems with this technique; however, Dr. Lyle George reports three cases of demyelination following xylazine epidural that caused these animals to be permanently paralyzed. He suggests that combination of lidocaine or carbocaine caudal epidural with systemic administration of xylazine is as effective as xylazine epidural administration and has less potential for serious side effects.
Epidural administration of opioids is another option for practitioners. Because they cause analgesia, but do not interfere with motor function, animals are less likely to become ataxic or recumbent. Also, the duration is longer (~12 hours for morphine). The disadvantages are that the analgesia is not as potent as lidocaine or carbocaine, and maximum effects of a morphine epidural may not occur for 2 hours to 3 hours or longer. Caudal epidural administration of morphine might be indicated for relief of pain in the perineum and to help reduce straining. Lumbosacral epidural administration of morphine may reduce pain during and after standing surgical procedures and may be used for pain relief in the rear limbs and pelvis. For maximum effects during surgery, lumbosacral epidural administration of morphine should be administered at least 2 hours to 3 hours prior to surgery, and routine local anesthetic techniques to anesthetize the flank should still be employed. The dose of morphine for epidural injection is 0.1 mg/kg diluted in 20 mls of sterile saline.
The combination of morphine and xylazine might be synergistic when administered epidurally.
Morphine (40 mg to 100 mg) can be injected directly into a joint, or used in the regional intravenous technique in the foot. Morphine used in this manner may not totally eliminate pain, but may reduce the amount of other analgesics needed, such as non-steroidal anti-inflammatory drugs, or NSAIDs.
Dr. Navarre works as an extension veterinarian with Louisiana State University's Department of Veterinary Science.