Local anesthesia in cattle
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.