Local and regional anesthesia provide many benefits in food animal medicine. General anesthesia in ruminants has inherent risks such as bloat or cardiopulmonary depression. Local and regional nerve blocks can provide a safe, effective, and humane alternative to perform surgical procedures. Additionally, general anesthesia requires more physical labor and equipment compared to local and regional anesthesia. Most local and regional anesthetic techniques are not difficult to perform and require only minimal expense and equipment. When used in conjunction with sedation and physical restraint, local anesthesia can improve the efficacy and efficiency of a procedure while increasing the safety for the surgeon and the animal. When local or regional nerve blocks are used in conjunction with general anesthesia, the dose requirement of anesthetic drugs can be reduced, enhancing animal safety and ensuring a smoother recovery. Furthermore, anesthetizing pain receptors helps to prevent post‐surgical pain until other analgesics can be administered.

Local and regional anesthesia should be performed with the animal in proper physical restraint and under aseptic technique. The area should have the hair removed and surgical preparation should be performed.

8.1 Local Anesthetic Agents

Numerous anesthetic agents are available to produce local and regional anesthesia. Differences in cost, toxicity, potency, and tissue drug residues must be considered during the selection process (see Chapter 12). The most commonly used agents in food animal medicine are 2% lidocaine hydrochloride and 2% mepivacaine hydrochloride, which are less expensive and have lower potential for toxicity than other options. Lidocaine induces more swelling and local irritation than mepivacaine [1].

It is important to keep in mind the potential for lidocaine toxicity. The maximal safe dose for cattle is 10 mg/kg of body weight. Small ruminants are more sensitive to lidocaine, with a maximal safe dose of 6 mg/kg of body weight used conservatively [2]. A pharmacokinetic study determined that a greater dose of 7 mg/kg was not only tolerated but considered safe for small ruminants [3]. When anesthetizing larger body surfaces or using lidocaine in smaller animals, the 2% lidocaine can be diluted with sterile saline at a 1 : 1 dilution to decrease the amount of lidocaine administered [4].

Adding sodium bicarbonate or epinephrine to injectable lidocaine solution can enhance the ease of administration and efficacy of lidocaine. Sodium bicarbonate (8.4% solution) can be added to decrease the initial discomfort associated with lidocaine administration. Mixing 2% lidocaine with 8.4% sodium bicarbonate at a 9 : 1 ratio decreases the discomfort of lidocaine administration [5, 6]. Lidocaine has vasodilatory properties, causing the undesirable effect of increased blood flow and hemorrhage when tissues are incised at the site of lidocaine administration. The addition of epinephrine to lidocaine at a ratio of 1 : 200 000 decreases the vasodilatory effect, reduces systemic absorption, and, as a result, prolongs the duration of the local anesthetic effect of lidocaine [7]. It is important to note that epinephrine should not be used in close proximity to wound edges or in the subarachnoid space due to the risks of tissue necrosis or spinal cord ischemia [8].

Local anesthetic agents prevent the initiation of the neuronal cell membrane action potential of the nerve fibers by blocking the voltage‐gated sodium channels. It is important to consider the condition of the tissue when administering local anesthetics. Inflamed and infected tissues are more acidic and do not respond to anesthetic agents as normal healthy tissues do. The efficacy and duration of anesthesia is decreased in these tissues due to greater ionization of the local anesthetic. Neonates and pregnant females are at a greater risk of anesthetic toxicity because of decreased cholinesterase activity [9, 10]. Additionally, pregnant females have greater cardiac output and higher plasma concentrations of α‐1‐acid glycoprotein, which increases the uptake of local anesthetics, reaching levels potentially high enough to be harmful to the neonate [11, 12]. Accidental intravenous administration of a local anesthetic agent is the most common reason for overdosing and development of toxicity. In severe cases, cardiac arrest occurs. Other signs of overdose include twitching, collapse, convulsions, coma, and methemoglobinemia [11,13–15].

8.2 Anesthesia for Dehorning

Dehorning or disbudding of large ruminants is commonly performed with local anesthesia of the horns along with physical restraint in a squeeze chute and head restraint. A cornual nerve block provides anesthesia to the horn and surrounding skin. This area is innervated by the cornual branch of the lacrimal nerve (also known as the zygomaticotemporal nerve). The lacrimal nerve originates from the ophthalmic branch of the trigeminal nerve. The cornual nerves runs dorsally through the periorbital tissues continuing along the frontal crest until it reaches the horn base. In cattle, a small amount (5–10 ml) of 2% lidocaine is deposited superficially along the zygomatic arch, halfway between the horn base and the lateral canthus of the eye (Figures 8.1 and 8.2). A depression can be palpated in this area. It is important to remember that full local anesthetic effect does not occur until 10–15 minutes after administration. Depth of administration varies from 1.0 to 2.5 cm, depending on the size of the animal [16]. A similar technique is used to block the cornual nerve in small ruminants, except that the palpable depression below the zygomatic arch is much smaller (Figure 8.3). Additional lidocaine can be administered at the caudal aspect of the horn base in a ring block. This is necessary in ruminants with a larger horn base, as superficial branches of the second cervical nerves need to be desensitized for a complete local anesthesia for dehorning [17–19].

Blocking the cornual branch of the infratrochlear nerve enhances the local anesthesia during dehorning of small ruminants and cattle. The nerve branches are located halfway between the medial canthus of the eye and the middle of the horn base (Figures 8.3–8.5) [16]. Lidocaine (2%) is administered along the dorsomedial aspect of the orbit, parallel to the surface of the skin (Figures 8.4 and 8.5, Video 8.1). It is best to distribute lidocaine using a fanning technique or as a line as this nerve often has many branches. A 20‐ or 22‐gauge needle is used to perform this block in small ruminants, as a larger gauge needle makes it difficult to administer lidocaine solution. Compared to cattle, the volume of lidocaine used in small ruminants is much lower, and approximately 2–3 ml of 2% lidocaine should be used for each site [16].

8.3 Anesthesia for the Eye and Eyelids

Multiple nerves control the sensory and motor function of the structures within and around the orbit. Sensory fibers of the ophthalmic branch of the trigeminal nerve provide sensory innervation of the globe, third eyelid, conjunctiva, and eyelids. The motor function of the extraocular muscles is supplied and innervated by different cranial nerves. The trochlear nerve innervates the dorsal oblique muscle. The abducens nerve supplies the lateral rectus and retractor oculi muscles. The oculomotor nerve supplies the remaining ocular muscles (inferior oblique, superior rectus, medial rectus, inferior rectus, levator palpebrae superioris, ciliary muscle, and sphincter muscles). The eyelids are innervated by the auriculopalpebral nerve [16, 20].

Blocking the auriculopalpebral nerve, a branch of the facial nerve, abolishes the motor function of the eyelids [21]. The nerve can be palpated as it passes over the zygomatic arch. In cattle, 5–10 ml of 2% lidocaine is administered subcutaneously along the zygomatic arch (Figure 8.2) [16]. The purpose of this block is to allow for manipulation of the eyelids and examination of the cornea, and it can be used in conjunction with a topical anesthetic if painful corneal disease exists [21]. A ring block using a 20‐ to 25‐gauge needle with 10–20 ml of 2% lidocaine is used for surgical procedures on the eyelid such as eyelid laceration repair or mass removal.

Anesthesia for surgical procedures on the eye, including enucleation or exenteration, is provided by administration of a retrobulbar or Peterson eye block. Appropriate physical restraint and sedation aid in administration of these blocks. Commonly preferred sites of administration of retrobulbar injections include injections through the upper and lower eyelids or medial and lateral canthus (Figure 8.6). To reach the necessary depth for efficient block, a 7.5–12‐cm, 18‐gauge needle is needed for cattle and a 3.75‐cm, 22‐ to 20‐gauge needle is adequate for small ruminants. The globe must be shielded by the surgeon’s finger to prevent accidental puncture of the globe. When administering at the medial canthus, the needle must be advanced through the conjunctiva of the medial canthus rostrally to the third eyelid and dorsomedially to the globe (Video 8.2). The bony orbit can be felt as the needle is passed caudally. Injection of the anesthetic can be performed as the needle is advanced caudally or the entire volume can be administered once the needle is fully seated in the orbit. The process is repeated dorsally, ventrally, and laterally to the globe. For the dorsal and ventral locations, it is common to insert the needle through the eyelids. In cattle, 20–25 ml of 2% lidocaine is administered to facilitate proptosis of the globe indicating proper retrobulbar administration. Possible detrimental effects of the retrobulbar injections include optic hemorrhage, optic nerve damage, increased globe pressure, and accidental administration of lidocaine into the optic nerve meninges [22].

While perceived to be less reliable due to decreased distribution of a local anesthetic than the retrobulbar nerve block [23], the Peterson eye block offers some advantages for ocular surgical procedures and is often combined with an auriculopalpebral block. The anatomic landmarks for the Peterson eye block are identified as the deviation created by the supraorbital process cranially, coronoid process of the mandible caudally, and zygomatic arch ventrally (Figures 8.2 and 8.7) [8, 24]. The skin is desensitized with 5 ml of 2% lidocaine. A 2.5‐cm, 14‐gauge needle can be placed into the deviation to serve as a guide cannula. A 10–12‐cm, 18‐gauge needle with a slight curve (Figure 8.8) is advanced through the cannula horizontally and posteriorly until the needle connects with the coronoid process of the mandible (Video 8.3) [25]. The authors prefer not to use a guide needle for this technique but advance the needle as described. The needle is walked off the front of the process and advanced to the pterygopalatine fossa. The typical depth is approximately 7.5–10 cm in most cattle [25]. Care must be taken not to rotate the curved needle out of the horizontal plane as this could result in detrimental effects on the patient. Once the needle encounters bone a second time, the needle is withdrawn by several millimeters, the syringe is aspirated, and 10–30 ml of 2% lidocaine is deposited. Aspiration and needle withdrawal ensure that injection of lidocaine into the optic nerve and ophthalmic artery does not occur [24]. When administered successfully, the Peterson technique blocks the oculomotor, abducens, and trochlear nerves as well as the ophthalmic, maxillary, and mandibular branches of the trigeminal nerve.

Successful retrobulbar and Peterson’s eye blocks inhibit the ability of the eye to blink for several hours. Sterile saline should be used to irrigate the globe to prevent damage to the cornea during the procedure. Ocular ointment should be applied to prevent detrimental effects associated with dry eye following the completion of the surgery. These protective measures must be taken until the return of motor function of the eye to prevent idiopathic keratoconjunctivitis. Exposure to direct sunlight, dust, or wind should be minimized. If the eye proptoses during the post‐surgical phase, gentle traction on the eyelids can replace the globe back into the bony orbit.

Anesthetic technique for enucleation, exenteration, and proptosis in small ruminants and camelids is similar to the technique used in cattle. The use of 5‐cm, 20‐ or 22‐gauge needles is more appropriate in these species. The authors strongly recommend that small ruminants and camelids are sedated to ensure compliance of the animal during these procedures. Please refer to Chapter 3 for options of standing sedation and chemical restraint protocol.

8.4 Nasal Anesthesia

The infraorbital nerve provides sensory input to the nose. It originates from the maxillary branch of cranial nerve 5 (trigeminal nerve). This nerve is relatively flat as it passes through the infraorbital canal and is then covered by the levator nasolabialis muscle [17].

Nasal anesthesia is used for placement of nose rings in cattle or the repair of nasal lacerations. This can be accomplished by injecting 20–30 ml of 2% lidocaine bilaterally at the infraorbital canal (Figures 8.2 and 8.9) where the infraorbital nerve exits. The canal can be challenging to locate but lies in front of the facial tuberosity between the second maxillary premolar and the nasomaxillary notch [16].

8.5 Anesthesia for Reproductive Procedures and for Cessation of Straining

Caudal epidural, continuous caudal epidural, and pudendal nerve blocks are routinely performed to provide anesthesia to the perineal area. All of these techniques provide relief of rectal tenesmus and they are cost‐effective and require only minimal training to perform them successfully [16]. The sacral paravertebral nerve block is another option can be used to relieve tenesmus while maintaining the motor function of the tail.

8.5.1 Caudal Epidural Anesthesia

Caudal epidural anesthesia is easily performed in ruminants with a small amount of local anesthetic, which can be administered in the epidural space of either the sacrococcygeal space or the first and second intercoccygeal space (Figure 8.10). A caudal epidural anesthesia at the sacrococcygeal space desensitizes sacral nerves 2 through 5, whereas when performed at the first and second intercoccygeal space, the block desensitizes sacral nerves 3 through 5. Increasing the volume of anesthetic administered enhances the cranial distribution of the anesthetic, and nerves cranial to sacral nerve 2 may become affected. The space used for injection is palpated by moving the tail up and down, and palpating a depression between the movable vertebrae. The cranial space most used for injection may be ossified in older cows [18], and both spaces may be affected by ossification or fibrosis in embryo donor animals due to repeated caudal epidural administration.

The space used for injection is easily located when standing directly behind the animal. The injection site should be aseptically prepared prior to needle placement to prevent the risk of ascending infection. The recommended dose of 2% lidocaine is 0.5 ml per 45 kg (99 lb) of body weight for cattle [24]. A 3.75–5‐cm, 18‐gauge needle is advanced perpendicularly through the skin on the midline [16]. A small amount of lidocaine is placed in the hub of the needle until a meniscus is formed (known as the hanging drop technique). The needle is then advanced into the epidural space. When negative pressure in the epidural canal pulls the lidocaine solution from the needle hub, this confirms proper needle placement [24]. Occasionally, this suction by the negative pressure in the epidural space can result in an audible sound. When the block is successful, the tail becomes flaccid and straining stops. Uterine motility is unaffected by caudal epidural injection of lidocaine, thus administration of a tocolytic agent, such as epinephrine, is necessary if uterine relaxation is desired. Recumbency can be achieved by using high‐volume caudal epidural anesthesia if the lumbosacral space is not easily palpable or administration is not feasible. High‐volume caudal epidural anesthesia is performed as described above, but instead uses 1 ml of 2% lidocaine per 45 kg (99 lb) of body weight [26]. High‐volume caudal epidural anesthesia produces anesthesia reaching the ventral flank area just behind the umbilicus. This technique can be used for abdominal surgeries such as umbilical resection, ventral‐midline cesarean sections, or abdominal exploratory surgeries. Cranial distribution of lidocaine in the epidural canal can be affected by arthritis or fat deposition in older or obese animals, respectively [8,27–29]. Abdominal analgesia can be enhanced by using local or regional anesthetic techniques (see section 8.6).

Analogous to cattle, the caudal epidural anesthesia technique can be used for surgery of the caudal reproductive tract, reproductive emergencies, or surgery of the gastrointestinal tract in small ruminants and camelids [30]. In sheep with docked tails, the caudal epidural injection may be impossible to administer, leaving lumbosacral epidural anesthesia as the only option available. In goats and camelids, a 3.8‐cm, 20‐gauge needle is inserted at a 45° angle to the vertebrae at the most cranial movable space. The hanging drop technique, described above, is used to ensure correct placement of the needle. The recommended dose range of lidocaine is 1 ml/15–50 kg (33–110 lb) of body weight [30]. The authors commonly use 1 ml/10–20 kg (22–44 lb) of body weight. Bupivacaine (0.5%) has been used at a dosage of 1 ml/4 kg (8.8 lb) of body weight [31]. Complications associated with prolonged recumbency are more commonly encountered when using bupivacaine, and this drug should be used with caution [30, 31].

In animals that require tail docking or amputation, either for cosmetic purposes or as a result of trauma, different anesthetic techniques are more appropriate for different species. In sheep, a ring block is more efficacious than a caudal epidural anesthesia for tail‐docking procedures [30]. The ring block should be placed proximally to the site of amputation using lidocaine at a dose of 6 mg/kg of body weight. In cattle and goats that have incurred tail trauma, a caudal epidural anesthesia effectively allows amputation. In the authors’ experience, if amputation is required in an adult small ruminant, sedation is helpful to complete the procedure.

In cases in which long‐term desensitization is required, an alcohol epidural injection can be performed with anesthesia resulting from demyelination of the nerve roots. A mixture of 70–95% ethyl or isopropyl alcohol with equal part of lidocaine is administered at similar volumes as described for lidocaine epidural anesthesia technique. This alcohol epidural anesthesia results in desensitization of the pelvic and perineal area as well as paralysis of the tail for weeks to months. Return of sensation depends on the rate of remyelinization of the nerve roots. Common complications of this technique are associated with chronic paralysis of the tail such as inflammation and fly strike of the perineal area. Overdose of alcohol results in longer duration or permanent pelvic limb paralysis [8].

8.5.2 Continuous Caudal Epidural Anesthesia

A continuous caudal epidural anesthesia can be utilized to facilitate pain management or to provide relief in animals with chronic vaginal or rectal prolapse that have continuous tenesmus after the administration of a traditional caudal epidural anesthesia. A catheter is placed and maintained in the epidural space for repeated administration of local anesthetics or analgesics. This procedure requires a Tuohy needle (available from various manufactures in different sizes, e.g. B. Braun Medical, Bethlehem, PA, USA), which is a specialized epidural needle with a slightly curved tip and a stylet (Figure 8.11). The needle is advanced cranially into the space between the first and second coccygeal vertebrae at a 45° angle (Figure 8.12). The stylet is removed, and 2 ml of anesthetic is administered to ensure the needle is in the epidural space. A closed‐tip nylon catheter is placed in the needle and advanced 2–4 cm cranially to the end of the needle. The Tuohy needle is removed and the catheter is maintained in place. An adapter is connected to the catheter and sutured to the skin of the tail head to stabilize the catheter. Anesthetic drugs or analgesics may then be administered as needed via the catheter [16, 24].

Lidocaine, xylazine, procaine, morphine, and ketamine have all been used for epidural anesthesia [1932–35]. Lidocaine doses ranging from 0.1 to 0.25 mg/kg alone or in combination with other drugs can be used. Ketamine alone or in combination can also be used at dosages of 0.25–0.7 mg/kg. The use of lidocaine and ketamine in combination provides epidural anesthesia twice as long as that obtained using either drug alone and has been used successfully for surgery of the paralumbar fossa [33, 36]. Xylazine at a dose of 0.1 mg/kg of body weight diluted with lidocaine to a final volume of 0.5–0.6 ml/kg was used for an umbilical surgery with success [34]

Only gold members can continue reading. Log In or Register to continue

Share this:

• Click to share on Twitter (Opens in new window)
• Click to share on Facebook (Opens in new window)

Related

Related posts:

1. Commonly Used Preanesthetics
2. Anesthetic Management of Specific Procedures
3. Euthanasia of Farm Animals
4. Perioperative Monitoring and Management of Complications

Stay updated, free articles. Join our Telegram channel

Join