L Block

The L block is a type of field block used to desensitize the flank for standing flank laparotomies. Local anesthetic is deposited in an inverted L configuration in the flank (Fig. 12-1). The anesthetic must be deposited in several layers (i.e., subcutaneous tissue and all muscular layers of abdominal wall). Large volumes of local anesthetic are required; often up to 100 ml of 2% solution is necessary in adult cattle. The anesthetic is deposited with an 18-gauge (ga) × – to 3-inch needle. Before beginning the surgical procedure, allow at least 10 to 15 minutes for the anesthetic to diffuse and take effect. The inverted L essentially forms a wall of anesthesia that protects the surgical field (Fig. 12-2). It is the simplest technique for desensitizing the flank and therefore is commonly used.

Paravertebral Block

This technique uses multiple specific nerve blocks to create a large region of flank anesthesia. Innervation of the flank arises from the spinal nerves of T13, L1, and L2 spinal segments. These nerves can be blocked near their exit from the vertebral column at a “paravertebral” location. The two main ways to approach these nerves are from (1) a dorsal approach near the intervertebral foramina (Cambridge, Farquharson, or proximal paravertebral method) or (2) a lateral approach near the tips of the transverse processes of the lumbar vertebrae (Magda, Cornell, or distal paravertebral method) (Fig. 12-3). Cattle require a 16- to 18-ga × 3- to 6-inch length needle for the proximal paravertebral (dorsal) approach. However, some clinicians prefer to first place a 14-ga × 1-inch needle as a trocar through the skin and muscle layers, then insert an 18-ga needle through the 14-ga needle to actually deliver the anesthetic. Up to 20 ml of anesthetic is necessary for each of the three injection sites in cattle.

For the distal paravertebral (lateral) approach, an 18-ga × – to 3-inch needle is sufficient for cattle. From 10 to 20 ml of anesthetic is deposited at each of the three injection sites.

The paravertebral block desensitizes all layers of the flank, from skin down to the peritoneum. Note that with the proximal paravertebral approach, once the block takes effect, paralysis of the longissimus muscle along the spine may cause temporary lateral curvature of the spine (bowing or scoliosis) toward the side of the block. This curvature may create some gaping of the skin incision, making suture closure more difficult. The distal paravertebral approach should not create scoliosis.

Cornual Nerve Block

The cornual nerve block is used for desensitization of the horn and horn base for dehorning surgery.

Cattle have a single nerve supply to each horn. The cornual nerve emerges from the orbit and ascends toward the base of the horn just below the temporal ridge of the frontal bone. Local anesthetic (≈ 3–5 ml in calves, 5–10 ml in adults) is deposited with an 18- to 20-ga × 1- to -inch needle just ventral to the temporal ridge at a site approximately halfway between the horn base and the lateral canthus of the eye. The nerve is covered only by skin and a thin layer of muscle at this location; depth of needle penetration is 1 cm in calves to 2.5 cm in large adults (Fig. 12-4).

Adult cattle with well-developed horns may require a second injection of several milliliters of anesthetic at the base of the horn, along the caudal aspect, just beneath the skin.

Intravenous Regional Analgesia (Bier Block)

Intravenous (IV) analgesia is considered superior to specific nerve blocks and ring blocks for most surgical procedures on the distal limbs. The technique uses an IV injection of local anesthetic, distal to a previously placed tourniquet. The anesthetic diffuses out of the veins and blocks the nerves in the area. Although it is seldom performed on standing animals, it is a common method of local analgesia for surgical procedures on awake, sedated animals in lateral recumbency.

The animal is restrained, sedated, and cast (placed in recumbency) for administration of this form of regional anesthesia. A tourniquet is applied at the desired level on the limb, which is determined by the location of the surgical procedure. For procedures on the feet, the tourniquet is placed at midcarpus or midtarsus. For more proximal procedures, the tourniquet is placed just proximal to the carpus or tarsus. Rubber tubing or other elastic strapping material is suitable for the tourniquet. Cotton or foam padding should be used beneath the tubing for additional protection of underlying tissues, especially where the tourniquet crosses superficial tendons. Proximal to the tarsus, the grooves in front of the common calcaneal tendon should be “filled in” on both sides of the leg with a roll gauze or other soft padding before placing the tourniquet. This step is necessary to achieve complete arterial occlusion.

Any large superficial vein can be used, but generally the dorsal metacarpal (metatarsal) or palmar (plantar) metacarpal (metatarsal) veins are used (Fig. 12-5). The site should be clipped and prepped. Lidocaine without epinephrine (2%) or mepivacaine (2%) is injected intravenously, with the needle directed distally; the backpressure creates some resistance to injection. Up to 30 ml can be administered. After the needle is withdrawn, digital pressure should be placed over the injection site for longer than normal to prevent hematoma formation.

Anesthesia is sufficient for surgery in 10 to 15 minutes and persists as long as the tourniquet is kept in place. A tourniquet may cause complications such as tissue necrosis, pain, and swelling if it is left in place longer than 2 hours. The tourniquet should be released gradually to prevent a bolus release of local anesthetic drug into the general circulation and possible resulting hypotension. The anesthetic effects on the distal limb disappear rapidly within 5 to 10 minutes after the tourniquet is the released.

Caudal Epidural Analgesia

Caudal epidurals are commonly used in ruminants, especially for obstetrical procedures and treatment of prolapses of the uterus, vagina, and rectum. When the epidural is properly performed, the anus, perineum, vulva, caudal vagina, and caudal aspects of the thighs are desensitized, which decreases pain and straining by the animal. Motor control of the hindlegs is usually retained, but occasionally hindlimb ataxia occurs if excessive anesthetic diffuses cranially.

The technique is similar to that described in horses. The procedure is performed through the dorsal aspect of the tail base, at the first intercoccygeal space (the sacrococcygeal space is another possibility but is more difficult to identify and less commonly used). To identify the first intercoccygeal space, manipulate the tail up and down while palpating the dorsal aspect of the tail base for the first obviously movable articulation (joint) caudal to the sacrum (Fig. 12-6). The area is clipped and sterilely prepped, and aseptic technique is used for the procedure. In general, blocking the skin and subcutaneous tissue is not necessary, but a small bleb of subcutaneous anesthetic can be placed with a 25-ga needle if desired. For placement of the epidural anesthetic, cattle require an 18-ga × – to 3-inch needle. The needle enters on dorsal midline at a 45-degree angle, and anesthetic is deposited into the epidural space. Lidocaine 2% without epinephrine is most commonly used (1 ml/100 kg body weight, or ≈5–6 ml in adult cattle). Mepivacaine 2% also is suitable, and xylazine (Rompun) can be combined or used as the sole agent.

The anesthetic generally takes effect in 10 to 20 minutes and lasts 1 to 2 hours on average. If prolonged anesthesia is necessary, a small-diameter epidural catheter (commercially available) or similar sterile medical tubing can be placed into the epidural space to provide continuous caudal epidural anesthesia. The catheter is placed and threaded cranially along the epidural space as in horses (Fig. 12-7). The catheter is placed and maintained aseptically. The end of the catheter is protected with an injection cap, and the exposed portion of the catheter should be secured to the skin. Small doses of lidocaine can be given every few hours as needed for pain or straining. A protective gauze bandage is advisable between uses. This technique spares the discomfort and tissue trauma from repeated standard epidurals. Disadvantages include kinking of the catheter and plugging of the tip with tissue or fibrin. Continuous caudal epidural anesthesia is also used successfully in equines.

Cranial epidurals are generally avoided in cattle. They are performed at the lumbosacral space, using landmarks similar to lumbosacral cerebrospinal fluid centesis. Cranial epidurals are technically more difficult to perform and have more potential complications than caudal epidurals, including accidental injection into the subarachnoid space/cerebrospinal fluid. Posterior paralysis, including the hindlimbs, occurs and produces recumbency. Animals may require assistance standing as the anesthesia begins to wear off. They are prone to “splay-legged” recoveries with overabduction of hindlimbs and resulting damage to the pelvis and inner thigh muscles.

Regurgitation

Ruminants are prone to regurgitation. The contents of the rumen/reticulum may be regurgitated during heavy sedation or general anesthesia. Aesthetic drugs relax the smooth muscle sphincters that normally protect both ends of the esophagus, making regurgitation more likely. The amount of regurgitated material often is voluminous. In adult cattle, gallons of rumen liquid may be expelled in a matter of seconds. The primary risk associated with regurgitation is aspiration of this material into the trachea and lungs, leading to aspiration pneumonia.

To minimize the risk of regurgitation, the most important principle is to reduce the size of the rumen and decrease pressure inside the organ. In adult cattle, food is withheld for 12 to 36 hours, and water is withheld for 6 to 12 hours before general anesthesia. In calves that are consuming solid food material, fasting for 2 to 4 hours is sufficient (withhold food only; water is permitted). In very young ruminants, the rumen/reticulum has little function, and the risk of regurgitation is minimal. Fasting of neonates may cause hypoglycemia and is not recommended.

Other precautions include the following:

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