Introduction

Special considerations exist for general anesthesia in farm animals, such as cattle, small ruminants, camelids, and pigs, due to their unique anatomy and physiology as well as their normal behaviors. Cattle, sheep, and goats have a stomach that is divided into four compartments, whereas llamas and alpacas have three compartments. Though anatomically different, the digestive system of these animals functions similarly. Significant size and body weight differences exist, varying from 2–3 kg immediately after birth to 500–1000 kg when fully grown. Accurate dosing of injectable drugs and appropriately sized anesthetic equipment and accessories should be available. Ruminants are generally docile and tolerate physical restraint well, allowing many minor surgical procedures to be performed in the conscious patient with regional anesthesia. Because camelids and swine are less tolerant of physical restraint, general anesthesia is more frequently performed for even minor surgical procedures.

Farm animals share nociceptive pathway anatomy and physiology with other mammals and, therefore, have the ability to process information similarly. Analgesia is an essential part of surgical and anesthetic management. When surgical procedures require general anesthesia, balanced anesthetic techniques should be employed to provide unconsciousness, amnesia, and muscle relaxation. Minimizing the stress response induced by handling, surgery, and anesthesia is also important.

Anesthesia and surgery can be costly relative to the value of the animal whose ultimate purpose is slaughter for human consumption. In addition, anesthetics, sedatives, and analgesics commonly used in farm animal practice may not have regulatory approval for use in ruminants, camelids, and swine [1,2]. In the United States, the Animal Medicinal Drug Use Clarification Act (AMDUCA) of 1994 codifies the requirements for extra‐label drug use (ELDU). In general, ELDU is permitted when animal health is threatened or death may result if a condition is untreated; however, the prescribing veterinarian is responsible for advising owners to ensure that no residual drug reaches processors or consumers [3].

Many anesthetics have short plasma elimination half‐lives (t_½) and achieve general anesthesia at low tissue concentrations due to their potency. Nevertheless, veterinarians should consult the Food Animal Residual Avoidance Databank (FARAD) for guidance on estimated meat and milk withdrawal intervals for ELDU of analgesics, tranquilizers, and injectable anesthetics as well as for updates on drugs where extra‐label use is prohibited [1,2,4,5] (Table 53.1). Practitioners working outside the United States should consult the appropriate regulations for their practice area to determine the specific requirements for drug use in animals intended for food production. Camelids are considered fiber animals, so concerns for withdrawal times and violative drug residues are minimal.

In addition to use as food, farm animal species of all types are often kept as companion animal pets. In particular, pot‐bellied pigs have become popular house pets in the last two decades. There are many anatomic and physiologic characteristics of pigs that make anesthetic management challenging. Although the commercial value of these pets may not be extraordinary, their owners are often willing to spend more for veterinary care. Although it is unlikely for a companion animal to end up in the food chain, the possibility of a pot‐bellied pig being slaughtered for food consumption does exist [6]. Therefore, it is important that owners be educated about the potential drug residues in the meat. Special considerations for laboratory swine are discussed extensively elsewhere [7,8].

Drug administration

Oral administration of a tranquilizer or sedative is sometimes needed to facilitate capture and/or reduce stress associated with restraint. In ruminants and camelids, oral medication is absorbed primarily via the rumen. Absorption is affected by motility and the pH of the ruminal fluid. Reticuloruminal motility is primarily controlled by the medullary gastric center in the brain. Physical conditions like central nervous system depression, pain, fear, excitement, pyrexia, endotoxemia, hypocalcemia, and extreme wall distension (e.g., rumen tympany) tend to reduce rumen motility. Analgesics and anesthetics like opioids, α₂‐adrenergic receptor agonists, and general anesthetics also decrease gastric motility, resulting in prolonged gastric emptying time and enhanced drug absorption [13]. The pH difference of saliva (8.2), plasma (7.4), and ruminal contents (5.5–6.5) affects drug absorption by altering drug ionization. Ionization and the amount of drug in saliva, plasma, and the rumen are constantly changing, making it difficult to predict bioavailability and distribution of orally administered drugs (e.g., tranquilizers, sedatives, or injectable anesthetics) [13]. In monogastric animals like pigs, the pH in the stomach is acidic (1.5–2.5) [14]. As a result, absorption from the gastrointestinal tract is more predictable in these species leading to greater bioavailability of orally administered sedative drugs. Oral administration of trazodone (8–10 mg/kg) and gabapentin (20 mg/kg) can be given to pet miniature pigs the night before and again 2 h prior to the veterinary visit to reduce stress and ease restraint for minor procedures such as hoof trim [15]. Intranasal administration of a sedative such as midazolam (0.4 mg/kg) has been reported to produce calming effects in pigs with a bioavailability of 64% and a terminal t_½ of 145 min, compared to 158 min following intravenous (IV) administration. Rapid anxiolytic effects and sedation occurred within 3–4 min following administration of either 0.2 or 0.4 mg/kg [16].

The external jugular veins of ruminants are easily palpable and visualized when occluded. A 14 gauge, 2–3 inch needle can be placed in the jugular vein of adult cattle for administration of injectable anesthetics and fluids during surgery. A 14 gauge, 5¼‐inch over‐the‐needle indwelling catheter may be placed and secured for longer duration IV medication or fluid therapy. A surgical cut‐down can be performed to facilitate insertion of the catheter through the thick skin of adult cattle. A 16 or 18 gauge IV catheter is appropriate for calves and small ruminants. The technique for IV catheterization in smaller ruminant species is similar to that used in calves. However, skin incision is generally not needed in smaller ruminants and the jugular vein is easily visualized when occluded.

Venipuncture and catheterization can be difficult in camelids because these animals have thick fiber coats and neck skin. More importantly, they do not have a jugular furrow, and the external jugular veins lie deep to the sternomandibularis and brachiocephalicus muscles, ventral to the cervical vertebral transverse processes, and superficial to the carotid artery and vagosympathetic trunk within the carotid sheath for most of their length [17–21]. External jugular veins of camelids are not always visible even with occlusion of the vessels, particularly in adult males. Camelids, like other long‐necked animals, have four or five jugular vein valves located at irregular intervals to prevent blood pooling in the head when the animal grazes. The presence of these valves can hinder the advancement of an IV catheter, even when correct catheter placement is confirmed by blood flowing into the stylet of the catheter. The right external jugular vein is preferred for venipuncture and catheterization to avoid damage to the esophagus, which runs on the left side of the neck. The landmarks for venipuncture in camelids are the cranial portion of the neck at the level of the mandible, on the caudoventral aspect of the neck, medial to the fifth cervical vertebral transverse process [19] (Fig. 53.1). Care should be taken to verify that a catheter is not accidentally placed in the carotid artery before injection of medications.

Venous blood of camelids appears to be bright red and, thus, may be mistaken for arterial blood. It has been suggested that the adaptations essential to high altitude existence are responsible for the bright red color of camelid venous blood, including the elliptical shape of their red blood cells, the relatively high concentration of hemoglobin in red cells, and a left‐shifted oxygen–hemoglobin dissociation curve [22]. Attention to an absence of characteristic pulsatile arterial flow will help prevent inadvertent carotid arterial injection [19]. A 14 gauge indwelling catheter is appropriate for adult camelids, and a 16 or 18 gauge catheter is suitable for crias. The catheter should be secured with suture or a bandage. Skin cut‐down is helpful in passing the catheter into the jugular vein [23,24]. Distension of the jugular vein with sterile saline facilitates advancement. If distension of the vessel is unsuccessful, relocate the placement site 1–2 cm cranially or caudally from the previous site [24]. The auricular (ear) vein is an alternative site for IV injection using a 25 gauge needle or butterfly catheter to deliver a small volume of sedative drug to cooperative camelids.

Pigs usually resist physical restraint. They are easily stressed when restrained and generally respond by constant struggling and vocalization [25–27]. Preanesthetic physical examination and evaluation should be as stress‐free as possible, especially since some pigs are susceptible to the genetic disorder, malignant hyperthermia (MH), or porcine stress syndrome (see below). Hog boards or crowd/sorting boards (36.5 × 30.5 inches) can be used to confine a pig to a small area in order to facilitate physical examination or drug administration by limiting the pig’s movement. A “forking” technique has been used in pigs to reduce anxiety, produce calming effects, and minimize stress prior to intramuscular (IM) injection of drugs [28]. IM administration of midazolam (0.1–0.5 mg/kg) [15,29]; midazolam (0.1–0.3 mg/kg) and butorphanol (0.1–0.3 mg/kg) [30]; midazolam (0.2 mg/kg), butorphanol (0.2–0.4 mg/kg), and xylazine (1 mg/kg) [29]; or dexmedetomidine (0.01–0.04 mg/kg) [30] have been administered for preanesthetic chemical restraint and sedation.

In general, rapid IM administration tends to be less stressful than IV administration for pigs. Most pigs have a thick layer of subcutaneous fat, especially pot‐bellied pigs. The adipose tissue layers in the neck and rump are particularly thick. Accurate deposition of preanesthetics or anesthetics into the muscle layer requires a needle of adequate length. Too short a needle will result in depositing the drug into the adipose tissue and slowed onset of action. It is recommended that the IM injection be made in the divot behind the base of the ear near the lateral aspect of the second cervical vertebra where the fat layer overlying the muscle is thin [31]. This author prefers a 23 gauge, 1½ inch needle for IM injection in these muscles as it appears to induce less pain and pigs often do not respond. At the cervical or thoracic areas of the trapezius muscle, at least a 2 inch long needle will be required to ensure drug injection into muscle tissue. Inconsistent effects are observed more frequently if shorter needles are used in these areas [25]. Alternatively, IM injection can be made at the semimembranosus and semitendinosus muscles located just above the hock or in the caudal portion of the biceps femoris muscle [25]. In pigs intended as food, IM injection into the “ham” muscles including gluteal, semimembranosus, and semitendinosus muscles is not recommended because of the potential for muscle inflammation and fibrosis [32].

IV injection of preanesthetics or anesthetics in pigs can be very difficult, particularly in pot‐bellied pigs, because of the lack of visible superficial veins. On occasion, an auricular vein can be used for IV injection if the pig cooperates or is easily restrained (Fig. 53.2). Ideally, deep sedation and immobilization is achieved first with IM injection. The jugular vein or anterior vena cava can be used for IV injection, but this technique is difficult due to the thick neck and abundant jowls.

Auricular veins, especially lateral auricular veins, are common sites for IV injection because they are superficial and easily accessible. However, in dark‐colored ears, IV injection remains challenging (e.g., pot‐bellied pigs). In larger adults, the central dorsal auricular vein is often utilized. An 18 or 20 gauge, 1–1½ inch hypodermic needle, indwelling catheter, or butterfly catheter can be used, while a 21 or 23 gauge needle is suitable for smaller ears. A butterfly catheter has a shorter needle and tends to stay in the vessel better than hypodermic needles, especially when the animal struggles during the injection. An indwelling catheter can be placed in an auricular, cephalic, or superficial abdominal/mammary vein if postoperative IV medication and/or fluid therapy are needed. In debilitated pigs, when repeated administration of medications or continuous fluid therapy is required, a central venous catheter can be placed in the jugular vein via the Seldinger technique using a large animal long‐term venous catheterization kit (e.g., Mila International Inc.) with the pig under general anesthesia (Fig. 53.3) [33].

Intramedullary cannulation has been used for fluid and drug administration when other vascular access cannot be established. An 18 gauge cannula can be placed into either the greater tubercle of the humerus or the trochanteric fossa of the femur. Although this technique is performed easily in immature pigs, rate of fluid administration may be limited in older pigs due to the presence of fat and fibrosis of the medullary canal [34].

Positioning during anesthesia

Neuromyopathy has been associated with improper positioning and inadequate padding of the surgery table. Myopathy does not occur readily in large ruminants, but nerve paralysis has been observed. Adult cattle should be placed on a 10‐cm‐thick high‐density foam pad, whereas a 5‐cm‐thick foam pad is sufficient for calves, small ruminants, and camelids. When placing adult cattle in dorsal recumbency, the animal should be balanced squarely on its back with the gluteal areas bearing equal weight. All limbs should be flexed and relaxed. In lateral recumbency, an automotive inner tube (valve stem pointed down) can be placed under the elbow of the dependent (lower) forelimb to prevent radial nerve paralysis. The dependent forelimb is placed through the inner tube and the tube positioned directly under the shoulder, which prevents pressure on the radial nerve as it traverses the musculospiral groove of the humerus. The bony point of the shoulder should be positioned within the hollow center of the inner tube. Non‐elastic or duct tape can be wrapped over the part of the inner tube not under the weight of the shoulder (opposite the valve stem) to prevent the tube from collapsing under the shoulder thereby ensuring support.

Once the inner tube is in place, pull the dependent forelimb anteriorly so the weight of the thorax rests on the triceps but not the humerus. Both uppermost limbs (front and hindlimbs) should be elevated and parallel to the table surface to minimize the pressure on the radial, femoral, or peroneal nerve of the dependent limb and prevent nerve paralysis. The head and neck should be at a slightly extended position (Figs. 53.4 and 53.5). The dependent eye needs to be protected by administration of ophthalmic ointment and ensuring closure of the eye to minimize the risk of corneal ulcer [35,36]. For camelids, similar precautions should be instituted to prevent radial nerve paralysis. A padded, supporting “H block” made of styrofoam with duct tape around the exterior can be used to support the upper, non‐dependent limbs and keep pressure off the lower, dependent limbs. In dorsal recumbency, overflexion or overstretching of the limbs to clear the limbs from the operative site with ropes should be avoided. Camelids have prominent eyes and administration of ophthalmic ointment and additional padding with gauze or soft towels under the dependent eye will minimize the potential for corneal laceration or ulceration [37].

Cardiovascular system

Cardiopulmonary depression associated with commonly used anesthetics tends to result in hypoventilation and hypoxemia [38]. Furthermore, compression of the great vessels such as the vena cava by the weight of abdominal viscera while in lateral or dorsal recumbency can result in decreased venous return, cardiac output, arterial blood pressures, and tissue perfusion [39]. Studies have shown that ventilation, cardiac output, and arterial blood pressure are better maintained when animals are positioned in sternal recumbency [40,41

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