Standing Surgery

Most large animal surgical procedures are performed on standing, awake patients. Most of these procedures are performed for treatment and repair of traumatic injuries, such as lacerations and punctures. Other common procedures are castration, female reproductive surgery, laser and endoscopic surgery of the upper respiratory tract, and minor hoof and lower leg procedures. Less common are abdominal and thoracic procedures, ophthalmic surgery, and sinus and dental procedures. Many of these procedures can be performed on the farm, saving the expense and risks associated with transportation.

Generally, the primary benefit of standing surgery is avoiding the risks associated with general anesthesia, especially recovery from general anesthesia. Recovery from general anesthesia is the single largest risk for surgical patients, even the healthy elective surgery patient. Also, some surgical procedures are technically easier to perform on a standing patient for anatomical or physiologic reasons. It is in the patient’s best interest to use a standing procedure whenever possible.

Criteria and indications for standing surgery include the following:

Standing surgery has drawbacks, however. Surgeon comfort is often compromised, especially if operating on the lower legs (Fig. 8-1). The surgeon’s visualization of the surgical field is often compromised because of the inability to use retractors and often poor lighting. It is very difficult to drape and maintain a sterile surgical field and to control contamination of instruments, especially under farm conditions (Fig. 8-2). Finally, because the patient is awake, the patient can move, presenting a danger to itself, personnel, and equipment. Even with heavy sedation and physical restraint, the possibility of motion must always be considered and precautions taken.

Preparation for Standing Surgery

Patient preparation depends totally on the procedure and whether it is being performed on an emergency or elective basis. If the procedure is scheduled in advance, it is preferable to restrict the patient’s food intake. Most procedures use some form of chemical restraint. All of the drugs used to produce sedation and tranquilization cause some degree of depression of gastrointestinal (GI) motility. Food restriction can reduce bulk in the intestines and reduce intake of highly fermentable foodstuffs. Clinicians have individual preferences for how to restrict food intake, but generally grain is withheld for 12 hours, and only small amounts of hay are allowed until 2 to 6 hours before the procedure. Water is not withheld. If the procedure is being performed on an emergency basis, the client should be instructed to remove all hay and grain immediately until the veterinarian arrives and evaluates the situation.

Technician Note

Preanesthetic fasting consists of withholding grain for 12 hours; only small amounts of hay are allowed until 2 to 6 hours before the procedure. Water is not withheld.

Optimally the procedure should be performed in a clean, dry, dust-free area. Drafts should be prevented because they can blow dust and debris into the surgical field. Noise and motion must be minimized to prevent arousing the horse.

Surgical instruments must be available to the surgeon yet out of the way of the horse if it moves. Every attempt should be made to elevate the instruments above ground level, where contamination by dust and debris is most likely. A hay bale, stool, or overturned bucket covered with a towel are simple ways to get instrument packs off the ground. The instrument pack can then be opened and used by surgical team members.

Restraint of the patient depends on the location of the procedure, duration of the procedure, “pain factor” of the procedure, temperament of the horse, facilities, and skill and number of available personnel. Physical restraint may range from a halter and lead shank to mechanical devices such as twitches and ropes. Chemical restraint (sedation or tranquilization) is often used; sometimes heavy sedation must be used (Fig. 8-3). Every procedure and patient is different; therefore, restraint must be tailored for the situation.

The pharmacology and effects of drugs used for restraint and analgesia are thoroughly discussed in numerous excellent references on anesthesia and pharmacology. The clinical effects of the most commonly used chemical restraining agents are listed in Box 8-1.

BOX 8-1   Clinical Effects of Commonly Used Chemical Restraining Agents

Acepromazine (Tranquilizer)

• Peripheral vasodilation

• Tendency for hypotension, especially if animal is dehydrated or debilitated

• Tendency for mild hypothermia

• Delayed onset of action (20–30 minutes after IV injection)

• Long duration of action (3–4 hours)

• May result in prolonged period of ataxia

• Penis may remain extended for prolonged time; subject to trauma

• Lowers seizure threshold of the brain

• Tranquilization is not profound; animal can still respond vigorously to stimuli

• No analgesic properties

• Respiratory rate decreases but tidal volume increases; net effect maintains relatively normal ventilation

Xylazine (Sedative-Hypnotic)

• Intramuscular route is irritating to foals; warn owner of irritation and swelling at injection site. Avoid neck if foal is nursing.

• Rapid onset of action (≈1–2 minutes after IV injection)

• Good muscle relaxation; shifts weight to forelimbs, lowers head/neck

• Animal can still respond to stimuli by kicking with hind limbs; use caution

• Good analgesia (for 20–30 minutes)

• Bradycardia, often with second-degree A-V block

• Second-degree A-V block usually is transient (5–8 minutes)

• Reduce dosage if patient has low resting heart rate or preexisting second-degree A-V block

• Peripheral vasoconstriction

• Helps maintain blood pressure

• Decreases respiratory rate

• Increases urine output (up to 10×) for 2–4 hours

• Causes sweating

• Decreases gastrointestinal motility

• Increases blood glucose up to 40%

Detomidine (Sedative-Hypnotic)

Similar to xylazine, except

• Longer duration of action (≈60 minutes)

• More potent analgesia

• More profound sedation

• Bradycardia may be more severe

Butorphanol (Opiate)

• Opiates cannot be used alone in equids; must first give a tranquilizer or sedative-hypnotic and allow it to take effect before giving the opiate

• Profound sedation and excellent analgesia when used in combination with sedative-hypnotics; xylazine/butorphanol is most often used

• Minimal cardiovascular effects

• Minimal or no adverse respiratory effects

• Controlled substance

A-V, Atrioventricular; IV, intravenous.

Control of Pain

Most surgical procedures either create pain, address a painful condition, or both. Analgesia must be provided to minimize patient discomfort. Many of the drugs used for chemical restraint have analgesic properties but are insufficient to control moderately or markedly painful procedures. Therefore, local anesthesia is usually employed for pain control.

Local anesthesia may be applied in several ways.

Nerve Blocks

If a nerve can be reached with a hypodermic needle, then local anesthetic drugs can be deposited over the nerve. The anesthetic diffuses into the nerve and interferes with impulse transmission for variable periods of time. The effects of blocking a nerve must be known in advance, in order to select the proper nerves to desensitize the surgical area and to prevent unwanted anesthesia of important structures. Anatomical and sensation “maps” for the major nerves are available and help guide the clinician in selecting which nerves to anesthetize. Local anesthetic drugs are primarily chosen according to their duration of action, matched with the anticipated length of surgery.

Field Blocks

These blocks are usually performed to desensitize the skin and subcutaneous tissue around a surgical area, without blocking specific nerves. Rather, a line of anesthetic is deposited subcutaneously around the perimeter of the surgical area (“line block”) to produce an area of desensitization. If deeper anesthesia is required, the depth of the line can be extended into underlying muscle tissue (if available). This type of block may produce small or large areas of anesthesia, depending on where the anesthetic is placed. The line block is very useful for suturing lacerations. Large field blocks are commonly used for abdominal surgery through the flank.

Epidural Anesthesia

Caudal epidural anesthesia is routinely used for analgesia of the tail, perineum, anus/rectum, vulva, and vagina. Caudal epidurals are also used to decrease straining associated with obstetric procedures for dystocias and other reproductive procedures.

Three classes of drugs can be used for caudal epidurals:

1. Local anesthetics block not only sensory fibers but also motor fibers and sympathetic fibers. Loss of motor control can cause ataxia and even collapse of the hindlimbs; this is a rare occurrence but can be disastrous if it does occur.

2. Alpha-2 agonists selectively block sensory fibers, with minimal effects on hindlimb function.

3. Opioids selectively block sensory fibers, with minimal effects on hind limb function.

Caudal epidural anesthesia is easily performed on large animals. The site of injection is between the first and second coccygeal vertebrae, on the dorsal midline (Fig. 8-4). An estimate of this location is made by moving the tail up and down while palpating for the first movable intercoccygeal space caudal to the sacrum. The patient should be restrained, and personnel should stand to the side of the patient to avoid being kicked.

Technician Note

Location of the caudal epidural injection site is found by lifting the tail up and down while palpating for the first movable intercoccygeal space caudal to the sacrum.

The area at the base of the tail and caudal sacrum are clipped and surgically prepped. The anesthetist should wear sterile gloves. The clinician inserts an 18-gauge (ga) × -inch needle (large patients may require an 18-ga × -inch spinal needle) at a 90-degree angle to the skin and advances it into the epidural space. Anesthetic solution is injected with a sterile syringe. Because sterile injection technique is required, the anesthetic solution should be taken from a new, previously unopened container. Sometimes the needle is left in place in case the patient needs more anesthetic later in the procedure. The needle should not be withdrawn until the clinician approves because bleeding and swelling (after removing the needle) can make repeating the epidural procedure difficult. Complications are unusual, and aftercare involves imply cleaning the area and applying an antibiotic or antimicrobial ointment to the puncture site.

Epidurals are not performed more cranially than the first intercoccygeal space in awake patients. This is because the risk of creating ataxia and collapse of the hindlimbs increases as the site of administering the epidural moves cranially.

Technician Note

Epidurals are not performed more cranially than the first intercoccygeal space in an awake patient because of the risk of ataxia and collapse.

General Anesthesia

General anesthesia can be technically simple or complex but never without risk. The procedure must be respected for the risks to which both patient and staff are potentially exposed, and the risks must be carefully evaluated. Much of the risk is due to patient factors such as size, weight, and temperament. The physiologic effects of preanesthetic and anesthetic drugs also pose certain hazards that are unique to large animal patients.

There is no single “correct” approach to large animal anesthesia. Clinicians and hospitals vary greatly with regard to all phases of administering general anesthesia: equipment, patient preparation, induction, maintenance, and recovery. Despite these variations, some generalizations can be made and are discussed in further detail here. For specific information on the pharmacology and in-depth mechanics of large animal anesthesia, the reader is referred to the many textbooks that have been written on the topic of veterinary anesthesia.

Preanesthetic Preparation

For preparation of an equine patient for general anesthesia, the following principles should be considered.

Ventilation Problems under General Anesthesia

When the patient is recumbent, the weight of the intestines presses against the diaphragm and thoracic cavity and prevents the lungs from expanding normally. Oxygen levels in the blood fall and carbon dioxide levels rise; these abnormalities worsen with time. These effects occur in lateral recumbency and are most severe in dorsal recumbency.

Ventilation may be improved by reducing the weight of the intestinal tract. Fasting the patient is the most efficient way to reduce the weight of intestinal contents. Grain and hay are usually withdrawn for at least 12 hours before general anesthesia, although clinicians vary in specifying time frames for feed withdrawal. Water is seldom withdrawn more than 2 hours before anesthesia because of the possibility of dehydrating GI contents (and causing subsequent GI colic) and possibly dehydrating the circulatory system of the patient. Most clinicians do not withdraw water at all before premedication for the actual procedure.

Foals are allowed to nurse until approximately 30 to 60 minutes before induction of general anesthesia. This allows time for the stomach to empty, which is important because foals, unlike adults, may regurgitate after induction. If the foal is eating solid food, it should be removed for 3 to 4 hours before induction. To prevent nursing the mare, a muzzle must be used on the foal. Foal muzzles can be purchased or made from half-gallon milk cartons by cutting the carton in half. Two nostril openings are made, and the muzzle is secured behind the foal’s ears with gauze or twine straps (Fig. 8-5).

Technician Note

Water is typically not withheld before equine surgery.

Anticholinergics Not Routinely Used for Preanesthetic Medication

Most anesthetic drugs cause some degree of depression of GI motility. Anticholinergic drugs, especially atropine, may have marked effects on depressing GI motility. The additive effect of anticholinergics and other anesthetic drugs may result in prolonged ileus, leading to GI colic. The administration of anticholinergics to decrease saliva production, as used in small animals, is not considered necessary in horses.

Technician Note

Use of anticholinergics depresses GI motility.

Anticholinergics are, however, very useful for treating bradycardia and second- and third-degree atrioventricular (A-V) block. These drugs should be available for emergency use. Anticholinergics are sometimes used during ophthalmic surgery, when manipulation of the eyeball may create excessive vagal tone.

Minimizing Anesthetic Time

Because the risks of general anesthesia increase with time, every effort should be made to decrease anesthetic time using the following steps.

Surgical Clip/Prep

Perform as much of the surgical clipping and prepping as possible before induction of anesthesia. The disposition of some patients may make this impossible; clippers should be available after induction for these patients. Note that razors are often used for final removal of hair. However, use of razors on awake patients is difficult and often results in cuts and abrasions of the patient’s skin. Therefore, if razors are necessary, they should be made available for use after induction of anesthesia.

Technician Note

If the surgical prep requires use of a razor, it should be used after induction to prevent cuts and abrasions.

Intravenous Catheterization

If intravenous (IV) catheters are to be used, they should be placed before induction.

Preparation of Equipment and Supplies

All surgical instruments and supplies should be assembled and brought to the surgical area. Equipment should be checked for proper function. Anesthetic supplies and equipment, including IV fluids, must also be anticipated. If gas anesthesia is to be used, the anesthetic machine should be checked for leaks and properly prepared.

Patient Positioning

Positioning should be discussed with the surgeon so that the operating table and patient padding and support can be set up before the induction.

Mouth Cleansing

The patient’s mouth may harbor feed material and other debris. Feed material tends to accumulate between the cheek teeth and cheek tissue. This material may be pushed into the trachea during endotracheal intubation. Always flush the patient’s mouth with a dose syringe or water hose if endotracheal intubation is anticipated.

Technician Note

The equine mouth should be flushed with a dose syringe or water hose if endotracheal intubation is anticipated to prevent feed material from being pushed into the trachea.

Preventing Contamination of the Surgical Room

The patient should be thoroughly cleaned to prevent contamination of the surgical room. Thorough grooming should always be performed; the patient is brushed and bathed as needed. The tail is often braided or taped to keep tail hairs out of the surgical field and to minimize contamination if the horse defecates while under anesthesia. The hooves should be cleaned and scrubbed with soapy water. If the patient is wearing horseshoes, they are usually removed at this time. The feet should be covered either before the animal enters the surgery room or immediately after the animal enters the surgery room. Latex examination gloves, rectal sleeves, plastic bags, and bandaging tape are commonly used for this purpose.

Occasionally it may be desirable to leave horseshoes on, especially if they are being used to correct or treat a medical condition. In such cases, the hooves should still be cleaned and some type of protective boot or hoof bandage placed over the shod hoof before anesthetic induction (Fig. 8-6).

Technician Note

During preparation, it is important to cover the hooves before entering the surgery room.

Protecting the Lower Legs during Induction and Recovery

Leg wraps or bandages should be placed on the horse to protect the tendons and ligaments of the lower legs. If removal of leg protection is necessary for the surgical procedure, the wraps should be replaced for recovery from anesthesia.

Technician Note

Leg wraps are placed to protect the legs during surgery and recovery.

Protecting the Eyes

The wide-set anatomical location of the equine eyes makes them susceptible to trauma. Halters with metal buckles/billets should be removed and replaced with a halter that does not have metal attachments. Padded walls or helmets can be placed to further protect the eyes. The head should be controlled during induction to prevent it from striking the floor or walls as the horse falls to the ground. Sharp objects and corners should be removed from the induction/recovery area or covered with padding if possible. In field settings, the lower (down) eye should be protected from the ground surface with a towel.

Technician Note

In a field setting, the down eye should be protected from the ground surface with a towel.

Preoperative Patient Examination and Laboratory Evaluation

The depth and extent of the physical examination and laboratory testing depend on the patient’s condition and the nature of the surgical procedure. The clinician prescribes the necessary laboratory tests to be performed. At a minimum, a physical examination should include temperature, pulse, and respiration; heart/lung auscultation; and body weight assessment before any general anesthetic episode. The patient’s temperament and manageability also should be assessed because these individual characteristics may influence the choice of induction and recovery techniques.

Correcting Fluid, Electrolyte, and Acid–Base Imbalances before Induction of General Anesthesia

Fluid therapy before induction is important to correct dehydration and other imbalances. Sometimes, the emergency nature of a case precludes complete correction of imbalances. In these cases, as much therapy as possible should be provided before induction and the therapy continued once the patient has been successfully induced.

Induction and Maintenance of General Anesthesia

General considerations for induction and maintenance of general anesthesia include the following.

Prevent Compartment Syndrome

Compartment syndrome is not uncommon in recumbent large animals, regardless of the cause of recumbency. All recumbent animals are susceptible to compartment syndrome, and the heavy body weight of large animals exacerbates the problem. Draft horse breeds and heavily muscled individuals are predisposed to developing the condition. Muscle “compartments” refer to muscles and muscle groups in the body that are encased in a dense connective tissue called “fascia,” which has little elasticity. Muscles that are enclosed in these nonelastic envelopes are prevented from swelling outwardly by the fascia.

The four clinically significant muscle compartments are the

1. gluteals

2. triceps

3. masseters

4. quadriceps

Normally, arteries pump blood into a muscle compartment to nourish the muscle, and veins and lymphatic vessels drain blood and tissue fluid (lymph) out of the compartment. In recumbent animals, however, the vessels may be sandwiched between the animal’s own body weight pressing down from above and the ground surface below. Compartment syndrome begins with the collapse (partial or complete) of the veins and lymphatics that drain the compartment. These vessels have very low pressure inside their walls, and they easily collapse when external pressure (such as body weight) is applied to them. Arteries, on the other hand, have much higher internal pressures and can resist higher compressive forces. Therefore, arteries remain open longer than do veins and lymphatics.

As the arterial supply continues to pump blood into the compartment, the pressure inside the compartment starts to rise because the blood and tissue fluid cannot drain adequately through the collapsed veins and lymphatics. A vicious cycle of increasing pressure and increasing collapse of vessels inside the compartment develops. Eventually, the muscle and nerve cells in the compartment begin to suffer because they cannot receive proper nutrition or eliminate their cell waste. Various degrees of muscle and nerve dysfunction result, and the cells may even die if the condition is not successfully treated. The severity of compartment syndrome often correlates with the length of time that the conditions persist.

Compartment syndrome is the primary cause of postanesthetic myopathy (muscle dysfunction) and neuropathy (nerve dysfunction). Postanesthetic myopathy/neuropathy is the leading cause of injury and death in healthy horses undergoing elective surgery. Incidence rates as high as 5% of general anesthesia cases have been reported. General anesthesia increases the risk of compartment syndrome because the drugs used to induce and maintain anesthesia usually depress the cardiovascular system and lower blood pressure. Blood pressure is the only pressure inside a vessel that resists external compression; therefore, maintaining adequate blood pressure is the primary protection against developing the condition.

Technician Note

Postanesthetic myopathy/neuropathy is the leading cause of injury and death in healthy horses undergoing elective surgery.

Compartment syndrome may affect only one compartment or multiple compartments, and it may have mild, moderate, or severe effects. Clinical signs may include difficulty standing or inability to stand following general anesthesia, palpable hardening of the affected compartment, paresis/paralysis, and lameness. Acute renal failure or even complete renal shutdown occasionally may result from damage to kidney tubules from myoglobin, which is released from damaged skeletal muscle cells. Myoglobin may cause dark urine; however, dark urine is not seen in every case. The condition is quite painful, and increased heart rate, respiratory rate, sweating, and anxiety may be observed. Although the syndrome is usually observed on the patient’s “down” side, it may also affect the “up” side if the limbs are not positioned properly or if prolonged hypotension is allowed to occur.

Severely affected animals may have irreversible damage to muscles, nerves, or kidneys and may cause severe damage to themselves by trying to stand on legs that cannot support their weight because of muscle and/or nerve damage. Euthanasia is not an uncommon ending for many affected animals; complications result in euthanasia in up to 25% of cases.

Technician Note

Complications in 25% of compartment syndrome cases result in euthanasia.

Compartment syndrome is a serious and realistic concern and is frustrating for clinicians and anesthetists. There is no reliable method for predicting or detecting the problem while the patient is under general anesthesia, and usually the problem does not become apparent until the patient tries to stand in the recovery area. Even when all precautions to prevent the syndrome are strictly followed, some individuals nonetheless develop the condition. Still, the incidence of compartment syndrome can be significantly reduced by following certain precautions:

• Minimize anesthetic time: The incidence of compartment syndrome increases with anesthesia time, especially in procedures lasting longer than 1 hour. As much prepping of the patient and setup of the surgical area as possible should be done before induction.

• Maintain anesthesia only as deeply as necessary: Blood pressure drops as the depth of anesthesia increases, increasing the risk to the patient.

• Use adequate padding: Facilities vary with regard to the type of padding used, but thick foam mattresses (15–20 cm thick), conventional mattresses, air mattresses, and waterbed mattresses can be used successfully (Fig. 8-7).

• Position the patient properly:

• Lateral recumbency: Place the forelimbs in a “staggered” position by pulling the lower forelimb forward (cranially) to rotate the shoulder girdle. This position decreases the body weight pressing down on the triceps muscle. The hindlimbs are not staggered. Elevate the upper limbs (both forelimb and hindlimb) to a horizontal position, parallel to the ground. Do not elevate the upper limbs above this level (Fig. 8-8). Protect the masseter muscle by removing or loosening the halter after induction and padding the area supporting the head.

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