Swine Anesthesia

Anesthesia and Analgesia in Swine {4160}

Special considerations (including malignant hyperthermia and everything you'd ever want to know about cardiopulmonary bypass)

I. Anatomic and Physiologic Characteristics

When considering the hemodynamic effects of anesthetics on studies using different stocks, it is not appropriate to compare pigs by body weight without indexing body surface area. Mini breeds are more mature than domestic swine at the same body weight.

Swine are prone to vasospasm and many peripheral vessels are not readily visualized. Swine are also highly susceptible to ventricular arrhythmias and have fragile pulmonary tissue. They have a left hemi-azygous vein that drains the intercostal vessels into the coronary sinus.

[Note: One of the authors of this chapter in the blue book is a well-known cardiovascular researcher who uses swine extensively, which may account for the emphasis on cardiovascular issues].

II. Anesthetic Delivery

	IM-thigh and neck
	Sub Q- neck or flank - the pig is a fixed skin animal.
	IV- lateral and medial ear veins (on the dorsal ear margins); cephalic veins located on the cranial aspect of the foreleg between the carpus and the elbow joint can be palpated if a tourniquet is placed around the elbow but not if held off and rotated as is commonly done in the dog.
	Cranial vena cava-thoracic inlet on right side.
	Method of endotracheal intubation: use lidocaine to reduce laryngospasm. Use 195 mm laryngoscope blade.

Top

III. Inhalation Anesthesia

A. Nitrous Oxide: Contraindicated alone due to low potency. Primary advantage is derived from the ability to reduce requirement of other anesthetic agents and reducing cardiovascular depression, although this works better in humans.

B. Halothane

	A potent myocardial depressant.
	Coronary blood flow, oxygen consumption and myocardial metabolism decrease with increasing halothane concentrations.
	Halothane compared to isoflurane is more depressant to global ventricular function.
	Halothane has a lower margin of safety compared to isoflurane.
	Halothane increases the sensitivity to arrhythmogenic effects of epinephrine.

Following chronic occlusion of a coronary artery, areas of myocardium develop dependence on collateral blood flow. "Coronary (or myocardial) steal" results when coronary vasodilation distributes myocardial blood flow away from these collateral-dependent zones to a normally perfused area, resulting in ischemia to the flow-limited area.

C. Isoflurane

	Rapid induction and recovery due to lower blood gas solubility.
	Greatest margin of cardiovascular safety.
	Use of nitrous maintains heart rate, cardiac output, and myocardial oxygen uptake to near conscious animal levels.
	Neonates develop severe bradycardia and hypotension.
	Isoflurane is potentially dangerous to “steal prone” patients.
	Preferred in liver transplant surgery. Does not cause hepatic injury.
	Considered the inhalant anesthetic of choice in swine.

D. Enflurane

At high-inspired concentration can cause muscle rigidity and seizure activity, and should therefore be avoided in swine.

E. Desflurane

The MAC is around 8%, whereas in humans it is 7%; this is consistent with the observation that MACs in swine are somewhat higher than in humans. The cardiovascular effects are similar to isoflurane: vasodilation, hypotension, and dose-dependent myocardial depression. Undergoes little or no metabolism. Has been identified as a trigger for malignant hyperthermia. Given that it is much more expensive than isoflurane, it is difficult to justify its use.

Top

IV. Injectable Anesthetics and Tranquilizers

A. Tranquilizers, Sedatives, and Anticholinergics

Phenothiazines (acepromazine): produce an a-adrenergic blockade in higher doses (>1.1 mg/kg), which may be a contraindication for their use in some cardiovascular studies.

Benzodiazepine tranquilizers (diazepam, midazolam): provide good hypnosis and sedation; have been associated with hypothermia with prolonged use which can be life-threatening. Flumazenil is a reversal agent which has not been used yet in swine. Midazolam (Versed) has been used as a sole agent for 20 min of deep sedation with minimal cardiovascular and respiratory depression.

Butyrophenone tranquilizers: droperidol and fluanisone are not routinely used as sole agents. Azaperone, labeled for prevention of fighting, has been used as a preanesthetic (2-4 mg/kg) or in higher doses (5.3-8 mg/kg) as an immobilizing agent with minimal cardiovascular depression.

a-adrenergic agonists: xylazine and medetomidine; usually used with other agents.

Xylazine (2 mg/kg) has only transient analgesic activity in swine, produces hypotension and 1-3 degree heart block. It is not satisfactory as a sole agent.

Medetomidine (0.2 mg/kg IV)-described as the best agent in LAS 46(2). Reversed by atipamezole.

Anticholinergics: dry secretions and vagolytic effect during intubation. Atropine or glycopyrrolate.

B. Dissociative Agents

The most commonly used injectable anesthetic agents in swine. Provide rapid, safe immobilization with minimal depression of the cardiovascular system. Poor muscle relaxation and little visceral analgesia so used in combination with other agents.

	Ketamine: characterized by catatonia. Unsatisfactory as a sole agent because of inadequate analgesia and muscle relaxation.
	Ketamine-Ace (or azaperone)- 30 min chemical immobilization, good preanesthetic
	Ketamine-Midazolam- 45min immobilization, profound hypothermia, requires 1-4 hrs before righting reflex.
	Ketamine-Medetomidine- 30min deep sedation; profound, but reversible hemodynamic depression.
	Ketamine-Xylazine- immobilization or minor surgery. Analgesic effect of xylazine only lasts 5 minutes.
	Ketamine-Xylazine-Oxymorphone- provides short term chemical restraint for minor surgery.
	Ketamine-Climazolam (or diazepam)- immobilization
	Tiletamine-zolazepam-20 min immobilization suitable for minor surgery. This combo produces hypothermia and cardiovascular depression.

Most of the IM preparations provide only 20-30 min of anesthesia suitable for minor procedures or for induction prior to inhalants. Of all the combos, only ketamine-xylazine, ketamine-midazolam, ketamine-xylazine-glycerol guaiacolate and Telazol-xylazine abolish the swallowing reflex for endotracheal intubation.

C. Barbiturates

The pig is prone to apnea, so use dilute solution. Continuous infusion rather than IV boluses better for cardiopulmonary depression. Ultra short- Induction of anesthesia and intubation use thiopental or thiamylal. Intermediate- Pentobarbital can be used for prolonged anesthesia. A cardiovascular depressant, metabolized by the liver, may have prolonged recovery time.

Barbiturates can delay episodes of malignant hyperthermia and are considered to be the most safe agents to avoid the condition, along with several other IV agents (droperidol, diazepam, midazolam, etomidate, ketamine, propofol, narcotics). Muscle relaxants, local anesthetics and nitrous oxide are also safe to give.{4160}

D. Opioids

Used with other agents to enhance analgesia, produce balanced anesthesia and in large doses stabilize the heart for surgery. Protects from arrhythmias. Fentanyl, as a bolus and sufentanil, as IV infusion reduces muscle rigidity. Do not use with paralytics until animal is adequately anesthetized. Naloxone reversal.

Fentanyl-droperidol produces short-term immobilization suitable for pre-anesthesia and minor surgery. Or combine with ketamine for 30 min of anesthesia.

E. Miscellaneous Injectables

	Saffan – alphaxalone + alphadolone- a short acting IM anesthetic. Contraindicated with barbiturates.
	a-chloralose- minimal cardiac depression, spares baroreceptors; use is questioned now because of poor analgesia, use high-dose narcotics or propofol
	Etomidate-as a sedative, has minimal analgesia and muscle relaxation. Combine with azaperone or with ketamine for induction.
	Propofol-an IV hypnotic agent with minimal cardiovascular depression. Adequate for major surgery if given at higher dosages (bolus 0.83-1.66 mg/kg IV, then infusion at 14-20 mg/kg/hr). Safe in malignant-hyperthermia susceptible swine at 12 mg/kg/hr.
	Metomidate-other combos are better

Top

V. Regional Anesthesia

To minimize stress and achieve immobilization, regional anesthesia should be performed using sedatives and with the animal physically restrained in a sling. Lidocaine - 90-180 min, bupivacaine - 180-300 min.

The most commonly used form of regional anesthesia in farm swine is epidural analgesia at the lumbosacral space. Paravertebral analgesia used for postop pain after thoracotomy.

Top

VI. Analgesia

Most analgesics have a short half-life in swine.

	Fentanyl (0.05 mg/kg IM q2hr or 50-100mg/kg/hr IV infusion). Fentanyl patches provide an inconsistent plasma level of drug, but the levels are in the range considered to be effective in humans.{4502}
	Sufentanil (5-10mg/kg IM q 2hr or 15-30 µg/kg/hr IV infusion)
	Meperidine (2-10 mg/kg IM q 4 hr)
	Oxymorphone (0.15 mg/kg IM q 4 hr)
	Pentazocine (1.5-3 mg/kg IM q 4 hr)
	Morphine causes excitement in non-painful swine, as in cats.
	Butorphanol (0.1-0.3 mg/kg IM q 4-6hr) is longer acting and has few side effects.
	Buprenorphine (0.05-0.1 mg/kg) is the postoperative analgesic of choice; effective for 8-12 hrs in these higher doses. Another reference involving analgesiometric skin-twitch latency testing showed that doses of 0.01mg/kg IV provided 6 hours of analgesia, while doubling the dose increased the time to beyond 10 hours.{4503}
	For musculoskeletal pain- phenylbutazone (10-20 mg/kg PO q 12 hr), aspirin (enteric coated due to gastric ulcer predisposition). NSAIDS ketoralac (1 mg/kg IM) or ketoprofen (1-3 mg/kg PO q 12hr) have been reported effective.

Top

VII. Intraoperative Support and Monitoring

Focus is to maintain pulmonary and cardiovascular systems and maintain body temperature. ECG monitoring is recommended to detect dysrhythmias. Normal swine have a prolonged Q-T interval.

A. Cardiac Arrhythmia

Swine are prone to development of fatal cardiac arrhythmias secondary to cardiac surgery, vagal manipulation etc. Mini swine may be less susceptible because they are older per kg of body weight. Bretylium (3-5 mg/kg IV q30 min slow IV) or lidocaine (0.3 mg/kg/hr) can be used to prevent. Pig skin has high electrical resistance so set the paddles on high (10 joules internal or 200-400 joules external).

Assessment of anesthesia- jaw tone, absence of pedal reflex in response to painful stimuli and a stable heart rate. Normal HR=70-150, ocular reflexes are not reliable.

B. Support

Lactated Ringer's solution at 5-10 ml/kg/hr. The pig is very prone to hypothermia especially in prolonged anesthesia. Also prone to pulmonary edema and pooling of blood in the abdominal viscera.

C. Post-surgical Care

Extubation should be performed only after a strong swallowing reflex is present. Post-op antibiotics rule of thumb: when using human meds-use pediatric dose and dosing schedule.

Top

VIII. Special Anesthetic Considerations

A. Malignant Hyperthermia

A genetic condition occurring in Poland China, Landrace and Pietrain breeds, but not in miniswine. (the mnemonic is You Poor Dumb Little Pigs=Yorkshire, Poland China, Duroc, Landrace, Pietrain).

MH is a “fulminant and hypermetabolic state of skeletal muscle” induced by volatile inhalation anesthetics, succinylcholine and perhaps stress and exercise.

Pathophysiology: uncontrolled increases in intracellular calcium. This leads to: 1) activation of contractile elements, 2) hydrolysis of ATP, 3) heat production, 4) oxygen consumption, 5) CO₂ and lactate production, 6) uncoupling of oxidative phosphorylation, 7) eventual cell breakdown and release of intracellular contents.

Recognition: 1st cardinal sign- increase in end-tidal CO₂; a rise of 5-10mm Hg over baseline is highly suspect. Concurrent tachycardia and tachypnea plus an increase in temperature should prompt an investigation. Within 5-10 minutes, increased PaCO2 over 45-50mm, decreased pH <7.25, base deficit <-8, and hypercarbia occur because of the increased metabolic rate.

Triggers: succinylcholine, halothane, isoflurane, enflurane, desflurane, etc. Barbiturates can delay an episode and are considered safe, as are most injectable agents (droperidol, diazepam, midazolam, ketamine, propofol, narcotics, muscle relaxants), local anesthetics, nitrous oxide, atropine, and glycopyrrolate.

Prevention and management: Dantrolene (3.5-5 mg/kg IV 1 hr preop and repeated hourly) is the drug of choice and is most effective when given prior to the trigger, when muscle perfusion is still adequate. It can also be given orally at 5 mg/kg 10-12 hours prior to surgery and repeated 2 hours preop. It is a hydantoin derivative acting distal to the end plate within the muscle fiber, decreasing the release of calcium from sarcoplasmic reticulum. Cardiac arrest during an episode of malignant hyperthermia is the result of hyperkalemia and acidosis. Treat the acidosis and hyperkalemia to relieve the dysrhythmias, possibly including bretylium. Calcium chloride should be used only as a last resort. Calcium channel blocking agents should not be used as they are synergistic with dantrolene and lead to cardiovascular collapse.

Treatment of a Malignant Hyperthermia Emergency
(Malignant Hyperthermia Assoc. of the US hotline 209-634-4917)

Stop all triggering agents immediately.

Hyperventilate with 100% oxygen using a new circuit and soda lime.

Give 2.5 mg dantrolene and continue until all signs resolve (total dose up to 10-20 mg/kg). Maintain IV dantrolene at 1 mg/kg every hr.

Correct metabolic acidosis (i.e. HCO₃ 1-2mEq/kg stat) and follow blood gases.

Correct hyperkalemia: give HCO₃ or glucose (0.5 gm/kg) and regular insulin (0.15 µ/kg).

Cool the animal: iced saline, body cavity lavage

If arrhythmias persist, give 3mg/kg procainamide to start; maximum of 15mg/kg.

Monitor blood gases, urine output, temperature, end-tidal CO₂, K⁺, Ca²⁺, lactate, CK, PT, platelets.

Monitor for recrudescence and late complications

Convert to oral dantrolene when awake and stable, at 1 mg/kg PO q 6 hr x 24 hr.

B. Cardiopulmonary Bypass

Two types of oxygenators: bubble (non-survival) and membrane (survival). Membrane oxygenators have less incidence of microemboli and blood trauma.

Basics- pump priming prior to bypass, myocardial protection- 3 methods- 1) chemical cardioplegia, 2) electrical fibrillation and 3) hypothermic cardioplegia. Anticoagulation, pump flow rates and monitoring, central venous pressure, weaning off pump and anesthetic management.

Progressive sustained rise in CVP is a hallmark for cardiac failure or peripheral vasoconstriction if colloids were not used.

C. Pregnant Animals/Fetal Surgery

In dorsal recumbency the gravid uterus can compress the aorta. Keep the environment and the sow warm to avoid fetal distress. The sow has a diffuse epitheliochorial placenta so most anesthetics can cross the placenta.

Top

Comments? Send an email to rodgers@uky.edu