Anesthesia and Analgesia in Dogs, Cats, and Ferrets{4159}

Introduction

The state or condition referred to as anesthesia, or particularly general anesthesia, includes the component conditions of unconsciousness, amnesia, lack of sensation to potentially noxious stimuli, and muscle relaxation.

Anesthetic protocols should include consideration of sedation; pharmacological restraint (occasionally appropriate as an alternative to general anesthesia); induction and maintenance of general anesthesia; perioperative physiological management or support; anesthetic monitoring and plans for responding to anticipated physiologic changes; and, in chronic studies, the recovery from anesthesia and postoperative care.

For acute studies (those with no recovery from anesthesia) there may be several additional options available in the choice of pharmaceuticals.  Drugs that may not afford suitable recovery (e.g., chloralose) may be quite appropriate for research protocols in which recovery is not planned.

Anesthesia and Analgesia in Dogs and Cats

Preoperative considerations

Anesthetic delivery

Preanesthetics

Intravenous agents

Inhalation agents

Monitoring

Special considerations

Top of page

I. Preanesthetic Evaluation

A. Historical Information and Vendor Health Profile

Review of documentation accompanying animals on receipt and review of daily records may aid in the identification of abnormalities.

B. Physical Examination

Two benefits of preanesthetic evaluation are the establishment of baseline data and the identification of any pre-existing physical or physiological abnormalities.

C. Laboratory Data and Parasitology Screen

The extent of medical profiling needed varies with the source of the animal, intended use, and the standards set for each research program and/or the facility.  Quarantine or perhaps a less restricted, but specified holding time, is suggested to verify health status.

Back to dogs and cats

II. Delivery of Anesthetics and Adjunctive Drugs

A. Venous Access Sites and Intravenous Catheter Placement

Cephalic and saphenous veins commonly used in dogs and cats.  Lateral saphenous in dogs, medial saphenous in cats.  Jugular vein is used for central venous access.  Jugular vein catheters are less subject to positional problems, but may be more often associated with significant thrombosis.

B. Sites for Intramuscular and Subcutaneous Drug Delivery

IM injections in dogs and cats have most often been delivered to the caudal muscles of the thigh, in the semimembranosus or semitendinosus muscles.  Superior absorption has been documented with injection into either the cranial thigh muscles (quadriceps) or the lumbar spinal epaxial muscles. When using the caudal thigh muscles, injection into the fascial planes decreases drug absorption and allows for tracking along the fascial planes, potentially to the area of the sciatic nerve.

Uptake of drugs following SQ injection may be variable and influenced by the state of hydration and local perfusion. Interscapular, lateral thoracic, and lumbar dorsal regions are used.

C. Chemical Restraint

Used when chemical restraint is needed for minor procedures.  Acepromazine is often used.  The  a₂-adrenergic agonist sedatives, xylazine and medetomidine, may be used in dogs.  Ketamine can be used in cats.

D. Induction of Anesthesia with Inhalant Anesthetics

Halothane and isoflurane are suitable for inhalation induction of anesthesia using a mask or chamber.  Methoxyflurane is not due to the high solubility that results in an unacceptably long transition from awake to asleep with associated delirium.

Healthy, young animals may need a CNS depressant to help with induction.  Excitement leads to catecholamine release and associated hemodynamic effects including increased risk of arrhythmias.

Two basic approaches to mask induction: 1) gradual step-up in delivered concentration of the gas, and 2) an immediate administration of high partial pressures of anesthetic.  The first method is generally indicated.

E. Techniques of Endotracheal Intubation

Airway control is fundamental to safe anesthetic management.  Cat and ferret laryngeal anatomy is similar to that of young children, so they are used for training in pediatric intubation techniques.  Cuffed Murphy or uncuffed Murphy or Cole tubes are typically used.  The cuff of the tube is inflated with just enough pressure to prevent leaks when the anesthetic breathing bag is squeezed to a pressure of 15 cm water.  This “minimum no-leak volume” technique will allow for positive pressure ventilation, prevent aspiration, and will maintain circulation to the tracheal mucosa.  Excessive cuff pressure may result in mucosal ischemia and necrosis or even tracheal rupture.

Back to dogs and cats

III. Preanesthetic Medications

A. Anticholinergics

The vagal parasympathetic nervous system tone increases with certain anesthetic and surgical procedures, including pharyngeal instrumentation (endotracheal intubation), visceral traction or manipulation, ocular procedures, and manipulation of the carotid sheath or vagus nerve.  Vagal tone also increases with the administration of vagotonic drugs such as most opioids and a₂-agonists.  Anticholinergics are used to prevent bradycardia and to decrease salivation.  Atropine is more widely used than glycopyrrolate and will more substantially raise heart rate.  Glycopyrrolate provides more control of salivation and may have a longer duration of action.  Risk of tachyarrhythmias and a bronchodilating effect that increases dead-space ventilation.

B. Tranquilizers and Sedatives

Acepromazine is the most common preanesthetic tranquilizer used.  Hypotension and prolonged tranquilization are frequent effects of acepromazine.  Diazepam is a valuable alternative for more rapid recovery and is generally less hypotensive. 

Xylazine is an a₂-adrenergic agonist.  Medetomidine is an almost pure a₂-agonist.  Can produce mild to marked vagal bradycardia and reduce cardiac output.  Pretreatment with atropine usually prevents xylazine induced bradycardia but does not prevent the decrease in cardiac output.  Anticholinergics work better as a preventive measure than as a treatment for bradycardia.

C. Opioids in Anesthesia

Used to control surgical and post-operative pain.  Most useful are: morphine, meperidine, oxymorphone, butorphanol, and buprenorphine.

Morphine: sedation and a considerable degree and duration of analgesia in dogs.

Meperidine: short-acting and maintains heart rate.

Both morphine and meperidine cause histamine release and subsequent hypotension and therefore are not injected IV.

Oxymorphone: longer duration of analgesia with little risk of histamine release.  Anticholinergics are used to prevent bradycardia.

Butorphanol/buprenorphine: agonist/antagonist opioids.  Both cause limited sedation.  Buprenorphine has a longer period of analgesia, but is difficult to reverse due to its high affinity for opioid receptors.

Back to dogs and cats

IV. Intravenous Agents for Induction or Maintenance of General Anesthesia

Used to induce anesthesia quickly , control the airway, avoid delirium associated with mask or chamber induction, avoid decreases in BP associated with inhalants, and to decrease exposure of personnel to waste gases.  

A. Barbiturates

Pentobarbital is used infrequently because of its narrow margin of safety, poor analgesic properties, and the long and occasionally violent period of recovery.

Thiobarbiturates are best used for induction or for very brief anesthesia.  Thiamylal is no longer available.  Thiopental (ultra short acting) is often used.

Methohexital (ultra short acting) is recommended for sight hounds.

Combination techniques using IV lidocaine and thiopental have been recommended for dogs to reduce the dose of barbiturate needed and to minimize some of the cardiac side effects.

B. Propofol

Propofol (diisopropylphenol) is a non-barbiturate, non-dissociative injectable sedative-hypnotic.  Characteristics of rapid elimination with full recovery.  Used for: 1) induction, 2) very brief procedures with rapid full recovery, and 3) maintenance of anesthesia by controlled IV infusion with rapid and full recovery.  Apnea commonly occurs after bolus administration in dogs and cats.  Decreased BP but myocardial contractility and heart rate are minimally affected.  Used in combination with opioids for animals with compromised cardiovascular function.  Only has minimal analgesic properties and therefore must be augmented with opioids or a₂-agonists if a painful procedure is to be performed.

C. Etomidate

Etomidate is an intravenous, ultra short acting imidazole derivative anesthetic.  Preserves cardiac and respiratory function.  Rapid recovery after bolus or continuous infusion.  Suppresses the adrenal cortical stress response to anesthesia and surgery.  Disadvantages: pain on injection and high cost.

D. Dissociatives

Increase cardiac output by increasing heart rate.  Improves circulation but causes increased myocardial work.  Elimination and recovery require hepatic metabolism.  Recovery can be accompanied by delirium.

Ketamine alone should only be used for diagnostic or minor surgical procedures.  Used in combination with diazepam (dogs and cats), midazolam (dogs), and medetomidine (cats).

Telazol® is a combination drug containing the dissociative tiletamine and the benzodiazepine zolazepam.  Most useful for substantial restraint or light general anesthesia.  Risk of substantial respiratory depression at higher doses.

Combinations of Telazol with ketamine and xylazine have been developed which offer advantages for general anesthesia. 

E. Chloralose

Frequently used for cardiovascular physiological studies because the hemodynamic profile in dogs given the agent is similar to that of conscious animals.  Useful for noninvasive, non-survival, cardiovascular procedures that require light anesthesia for 4-8 hours.  Should always be used in combination with a thiobarbiturate, methohexital, tranquilizer, or opioid to eliminate the tonic convulsions that occur.  Morphine/chloralose combination has been found to produce a more normal hemodynamic profile than pentobarbital or fentanyl used with chloralose.

F. Reversal or Antagonism of Injectable Anesthetics

Xylazine can produce mild to extreme bradycardia and reduce cardiac output.  Advanced second degree heart block with rates less than 30 BPM can occur.  Yohimbine can reverse the side effects of xylazine and other a₂-agonists.

Naloxone reverses any of the opioids used in small animals.  May need repeated doses to reverse oxymorphone, if it was given in repeated doses.

Butorphanol can be used for partial reversal of the depressant effects of other opioids while preserving analgesia. 

Flumazenil reverses benzodiazepine tranquilizers such as diazepam and midazolam.

V. Balanced Anesthesia

Tranquilizers, sedatives, opioids, or other CNS depressants are components of “balanced” anesthesia in which their use reduces dosages of injectable or inhalant anesthetics, thereby minimizing the more adverse effects of high dosages of any one drug.

Back to dogs and cats

VI. Overview of Inhalation Anesthesia

Safety of inhalant anesthesia over injectable is attributed to supplemental oxygen.  Halothane and isoflurane have the widest application in dogs and cats.  Newer inhalants (desflurane and sevoflurane) have been described, but these are not yet in common use for research protocols.

The utility of nitrous oxide is limited by its low potency.  Relatively little reduction in other anesthetics is achieved through the use of nitrous oxide in animals.  Other disadvantages: reduction in delivered oxygen concentration, potential for diffusion hypoxia on recovery, diffusion of nitrous oxide into closed body cavities.   Can, however, facilitate mask or chamber induction, and it produces a sparing effect on cardiovascular function.

Isoflurane provides almost ideal characteristics of rapid induction and recovery with reduced potential toxicity and acceptable levels of cardiac suppression.  Has similar vapor pressure to halothane.  Can use most halothane vaporizers to deliver isoflurane.

VII. Hemodynamic Effects of Inhalation Anesthesia

The cardiopulmonary depressant effects of anesthetics are generally dose related.  All of the inhalants cause a decrease in ventilation and BP.  The notable hypotension is due to a combination of dilatation of the vascular smooth muscle and decrease in cardiac output.  With isoflurane, cardiac output remains near normal at clinically useful concentrations.  There is a dose dependant peripheral vasodilation that leads to a drop in BP, as with halothane.  Unlike halothane, isoflurane produces a less pronounced effect on both the myocardium and the pumping function of the heart at equipotent anesthetic concentrations.  Isoflurane does not sensitize the heart to catecholamine induced arrhythmias.

VIII. Methods of Induction of Inhalation Anesthesia

The very low solubility of isoflurane allows for a very rapid induction and recovery.  It is by far the most suitable anesthetic for mask or chamber induction.  Rapid recoveries from halothane and isoflurane are usually smooth and free of excitement and delirium, particularly if a preanesthetic tranquilizer or sedative is used. 

Back to dogs and cats

IX. Intraoperative Monitoring and Physiological Support

Basic monitoring is directed at physical changes that may reflect the development of toxic side effects of anesthetics or may signal deterioration or inappropriate anesthetic depth.  Priorities in monitoring address circulation, ventilation, oxygenation, and neurological status.  Anesthesia records are important. 

X. Postoperative Care

A. Circulatory Complications

Common circulatory complications are hypotension and cardiac dysfunction.  Most common causes of hypotension are hypovolemia and ventricular dysfunction.  Dysrhythmias in the absence of cardiac manipulation are usually suggestive of hypoxemia or respiratory acidosis.  A rapidly developing bradycardia in the face of deteriorating tissue perfusion is indicative of severe hypoxemia.  Tachycardia is usually seen during post-op recovery and is usually the result of pain, hypovolemia, and hypercarbia.

PVC’s and ventricular tachycardia are considered ominous signs and should be treated with a lidocaine bolus followed by continuous infusion.  Atropine is used to increase heart rate, and beta blockers (propranolol, esmolol) to eliminate excessive sympathetic activity and tachycardia and to treat dysrhythmias (PVC’s and ventricular tachycardia).

B. Respiratory Complications

Commonly occur in the immediate postoperative period.  Caused by accumulation of carbon dioxide and a residual neuromuscular block, which contributes to hypoventilation and hypoxemia.  Alveolar hypoventilation is considered when PaCO₂ exceeds 45 mm Hg and is best diagnosed by the analysis of arterial blood gases. 

Clinical signs of arterial hypoxemia (PaO₂ below 60 mm Hg) include restlessness, tachycardia, tachypnea, and may later progress to bradycardia, cyanosis, and severe depression.

Airway obstruction commonly occurs during the postoperative period in animals that have not remained sufficiently conscious to protect their airway.

C. Hypothermia

Postoperative hypothermia is commonly encountered due to many factors, including vasodilation, intraoperative heat loss from surgical wounds, and from the high flows of unhumidified gases in the breathing circuits.  Exposure to cold temperatures in the OR and unwarmed IV fluids will contribute to significant heat loss.  Young animals more affected than older.  Decreased platelet function and clotting factor activity can cause increased bleeding.  Shivering increases oxygen demand.

D. Renal Failure

Seen as a decrease in urine production to less than 0.5 ml/kg/hr.  Causes: decreased renal blood flow, intraoperative insult to kidneys due to hypoxia, renal vascular injury, or toxic drugs. Treatment: placement of urinary catheter, restore adequate cardiac output, diuretics, renal vasodilators (dopamine).

E. Hemorrhage

Bleeding in the postoperative period is due to inadequate surgical hemostasis or coagulation disorders.

XI. Postoperative Analgesia

Severity of postoperative pain will vary depending on the site of surgery with thoracotomy, upper abdominal, and orthopedic surgeries considered to be the most painful.

Goal of pain management is to decrease the animal’s experience to noxious stimuli and to restore a more normal physiologic state to the components affected by pain.  Pain may interfere with respiration, which results in atelectasis, and arterial hypoxemia.

Assessing pain in animals is difficult.  To overcome the uncertainty of pain assessment, and to facilitate appropriate analgesic administration, quantification of pain may be necessary.  Calculation of pain score has been widely used in pediatrics.  Numerical rating scales allow a greater degree of sensitivity.

Opioids produce profound analgesia with mild sedation. 

Epidural administration of opioids is an effective and safe alternative to parenteral opioids.  This method of producing a selective blockade of pain transmission has been shown to cause minimal depression of the sympathetic nervous system or interference with neuromuscular function.

A transdermal drug delivery system (fentanyl) offers a method of administering a drug in a slow and continuous fashion, resulting in a fairly constant plasma drug concentration for a prolonged period.

Rationale of using NSAIDs is that soft tissue inflammation may be a potent factor in postoperative pain.  Disadvantage is that most NSAIDs must be given orally.  Do not provide sufficient analgesic properties and are limited to adjunct therapy for acute states. Aspirin can be used to control short term (2-4 hr) moderate somatic pain. Carprofen is 5x as potent as ibuprofen and can be given orally or IV. Ketorolac inhibits platelet function for 24-48 hr. Local anesthetics are used for nerve blocks.  Lidocaine and bupivacaine are used.

Back to dogs and cats

XII.  Special Considerations

A. Neonatal and Pediatric Animals

Subject to hypothermia due to immature thermogenesis and relatively great surface area to body mass ratio.  Tendency to develop hypoglycemia.  Neonates are dependent on maintaining heart rate in order to maintain cardiac output.

B. Cesarean Section

Because all drugs capable of crossing the blood brain barrier (anesthetics) also cross the placental barrier, principles of neonatal anesthesia should be considered.

C. Geriatric Animals

The limited reserve of the geriatric animal for hepatic and renal metabolism and the elimination of drugs are important considerations in anesthetic management.  Useful to select anesthetics that can be reversed, or can be totally eliminated by support ventilation (isoflurane), or drugs that have neither intrinsic toxicity nor significant adverse effects should their effects persist (glycopyrrolate, diazepam, butorphanol), or have very rapid metabolic elimination (propofol).

D. Extracorporeal Bypass

Although swine and ruminants are more popular models, dogs have been used in cardiopulmonary bypass and extracorporeal membrane oxygenation procedures.

E. Prolonged Surgical Procedures

Any anesthetic procedure of longer that 3-4 hours may warrant special attention.  Airway management, IV fluid volume, urine output and maintenance of body temperature are important.

F. Breed Considerations

Breed specific differences in drug metabolism have been noted in “sight hounds”, including greyhounds.  Inefficient hepatic biotransformation and altered distribution of injectable anesthetics make barbiturates less suitable for these dogs.

G. Ocular and Intraocular Surgery

Two goals: 1) avoid increases in intraocular pressure, and 2) support cardiovascular function.  The significant potential for oculocardiac reflex vagal stimulation suggests the use of anticholinergics.

H. Cardiac Dysfunction

Both dogs and cats are used as cardiovascular disease models.  Anesthesia must be adjusted to meet the disease condition.

Back to dogs and cats

Anesthesia and Analgesia in Ferrets

Preoperative considerations

Delivery methods

Parenteral anesthetics

Inhalants

I. Introduction

Ferrets have been used as animal models for bacterial and viral diseases, reproduction, and GI research.  Also for pediatric intubation training.

II. Unique Features of Ferret Anesthesia

May not be docile, so sedation or general anesthesia is frequently necessary.  Because of their small size and rapid metabolic rate, intraoperative and postoperative supportive techniques are often the same as those used for rabbits and large rodents.

III. Preoperative Ferret Evaluation and Care

Best to purchase purpose bred, SPF ferrets.  Quarantine for 7 days.  Screening measures include PE, hematologic and biochemical screening, and fecal exam.  Blood is most frequently drawn from the cephalic, tarsal, or, less frequently, coccygeal venipuncture or by toenail clipping.  (Note from Ed: I have only been successful in bleeding ferrets from the jugular vein for CBC/Chemistry or toenail clip for PCV.  The other veins are too small or inaccessible, even in anesthetized ferrets.)

For chronic studies, OHE is performed on females to prevent aplastic anemia.  Vaccinated for feline (?) and canine distemper.

Back to ferrets

IV. Methods of Anesthetic Delivery and Equipment

Same as for small cats.  Endotracheal tubes, size 3-4 mm inside diameter, can be used.  Susceptible to laryngeal spasm.  Pediatric non-rebreathing circuits should be used.  IM injections can be given using a 25-27 gauge needle into the semimembranosus/ semitendinosus muscles. IP injections are also used.  It is important not to give IP injections into the upper left abdominal quadrant due to the large spleen.

V. Sedatives and Tranquilizers

Acepromazine, xylazine, diazepam, and ketamine are used.

Back to ferrets

VI. Parenteral Anesthetics

A. Barbiturates: Pentobarbital is most frequently used (IP).

B. Dissociatives

C. Neuroleptanalgesics: Hypnorm is the neuroleptanalgesic of choice for ferrets in Europe.  Innovar-vet has not been described in ferrets.

D. Alphaxalone-Alphadolone (Saffan, Althesin)

This combination steroid anesthetic, which is only available outside of the U.S., has been frequently used in ferrets.  Sedation to light surgical anesthesia.  Moderate muscle relaxation, good analgesia, prolonged recovery (1-2 hrs).

E. Urethane: Used for acute (non-recovery) experiments.

Back to ferrets

VII. Inhalant Anesthesia

See dog and cat section above.

VIII. Regional Anesthesia: Not used due to fractious nature of ferrets.

IX. Monitoring of Anesthesia

Same as for cats

X. Intraoperative and Postoperative Supportive Care

Volume deficits can be corrected by SQ, IP, IV or intra-osseous administration of warmed fluids.  Ferrets can be gavaged with a 3.5 - 5.5 French red rubber feeding tube.  Difficult to keep bandages or jackets on ferrets.  E-collars can be used, but must be removed several times a day to allow the ferret to eat, drink, and groom. 

XI. Acute and Chronic Analgesic Therapy

No published guidelines for analgesic management of ferrets.

Back to ferrets

Back to dogs and cats

Top of page

©1999, Janet Becker Rodgers, DVM, MS

All rights reserved.

Comments? Send an email to rodgers@uky.edu