CHAPTER 29. Anesthesia
Deborah V. Wilson
GENERAL INFORMATION
I. Anesthesia in the horse presents several challenges
A. The horse has a pronounced flight-or-fight response, meaning that induction and recovery can be violent and difficult. For this reason, adequate preanesthetic sedation is important in this species
B. The horse possesses several anatomic characteristics that contribute to commonly encountered problems in gas exchange and recovery
1. The horse is a large animal with relatively small, fragile limbs
2. The long, sloping diaphragm and poorly lobulated lungs mean that much of the equine lung is vulnerable to compression by the abdominal contents when the horse is recumbent
3. When anesthesia is induced and the horse becomes recumbent, marked abnormalities in ventilation-perfusion matching and intrapulmonary shunting occur
II. Most anesthesia is short in duration and occurs in the field
A. Short and simple procedures using injectable anesthetics are usually well tolerated
B. As the length and complexity of the procedure increase, the chance of anesthetic-related morbidity or mortality increases
C. When inhalant anesthetics are used to maintain anesthesia, myocardial depression and hypotension are commonly encountered (Table 29-1)
| Variable | Normal Value |
|---|---|
| Respiratory rate (beats/min) | 5-12 |
| Heart rate (beats/min) | 25-50 |
| Tidal volume (mL/kg) | 10-20 |
| Mean arterial blood pressure (mm Hg) | 60-100 |
| CVP (cm H 2O) Lateral dorsal recumbency | 10-30 |
| 5-10 | |
| End-tidal carbon dioxide tension (mm Hg) | 50-70 |
ANESTHETIC AND SEDATIVE AGENTS
I. To produce standing restraint, or to calm and relax a horse before anesthesia, four different classes of compounds are used: The α 2 agonists, the phenothiazine tranquilizer, benzodiazepines, and mixed opioid agonists-antagonists. These compounds are frequently combined
II. α 2 Agonists
A. Sedatives with analgesic properties
B. Include xylazine, detomidine, medetomidine, romifidine
1. Xylazine is approved for use in the horse, not for use in meat production
2. Detomidine is approved for use in the horse, excluding breeding stock and meat production
3. Romifidine is approved for use in the horse
4. Detomidine and romifidine are more potent and longer lasting than xylazine
C. Actions
1. Produce stimulation of the presynaptic and postsynaptic α 2-adrenoreceptor, causing a decrease in release of catecholamines
2. Profound muscle relaxation
3. Potent visceral analgesics
4. Produce peripheral vasoconstriction and increased blood pressure
5. Associated decrease in heart rate with first- and second-degree atrioventricular (AV) block
6. Decreased cardiac output, respiratory rate
7. Diuresis
D. Disadvantages
1. Produce elevated blood glucose, decreased plasma insulin, decreased hematocrit
2. Cause decreased intestinal motility, may reduce intestinal blood flow
3. Will produce sweating, penile prolapse, congestion of nasal mucosa, relaxation of upper airway, occasional snoring
4. Depresses thermoregulation in foals
5. May make horses more irritable and likely to bite or kick even though they appear sedate
E. Produce regional analgesia via epidural injection as well
F. Lack of oxytocic effect on the pregnant uterus
G. Onset of effect: 2 to 5 minutes after intravenous (IV) injection, 15 to 20 minutes after intramuscular (IM) injection
H. Subtle effects of the drugs can persist for 24 hours
I. Xylazine does not appear to lower the arrhythmogenic dose of epinephrine
J. Can be combined with butorphanol or acepromazine or both to potentiate the sedative effects
K. Detomidine is effective when given transmucosally but not if swallowed; onset is 40 minutes
M. α 2 Antagonist
1. Yohimbine approved for use in the dog only
2. Tolazoline approved for use in the horse, not for meat production
3. Atipamezole approved for use in the dog only
4. Competitive antagonists
5. May cause transient hypoxemia
6. Reverses the cardiovascular effects of all the α 2 agonists
7. Administration soon after ketamine or telazol may cause excitement, delirium, and muscle contractions and is not recommended
III. Phenothiazine tranquilizers
A. Include promazine, acetylpromazine
1. Approved for use in the horse, dog, and cat
2. Dopaminergic antagonist
B. Effects
1. Depresses alertness
2. Produces abolition of conditioned avoidance responses
3. Dose-dependent blockade of peripheral α 2-adrenoreceptor function
4. Produces potent peripheral vasodilation, decreased blood pressure, decreased central venous pressure, reduced hematocrit; may reduce cardiac output
5. Mild anticholinergic, antiemetic, antiarrhythmic effects
6. Depresses thermoregulation
7. Inhibits platelet function
8. Causes relaxation of the retractor penis muscle in stallions and geldings and reported to cause priapism occasionally. Be sure that the penis retracts back into the prepuce within a few hours of drug administration or intervention (hydrotherapy, benztropine, sling, support) is necessary
C. Effective after oral, IV, or IM administration
1. Increasing the dose usually does not increase sedation
2. Intracarotid injections may prove lethal
3. Show little effect in excited horses (effects can be overridden)
D. Contraindications
1. Neonates
2. Hypovolemic patients
3. Surgical colics
4. Anemic patients
5. Do not use in cases of organophosphate poisoning
E. Treat overdose with vasoconstrictor, fluid therapy, but not epinephrine
F. Onset of effect is 5 to 20 minutes following IV injection; similar following IM injection
G. Antagonist. None available
IV. Benzodiazepine tranquilizers
A. Extra-label use in the horse
B. Not used for routine standing restraint
C. Cause decreased anxiety, muscle relaxation
D. Potent antiseizure agents
E. Seldom used alone
F. Will produce mild sedation in foals
G. IV injection of large doses of diazepam alone in adult horses will cause stimulation, increased motor activity, then muscle relaxation, ataxia, possible recumbency
H. These drugs potentiate the duration and quality of anesthesia if administered with ketamine
I. Antagonist
1. Flumazenil (Romazicon)
2. Commercially available benzodiazepine antagonist
3. Extra-label use in the horse
4. Not very potent
V. Opioid agonists-antagonists
A. Butorphanol is the main opioid agonist-antagonist used in the horse
1. Butorphanol is considered a mild analgesic
2. Butorphanol is Food and Drug Administration (FDA) approved for use in the horse, not for meat production
B. Ceiling effect for analgesia
C. May produce some central nervous system excitement; combine with a sedative or tranquilizer
D. Potentiates the restraint when combined with sedative or tranquilizer
E. Morphine
1. Not FDA approved for use in the horse
2. Epidural morphine has been shown to produce effective analgesia in horses with hindlimb pain
F. Meperidine (Demerol)
1. Not FDA approved for use in the horse
2. Meperidine is a more potent analgesic with short duration of effect
G. Methadone
1. Similar to butorphanol in analgesic effect and durtation of effect
H. Side effects tend to be dose dependent
I. Antagonist
1. Naloxone is a potent opioid antagonist
2. Reverses all μ-receptor effects
3. Extra-label use in the horse
VI. Anticholinergics
A. Include atropine, glycopyrrolate
1. Neither atropine nor glycopyrrolate are specifically approved for horses
2. Not used routinely
C. Causes reduced peristalsis and propulsive activity of the intestinal tract, which may cause colic, especially in recently fed horses
D. Colic is the most commonly encountered side effect that limits the use of atropine in the horse
E. Mydriasis may persist for 24 hours, so wait this interval before exposure to full sunlight
F. Remember that atropine ophthalmic ointment contains a lot of atropine that is readily taken up by the systemic circulation
INDUCTION AGENTS
