Anesthetic Procedures and Techniques in Horses

Anesthetic Procedures and Techniques in Horses

“The little neglect may breed mischief … for want of a nail the shoe was lost; for want of a shoe the horse was lost; and for want of a horse the rider was lost.”

BENJAMIN FRANKLIN

Overview

Horses may be the most difficult domestic species to safely anesthetize. Mortality rates for horses are significantly higher than for cats and dogs, ranging from 1% to 2% in horses compared with about 0.1% in dogs and cats. Individual equine temperament varies considerably and has significant influence on the dose of drug required and selection of the anesthetic technique. The ability to anticipate drug effects and drug actions in horses is the single most important aspect of safe and effective equine anesthesia. Appropriate preanesthetic medications in conjunction with boluses and infusions (total intravenous [IV] anesthesia) of anesthetic drugs are combined with physical restraint to induce general anesthesia in horses. Anesthetic techniques are designed to produce rapid and safe induction to and recovery from recumbency and to maximize muscle relaxation and analgesia while maintaining optimal cardiopulmonary status. Horses frequently benefit from assistance in regaining and maintaining a standing position after general anesthesia.

General Considerations

I Preparing the horse for surgery

A Withhold food for approximately 4 to 8 hours before surgery except in foals

1. Do not withhold water

B Perform a complete physical examination with emphasis on cardiorespiratory function

C Determine body weight using a scale or a girth weight estimator tape

1. Note body type (e.g., lean/racing, conditioned, draft)

D Groom the horse and wipe with a moist cloth to remove dander and debris

E Clean the feet before induction; pull or pad shoes to prevent injury

F Prepare (clip and scrub) the surgical site before induction (if possible) to reduce anesthesia time

1. Sedation may be required to complete surgical preparation

G Place and secure an IV catheter in a jugular vein before induction (Fig. 22-1 A, and B)

FIG. 22-1 A, Types of intravenous (IV) catheters: over-the-needle (left) and through-the-needle (right). B, Placement of an IV catheter in the horse. Do not attempt to withdraw an over-the-needle catheter back onto the needle once it has been advanced. This could lead to catheter shearing and loss of the catheter tip into the horse’s blood stream.

H Administer preanesthetic medications (antibiotics, analgesic drugs) 5 to 30 minutes before induction

I Rinse the mouth thoroughly with water before induction

II Proper positioning and appropriate padding of the head, shoulder, and hip minimize the incidence of neuropathies and myopathies

III All spontaneously breathing horses develop an acid-base disturbance under anesthesia, particularly respiratory acidosis (increased PaCO₂) or hypoxemia (PaO₂ < 60 mm Hg) when breathing air

IV Controlled positive pressure ventilation can be used to maintain normal PaCO₂ and may help to maintain arterial blood oxygenation (PaO₂)

V Prevent hypotension and hypoxemia to help avoid perioperative complications, including myopathy

VI Horses should be closely monitored during the recovery period and, if needed, assisted to a standing position

A Nasotracheal intubation may be required in horses that snore or demonstrate signs of upper airway obstruction

B Alternatively, phenylephrine can be sprayed into each nostril to decrease nasal congestion, but this therapeutic technique is frequently ineffective

Preanesthetic Evaluation

I Review history

II Conduct a physical examination

A Determine age, weight, sex, body type, physical condition

B Temperament of the horse and concurrent disease (see Chapter 2)

C Emphasize evaluation of the cardiopulmonary system; check for subclinical respiratory disease

1. Place a large plastic bag over the nostrils and allow the horse to rebreathe exhaled air while auscultating the horse’s lung sounds

D Determine the degree of lameness or ataxia if present

1. Severely lame horses should be sedated at the site of induction to anesthesia to avoid accidents

2. Some horses may require assistance after sedation

E Review the current and previous drug history

F Consider the consequences (e.g., tissue trauma, pain, blood loss, duration) of the surgical procedure to be performed

G Prepare an anesthetic care plan

III Conduct a laboratory evaluation

A Routine evaluation

1. Complete blood count (packed cell volume, white blood cell count)

2. Total protein (hydration status)

B Suggested further evaluation

1. Serum electrolytes

2. Serum chemistries (lactate, muscle enzymes)

3. Acid-base balance (pH, PaO₂, PaCO₂)

Preanesthetic Medications (Fig. 22-2, A)

I Xylazine (used to produce sedation, analgesia, and muscle relaxation)

A Dose: 0.4 to 1.0 mg/kg of body weight IV, 1 to 2 mg/kg intramuscularly (IM)

B Onset of action: within 2 to 3 minutes after IV administration and within 10 to 15 minutes after IM administration

C Duration: 30 minutes after IV administration and 60 minutes after IM administration

D Sinus bradycardia and first- and second-degree atrioventricular block may occur

E Head-down posture with significant ataxia and muscle relaxation

1. Some horses develop variable degrees of upper airway obstruction (they snore)

F Horses are arousable and may overreact to sudden movement or sounds (startle response)

1. Occasional disinhibition (excitement, aggression) has occurred

II Detomidine (similar to xylazine)

A Dose: 10 to 20 µg/kg IV; 20 to 40 µg/kg IM; 60 µg/kg buccally

B Approximately two times longer duration of action than xylazine

C Smaller volume of drug required (compared to xylazine or romifidine)

D Onset of effect about 30 to 45 minutes after sublingual administration

E Head-down posture with significant ataxia and muscle relaxation

F Horses are arousable, and occasional disinhibition (aggression) is reported

III Dexmedetomidine (similar to xylazine)

A Dose: 3 to 15 µg/kg IV

B Short duration of effect and can be administered by infusion to sustain effect

C Significant ataxia may occur

IV Romifidine (similar to xylazine)

A Dose: 90 to 120 µg/kg IV; 80 to 120 µg/kg IM

B Reduced ataxia and head drop compared with xylazine and a longer duration of effect

V Acepromazine (used to produce a calming effect)

A Dose: 20 to 80 µg/kg IM; 10 to 40 µg/kg IV

B Onset of action: within 10 to 20 minutes after IV administration

C Duration: 2 to 4 hours

D Decreases in arterial blood pressure may last for 12 hours

E Potential for persistent penile paralysis

F Mild to moderate ataxia, no analgesia but may potentiate the analgesic effects of analgesic drugs (opioids, α₂-agonists)

VI Chloral hydrate (infrequently used to produce or enhance sedation); 20 to 80 mg/kg IV (see Chapter 8)

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Tags: Handbook of Veterinary Anesthesia

Sep 6, 2016 | Posted by admin in SUGERY, ORTHOPEDICS & ANESTHESIA | Comments Off

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