Equine Anesthesia

• Lisa S. Ebner, DVM, MS, DACVAA, CVA teaches Equine Anesthesia as part of CVM 746.

Lecture Objectives

• Understand the anesthetic considerations for an equine patient undergoing general anesthesia.
• Identify drugs commonly used in equine anesthesia and their specific side effects.
• Know the equipment needed for administering inhalant anesthesia in horses.
• Understand the key factors for safe induction, maintenance and recovery in equine patients.
• Know acceptable vital sign values under general anesthesia and how variations can affect patient outcome.

History of Equine Anesthesia

• Previously, equine anesthesia involved heavy physical restraint and herbal remedies, including hobbles, casting harnesses, and ropes.
• Chloroform became widely used in veterinary medicine in 1845, followed by ether in 1847.
• Equine anesthesia began evolving into a more scientific process after 1950.
• The shift occurred with the development of new drugs, textbooks, and controlled studies in horses.
• Succinylcholine, a depolarizing neuromuscular blocking drug, was used for short surgical procedures in 1955.
• Halothane was first used in horses in 1957.
• Guaifenesin was used from the late 1960s to reduce the dose of thiopental and subsequent cardiovascular depression.
• Xylazine was introduced in the early 1970s.

Preoperative Evaluation

• Take a patient history, including all dietary supplements and drugs.
• Consider concurrent medications and any physical exam findings.
• Collect laboratory data when possible.
• Assign an ASA Physical Status (PS) score.
• It is generally recommended to withhold food 3-6 hours before anesthesia without over stressing the horse, though fasting is controversial.
• There is no need to fast nursing foals.
• Procedures should be postponed if the animal has a respiratory disease.

Patient Preparation

• Establish IV access prior to anesthesia.
• Flush food debris out of the mouth.
• Clean feet, and remove or tape over shoes.
• Administer perioperative antibiotics, anti-inflammatories, and tetanus prophylaxis 30 minutes before induction.
• Consider the potential for cardiovascular compromise or drug interactions.
• Clip hair over the surgical site, if possible.
• Provide fluid resuscitation prior to anesthesia if the patient is dehydrated or in shock.

Considerations for Equine Anesthesia

• Safety for staff and patient is a priority.
• Take into account the horse's behavior with adequate staffing and facilities.
• There is an increased risk of anesthesia in horses; the anesthetic mortality rate is approximately 1.94% including colic, and ~1% not including colic.
• Understand anatomy and physiology, obligate nasal breathers
• Have an understanding passage of the nasal passage, edema
• Be aware of the high likelihood of V/Q mismatch and hypoxemia
• Consider GI tract concerns.
• Development of myopathy and/or neuropathy can occur.

Increased Anesthetic Risk

• Fracture repair
• Very young or old age
• Colic and/or emergency surgery
• Surgery between midnight and 6 am
• Inexperienced surgeon
• Long duration of anesthesia
• Trauma, dehydration, stress, poor general state or systemic disease
• Pregnancy
• Drug choice
• Breed predisposition

Beyond the ASA PS System

• CHARIOT is a newer tool that augments the ASA Physical Status system for pre-anesthetic risk assessment by accounting for factors unique to equine patients.

Respiratory Considerations

• Horses are prone to compression atelectasis when placed in dorsal recumbency.
• All anesthetic drugs depress respiratory drive, muscle function, ventilatory rate/volume, and the response to hypercarbia and hypoxia.
• Inhalant anesthetics alter pulmonary blood flow by abolishing hypoxic pulmonary vasoconstriction.
• Blood flow is shunted to areas that are perfused, but not well ventilated, worsening hypoxemia and contributing to V/Q mismatch.
• Nasal edema is expected after prolonged surgery, especially if the head was below the level of the heart.

Cardiovascular Considerations

• A 2° AV block is generally normal and due to inherently high vagal tone.
• Horses have a large SA node, so a wandering pacemaker is common.
• Atrial mass in horses is large, so a biphasic P wave is normal.
• The large atrial size predisposes horses to re-entrant rhythms.
• Atrial fibrillation is the most common pathologic arrhythmia in horses.

Atrial Fibrillation

• ECG characteristics include no P waves, "F" waves, and variable QRS responses.

###Premedication Drugs

• Acepromazine (0.02-0.05 mg/kg) is used as an adjunct to other sedatives in excitable horses.
• Exercise caution when administering to breeding stallions due to the potential for penile prolapse; it isn't adequate anxiety reduction.
• Alpha2 agonists are commonly used for sedation, muscle relaxation, and analgesia.
• Xylazine (0.5-1.0 mg/kg IV; 1-2 mg/kg IM) - large dosage difference between IV and IM use.
• Detomidine (5-20 µg/kg IV; 20-40 µg/kg IM; 60 µg/kg BM).
• Dexmedetomidine 2–5 µg/kg IV can be used as a CRI during anesthesia.
• Romifidine (80-120 µg/kg IV or IM).

Opioids

• Less likely to cause excitement when co-administered with alpha2 agonists, or acepromazine.
• Butorphanol 0.01-0.04 mg/kg: IV or IM
• Morphine 0.04-0.1 mg/kg: IM
• Hydromorphone 0.04 mg/kg: IV
• Buprenorphine 5-10 mcg/kg: IV
• Paradoxical excitement, nausea, and vomiting are potential side effects in horses.

Benzodiazepines

• Used rarely with horses as a drug, but can with foals.
• Diazepam 0.02-0.1 mg/kg: IV
• Midazolam 0.02-0.1 mg/kg: IV

IV Induction Drugs

• Ketamine (2.2 mg/kg) + benzodiazepine (0.05 mg/kg).
• Ketamine (2 mg/kg) + guaifenesin (50-100 mg/kg).
• Ketamine alone (2.2-2.5 mg/kg) following heavy alpha2 agonist sedation.
• Ketamine can cause an apneustic breathing pattern.
• Propofol (2 mg/kg) is not commonly used due to cost, large volume, and poor quality induction.
• Telazol (1–1.25 mg/kg) provides excellent induction following alpha2 agonist sedation, but recovery is not as smooth.
• Alfaxalone has been studied in horses, but is very expensive.

Maintenance Drugs

• Isoflurane MAC = 1.31%.
• Sevoflurane MAC = 2.31%.
• End-tidal concentration of inhalant about 30% greater than MAC is needed to prevent movement during surgery.

TIVA ("Triple Drip")

• Administer 1-2 ml/kg/hr, and its use is generally recommended <= an hour.
• Too much muscle relaxation, bad recovery, and hemolysis can happen if Guaifenesin (GG) concentration is > 15%.
• Other α2 agonists or potentially midazolam may be used if GG is not available. A common recipe for a triple drip includes the following in 500mL of 5% dextrose:
• 25 gram GG = 5%
• 500-600 mg ketamine
• 250-300 mg xylazine

Adjunct Drug

• Lidocaine CRI improves analgesia and GI motility, and reduces MAC by ~25%.
• Morphine-Lidocaine-Ketamine: can reduce MAC by 50%.
• Dexmedetomidine CRI can be used for prolonged standing sedation, PIVA, and TIVA, providing a good PK profile.
• Locoregional techniques

Fluid Therapy

• Hypovolemia and/or dehydration should be corrected prior to anesthesia.
• Replacement fluids (isotonic) @ 5–10 ml/kg/hr.
• Hypertonic saline (7.2%) @ 2-4 ml/kg over 15 minutes for emergency resuscitation of hypovolemic patients.
• Be sure to follow with isotonic crystalloids.
• Colloids: Synthetic (hetastarch, vetstarch) or natural (whole blood or plasma).
• Blood volume is ~8% of body weight (kg) in adults and ~9% in foals.
• Foals are not able to compensate for increased fluid load and may get edema.

Equipment

• Scavenging can be active or passive, but f/air (charcoal) canisters become ineffective at the high gas flow rates typically used for adult horses.
• Use a hydraulic lift on surgery table with adequate protective padding and soft ropes to tie on limbs.
• Use an oxygen demand valve and padded recovery stall, sling, overhead hoist, ropes for the head and tail, and correct size halter.
• Protect the eyes.

Induction Period

• Plan for a designated space for induction and keep the owner at a safe distance.
• Explain to the owner what is going to happen to their horse and keep the noise level down.
• The patient should be adequately sedated.
• Announce to the room when administering induction drugs.
• The person on the head has the most responsibility; administer all drugs at once, and do so cautiously during onset time.
• If using an induction stall with swing gate: keep the head steady until the patient begins to buckle at the knees, then push head up to make the horse "dogsit" before coming down on the front legs.
• If doing a freefall induction: once the horse begins to buckle and relax before falling, turn the head in the OPPOSITE direction of how you want the horse to lay.
• Blind intubation can be performed in lateral or in sternal recumbency; never force the tube.
• Mouth gag equipment should include a 5" length of PVC pipe with +/- elasticon tape covering.
• Average adult (400-500 kg): 26mm
• Larger horses (>500 kg): 30mm
• Newborn foal (40-50 kg): 10-11mm
• Donkey: 14-16mm
• Avoid gas sterilization of tubes with ethylene oxide, which can cause severe burns of the respiratory tract.
• Use a large syringe (60cc) to inflate the cuff.
• Use water soluble lubricant, especially for nasotracheal intubation.

Maintenance

• TIVA is recommended with the patient intubated and on oxygen.
• With inhalant anesthesia, use a higher oxygen flow rate (20 mL/kg/min) and vaporizer setting in the beginning.
• Once an adequate plane of anesthesia is reached, reduce vaporizer and oxygen setting to maintenance levels (5-10 mL/kg/min).

Monitoring

• Always be prepared for the patient to become light and start to move
• Communicate to surgeon to stop stimulation.
• Make available hands to safely hold patient on table
• Administer a rapidly acting injectable anesthetic.
  • Ketamine 100-300 mg IV
  • Propofol up to 200 mg IV
• Increase vaporizer and O2 flow rate, and give IPPV.
• Add a local anesthetic block, if possible.

Physical Signs of Anesthetic Depth

• “Eye signs” - palpebral and corneal reflexes, lacrimation, nystagmus, position of the eyeball.
• Which drug makes the reflexes less reliable?
  • Ketamine
• Muscle tone, movement of limb
• Swallowing
• Ear movement
• Anal sphincter tone
• Response to surgical stimulation
• Shivering, stretching
• Respiratory rate, heart rate and BP are less reliable during surgical plane of anesthesia.

Monitoring

• Physically or visually monitor
• Palpate pulse and assess strength
• Auscultate the heart
• Capillary refill time
• Mucous membrane color
• Respiratory rate and pattern, inspiratory effort, degree of chest wall and abdominal movement
• "Eye signs"
• Movement of the limbs, ears, tensing of neck muscles, swallowing
• Lead placement for obtaining a base-apex electrocardiogram

Hypotension

• In 1987, Grandy et al. reported a direct link between arterial hypotension and post-anesthetic myopathy.
• Normal on recovery with MAP of >70 mmHg
• Incidence of hypotension is reduced with the use of TIVA or PIVA.
• Neonatal foals have a decreased vascular tone so a MAP closer to 60 mmHg is acceptable.
• Treat by administer positive inotrope (Dobutamine ~0.5-3 μg/kg/min) or ephedrine (0.05-1.0 mg/kg, IV), correct volume deficits, or change anesthetic regiment to PIVA.

Recovery

• Communicate with the surgery team to coordinate movement to the recovery area.
• The recovery period is potentially life-threatening.
• Administer a dose of alpha2 agonist (e.g. xylazine 50-100mg, IV) as the inhalant is discontinued.
• This is not necessary if triple drip was used for maintenance.
• Use a demand valve to deliver IPPV until spontaneous ventilation resumes.
• Place a nasal tube, or phenylephrine in nostril(s) to ensure a patent airway.
• Keep the room Quiet and dark with a cotton in ears and a towel over the eyes
• Maintain a “safe” position on the dorsal side of neck and control the head at all times.
• Remove deflated ETT when the patient swallows and remove the mouth gag.
• Time for recovery varies by patient, procedure duration, drugs used, and body temperature.
• Generally, allow about 30 minutes after TIVA and 1 hour after longer (>60 min) of inhalant anesthesia time.
• Recovery >90 minutes would be considered prolonged.

Keys to Equine Anesthesia Success

• Recognize that it is a “risky business”
• Develop horse sense
• Be knowledgeable in pharmacology and equine physiology
• Keep MAP > 70-80 mmHg
• Ensure adequate padding and positioning
• Maintain a patent airway
• Keep surgery < 3 hours
• Prioritize safety and be prepared for complications.
• 92% of anesthetic complications occur in recovery period, so be prepared at this time.