Equine Anesthesia

Introduction

• Higher morbidity and mortality associated with general anesthesia (1:100) in comparison to small animals (1:1000) or human (1: 200,000) • No change of the risk ratio for the last 30 years, but the duration of surgery extended.
• Unique anatomic and physiologic characteristics presents additional challenge
• More pronounced cardiovascular depression (hypotension and reduced cardiac output) at equipotent MAC than other species such as dogs and cats
• The size, weight temperament and tendency to panic of the adult horse introduce the risk of injury to the patient and to the personnel.
• Prolonged recumbency is unnatural in the horse
• When a horse is placed in dorsal recumbency, the weight of the abdominal contents presses on the diaphragm and limits lung expansion, leading to hypoventilation. If the drugs used to produce anesthesia depress cardiovascular function, these changes will be exaggerated due to a ventilation-perfusion mismatch.
Standing chemical restraint and preanesthetic agents
• Due to higher risk associated with general anesthesia in this species, standing chemical restraint can offer safer alternative for many procedures
• Neuroleptanalgesia (neuroleptics + opioids) or sedative/opioid combination are most popular
• Produced by the concurrent administration of a sedative/tranquilizer and a narcotic analgesic (e.g. detomidine and butorphanol; acepromazine and morphine)
• Better restraint and analgesia (the combination is synergistic, not merely additive)
• Many procedures can be performed which would not be possible with the tranquilizer or sedative alone
• Dose sparing effect on both drugs
• Better cardiovascular preservation
• Can provide satisfactory working condition for minor surgery when combined with local anesthesia
• Less expense, less risk, less logistics
• This combination can also be effective as preanesthetic medication to produce reliable sedation (e.g. xylazine and butorphanol combination) • A good preanesthetic sedation facilitate smooth induction and has anesthetic sparing effect during maintenance
Acepromazine

• Major tranquillizer
• Hypotensive
• Anti-arrhythmic
• Stallion: penile priapism
• Requires at least 20 min for good effect even after IV injection, and 30 to 45 min when given IM
• Prolonged duration
• 0.025 – 0.05 mg/kg
• Premedication dose of 0.04 mg/kg IV has minimal cardiovascular effect in healthy horses