Chapter 24 24.1 Risks associated with equine anaesthesia 24.6 Induction of general anaesthesia in adults 24.7 Prolonging anaesthesia with injectable drugs 24.8 Total intravenous anaesthesia 24.12 Epidural anaesthesia and analgesia 24.13 Complications of general anaesthesia • The risks of anaesthesia increases with age of the horse, type of surgery (highest risk with fracture repair), emergency procedures, and duration of surgery. • Anaesthesia maintained with inhalational drugs is thought to impart a higher risk than anaesthesia maintained with injectable anaesthetics, and this is probably due to greater cardiovascular depression induced by inhalational drugs. • A physical examination should be performed, and the horse’s medical history obtained. The cardiovascular and respiratory systems should be the focus of the physical examination. Conditions that may affect recovery or induction, such as musculoskeletal injury or neurological disease, should also be considered and, if necessary, plans should be implemented to assist the horse during induction and recovery. • Laboratory tests are not generally necessary for healthy horses but are warranted for some horses undergoing general anaesthesia, especially for an emergency procedure. • Whether horses should be fasted is controversial. Fasting may not be necessary in most cases, but the mouth should be flushed with water to remove debris. This is especially important if tracheal intubation is intended. • A jugular catheter should be placed if general anaesthesia is intended or when prolonged sedation, using a constant infusion of drugs, is planned. • Preoperative stabilization of the cardiovascular system with intravenously administered fluids is necessary for some horses undergoing emergency surgery (e.g. intestinal accident). • Phenothiazines are purported to lower the seizure threshold, but this property is not proven. • The most commonly used member of the group is acepromazine. • Acepromazine has a number of physiological effects: Produces a mild tranquillizing effect. Acepromazine alone, however, does not give reliable restraint. Decreases the minimum alveolar concentration (MAC) of volatile anaesthetics. Decreases release of histamine from mast cells. Has cardiovascular effects, including an anti-arhythmogenic effect and vasodilation. Because of its vasodilating actions, acepromazine is not recommended for use in dehydrated horses or for use in those with cardiovascular compromise, such as may occur during endotoxaemia. Analgesia is not considered to be significant. Penile paralysis and priapism are potential, adverse side-effects. Keeping the dose to a minimum may reduce the likelihood of either of these complications occurring. • The dose for acepromazine is 0.02–0.05 mg/kg, i.v. or i.m. There is no benefit in increasing the dose beyond this range in an attempt to induce more profound tranquillization. • The onset of action is approximately 15 min after i.v. administration and 30 min after i.m. administration. • Acepromazine can be combined with an alpha2 agonist (e.g. xylazine) and an opioid (e.g. butorphanol) for more profound sedation and analgesia. • A variety of physiological effects result from the administration of alpha2 agonists and include: Sedation, the degree and duration of which is dose-dependent. Analgesia (somatic and visceral). A decrease in the MAC of volatile anaesthetics. Cardiovascular changes, including bradycardia (often accompanied by a second-degree heart block), hypertension followed by hypotension, and a decrease in cardiac output. Hyperglycaemia, resulting from an inhibition of insulin release. Contraction of the gravid uterus, but this does not seem to create a problem. Nevertheless, advising the owner of the risk of sedation of a mare in late pregnancy is prudent. • Depending on the degree of sedation required, i.v. bolus injections in the following dose ranges can be used: • When used in combination with an opioid, the horse must be sedated with the alpha2 agonist prior to opioid administration, to avoid opioid-induced excitement. An opioid is combined with an alpha2 agonist only when heavy sedation or additional analgesia is required. Doses of the individual drugs should be tailored to achieve the desired effect. Bolus injections (i.v.) of the combinations, in the following dose ranges, can be used: Xylazine (0.5–1.0 mg/kg) plus butorphanol (0.02–0.05 mg/kg) Xylazine (0.5–1.0 mg/kg) plus morphine (0.15 mg/kg) Detomidine (0.005–0.015 mg/kg) plus morphine (0.15 mg/kg) Romifidine (0.03–0.08 mg/kg) plus butorphanol (0.02–0.05 mg/kg) Medetomidine (0.0035–0.005 mg/kg) plus butorphanol (0.02–0.05 mg/kg) Medetomidine (0.0035–0.005 mg/kg) plus morphine (0.15 mg/kg). • An i.v. catheter is placed, and the CRI of drugs is generally administered by adding the drugs to a 1 L bag of fluids (e.g. 0.9% NaCl). • A loading dose (Ld) of the drug(s) is given before the CRI is started. Doses in the following ranges can be used: Detomidine: Ld = 0.005–0.01 mg/kg i.v. and a CRI = 0.02 mg/kg/hour. Medetomidine: Ld = 0.005 mg/kg i.v. and a CRI = 0.0035–0.005 mg/kg/hour. Dexmedetomidine: Ld = 0.0025 mg/kg i.v. and a CRI = 0.0017–0.0025 mg/kg/hour. Romifidine: Ld = 0.03–0.1 mg/kg and a CRI = 0.018 mg/kg/hour. Xylazine: Ld = 0.5–1 mg/kg i.v. and a CRI = 0.5–1.0 mg/kg/hour. • An alpha2 agonist can be combined with an opioid for CRI. In such cases, the dose of each drug may have to be reduced to avoid over sedation. Detomidine: Ld = 0.005 mg/kg i.v. and a CRI = 0.02 mg/kg/hour, Butorphanol: Ld = 0.02 mg/kg, i.v. and a CRI = 0.012 mg/kg/hour. Detomidine: Ld = 0.005 mg/kg, i.v. and a CRI = 0.02 mg/kg/hour, Morphine: Ld = 0.15 mg/kg, i.v. and a CRI = 0.15 mg/kg/hour. Medetomidine: Ld = 0.005 mg/kg, i.v. and a CRI = 0.0035–0.005 mg/kg/hour, Morphine: Ld = 0.15 mg/kg, i.v. and a CRI = 0.15 mg/kg/hour. • If necessary, sedation or analgesia may be augmented during the CRI by giving a bolus of approximately • Older foals are sedated in a manner similar to that of adults but may require higher doses to achieve the equivalent effect. • These can be combined for painful procedures (e.g. joint lavage). • Ketamine (2–3 mg/kg) can be mixed with diazepam or midazolam (0.05–0.1 ml/kg), and the mixture can be dosed to effect to produce a light plane of anaesthesia. The advantages of this regimen over a benzodiazepine alone are the provision of analgesia and faster recovery and reduced ataxia from using a lower dose of the benzodiazepine. • Premedication with a low dose of an alpha2 agonist (e.g. xylazine 0.25–0.5 mg/kg, i.v.) augments the aforementioned drugs and may be necessary for stronger foals.
Sedation and anaesthesia
24.1 Risks associated with equine anaesthesia
24.2 Patient preparation
24.3 Drugs used for sedation
Phenothiazines
Clinical use of acepromazine
Alpha2 adrenergic agonists
24.4 Sedation of adult horses
Sedation using constant rate infusions (CRI)
of the loading dose of the alpha2 agonist.
24.5 Sedation of foals
Benzodiazepine and ketamine
Sedation and anaesthesia
