Sedation of Adult Horse

Short-term sedation is achieved by administering a bolus of an alpha2 agonist, with or without an opioid. 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.

Standing sedation

Sedation is necessary for a variety of surgical and non-surgical procedures in the standing horse. Standing sedation is more and more frequently done as it is less risky to the horse, though more risky to staff. Standing sedation is useful as bleeding is less for some operations (like nasal surgeries). If profitable- standing is better.

Indications

Stomatology, diagnostic imaging, farriery, sinus surgeries, urogenital, orthopoedic

Drugs 

1. Alpha2 adrenergic agonists

   1. Alpha2 adrenergic agonists drugs include xylasine, detomidine, metedomidine. They are the most commonly used drugs for sedation of horses for standing procedures. They can be used alone or combined with other drugs, such as opioids or acepromazine.

   2. Alpha2 agonist have profound, dose-dependent, sedating, and analgesic effects. The receptors are found throughout the central and peripheral nervous system and in many organs. 

   3. Effect: sedation (degree and duration is dose-depended), analgesia (somatic and visceral), cardiovascular changes (bradycardia, hypertension followed by hypotension, and decrease in cardiac output), hyperglycemia (from inhibition of insulin release).

   4. Onset of peak effect is about 5 min, i.m administration: onset of peak is about 15min

2. Phenothiazines 

   1. Acepromazine is the most common phenothiazine. It produces a mild tranquillizing effect, but don’t give reliable restraint alone, thus often used in combination with alpha2 agonist.

   2. It decreases the minimum alveolar concentration (MAC) of volatile anaesthetics and decreases the release of histamine. Also has cardiovascular effects, including an anti-arrhythmogenic effect and vasodilation. Because of its vasodilating actions, it is not recommended for use in dehydrated horses or for those with cardiovascular compromise.

   3. Dose: 0.02-0.05 mg/kg i.v. or i.m.

3. Opioids

   1. Includes butophanol, morphine, buprenorphine, meperidine.

4. Benzodiazepines

   1. They are not used alone for sedation, as they are likely to cause dysphoria or excitement. iv. Includes diazepam and midazolam. Not used to sedate adult horses because of their muscle-relaxing and ataxia-inducing properties. It can be used to sedate very young foals.

Methods

Standing sedation: IV boluses (prolonged with further IV bolus)

Standing surgery: IV bolus + controlled rate infusion (CRI)

Risk Factors

• Poor patient temperament (restless, fractious, aggressive)

• Noxious stimulus from the surgical procedures

• Irregular or slippery floor surfaces (not good with shoes horse)

• Unexpected environmental sounds or stimuli

• Inadvertent overdoses of medication by epidural or systemic route

• Inadvertent intra-arterial (e.g. Intra-carotid) drug administration

  • Usually high pressure in artery -> haematoma after removal of needle

  • If not sure if in vein or artery, remove and try again DO NOT INJECT

  • Can have very serious reactions to IA administration

• HUMAN FACTOR: look at what you're giving, mark your syringes

Sedation of Foals

Sedation and anaesthesia may be required in neonatal foals to allow diagnostic and therapeutic procedures to be carried out in a safe fashion. There are several differences in foals compared to adult horses that require consideration- circulatory system (transition from in-utero to newborn), metabolism of drugs, susceptibility to hypothermia and hypoglycaemia Avoid drugs that lower heart rate (xylazine and detomidine) and those that decrease preload (acepromazine) as they may produce diminished cardiac output and tissue perfusion. 

Neonatal foals become recumbent when sedated, and the foal should be supported until it assumes recumbence. 

1. Alpha2 agonist: can be given safely in healthy foals, but the dose should be kept to a minimum. Older foals are sedated in similar way as adults, but they may require higher doses. 

2. Benzodiazepines (diazepam and midazolam): should be administered slowly. Ataxia may be prolonged, and high or repeated dosing is not recommended. They are not analgesic. - Dose: 0.05-0.1mg/kg i.v. 

3. Benzodiazepines and ketamine: these can be combined for painful procedures. Produces a light plane of anaesthesia. 

4. Diazepam/midazolam (0.05-0.1mg/kg) + Ketamine (2-3 mg/kg)