Standing Sedation Techniques – Picking the Right Cocktail?

Introduction

Standing sedation is commonly used to perform surgical and medical procedures in horses when general anaesthesia is not warranted or desired. Advantages of standing sedation over general anaesthesia include lower complication rates [1], less cost, simpler monitoring and reduced time to complete the procedure in most cases. Disadvantages of standing sedation could be: excessive sedation which may result in ataxia and, rarely, recumbency; nonideal surgical conditions as the horse may still move; and potentially danger to personnel and horse if the sedation level is inadequate. A multimodal balanced approach by supplementing sedatives and tranquillisers with systemic analgesic or locoregional anaesthetic techniques will facilitate standing procedures [2]. Thus, the choice of the appropriate drug combination is essential to achieve an adequate level of sedation for the procedure.

Patient evaluation and preparation

Patient evaluation should be performed as it would be prior to general anaesthesia. Horses that are not used to handling or are in severe pain may not be the best candidates for standing sedation. Specific preparation may be required (e.g., fasting prior to laparoscopic surgery). The procedure should be carried out in a quiet location and the patient should become accustomed to this environment before any drug administration. This is important when drugs are administered by the oral route or i.m. to avoid delayed onset of action [3]. An i.v. catheter should always be aseptically placed to facilitate drug administration and to avoid perivascular or intracarotid injection. A urinary catheter may be helpful for procedures longer than 60 minutes as α2-agonists promote urinary production. The head should be supported with a stand placed in front of the horse. Demeanour, cardiovascular parameters (i.e., heart rate, capillary refill time, peripheral pulse quality), respiratory parameters, and degree of ataxia should be monitored to detect excessive sedation or drug-induced side effects. To improve adjustment of infusion, an algorithm has recently been published [4]. It is strongly advised to be always ready to convert to general anaesthesia in case it is needed.

Drug administration techniques and combinations

No single drug produces ‘ideal’ chemical restraint in every horse. Drugs are used in combination to optimise the onset, quality, and duration of sedation while minimising side effects. Drugs are either administered i.m., by i.v. bolus or by constant rate infusion (CRI) during the procedure. Intramuscular injection may be performed 30–45 minutes prior to the start of the procedure to reduce the horse’s stress, improve sedation and decrease infusion requirement. Sedation is mainly based on α2-agonists. An opioid should be included to improve quality of sedation/ analgesia. Low doses of either ketamine or lidocaine can be added as part of a balanced protocol to improve sedation and analgesia. Locoregional nerve block and epidural injection should be performed wherever appropriate. A summary of infusion rates for sedatives and analgesics, with suggestions on how to prepare a fluid bag for an i.v. infusion, are shown respectively in Tables 1 and 2.

Conclusion

Several factors need to be considered before making a final choice of agent, route of administration and local analgesic techniques.

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