Constant Rate Infusions for Analgesia: A Review of the Options

Provision of adequate analgesia is important in horses undergoing surgery or suffering from painful conditions. Horses undergoing invasive surgery or suffering from severe pain may benefit from a constant rate infusion (CRI) of an analgesic. The advantages of a CRI are that it can provide predictable plasma concentrations of the drug at steady state conditions, avoiding the variations that occur with intermittent bolus dosing and thus the more consistent analgesia can reduce the likelihood of ‘breakthrough’ pain. Knowledge of the pharmacokinetics is required to calculate infusion rates for individual drugs although it should be appreciated that many of the studies performed include healthy horses receiving only the drug being evaluated. Thus, monitoring the effects of the drugs and adjusting the rates as required is important when using CRIs in clinical cases as abnormalities in body composition, organ function and concurrent drug administration may alter the pharmacokinetics of the drug.

Drugs with analgesic properties that have been studied for administration as CRIs in horses include opioids, α2- adrenoreceptor agonists, ketamine and lidocaine. Some of these investigations have focused on using the drugs as anaesthetic adjuncts to reduce the amount of inhalational anaesthetics required (partial or supplemental intravenous anaesthesia) or as part of sedative protocols rather than their analgesic effects. Accurate dosing is essential so the use of a syringe driver or an infusion pump is recommended. The cascade must be applied when selecting drugs as some of the drugs discussed do not have a UK marketing authorisation or are being used ‘off label’ when administered by infusion. [...]