Imagine this clinical scenario: a four-year-old Warmblood has presented for a bilateral stifle arthroscopy under general anaesthesia. Equine anaesthesia is high risk, with a mortality rate of approximately 0.12 to 1 percent in healthy horses undergoing elective surgery (Johnston et al., 2002; Bidwell et al., 2007). Recovery is one of the highest risk periods as catastrophic injury can occur, such as limb fractures or subluxations, accounting for 71.4 percent of fatal recovery complications (Dugdale et al., 2016).
When discussing the risks of general anaesthesia with the owner, they ask whether there are ways to make recovery safer. Partial intravenous anaesthesia (PIVA) involves combining a volatile anaesthetic agent with additional analgesia, benefiting from the minimum alveolar concentration (MAC)-sparing effects (Ringer et al., 2007) and leading to reduced cardiovascular depression and a smoother and more controlled recovery period.
Partial intravenous anaesthesia (PIVA) involves combining a volatile anaesthetic agent with additional analgesia, benefiting from the MAC-sparing effects and leading to reduced cardiovascular depression and a smoother and more controlled recovery period
Although many drugs, such as lidocaine, ketamine and opioids, can also be used in PIVA protocols, alpha-2 agonists may provide additional benefits in terms of recovery quality due to their anxiolytic effects. You decide to explore the evidence of giving an alpha-2 adrenoceptor agonist as a constant rate infusion (CRI) intraoperatively and the effects on recovery from general anaesthesia.
Overall, eight prospective randomised clinical studies were critically appraised, of which seven were blinded and four were crossover designs. Crossover studies are less applicable to studies investigating anaesthetic recovery quality as recovery behaviour is learned, and recoveries will improve with multiple general anaesthetics (Valverde et al., 2013; Platt et al., 2017). Most of the studies involve client-owned horses undergoing elective anaesthesia, but there is also evidence from research horses undergoing research anaesthesia, mainly for cardiorespiratory studies. The sample sizes are small to moderate, which reduces the quality of the evidence found.
Recovery quality was measured through various factors, including successfully standing after anaesthesia without injury, how calm the horse is during recovery, how ataxic the horse is during and immediately after standing, the number of attempts to stand and the time taken until standing after the cessation of anaesthesia.
The available evidence showed that a CRI of an alpha-2 agonist leads to equal (3/8 papers) or better (5/8 papers) recovery scores in healthy horses undergoing general anaesthesia with isoflurane or sevoflurane
The available evidence showed that a CRI of an alpha-2 agonist leads to equal (3/8 papers) or better (5/8 papers) recovery scores in healthy horses undergoing general anaesthesia with isoflurane or sevoflurane, compared to those who have received volatile-only anaesthesia maintenance.
Marcilla et al. (2012) and Risberg et al. (2016) demonstrated that recovery scores were better for horses given a bolus of dexmedetomidine (3.5mcg/kg and 8mcg/kg respectively) followed by a CRI of dexmedetomidine at 1.75mcg/kg/hr when compared to a saline CRI. The study by Gozalo-Marcilla et al. (2013) showed no differences in recovery scores between groups, with group one receiving dexmedetomidine bolus at 3.5mcg/kg IV then a CRI at 1.75mcg/kg/hour, and group two receiving sevoflurane only with saline placebo.
Three papers investigated the use of a medetomidine CRI (Creighton et al., 2012; Tokushige et al., 2015; Simeonova et al., 2017), and all demonstrated that medetomidine at a bolus of 5 to 7 mcg/kg followed by a CRI of 1.75 to 5 mcg/kg/hour improved recovery quality.
Devisscher et al. (2010) showed that quality of recovery did not significantly differ between romifidine CRI and saline placebo CRI groups. The paper by Schauvliege et al. (2011) showed no differences between a CRI of detomidine and a saline CRI on recovery scores.
In summary, there is evidence of a moderate strength to show that a CRI of an alpha-2 agonist will improve recovery quality when compared to anaesthetic maintenance with a volatile agent only. Recommendations for using a CRI to improve recovery quality would be:
- Medetomidine at a bolus of 5 to 7mcg/kg, followed by a CRI of 1.75 to 5mcg/kg/hour (Creighton et al., 2012; Tokushige et al., 2015; Simeonova et al., 2017), or
- Dexmedetomidine as a bolus of 3 to 8mcg/kg IV followed by a CRI of 1.75mcg/kg/hour (Marcilla et al., 2012; Risberg et al., 2016)
There is little published evidence directly comparing recovery in horses undergoing volatile agent anaesthesia only and those receiving a CRI of alpha-2 agonists without sedation in recovery for either or both groups. Performing studies without further sedation for recovery can be challenging as horses need to be transported to recovery boxes, so further sedation is given for safety reasons to try to prevent a premature recovery. A bolus of sedation has been shown to improve recoveries (Santos et al., 2003); however, many patient factors, clinical factors and individual preferences help guide decision making as there is little evidence in this area.
A bolus of sedation has been shown to improve recoveries (Santos et al., 2003); however, many patient factors, clinical factors and individual preferences help guide decision making as there is little evidence in this area
Additionally, the studies where anaesthesia was performed without surgery (Creighton et al., 2012; Gozalo-Marcilla et al., 2013; Risberg et al., 2016; Simeonova et al., 2017) may not be very applicable to clinical scenarios as surgical factors are important at influencing recovery from anaesthesia.
Further evidence directly comparing medetomidine and dexmedetomidine infusions in larger population studies would be beneficial. Future research should also investigate the effect of further sedation on recovery after a CRI of an alpha-2 agonist in horses undergoing surgery in real-life clinical scenarios.