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recovery from infusion anaesthesia was observed in the majority of the horses in that study with sternal recumbency and standing occurring in 24 and 38 min, respectively (Young et al. 1993).
Detomidine (Dormosedan)1 is approved for use in horses in
the United States as a sedative and analgesic. Short-term anaesthesia in horses can be achieved with detomidine followed in 6–8 min by ketamine (Clarke et al. 1986; Matthews et al. 1991b). When compared with xylazine and ketamine anaesthesia, detomidine and ketamine induce better muscle relaxation but recovery can be unpredictable with this combination. On occasion, horses and ponies experienced a rough recovery in which several failed attempts to stand are made. This rough recovery is believed to be caused by the longer duration of sedation and muscle relaxation of detomidine outlasting those of ketamine (Clarke et al. 1986; Matthews et al. 1991b). Adding butorphanol (0.01–0.04 mg/kg bwt) to detomidine and ketamine not only increased the duration of recumbency but also improved the quality of recovery (Matthews et al. 1991b). A combination of guaifenesin (50 mg/ml), ketamine (2 mg/ml) and detomidine (0.005 mg/ml) was used for castrating ponies. Ponies were premedicated with 0.022 mg/kg bwt i.m. detomidine. Fifteen minutes later, anaesthesia was induced and maintained with the mixture. This combination produced good muscle relaxation and analgesia with minimal cardiopulmonary effects during surgery. Standing recovery occurred 25–40 min after termination of infusion (Thurmon et al. 1991). In pregnant pony mares, infusion of a mixture of detomidine (0.04 mg/ml), ketamine (4 mg/ml) and guaifenesin (100 mg/ml) was used to maintain surgical anaesthesia but 4 ponies required an additional single dose of ketamine. These mares were mechanically ventilated with 100% O2. Maternal arterial blood gas values and pHa remained within normal ranges. The heart rate was unaffected but the arterial blood pressure decreased by 10% during infusion. Blood gas values from uterine venous, fetal arterial and umbilical venous blood samples indicated that uterine and fetal perfusion were better maintained with this combination than with halothane. The quality of recovery was good and the mares stood within 60 ± 28 min after discontinuation of the infusion (Taylor et al. 2001). Another α2-agonist, romifidine (Sedivet)7, has also been
combined with ketamine to induce short-term anaesthesia. Kerr et al. (2004) reported that romifidine and ketamine combination induced similar anaesthetic effects as xylazine and ketamine combinations (Kerr et al. 2004). However, other investigators indicated that romifidine and ketamine combinations did not produce sufficient sedation and muscle relaxation to provide satisfactory anaesthetic effects (Hubbell 2013). Transient jaw tone, limb rigidity and mild muscle tremors were commonly observed following the initial induction period (Diamond et al. 1993; Taylor and Clarke 1999). Adding diazepam to the combination of romifidine and ketamine improves the quality of induction and results in better muscle relaxation. However, the duration of recumbency is slightly shorter with the combination of romifidine, diazepam and ketamine than with a xylazine and ketamine combination (H.C. Lin, personal observation). In horses sedated with romifidine (0.1 mg/kg bwt i.v.) or xylazine (1.1 mg/kg bwt i.v.) followed by diazepam (0.04 mg/kg bwt i.v.) and ketamine (2.2 mg/kg bwt i.v.) anaesthesia, lower heart rates and arterial blood pressures, more severe and longer lasting second degree atrioventricular blockade, and
longer duration of anaesthesia (20.8 ± 2.3 vs. 15.8 ± 1.6 min) were observed in romifidine-sedated horses (Kerr et al. 1996). A mixture of romifidine (20 mg), midazolam (15 mg) and ketamine (500 mg) in 50 ml of saline has been used to induce anaesthesia for castration. Horses were sedated with romifidine and anaesthesia was induced with midazolam and ketamine. A romifidine, midazolam and ketamine mixture was then used to maintain anaesthesia during castration. A third of the induction dose of midazolam and ketamine was administered when spontaneous movement in response to surgery occurred. Three horses required 6 additional doses of midazolam and ketamine to maintain surgical anaesthesia of 25–40 min. Smooth recovery to standing occurred in all horses within 24.6 ± 8.5 min after surgery (Bouts et al. 2002). The anaesthetic effects of medetomidine (Domitor)1
combined with midazolam and ketamine were studied in 14 horses. Immediately following induction of anaesthesia, the anaesthetic mixture composed of medetomidine, midazolam and ketamine was administered i.v. during castration. Total duration of anaesthesia was 38 ± 8 min and horses stood uneventfully at 33 ± 13 min (Yamashita et al. 2007). Administration of 50–75 mgof ketamine was demonstrated
to produce effective standing restraint and analgesia in sedated, colicky horses during preparation for surgery. This technique is particularly useful in fractious horses that are difficult to handle or those sensitive to hindlimb procedures (Bohanon 2005). Use of low doses of ketamine by CRI for long-lasting post operative pain management has been studied in chronic pain conditions and variable results have been reported. Matthews et al. (2004) and Branson (2011) reported that in horses with osteomyelitis, laminitis and severe burns receiving ketamine CRI (0.4–0.8 mg/kg bwt/h), effective pain relief was observed (Matthews et al. 2004; Branson 2011). However, other studies reported that a ketamine infusion rate of 0.8 mg/kg bwt/h provided ineffective analgesia, resulted in increased heart and respiratory rates. Signs of excitation were observed when the infusion rate was increased to 1.5–1.6 mg/kg bwt/h (Fielding et al. 2006; Davis and Davis 2013). Other side effects such as muscle fasciculation were also observed with ketamine CRI at a loading dose of 0.1–0.2 mg/kg bwt with infusion rate set at 0.5–1.0 mg/kg bwt/h (Muir 2011). It seems that ketamine may not be an effective analgesic on its own; however, it is very useful in a multimodal approach for pain management. When combined with tramadol (Tramadol hydrochloride)13 for 6 h pain relief in horses with chronic laminitis, ketamine (0.6 mg/kg bwt/h) enhanced the analgesic effect of tramadol (Davis and Davis 2013). Another popular combination (Trifusion) used for perioperative analgesia in horses include morphine (loading dose: 0.1 mg/kg bwt; infusion: 0.025 mg/kg bwt/h), ketamine (infusion: 0.6 mg/kg bwt/h) and lidocaine (loading dose: 1.3 mg/kg bwt; infusion: 3 mg/kg bwt/h) (Abrahamsen 2009). Acepromazine (0.002 mg/kg bwt/h) and detomidine (0.0044 mg/kg bwt/h) have been used in a 5 drug combination (Pentafusion) with the combination of morphine, ketamine and lidocaine. The analgesic effect produced by either Trifusion or Pentafusion has a slow onset of action. However, once a peak analgesic effect is achieved, the infusion rate of acepromazine and detomidine can be decreased by 50% to minimise their effect on gastrointestinal motility (Abrahamsen 2009). Table 1 summarises the use of ketamine combinations for anaesthesia in horses.
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