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in the study stood unassisted within 1 h following discontinuation of the infusion (Mama et al. 2005). In horses, the addition of butorphanol (Torbugesic1;
0.01–0.04 mg/kg bwt) enhances muscle relaxation and analgesia when using a xylazine and ketamine combination. Behavioural changes induced by butorphanol may be breed-dependent. Central nervous system stimulation characterised by hyperresponsiveness and spasmodic lip movements were reported in excitable individuals after butorphanol administration, whereas deep sedation and ataxia were observed in a Belgian stallion (Tranquilli et al. 1983). For extremely painful procedures, methadone (Methadone Hydrochloride)6, with and without acepromazine (PromAce)7 was administered to enhance analgesia during xylazine and ketamine anaesthesia (Parsons and Walmsley 1982; Fisher 1984). Benzodiazepines such as diazepam (Diazepam
injectable)8 or midazolam (Midazolam injection)9 were administered i.v. with ketamine for 20–25 min of anaesthesia (Hubbell 2013). When diazepam is combined with xylazine and ketamine, muscle relaxation is improved (Matthews et al. 1991b). Compared to guaifenesin, diazepam offers similar central muscle relaxation but provides practical advantages including availability in a commercial preparation, small volume, and ease of administration (Brock and Hildebrand 1990). Xylazine, butorphanol, diazepam and ketamine combinations can also be used to induce short-term anaesthesia, particularly in horses less sensitive to a xylazine and ketamine combination. Similarly, Temazepam (Restoril10; 0.044 mg/kg bwt), another benzodiazepine derivative, was used to prolong the duration of anaesthesia induced by xylazine and ketamine (Matthews et al. 1991a). Luna et al. (1992) described the use of an i.v. infusion of methotrimeprazine (Levomepromazine hydrochloride11 [0.5 mg/kg bwt]), midazolam (0.1 mg/kg bwt), 10% guaifenesin (100 mg/kg bwt) with or without ketamine (1.6 mg/kg bwt) to induce recumbency in 15 horses. Anaesthesia was maintained with halothane in 100% O2. Horses receiving ketamine required lower halothane concentrations to maintain surgical anaesthesia. Subsequently, standing recovery of these horses occurred more rapidly than those that did not receive ketamine (Luna et al. 1992). Hubbell et al. (2012) also reported the use of midazolam, ketamine and xylazine in isotonic solution for total i.v. anaesthesia (TIVA) in horses (Hubbell et al. 2012). For surgical manipulation and injection of palmar digital nerves in horses, xylazine (1 mg/kg bwt i.v.) for preanaesthetic sedation, midazolam (0.1 mg/kg bwt i.v.) and ketamine (2.2 mg/kg bwt i.v.) for induction of anaesthesia and continuous infusion of a combination of midazolam (0.12 mg/kg bwt/h), xylazine (0.96 mg/kg bwt/h) and ketamine (1.8 mg/kg bwt/h) for maintenance were recommended. Approximately 67% of the horses in that study required supplemental ketamine (0.2–0.4 mg/kg bwt i.v.) to prevent movement during the procedure (Hubbell et al. 2012). In foals premedicated with i.v. diazepam or midazolam, with or without butorphanol or morphine, TIVA can be maintained with ketamine, lidocaine (Lidocaine HCl)5 and morphine. However, reduction of the doses may be necessary to avoid accumulation of the drugs with subsequent prolonged recovery following a long period of infusion (Burns 2008). Ketamine (1.5–2 mg/ml) can be added directly to a 5% guaifenesin solution and the mixture administered as a rapid
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infusion or it can be administered as a bolus (1.5–2.2 mg/kg bwt i.v.) after administration of sufficient guaifenesin to produce limb weakness. Following premedication with xylazine and atropine (Atropine Sulfate)12, ketamine and guaifenesin rapidly induced anaesthesia with good muscle relaxation and analgesia followed by a smooth recovery in foals. In these foals, the mean induction and maintenance doses of ketamine were 2.3 and 12.1 mg/kg bwt/h, respectively (Hikasa et al. 1989). In horses undergoing various surgeries, infusions of guaifenesin (150 mg/ml) and ketamine (6 mg/ml) were administrated to reduce the dose requirement for halothane during anaesthesia. The infusion rate was adjusted to effect with an average rate from approximately 0.5 ml/kg bwt/h initially to 0.17 ml/kg bwt/h at the end of surgery. Infusion of guaifenesin and ketamine during halothane anaesthesia reduced halothane MAC value to 0.6% as compared to 1.24% of those anaesthetised with halothane alone. A greater stability of anaesthesia and reduced need for dobutamine for treatment of hypotension was reported in horses receiving guaifenesin and ketamine (Spadavecchia et al. 2002). Continuous infusion of a guaifenesin, ketamine and
xylazine combination (‘Equine Triple Drip’) is a safe and effective technique to extend the duration of anaesthesia following xylazine/ketamine induction in adult horses. This drug combination is prepared by adding 500 mg xylazine and 2.0 g ketamine to1lof5% guaifenesin in dextrose (Greene et al. 1986; Lin et al. 1993). In ponies anaesthetised for 2 h with ‘Equine Triple Drip’, arterial blood pressures were transiently decreased for the first 15–30 min after induction (Greene et al. 1986). Hypoventilation with mild hypercapnia was noted throughout the study (Greene et al. 1986). In nonsedated ponies and foals, anaesthesia can be induced with a rapid i.v. injection of 1.1 ml/kg bwt of ‘Equine Triple Drip’. Anaesthesia may be maintained by continuous i.v. infusion of 2–4 ml/kg bwt/h to effect, depending on the anaesthetic requirement. Standing recovery usually occurs within 25–30 min (Greene et al. 1986; Benson and Thurmon 1990; Lin et al. 1993). An α2-antagonist such as yohimbine (Yobine3; 0.125 mg/kg bwt) or tolazoline (Tolazine3; 2–4 mg/kg bwt) can be administered i.v. to hasten recovery (Benson and Thurmon 1990; Lin et al. 1993). In 1993, Young et al. reported the use of guaifenesin (100 mg/ml), ketamine (2 mg/ml) and xylazine (1 mg/ml) anaesthesia in horses undergoing various orthopaedic and soft tissue surgeries (Young et al. 1993). Note that 10% (100 mg/ml) of guaifenesin was used in that study rather than the 5% solution described above. The average anaesthetic infusion rate was 1.1 ml/kg bwt/h. The anaesthesia was characterised by active palpebral reflexes, variable degrees of nystagmus, occasional swallowing and ear movement. Swallowing was considered undesirable in horses undergoing laryngeal surgery but the infusion produced satisfactory anaesthesia for all other procedures. When compared to halothane anaesthesia, the arterial blood pressure was better maintained (mean blood pressure of 70–110 mmHg) in horses anaesthetised with a continuous infusion of this mixture. However, the heart rate was significantly lower during infusion anaesthesia than during halothane anaesthesia. This may be attributed to maintenance of the blood pressure by ketamine and the bradycardic effect of xylazine (Kerr et al. 1972). Less respiratory depression was observed in horses receiving infusion anaesthesia as reflected in lower PaCO2 and higher PaO2 than those anaesthetised with halothane. Good
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