HOW TO PERFORM FIELD ANESTHESIA AND PAIN MANAGEMENT


beyond the single redosing is discouraged because the quality of the anesthetic state and the recovery from anesthesia worsen in the experience of the author. The constant infusion of a combination of xylazine and ketamine is reported.7 Infusion of xy- lazine (35 ug/kg/min) and ketamine (120 ug/kg/min) maintained anesthesia for 60 minutes in a group of research horses not subjected to surgery.


Guaifenesin Recipes


One of the most popular methods of extending IV anesthesia for up to 60 minutes after xylazine- ketamine anesthesia is to use a guaifenesin recipe (combination). Guaifenesin is approved for use in the horse but is not marketed in the United States. Guaifenesin is available in 1-L bags from compound- ing pharmacies. Guaifenesin (5%) solution can be combined with xylazine and ketamine to produce a solution that is called “GKX” or “Triple Drip.”8 “Triple Drip” is formulated by taking1Lof5% guaifenesin and adding 1000 to 2000 mg of ketamine and 500 mg of xylazine. The combination is admin- istered to effect up to a rate of 2 mL/kg of body weight/hour. The combination produces excellent muscle relaxation and suitable analgesia. The combination is not usually used for induction of an- esthesia because of uncertainty when recumbency will occur. If an extended period of anesthesia is anticipated, the infusion can be started after induc- tion once the horse has been positioned for surgery. The degree of muscle relaxation and lack of move- ment are the best indicators of the depth of anesthe- sia. The quality of recovery is generally good if the anesthetic period is kept to less than 1 hour. “Tri- ple Drip” should not be used for anesthetics greater than 1 hour in duration unless oxygen supplemen- tation and respiratory support is provided.


Midazolam Replacement for Guaifenesin


Midazolam, a water-soluble benzodiazepine, can be used to replace guaifenesin in “Triple Drip” and


other2 agonist–ketamine combinations for extend- ing short-term field anesthesia.9–11 The usual quan- tities of xylazine (500 mg) and ketamine (1000–2000 mg) are added to1Lof isotonic fluids. Midazolam (25 mg or 5 mL/L) is added instead of guaifenesin. The resultant solution is dosed at the same rate as conventional “Triple Drip” (2 mL/kg/hr, IV). Smaller volumes (250–500 mL) can be formulated if the antic- ipated duration of anesthesia is shorter than 60 min- utes, reducing cost and wastage. The effects are similar to conventional “Triple Drip” and the cost may be less expensive depending on your source of guaife- nesin.


3. What if I Know I Need Additional Time Before I Induce Anesthesia?


There are two anesthetic regimens that produce 30 to 40 minutes of anesthesia from a single injection. The combination of xylazine and tiletamine-zolazepam can be used to produce good-quality anesthesia for


274 2018  Vol. 64  AAEP PROCEEDINGS


30 to 40 minutes.12 Tiletamine is a drug similar to ketamine and zolazepam is a benzodiazepine, simi- lar to diazepam and midazolam. Xylazine (1.1 mg/ kg) is administered IV in order to produce profound sedation and relaxation. Tiletamine-zolazepam (1.1 mg/kg) is given following the onset of full sedation. Induction is smooth and tends to be somewhat quicker than that seen with xylazine and ketamine. The quality of anesthesia is similar to that with xylazine-diazepam-ketamine in that muscle relax- ation is excellent. Respiration is depressed but remains adequate for the period of recumbency. Recoveries are not as crisp as seen with the xyla- zine-ketamine combination because of the greater degree of muscle relaxation. Detomidine and deto- midine-butorphanol have also been used prior to tiletamine-zolazepam. The addition of detomidine prolongs the anesthetic duration but the quality of the recovery suffers. The administration of a com- bination of ketamine (0.5 mg/kg), tiletamine-zolaz- epam (0.7 mg/kg), and detomidine (0.01 mg/kg) has been investigated for anesthesia for castration.13 The combination is prepared by reconstituting 500 mg of tiletamine-zolazepam powder with 4 mL of ketamine (100 mg/mL) and 1 mL of detomidine (10 mg/mL). The mixture has been administered af- ter xylazine sedation at a rate of 0.007 mL/kg, IV (app 3 mL/450 kg). The combination produces excellent induction to anesthesia with intraoperative arterial blood pressures higher than those seen with most other techniques. Duration of anesthesia is longer than xylazine and ketamine and recoveries usually require assistance.


4. Discussion


Xylazine and ketamine have been successfully used to produce short-term IV anesthesia in horses for over 30 years. The addition of diazepam or mida- zolam extends the period of anesthesia and aug- ments the quality of the anesthetic period by producing improved muscle relaxation. Longer pe- riods of anesthesia can be produced by administer- ing additional doses of xylazine and ketamine but guaifenesin or midazolam recipes incorporating xy- lazine and ketamine produce improved results in terms of the quality of the anesthetic period and smoother, but typically, longer recoveries from an- esthesia. The improvement in the quality of anes- thesia is primarily due to the augmented muscle relaxation and minimal additive sedation produced by the administration of guaifenesin or a benzodi- azepine, such as midazolam. As stated, the addi- tion of these drugs typically extends the duration of anesthesia but at a cost of prolonging the recov- ery period and causing the horses to be somewhat weaker and potentially have a degree of ataxia when they attempt to stand, compared to short- duration xylazine-ketamine anesthesia. Typically, such horses benefit from assistance, such as steadying the head and tail as the horse moves to regain its feet.


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