HOW TO PERFORM FIELD ANESTHESIA AND PAIN MANAGEMENT


complish these more complex and potentially more invasive surgical procedures frequently exceed those needed for the shorter, simpler procedures more commonly performed. The purpose of this presen- tation is to describe how to amplify the sedative and analgesic effects of basic sedation protocols and how to produce maximal standing chemical restraint for more extensive procedures in the field setting.


2. Review of Commonly Used Drugs


Phenothiazine Tranquilizers Phenothiazine tranquilizers are used to produce calming and relaxation.5 Phenothiazines block the action of neurotransmitters centrally and peripher- ally and cause alpha adrenergic blockade leading to arterial hypotension.6 The onset of action after parenteral administration occurs within 15 to 30 minutes but peak effects may not be seen for up to 45 minutes. The duration of sedation depends on the dose administered but frequently lasts for 6 to 10 hours (Table 1). Minimal muscle relaxation or ataxia occur. Acepromazine does not produce anal- gesia but may make analgesic drugs more effective. Increasing the dose of acepromazine does not usu- ally produce a greater effect but the duration of action is increased. In stallions and geldings, phe- nothiazine administration rarely causes persistent penile paralysis. Acepromazine is infrequently used as the primary drug for standing chemical re- straint because of the depth of sedation produced, the comparatively long time to peak effect, and the lack of analgesia. Acepromazine is used in some combinations because it provides a consistent level of background tranquilization that facilitates a more consistent level of overall restraint.7


Benzodiazepines


Benzodiazepines are primarily used to augment muscle relaxation and as anti-epileptic agents in the horse. Benzodiazepines do not produce significant sedation or analgesia. Both midazolam and diaze- pam can produce significant muscle relaxation, weakness, and apparent ataxia when given in high doses so caution should be exercised when they are used as part of a protocol for standing chemical restraint.8 Benzodiazepines are primarily used as adjuncts in dental procedures because they are re- ported to reduce chewing and tongue movement.9


2 Agonists


2 agonists produce sedation with muscle relaxation, ataxia, and analgesia when given orally, intravenously


(IV), or intramuscularly (IM) to horses.10 The phar-


macologic/pharmacodynamic effects of the 2 ago- nists are similar to one another with increases in arterial blood pressure and decreases in heart rate, cardiac output, and respiratory rate frequently seen. All produce analgesia, the duration of which is con- sistently shorter than the accompanying sedation. Xylazine, detomidine, and romifidine are approved


270 2018  Vol. 64  AAEP PROCEEDINGS


for use in the horse in the United States. Else- where, medetomidine and dexmedetomidine are used.1 In large part, the effects of the agents are qualitatively similar with differences centering on dose (due to receptor specificity and potency), time to onset, duration of action, and degree of analgesia produced. The level of sedation produced by ad-


ministration of an 2 agonist is more pronounced than that produced by phenothiazine administra-


tion. Horses that have received2 agonists assume a “head-down” or “saw horse” stance and frequently shift their weight from side to side. Occasionally, these effects are problematic, particularly when the horse has ataxia and the procedures include sam- pling of cerebrospinal fluid and/or cervical radiogra- phy. In such instances, small doses of shorter-


acting2 agonists, such as xylazine, may be the best option. Romifidine, in locations where it is avail- able, is another potential choice for horses with ataxia. IV administration produces a quicker onset of action, an increased intensity of effect, but a


shorter duration of effect. Infusions of 2 agonists for standing surgery are gaining popularity.6,11,12 The use of constant rate infusion reduces the ups and downs of repeat bolus administration and fre- quently lowers the total dose of drug administered.


Opioids


Opioids are used to produce analgesia and augment the effects of sedatives and tranquilizers.13 When administered alone to pain-free animals, opioids can cause nervousness and excitability, so prior sedation is required. Butorphanol is a synthetic opioid ago- nist/antagonist that is approved for use in the horse for the treatment of abdominal pain.14 Butorpha- nol is included in a number of combinations used for minor standing procedures because the dose of other


drugs (such as 2 agonists) can be reduced but it may not produce the level of analgesia and restraint required for more extensive procedures even when local anesthesia is included.15 The duration of ac- tion of buprenorphine is longer than that seen after butorphanol and the potential for restlessness or excitement is similar.12,16 A number of other opi- oids including morphine and meperidine have been used to produce standing chemical restraint.17,18 Horses may remain sensitive to touch so local anes- thesia should be incorporated with the technique. In the author’s experience, the level of augmented sedation and analgesia is greater with morphine and meperidine than it is with butorphanol or bu- prenorphine. Naloxone can be used to antagonize morphine or meperidine, if necessary. Renarcotiza- tion (excitement) occasionally occurs and is best treated by repeat administration of naloxone or tran- quilizer administration.


Ketamine


Ketamine infusions are beginning to be incorporated into standing restraint protocols in an attempt to increase analgesia.9,19 The dose administered ranges


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