Page 36 - Manual of Equine Field Surgery

32 PRESURGICAL PREPARATION AND ASSESSMENT

be considered when determining the dose of tio11.34 A 10% solution of guaifenesin in sterile

antagonist to administer. If in doubt, it is certainly water also produces minimal hemolysis.35 Less

appropriate to titrate the antagonist dose to concentrated solutions are less likely to cause

produce the desired degree of reversal. phlebitis or thrombus formation. 36 Perivascular

administration of guaifenesin can result in severe

Benzodiazepines tissue damage. If the solution is allowed to cool

substantially below room temperature, the guaife-

The benzodiazepines are used primarily for their nesin will precipitate out of the solution. It can be

muscle relaxant effects. In addition, they provide redissolved by warming the solution. Guaifenesin

some sedation, although their sedative effects undergoes hepatic metabolism and the metabo-

are minimal in horses. They are also used to lites are excreted in the urine.37 Accumulation of

treat seizures. The benzodiazepines function by metabolites, such as catechol, can lead to signs of

enhancing the effect of y-aminobutyric acid toxicity, including muscle stiffness, tremors, and

(GABA), an inhibitory neurotransmitter. This dyspnea. Guaifenesin is rarely used alone but is

results in sedation by depression of the limbic usually combined with an injectable anesthetic

system and in muscle relaxation by inhibition of agent such as a barbiturate or a dissociative anes-

internuncial neurons within the spinal cord.11 thetic such as ketamine.

Limited data are available on the physiologic effects

of the benzodiazepines in horses, but the cardio-

vascular effects are minimal in most species.!':":" OPIOIDS

Three benzodiazepines are currently in use in

equine anesthesia-diazepam, midazolam, and Opioids are potent analgesics; unfortunately, they

zolazepam. Zolazepam is part of a fixed drug often produce excitement when administered by

product, Telazol (Fort Dodge Animal Health), themselves to horses. This is especially true of the

which is a combination of zolazepam and tileta- full opioid agonists. The development of opioids

mine, a dissociative anesthetic that is discussed that are agonists at only some opioid receptors has

further in the dissociative anesthetic section later made the use of opioids in the horse easier and

in this chapter. Diazepam is supplied in a propy- more effective. Opioid receptors are commonly

lene glycol vehicle that makes intramuscular classified as mu (u), kappa (K), and delta (8)

injection painful and the rate of absorption from receptors. Mu receptor activation is generally

the injection site variable. Midazolam is water associated with profound analgesia as well as with

soluble and well absorbed after intramuscular some of the undesirable opioid effects such as

32
injection. All of the benzodiazepines appear to bradycardia, hypoventilation, and excitement.

undergo hepatic metabolism with the metabolites Kappa receptor activation produces analgesia that

excreted in the urine. Some of the metabolites of is not as intense as that associated with mu recep-

diazepam appear to have a significant pharmaco- tor activation but is also associated with fewer

logic effect. 32•33 Because these drugs are rarely used undesirable effects. Delta receptors are pri.J.narily

alone, the commonly used doses are included in thought to modulate mu receptor activity and

the later section on anesthetic combinations. produce analgesia. In general, the opioids have

minimal cardiovascular and respiratory effects.

Guaifenesin Small increases in heart rate, blood pressure, and

cardiac output were observed after full agonists

Guaifenesin, also known as glyceryl guaiacolate were administered, probably from the excitatory

(GG), is used for its muscle relaxant properties at effects of these drugs in the horses studied. The
38

the internuncial neurons in the spinal cord. The nonselective full agonists such as morphine and

cardiovascular and respiratory effects of guaifen- fentanyl have a very narrow margin between the

esin are minimal when the commonly recom- analgesic and the excitatory dose, especially in

mended clinical doses are used. It is usually pain-free animals.39 It is i.J.nportant to differenti-

supplied as a sterile powder that is dissolved to ate the behavioral effects of opioids in pain-free

form a 5% or lOo/o solution of guaifenesin. A 5% horses, such as those often used in research

solution is commonly dissolved in a 5% glucose studies, and their effects in clinically painful

solution to minimize hemolysis after administra- horses. In a study of the perioperative use of mor-