Page 44 - Manual of Equine Field Surgery

40 PRESURGICAL PREPARATION AND ASSESSMENT

used. A Touhy needle with a slightly curved tip take up to 20 minutes. The opioid most com-

allows easier placement of the catheter. After monly used for epidural injection is morphine;

the needle is in place, the catheter is advanced the dose usually used is 0.1 mg/kg. This dose can
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through the needle and cranially into the epidural provide analgesia without any motor effects for up

space. The opening on the needle should be ori- to 18 hours. Commercial preservative-free mor-

ented cranially, and this can be checked while the phine preparations are available that should be

needle is in the animal by observing the notch on used if possible, especially if multiple injections

the hub of the needle. This notch will be over are to be performed. Unfortunately, the currently

the opening at the distal end of the needle. The available preservative-free preparations are so

catheter should be advanced 5 to 10 cm cranially. dilute, the volume required is too large to be prac-

If local anesthetics are to be injected, this distance tical. When the more concentrated morphine

should be shorter, only 2 to 4 cm, to prevent an preparations are used, they should be diluted to a

impairment of motor innervation to the hind total volume of 10 mL with 0.9% sterile saline.

limbs. Once the catheter is in place, the needle is Xylazine can be administered epidurally by itself

withdrawn and the injection hub is attached to or with other drugs. It appears to have some weak

the catheter per the manufacturer's instructions. local anesthetic effects that are not reversed by CX.2-

The catheter is then secured to the horse, and the antagonists as well as analgesic effects produced

injection hub and site of entry of the catheter by adrenergic receptors in the spinal cord. The

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through the skin are covered. It is important to usual dose is 0.17 mg/kg. The author often adds

secure the catheter in such a way that prevents 1 mL of 100 mg/mL xylazine to 4mL of 20/o lido-

kinking or accidental removal. The catheter caine to be used for caudal epidural injection in a

should be flushed daily if no therapeutic injec- 450-kg horse.

tions are performed.

REFERENCES

Epidural Drugs

1. Tobin T, Ballard S: Pharmacological review-the
Classically, local anesthetics have been used for phenothiazine tranquilizers, J Equine Med Surg

epidural anesthesia. They provide complete de- 3:460, 1979.

sensitization and can affect rnotor innervation 2. Walker M, Geiser D: Effects of acetylpromazine on

as well. The dose of local anesthetic is based to a the hemodynamics of the equine metatarsal artery,

large extent on volume. The greater the volume of as determined by two-dimensional real-time and

drug injected into the epidural space, the farther pulsed Doppler ultrasonography, Am J Vet Res

rostral the drug will have an effect. More recently, 47:1075, 1986.

CX.2-agonists and opioids have been used alone or 3. Muir WW, Werner LL, Hamlin RL: Effects of

in conjunction with local anesthetics to prolong x:ylazine and acetylpromazine upon induced ven-

anesthetic effects and provide long-term analgesia tricular fibrillation in dogs anesthetized with thi-
amylal and halothane, Am J Vet Res 36:1299, 1975.
without the loss of motor function. Additionally, 4. Maze M, Hayward E, Gaba DM: Alpha I-ad-

the analgesic effects of opioids extend farther ros- renergic blockade raises epinephrine-arrhythmia

trally than the sacrococcygeal region and can threshold in halothane-anesthetized dogs in a

provide analgesia for hind limb and even abdom- dose-dependent fashion, Anesthesiology 63:611,

inal pain.82•83 A volume of 5 to 7 ml of local anes- 2004.

thetic is commonly used for a 450-kg horse. The 5. Geiser DR: Chemical restraint and analgesia in the

local anesthetics most commonly used in the horse, Vet Clin N A1n Equine Pract 6:495, 1990.

United States are 2o/o lidocaine, 2°/o mepivacaine, 6. Marroum PJ, Webb AI, Aeschbacher G, et al: Phar-

and 0.5% bupivacaine. The duration of effect can macokinetics and pharmacodynamics of acepro-

be variable, but lidocaine will typically provide 30 mazine in horses, Am J Vet Res 55:1428, 1994.

to 60 minutes of desensitization. Mepivacaine can 7. Parry BW, Anderson GA, Gay CC: Hypotension in

provide 90 to 120 minutes of desensitization, • the horse induced by acepromazine maleate, Aust
Vet J 59: 148, 1982
.
and bupivacaine can produce up to 4 l1ours of 8. Jones RS: Penile paralysis in stallions, J Am Vet Med

desensitization. The addition of epinephrine Assoc 149:124, 1966.

(1:100,000) will prolong the duration. The onset 9. Lucke JN, Sansom J: Penile erection in the horse

of blockade after local anesthetic injection may after acepromazine, Vet Rec 104:21, 1979.