Page 43 - Manual of Equine Field Surgery

Field Anesthesia 39

to stand. If the horse is trying to get up but is joint is located by moving the tail up and down

unable to remain standing, a small dose of and palpating for flexion. Once the joint has been

xylazine (0.2 to 0.4 mg/kg IV) may calm the ani- located, 2mL of a local anesthetic such as 20/o li-

mal and provide a quieter recovery. Some practi- docaine can be injected into the superficial tissues

tioners advocate holding the horse down until it is over the joint. An 18- or 20-gauge 2.5-inch (6.4-

able to get up, and this may be of benefit. It is best cm) spinal needle is used to access the epidural

done by kneeling on the horse's neck at the dorsal space. In large or heavily muscled horses, a 3.5-

aspect and holding the head to keep the horse from inch (8.9-cm) needle may be needed, and in many

swinging it up. Once the horse's attempts to get up horses, a standard 1.5-inch (3.75-cm) hypodermic

have become more vigorous, the head can be needle will be adequate (Figure 5-1). The needle

released and the horse allowed to stand. After it is is introduced perpendicular to the skin directly

standing, it is important to try and steady the horse over the center of the space on the midline. As the

to keep it from stumbling around and injuring needle is advanced and the epidural space is

itself. If it is standing but very unstable, a second entered, a loss of resistance will be felt. If the tip

person holding the tail may be of benefit. of the needle strikes the floor of the canal, it

should be withdrawn slightly. There should be no

resistance to the injection of fluid or air at this
EPIDURAL ANESTHESIA AND ANALGESIA point. An alternative method of determining

when the epidural space is entered is called the

Epidural anesthesia is an excellent method of pro- "hanging drop" technique. After the needle has

viding desensitization to the tail and perineal been advanced through the skin and into the soft

region of the standing horse. Local anesthetics tissue overlying the intervertebral foramen, the

traditionally have been used, but more recently stylet is removed (if present) and a small amount

other drugs such as opioids and a.2-agonists have of saline or local anesthetic is instilled into the

been used separately or with local anesthetics to hub of the needle. As the tip of the needle pene-

improve the desensitization provided by local an- trates the ligamentum flavum, the fluid runs

esthetics or provide long-term pain control. down the needle into the epidural space and the

fluid in the hub disappears.

Anatomy If repeated epidural injections are to be made,

an epidural catheter can be placed to make this
The spinal cord and meninges usually end in the

sacrum. Epidural injections are usually performed more convenient. Several commercial epidural

at the sacrococcygeal or first intercoccygeal joint. catheter kits that are suitable for equine use are

Either location is acceptable, and the injection site available. The needle insertion technique is the

can generally be determined by moving the tail up same as described above but a different needle is

and down and palpating for the most proximal

movable joint. The depth of the soft tissue over

the first intercoccygeal space is 3.5 to 8cm.81 The

nerves desensitized by the injection of a local

anesthetic in this area include the caudal and 52

to SS sacral spinal nerves. These provide nerve

fibers making up the pudendal, middle rectal, and

cau-dal rectal nerves. The 52 nerve also con-

tributes motor innervation to most of the hind

limb, and blockade of this nerve may cause hind

limb ataxia. For this reason, it is important to limit

the volume of local anesthetic injected because the

volume injected will determine the rostral extent

of the blockade. Figure 5-1 A sagittal section through the sacrococ-

cygeal region of an equine cadaver showing the needle

Technique placement for a caudal epidural injection. The upright

needle is 2.5 inches long and the more caudally inserted

After the hair is clipped, suitably cleaned, and dis- needle is 3.5 inches in length. The horse weighed

infected, the sacrococcygeal or first intercoccygeal approximately 500 kg when alive.