2007, John Wiley & Sons.
Proximal Paravertebral
The proximal paravertebral nerve block desensitizes the dorsal and ventral nerve roots of the last thoracic (T13) and first and second lumbar (L1 and L2) spinal nerves as they emerge from the intervertebral foramina. To facilitate proper needle placement of anesthetic, the skin at the cranial edges of the transverse processes of L1, L2, and L3 and at a point 2.5–5 cm off the dorsal midline is desensitized by injecting 2–3 ml of local anesthetic using an 18‐gauge, 2.5‐cm needle. A 14‐gauge, 2.5‐cm needle is used as a cannula or guide needle to minimize skin resistance during insertion of an 18‐gauge, 10‐ to 15‐cm spinal needle. Approximately 5 ml of local anesthetic is placed through the cannula to anesthetize the needle tract for further needle placement.
To desensitize T13, the cannula needle is placed through the skin at the anterior edge of the transverse process of L1 at approximately 4–5 cm lateral to the dorsal midline. The 18‐gauge, 10‐ to 15‐cm spinal needle is passed ventrally until it contacts the transverse process of L1. The needle is then walked off the cranial edge of the transverse process of L1 and advanced approximately 1 cm to pass slightly ventral to the process and into the intertransverse ligament. A total of 6–8 ml of local anesthetic is injected with little resistance to desensitize the ventral branch of T13. The needle is then withdrawn 1–2.5 cm above the fascia or just dorsal to the transverse process and 6–8 ml of local anesthetic is infused to desensitize the dorsal branch of the nerve.
To desensitize L1 and L2, the needle is inserted just caudal to the transverse processes of L1 and L2. The needle is walked off the caudal edges of the transverse processes of L1 and L2, at a depth similar to the injection site for T13, and advanced approximately 1 cm to pass slightly ventral to the process and into the intertransverse ligament. A total of 6–8 ml of local anesthetic is injected with little resistance to desensitize the ventral branches of the nerves. The needle is then withdrawn 1–2.5 cm above the fascia or just dorsal to the transverse processes and 6–8 ml of local anesthetic is infused to desensitize the dorsal branch of the nerves (Figure 17.2).
Figure 17.2 Proximal paravertebral.
Source: From [2], © 1986, Elsevier.
Evidence of a successful proximal paravertebral nerve block includes increased temperature of the skin; analgesia of the skin, muscles, and peritoneum of the abdominal wall of the paralumbar fossa; and scoliosis of the spine toward the desensitized side. Advantages of the proximal paravertebral nerve block include small doses of anesthetic, wide and uniform area of analgesia and muscle relaxation, decreased intra‐abdominal pressure, and absence of local anesthetic at the margins of the surgical site. Disadvantages of the proximal paravertebral nerve block include scoliosis of the spine, which may make closure of the incision more difficult, difficulty in identifying landmarks in obese and heavily muscled animals, and more skill or practice required for consistent results [2, 3, 5].
Distal Paravertebral
The distal paravertebral nerve block desensitizes the dorsal and ventral rami of the spinal nerves T13, L1, and L2 at the distal ends of the transverse processes of L1, L2, and L4, respectively. An 18‐gauge, 3.5‐ to 5.5‐cm needle is inserted ventral to the transverse process and 10 ml of local anesthetic is infused in a fan‐shaped pattern. The needle can then be removed completely and reinserted or redirected dorsal to the transverse process, in a caudal direction, where 10 ml of local anesthetic is again infused in a fan‐shaped pattern. This procedure is repeated for the transverse processes of the second and fourth lumbar vertebrae (Figure 17.3). Advantages of the distal paravertebral nerve block compared with the proximal paravertebral nerve block include lack of scoliosis, easier performance, and more consistent results. Disadvantages of the distal paravertebral nerve block compared with the proximal paravertebral nerve block include larger doses of anesthetic and variations in efficiency caused by differences in anatomic pathways of the nerves [2, 3, 5].
Figure 17.3 Distal paravertebral.
Source: From [2], © 1986, Elsevier.
Anesthesia of the Perineum
Anesthesia of the perineum is routinely performed for obstetric procedures and for relief of rectal tenesmus in the cow. Many urogenital surgeries in cows (i.e. replacement of vaginal/cervical prolapse) and bulls/steers (ischial urethrostomy, penile amputation) also require anesthesia of the perineum.
Caudal Epidural Anesthesia
Caudal epidural anesthesia is an easy and inexpensive method of analgesia that is commonly used in cattle. A high caudal epidural at the sacrococcygeal space (S5–Co1) desensitizes sacral nerves S2, S3, S4, and S5. The low caudal epidural at first coccygeal space (Co1–Co2) desensitizes sacral nerves S3, S4, and S5; as the anesthetic dose increases, nerves cranial to S2 may also become affected [6]. If possible, the hair should be clipped and the skin scrubbed and disinfected.
Standing alongside the cow, the tail should be moved up and down to locate the fossa between the last sacral vertebra and the first coccygeal vertebra (first freely moveable space) or between the first and second coccygeal vertebrae. An 18‐gauge, 3.8‐cm needle (with no syringe attached) is directed perpendicular to the skin surface. Once the skin is penetrated, place a drop of local anesthetic solution in the hub of the needle (hanging drop technique). The needle should then be advanced slowly until the anesthetic solution is drawn into the epidural space by negative pressure. The syringe may then be attached to the needle and anesthetic solution slowly injected with no resistance (Figure 17.4a). The dose of local anesthetic to be used is 0.5 ml per 45 kg body weight.
Figure 17.4 (a) Needle placement for caudal epidural. Source: Image courtesy of Dr. Douglas Hostetler.
Adapted with permission from Lumb and Jones’ Veterinary Anesthesia.
(b) Catheter placement for continuous flow epidural.
Source: From [1], © 2007, John Wiley & Sons. Adapted with permission from Lumb and Jones’ Veterinary Anesthesia.
An increased dose of local anesthetic can be used to facilitate other urogenital procedures including teaser bull preparation and ventral midline cesarean section. This is referred to as high‐volume caudal epidural anesthesia and is performed in the same way as the standard caudal epidural, only increasing the volume of local anesthetic used. The recommended dose is 1 ml per 5 kg body weight; however, volumes up to 0.5 ml/kg have been used without adverse effects. The patient will lose control of the hindlimbs, so recumbency is a must when using this technique. The effects on the hindlimbs may last up to four to six hours after administration. It is recommended that hobbles be placed on the hindlimbs and the patient
