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Nerve stimulation-guided thoracolumbar paravertebral block for flank laparotomy in a horse

Published:February 22, 2017DOI:https://doi.org/10.1111/vaa.12382
      A thoracolumbar paravertebral block (TPVB) is commonly performed in ruminants, however, the transverse processes of the thoracolumbar spine are difficult to palpate in some horses. Blind techniques for equine TPVB have been described in the past (
      • Skarda R.T.
      Local anesthesia and local anesthetic techniques in horses.
      ,
      • Moon P.F.
      • Suter C.M.
      Paravertebral thoracolumbar anaesthesia in 10 horses.
      ). This technique can be quite difficult to perform in a horse with a body condition score greater than 4 (
      • Henneke D.R.
      • Potter G.D.
      • Kreider J.L.
      • et al.
      Relationship between condition score, physical measurements and body fat percentage in mares.
      ). In humans, the use of electro-location of the thoracolumbar region has previously been described in detail elsewhere (
      • Naja Z.M.
      • Raf M.
      • Rajab M.E.
      • et al.
      A comparison of nerve stimulator guided paravertebral block and ilioinguinal nerve block for analgesia after inguinal herniorrhaphy in children.
      ). This letter reports the use of a nerve stimulator for a thoracolumbar paravertebral block for a standing laparoscopic procedure on a horse.
      A 5-year-old, 500-kg, thoroughbred mare was presented with a presumptive diagnosis of a granulosa cell tumor on the right ovary. A flank laparoscopy was scheduled in order to perform a right-hand side ovariectomy. The mare was restrained in a standing stock, and the TPVB injection and surgical site were aseptically prepared (Fig. 1). The mare was then sedated with detomidine (10 mg) plus butorphanol (10 mg), intravenously. Immediately after the initial bolus, a detomidine constant rate infusion was initiated to maintain moderate-to-deep sedation. The last thoracic (T18), first and second lumbar (L1 and L2) spinal nerves were then desensitized approximately 5 cm lateral from the dorsal midline using a nerve stimulator (Plexygon Nerve Stimulator; Vygon, Italy). The sites for desensitization were palpated by locating the third lumbar transverse process (L3), which is on a line between the most caudal extension of the last rib and perpendicular to the long axis of the spinal vertebrae (Fig. 1). The distance between the injection sites (from L3 to L2 and L1) was approximately 5 cm. The skin, subcutaneous and superficial musculature of each site (red circles; Fig. 1), was desensitized, using 5 mL of Lignocaine 2% and a 1.5 in (3.8 cm) 21- gauge needle. A long beveled –20 gauge – L.150- mm-insulated needle (Locoplex; Vygon, France) was then introduced vertically (perpendicular to the skin) until it impinged on the transverse process of L3 (depth ± 9 cm) when it was then walked off cranially until the intertransverse ligament between L3 and L2 was penetrated. At this time, we aimed to identify an evoked musculature contraction of the flank region using a 1-mA current. Initially, twitching of the lumbar muscles was seen as a result of direct muscle stimulation but as the needle was slowly advanced, an evoked contraction of the muscles composing the flank region (external and internal oblique abdominal muscles and transverse abdominal muscle) could be seen. The proximity with the root of L2 was confirmed when a positive twitch of the most caudal aspect of the flank (closer to tuber coxae) was present when the current was decreased to 0.5 mA. By using a low current intensity, we aimed to avoid unspecific direct muscular stimulation unrelated to the proximity to the paravertebral branches. It is easy to enter the peritoneum, and this is detected by a loss of resistance or aspiration of air. Should this happen, the needle should be withdrawn to a retroperitoneal position before the local anesthetic is deposited. After aspiration to rule out venipuncture, 5 mL of Mepivacaine 2% was infiltrated when at maximum positive stimulation and another 5 mL was infiltrated when retracting the needle 2 cm above this point. The procedure was repeated to desensitize nerves L1 and T18 at the transverse process of L2 and L1, respectively. The proximity with the root of T18 was confirmed by contraction of the most cranial aspect of the flank (closer to the last rib). Adequacy of the block was checked 15 minutes later by sensory loss of the surgical field to pinprick sensation. Lignocaine 2% (10 mL) was used to block the ovarian pedicule using a long needle through the laparoscopic port. The total amount of Mepivacaine and Lignocaine used was 30 and 25 mL, respectively. The block was not only deemed sufficient to desensitize the right flank for the laparoscopic ports but also to remove the right ovary through a 10-cm incision. During TPVB, the animal showed no discomfort to deep muscle injection, as anaesthetics drugs and desensitization of the most superficial tissue planes had been performed before block. A limitation of the current technique was that duration of the blockade was not measured and could have been influenced by the drugs used. Furthermore, we did not collect objective data on the extent of the desensitized area, which could have showed which dermatomes were blocked.
      Figure thumbnail gr1
      Figure 1Right thoracolumbar area of a standing adult thoroughbred mare with injection sites (red circles) for paravertebral block of T18, L1 and L2 root nerves. Notice that the injection sites are approximately 5 cm distant from midline (spinous processes of lumbar vertebra) and each injection site approximately 5 cm apart. The dotted line represents the landmark to locate the transverse process of L3 and it is located at the most caudal extension of the last rib and perpendicular to the long axis of the spinal vertebrae.
      Thoracolumbar paravertebral block can be used as an alternative to infiltration anesthesia or an epidural during standing procedures (
      • Moon P.F.
      • Suter C.M.
      Paravertebral thoracolumbar anaesthesia in 10 horses.
      ). The advantages of paravertebral anesthesia using nerve stimulation over the blind technique include not only smaller doses of local anesthetic but also smaller chances of having the third lumbar spinal nerve inadvertently desensitized, which would cause loss of motor control to the ipsilateral pelvic limb (
      • Moon P.F.
      • Suter C.M.
      Paravertebral thoracolumbar anaesthesia in 10 horses.
      ).

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