Continuous peripheral nerve block in the lower forelimb of the horse

Introduction. Managing pain effectively is one of the most important tasks that veterinarians perform on a daily basis in clinical practice. In horses successful treatment of severe pain originating in the forelimb is complicated mainly because regional analgesic techniques are not yet available and systemic administration of non-steroidal anti-inflammatory drugs and other analgesics (e.g. opioids) are often not very effective and/or associated with multiple adverse effects. Local anesthesia presents a relatively low-cost alternative technique of analgesia with only minimal systemic local anesthetic (LA) absorption, however, producing only brief periods of pain relief due to the short half-life of LAs. In human medicine a method of continuous peripheral nerve blockade (CPNB) has been developed and its application has gained increasingly more popularity among clinicians for treatment of virtually all types of severe surgical and procedural pain. Single dose or continuous perineural LA infusion holds considerable promise also in small animals to minimize intra- and postoperative discomfort. The aim of this study was to develop a technique for placing CPNB catheters along the palmar nerves in horses and to evaluate the nociceptive efficacy of LA infusion in the distal equine forelimb. Materials and methods. Feasibility and efficacy of CPNB catheter instrumentation in standing, sedated horses was tested in 13 animals, using a percutaneous “blind” technique with pre- and postultrasound control using a method previously developed in 42 forelimb specimens for CPNB catheter placement along the medial and lateral palmar nerves. Anatomical landmarks, potential risk for damage to surrounding tissues during catheter placement, satisfactoriness of the catheterskin fixation and incidence of complications (dislocation/kinking, infection) were evaluated. Five horses received initially or repeatedly boluses of 3 to 4 mL per catheter (bupivacaine 0.125% in 0.04% NaHCO3 and adrenaline 1:200,000 solution; and NaCl 0.9% [SAL] in 0.04% NaHCO3 and adrenaline 1:200,000 solution as control) followed by a constant rate infusion of 2 ml/h over a period of six days. Electrical and mechanical stimulation thresholds and response latencies for eliciting hoof withdrawal responses (HWR) were determined following either SAL or LA infusion in these horses using a constant current stimulator and a spring-loaded pinprick instrument, respectively. Results. Using an IV sedation protocol and local anesthesia (i.e. bilateral skin and palmar nerve block with 2% mepivacaine) medial and lateral CPNB catheters were inserted ~ 5 and 7 cm distal to the accessory carpal bone, respectively. After subcutaneous tunneling over 2.0-2.5 cm, a 20 G 8.9 cm Tuohy-Schliff needle was inserted over its entire length along each nerve and a closed-tip, polyamide catheter (20 G, 104 cm) advanced for ~10-12 cm. The free endings of the CPNB catheters were later secured to the skin and connected to light-weight, battery-driven ambulatory infusion pumps safely placed in horse boots, and legs were bandaged. Ultrasound examination during and following CPNB catheter placement confirmed correct placement without any obvious damage to nerves or adjacent structures (i.e. tendons, palmar blood vessels). Positive microbial contamination was detected in 8/20 CPNB catheters retrieved from the 5 study horses (20%) at the end of the experiment, with environmental contaminants that lack clinical significance predominating. Infusion of LA as compared to SAL solution increased electrical and mechanical stimulation HWR thresholds by approximately 4-5 times. Catheters were well tolerated for 6 to 10 days without major complications. Conclusions. The developed technique of placing and securing palmar CPNB catheters in the equine forelimb can be successfully applied under ambulatory conditions in the standing horse without causing major harm. However, in animals the frequency of complications associated with regional anesthetic techniques is largely unknown, thus additional studies are warranted to prove any superiority of the CPNB technique over other methods of pain management in the horse. Acknowledgment. This study was supported by Morris Animal Foundation (Grant DO5EQ-024; P.I. B.D.)