Regional nerve blocks are well-described viable alternatives to systemic anesthesia for many invasive procedures and surgeries. Local anesthetics block nerve conduction through sodium channel inhibition. Fiber size, the myelin sheath, drug volume, and drug concentration all influence the degree of nerve block; this can be modified by manipulation of local anesthetic volume and concentration. The efficacy of a combined paravertebral lumbar plexus/parasacral sciatic block was evaluated in this study; varying bupivacaine concentrations were injected at different points along L4 to L7 and at strategic points in the parasacral region (from the dorsal iliac crest to the ischial tuberosity) in 8 dogs. Patients were sedated with intravenous medetomidine; blocks were performed with bupivacaine 0.25% (0.2 mL/kg), bupivacaine 0.5% (0.2 mL/kg), bupivacaine 0.25% (0.4 mL/kg), and saline controls. Dogs were evaluated for sensation and motor movement in their hindlimbs using a previously described protocol. Bupivacaine at 0.05 mL/kg for each nerve root of the lumbar plexus (total volume = 0.2 mL/kg) was sufficient to produce a complete blockade at a concentration of 0.5%. The elevated concentration of local anesthetic allowed a higher local anesthetic input to the inner fibers of the nerve trunk, producing a longer and more effective block. Interestingly, the parasacral sciatic nerve block with bupivacaine 0.5% was not very successful; it is possible that a higher anesthetic volume was required to reach the nerves in this region with a paravertebral sciatic approach. Partial blockade of the tibial and peroneal nerves in the parasacral region was identified in 25% of dogs, suggesting that parasacral sciatic nerve blocks require increased volumes of anesthetic.

Commentary: Regional nerve blocks are beneficial because they reduce hospitalization times, costs, and recovery from surgical procedures that would otherwise require general anesthesia. They are also beneficial for extremity pain control in patients with moderate to severe pain, such as bone tumor pain. This study discusses the practical use of a paravertebral nerve block for hindlimb procedures instead of an epidural or general anesthesia. The block is easy to perform, and the use of a nerve stimulator improves placement accuracy of the injection needle and provides additional support for the practitioner. Future studies could further elucidate the volumes necessary to effectively block the nerves from the parasacral plexus in order to anesthetize the entire hindlimb.—Heather Troyer, DVM, Diplomate ABVP (Canine & Feline Practice)

Combined paravertebral plexus block and parasacral sciatic block in healthy dogs. Diego AP, Otero PE, Tarragona L, et al. VET ANAESTH ANALG 37:531-541, 2010.