Sensory nerve conduction and nociception in the equine lower forelimb during perineural bupivacaine infusion along the palmar nerves

Abstract

The purpose of this investigation was to study lateral palmar nerve (LPN) and medial palmar nerve (MPN) morphology and determine nociception and sensory nerve conduction velocity (SNCV) following placement of continuous peripheral nerve block (CPNB) catheters along LPN and MPN with subsequent bupivacaine (BUP) infusion. Myelinated nerve fiber distribution in LPN and MPN was examined after harvesting nerve specimens in 3 anesthetized horses and processing them for morphometric analysis. In 5 sedated horses, CPNB catheters were placed along each PN in both forelimbs. Horses then received in one forelimb 3 mL 0.125% BUP containing epinephrine 1:200 000 and 0.04% NaHCO(3) per catheter site followed by 2 mL/h infusion over a 6-day period, while in the other forelimb equal amounts of saline (SAL) solution were administered. The hoof withdrawal response (HWR) threshold during pressure loading of the area above the dorsal coronary band was determined daily in both forelimbs. On day 6 SNCV was measured under general anesthesia of horses in each limb's LPN and MPN to detect nerve injury, followed by CPNB catheter removal. The SNCV was also recorded in 2 anesthetized non-instrumented horses (sham controls). In both LPN and MPN myelinated fiber distributions were bimodal. The fraction of large fibers (>7 μm) was greater in the MPN than LPN (P < 0.05). Presence of CPNB catheters and SAL administration did neither affect measured HWR thresholds nor SNCVs, whereas BUP infusion suppressed HWRs. In conclusion, CPNB with 0.125% BUP provides pronounced analgesia by inhibiting sensory nerve conduction in the distal equine forelimb.

Figure 1

A — Representative NCS/EMG unit recordings from the median and ulnar nerves after percutaneous stimulation of the medial and lateral palmar digital nerves, respectively in an anesthetized horse, previously instrumented with CPNB catheters along the palmar nerves in its two forelimbs. At the time of recordings one forelimb had been continuously infused for 6 days with saline solution containing 0.04% NaHCO₃ and epinephrine 1:200 000 (SAL-infused limb), while the contra-lateral limb had been infused with bupivacaine 0.125% (in the same solvent) over a 6-day period (BUP-infused limb). The initial deflection at the far left side of all of the tracings represents the stimulus artifact and the first positive (downward) peak of the compound action potential (CAP), which is equivalent to the onset of negativity, indicates the arrival at the recording electrode of impulses traveling in the fastest conducting fibers, that is, the latency (16). Unlike in control limbs, no CAPs were recorded in BUP-infused forelimbs (upper right hand panels). B — In the SAL-infused forelimb sensory nerve impulse conduction from the medial digital palmar nerve to the median nerve was recorded a second time (left hand trace) and then after BUP 0.125% administration. Recordings to the right represent recordings performed sequentially at 6, 12, and 18 min following BUP bolus administration and beginning of BUP infusion. At 18 min (far right panel) the CAP was abolished.