A commonly recommended volume of local anesthetic administered adjacent to each palmar digital nerve of the horse when performing a palmar digital nerve block (PDNB) is 1 to 1.5 mL,1,2 although some clinicians have recommended depositing as much as 2 mL local anesthetic adjacent to each palmar digital nerve.3–10 A positive response (ie, resolution or amelioration of lameness) to anesthesia of the palmar digital nerves usually localizes lameness to the foot,10,11 but a PDNB has been shown, in some cases, to desensitize structures more proximal in the limb, such as the proximal interphalangeal joint10,12 and even the metacarpophalangeal or metatarsophalangeal joint.10,13
The potency of a local anesthetic is partly determined by the volume and the concentration of the anesthetic.14,15 The larger the volume, the longer the segment of nerve in contact with the anesthetic and the more profound the block, but several studies16,17 have shown that concentration is more important than volume in acquiring potency. The smaller the volume of local anesthetic, the shorter the segment of nerve in contact with the anesthetic, thus sparing anesthesia of more proximal nerve branches and thus unintended desensitization of tissue proximal to the nerve block. For the purpose of diagnostic anesthesia, the specificity of the nerve block might be increased by administering the smallest volume of local anesthetic that produces a substantial, observable improvement in lameness. Administering a potent local anesthetic allows a small volume to be injected.
Adding epinephrine to a local anesthetic increases the potency of the anesthetic, thereby decreasing the volume of anesthetic required to anesthetize a nerve,18,19 and intensifies the analgesic effect of a local anesthetic.20,21 We postulated that the volumes of 2% mepivacaine commonly recommended to desensitize the foot with a PDNB are excessive and that a smaller volume of 0.75 mL injected over each nerve would be just as effective as twice that volume in ameliorating lameness caused by foot pain. We also postulated that if the smaller volume of mepivacaine was not as effective as the 1.5-mL volume, adding epinephrine to the lower volume of mepivacaine would enhance the potency of the smaller volume so that its anesthetic effect would be equivalent to that of a 1.5-mL volume.
Methods
Animals
Six naturally lame, gelded horses, ranging in age from 4 to 24 years (mean ± SD, 16.8 ± 7.3 years) and body weight from 510 to 639 kg (mean ± SD, 565 ± 62 kg) from the university’s horse herd were enrolled in the study. Each of these horses (2 American Quarter Horses, 3 Thoroughbreds, and 1 Warmblood) had previously been determined, by using visual assessment and diagnostic anesthesia, to be lame because of pain isolated to a forefoot. Study procedures were reviewed and approved by our college’s IACUC (protocol No. 2023–5212).
Experimental protocol
The study had a crossover design. Horses were randomly assigned to 3 groups, with 2 horses in each group. Each group was subjected to each of the 3 treatments, allowing 48 hours between treatments. The horses were trotted in a straight line on an asphalt surface for 30 to 40 strides. Lameness was identified and quantified by using an inertial, sensor-based motion-analysis system. After scoring the baseline lameness, a medial and lateral PDNB was administered to the lame forelimb while restraining the horse with a nose twitch. After applying 70% isopropyl alcohol to the sites of injection, each palmar digital nerve of the lame limb was injected with 1.5 mL 2% mepivacaine alone, 0.75 mL 2% mepivacaine alone, or 0.75 mL of 2% mepivacaine with epinephrine (1:200,000). Local anesthetic was administered through a 25-gauge, 16-mm needle to deposit solution perineurally near the level of the dorsal margin of the cartilage of the foot. To create the 2% mepivacaine plus epinephrine solution, 0.1 mL of a 1:1,000 epinephrine solution (ie, 1 mg/mL) was added to 20 mL 2% mepivacaine to create a solution with a 1:200,000 dilution. Epinephrine was added to mepivacaine immediately before beginning each lameness trial requiring this treatment. A 3-mL syringe was used to administer the 1.5-mL volume of local anesthetic, and a tuberculin syringe was used to administer the 0.75-mL volume. One clinician (JS) administered all nerve blocks. Gait was reevaluated at 5-minute intervals for 15 minutes, beginning 5 minutes after the nerve block.
Objective lameness evaluation
Gait was analyzed by using a body-mounted inertial-sensor system (Q with Lameness Locator; Equinosis LLC). The horses were trotted in a straight line on an asphalt surface for 30 to 40 strides, led from the left by a handler with a lead shank attached to a halter. For these evaluations, each horse was instrumented with sensor devices placed and secured to the head, pastern of the right forelimb, and pelvis in accordance with the manufacturer’s recommendations and as described elsewhere.22–28 To keep the speed of movement consistent, the same handler was used for all evaluations.
The nature of the forelimb lameness was measured as the vector sum (VS) of the difference between the local maximum head height before right forelimb stance and the local maximum head height before left forelimb stance (MaxHDiff) and the difference between the local minimum head height during right forelimb stance and the local minimum head height during left forelimb stance (MinHDiff) (ie, VS = [MaxHDiff2 + MinHDiff2]1/2), both of which are measures of asymmetric head height during strides of the right and left forelimbs. The VS is an overall measurement of the magnitude of forelimb lameness and is given a positive value if MinHDiff is > 0, indicating the site pain causing lameness is in the right forelimb, or a negative value if MinHDiff is < 0, indicating the site of pain causing lameness is in the left forelimb. An estimate of the reference range of VS between evidence of forelimb lameness and no evidence of forelimb lameness, based on the 95% CIs of the y-intercepts of MaxHDiff and MinHDiff (both of which are ± 6 mm), is ± 8.5 mm.26 Data collected from vertical pelvic movement were not used because we were not concerned with hind limb lameness in this study. The described methodology,28–31 measures,22,25,32,33 and thresholds23–25 used to evaluate forelimb lameness of horses have been previously described.
Statistical analyses
where Y was the vector of observations’ X was the treatment design matrix (treatment with 0.75 mL mepivacaine, 1.5 mL mepivacaine, or mepivacaine/epinephrine; treatment sequence; and lameness evaluations at pretreatment and 5, 10, and 15 minutes post-treatment), β was the vector of fixed treatment effects, Z was the random effects design matrix (horse), μ was the vector of random block effects, and e was the vector of experimental error. A first-order, autoregressive covariance structure was used to account for the nonindependence of observations within horses repeatedly measured.34,35 Models were compared using the Akaike information criterion.34–36 Horse was included in models as a random effect.34–36 The Kenward-Roger correction was used for all models.34–36 The level of significance was set at ≤ 0.05. All statistics were performed using commercially available software (PROC MIXED, SAS, version 9.4; SAS Institute Inc).
Results
Treatments (0.75 mL mepivacaine, 1.5 mL mepivacaine, and 0.75 mL mepivacaine/epinephrine) were compared using a repeated measures mixed linear model as described in Methods. The model was not improved by treatment sequence or treatment*time of lameness evaluation interaction and was eliminated from the final model. Time of lameness evaluation (P = .045) was included in the final model. Treatment was forced into the final model as the primary interest of the study. The final model indicated that treatments were not significantly different (P = .202).
The effect of treatments on VS were then compared to pretreatment VS for each treatment. Treatment with 0.75 mL mepivacaine significantly reduced VS at all post-treatment time points (5 minutes, P = .0011; 10 minutes, P = .0010; 15 minutes, P = .0133; 5 to 15 minutes, P = .0006). Treatment with 1.5 mL mepivacaine also significantly reduced VS at all post-treatment time points (5 minutes, P = .0047; 10 minutes, P = .0030; 15 minutes, P = .0041; 5 to 15 minutes, P = .0012). Treatment with 0.75 mL mepivacaine with epinephrine also significantly reduced VS at all post-treatment time points (5 minutes, P = .0005; 10 minutes, P = .0003; 15 minutes, P = .0023; 5 to 15 minutes, P = .0002). Mean VS ± SD for each treatment and time of lameness evaluation is presented in Figure 1.
Mean ± SD decrease in the vector sum (VS) from baseline (0 minutes) for 6 horses with naturally occurring lameness localized to a forefoot at 5-minute intervals after administration of a palmar digital nerve block with 1.5 mL 2% mepivacaine (dotted line), 0.75 mL 2% mepivacaine (solid line), and 0.75 mL 2% mepivacaine plus epinephrine (dashed line). Each horse received each treatment with a 48-hour interval between treatments.
Citation: American Journal of Veterinary Research 85, 12; 10.2460/ajvr.24.04.0107
No adverse effects, acute or chronic, associated with the PDNBs performed with epinephrine added to the local anesthetic were observed when horses were examined during the study and several weeks after the study.
Discussion
Our hypothesis that a low volume of mepivacaine would be as effective in ameliorating lameness caused by digital pain as a higher, commonly recommended volume was found to be correct. Because our first hypothesis was correct, our second hypothesis, that epinephrine added to mepivacaine would improve the anesthetic effect of a less effective low volume, was invalid for the low volume investigated.
The results indicate that, for the selected time points of this study, depositing 0.75 mL of 2% mepivacaine adjacent to each palmar digital nerve was just as effective as depositing 1.5 mL in ameliorating lameness caused by pain in the foot. Fifteen minutes was chosen as the endpoint of the study because a substantial reduction in lameness is usually observed within 15 minutes after performing a PDNB.7,37–39
The small number of horses used in this study may cast doubt concerning the validity of the data. The sample size was based on numerous previously published studies of similar design where 6 horses proved sufficient to achieve 0.05 significance with 80% power.20,40,41
As expected, no adverse effects, such as white hair growth, swelling, or skin necrosis at the site of injection, associated with the PDNBs containing 3.75 µg epinephrine/injection were observed. In a previous study,20 local anesthetic containing 7.5 µg epinephrine injected over each palmar digital nerve in horses caused no apparent adverse effects. A review of human medical literature indicates that local anesthetics containing epinephrine at a concentration of 1:200,000 or less do not cause necrosis and are safe to use for regional anesthesia.42,43
Published volumes of local anesthetics administered for diagnostic anesthesia appear to be arbitrary, and we are not aware of any study attempting to determine the volume of a particular local anesthetic necessary to anesthetize a nerve to desensitize the region of the body innervated by that nerve. A local anesthetic must be deposited beneath the circumneural sheath, within the subcircumneural space, to rapidly anesthetize a nerve.38,44–49 The circumneural sheath is a sleeve containing a nerve and vasculature. The more local anesthetic deposited within the subcircumneural space, the further proximal within the sleeve the local anesthetic is forced, perhaps resulting in desensitization of structures proximal to the site at which the anesthetic was administered.11,47
In 1 report, a PDNB performed using an unusually large volume of 2.5 mL of 2% mepivacaine per nerve caused a subjective improvement in lameness greater than 90% in 15 horses, all of which had lesions within the metacarpophalangeal joint.13 Based on results of MRI, pain caused by lesions in the metacarpophalangeal joint, rather than pain in the foot, were assumed to be responsible for lameness. Other clinicians have had similar experiences with a PDNB failing to localize lameness to the foot, even when smaller volumes (eg, 1.5 to 2 mL) of local anesthetic were injected adjacent to each palmar digital nerve.10 The authors of those reports recommended intrathecal or intra-articular blocks to verify the site of pain to ensure that the proper anatomic site is examined when using an expensive imaging modality, such as MRI. We believe that administering the smallest effective volume of a local anesthetic, rather than commonly recommended volumes, could also be used to localize the site of pain more specifically.
Because local anesthetics may differ in potency, the smallest effective volume administered is likely to vary among local anesthetics. For instance, 2% lidocaine appears to be less potent than 2% mepivacaine, 0.5% bupivacaine, or 7.5% ropivacaine for anesthesia of the palmar digital nerves when administered at the same volume.39,50 It seems reasonable to assume that a small volume of a potent local anesthetic would better localize the site of pain than would a larger volume of a less potent local anesthetic. A large volume of local anesthetic may be warranted if alleviation of pain for humane reasons is the purpose of a nerve block. For diagnostic anesthesia, a balance should be struck between a low volume of local anesthetic that can accurately identify a region of the body that is the site of pain causing lameness, yet sufficiently alleviate pain to the extent that pain at other sites, if present, becomes apparent. As an example, a low volume of local anesthetic might accurately localize pain to the foot yet insufficiently alleviate foot pain to make lameness caused by pain in the other forefoot apparent.
Perhaps the 0.75-mL volume of mepivacaine per nerve we used in this study for a PDNB could be further reduced while still providing sufficient desensitization for accurate interpretation of the nerve block. The minimal effective volume of local anesthetics for diagnostic neural anesthesia of the horse should be a subject for investigation.
Acknowledgments
None reported.
Disclosures
The authors have nothing to disclose. No AI-assisted technologies were used in the generation of this manuscript.
Funding
The authors have nothing to disclose.
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