Article
Sensory nerve conduction and nociception in the equine lower forelimb
during perineural bupivacaine infusion along the palmar nerves
Laura Zarucco, Bernd Driessen, Massimiliano Scandella, Francesca Cozzi, Carlo Cantile
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% NaHCO3 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 mm) 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.
Résumé
L’objectif de cette étude était d’étudier la morphologie du nerf palmaire latérale (LPN) et médiale (MPN) et de déterminer la nociception et
la vélocité de conduction du nerf sensitif (SNCV) suite à la mise en place de cathéters pour un bloc continu du nerf périphérique (CPNB)
le long de LPN et MPN avec des infusions subséquentes de buvicaïne (BUP). La distribution des fibres nerveuses myélinisées dans LPN et
MPN a été examinée après la récolte de spécimens de nerf chez 3 chevaux anesthésiés et en les traitant pour analyse morphométrique. Chez
5 chevaux sous sédation, des cathéters CPNB ont été placés le long de chaque PN dans les deux pattes antérieures. Les chevaux ont par la suite
reçu dans une des pattes avant 3 mL de BUP 0,125 % contenant de l’épinéphrine 1:200,000 et 0,04 % de NaHCO3 par site de cathéterisation
suivi par une infusion à raison de 2 mL/h pendant une période de 6 jours, alors que dans l’autre patte avant des volumes égaux de saline
(SAL) étaient administrés. Le seuil de réponse du retrait du sabot (HWR) pendant une charge de pression de la région au-dessus de la bande
coronaire dorsale a été déterminé quotidiennement pour les deux pattes avant. Au jour 6 la SNCV du LPN et MPN de chaque membre a
été mesurée sous anesthésie générale des chevaux afin de détecter des dommages nerveux, suivi du retrait du cathéter du CPNB. La SNCV
a également été enregistrée chez 2 chevaux anesthésiés non-instrumentés (témoins simulés). La distribution des fibres myélinisées dans les
LPN et MPN était bimodale. La fraction fibres larges (. 7 mm) était plus grande dans le MPN comparativement au LPN (P , 0,05). La
présence de cathéters CPNB et l’administration de SAL n’a nullement affecté les seuils mesurés de HWR ni de SNCV, alors que l’infusion
de BUP a supprimé les HWR. En conclusion, un CPNB avec 0,125 % de BUP fourni une analgésie prononcée en inhibant la conduction
des nerfs sensitifs dans la patte avant équine distale.
(Traduit par Docteur Serge Messier)
Dipartimento di Patologia Animale, Facoltà di Medicina Veterinaria, Università degli Studi di Torino, Italy (Zarucco); Department of Clinical
Studies-New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, USA (Zarucco, Driessen); University
of California-Los Angeles, David Geffen School of Medicine, Department of Anesthesiology, Los Angeles, California 90095, USA (Driessen);
Dipartimento di Scienze Cliniche Veterinarie, Facoltà di Medicina Veterinaria, Università degli Studi di Milano, Italy (Scandella); Clinica
Veterinaria Milano Sud, Peschiera Borromeo, (Milan), Italy (Cozzi); Dipartimento di Patologia Animale, Profilassi ed Igiene degli Alimenti,
Facoltà di Medicina Veterinaria, Università degli Studi di Pisa, Italy (Cantile).
Address all correspondence to Dr. Bernd Driessen; telephone: +1-610-925-6130; fax: +1-610-925-6820; e-mail: [email protected]
Received July 14, 2009. Accepted October 1, 2009.
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Introduction
Pain therapy is an important aspect of equine orthopedic and
trauma surgery. While long-term epidural analgesia has become a
frequently used method of treatment for severely painful conditions
in the horse’s pelvic limbs (1), a similarly effective technique for
management of pain in the horse’s front limbs is lacking. Systemic
analgesia does often not provide sufficient pain relief and is commonly associated with significant side effects (2). If persistent,
mechanisms described as peripheral and central sensitization (‘windup’) alter the pain experience of the patient, transforming physiological pain into maladaptive pain. Based on current knowledge,
only blockade of afferent sensory impulse flow can reliably prevent
or curtail central sensitization (2).
To improve pain management in the equine, we searched for a
loco-regional technique more suitable than systemic analgesia for
treating horses suffering from severe acute or chronic pain in their
distal forelimbs. Continuous peripheral nerve blockade (CPNB)
is a treatment modality that has been long introduced in human
medicine and is currently widely applied in orthopedic and trauma
surgery. The technique entails continuous or intermittent low-dose
administration of local anesthetics (LAs) via catheters placed along
peripheral nerves, thus providing pain control during and following
surgery in the extremities while reducing the need of systemic medications (3,4). We recently developed a technique for percutaneous
placement of catheters along the palmar nerves (PN) in the standing,
sedated horse, proved the feasibility of applying the CPNB method
in horses, and discussed possible indications and complications of
this technique (5,6).
The main goals of the present study were 1) to extend the current
knowledge of the morphology of the palmar nerves of the horse to
the lateral (LPN) and medial palmar nerves (MPN) at the anticipated
locations of CPNB (close to the communicating branch) to possibly
better predict LA efficacy; 2) to study the effect of a continuous lowdose bupivacaine (BUP) infusion along the PNs on nociception, that
is, the hoof-withdrawal response (HWR) after mechanical stimulation; and 3) to evaluate whether long-term placement of CPNB
catheters negatively affects nerve impulse conduction velocity as
an early indicator of axonal neuropathy.
Materials and methods
Nerve morphometric study
Nerve samples were collected and processed for morphometric
analysis with the objective of studying myelinated nerve fiber (MF)
distribution in palmar nerves in the area where LAs would be
infused during CPNB.
Sample population and nerve harvest
After approval by the Institutional Animal Care and Use
Committee at the University of Pennsylvania, nerve samples were
harvested from 3 horses (1 Thoroughbred, 1 Warmblood, 1 Morgan;
2 geldings, 1 female; 19 to 27 years old and weighing 509 to 673 kg),
which for health reasons unrelated to the present study had to be
euthanized.
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In all 3 horses an intravenous (IV) catheter (14 ga, 13.3 cm;
Angiocath; Becton Dickinson Infusion Therapy Systems, Sandy,
Utah, USA) was placed under aseptic conditions in the left jugular
vein and the animals were sedated with IV xylazine (Rompun;
Bayer Corporation, Shawnee Mission, Kansas, USA), 0.6 mg/kg.
Anesthesia was induced 5 to 10 min later by administration of IV
diazepam (Diazepam; Abbott Laboratories, Chicago, Illinois, USA)
0.15 mg/kg and IV ketamine (Ketaset; Fort Dodge Laboratories,
Fort Dodge, Iowa, USA), 2.5 mg/kg. Animals were subsequently
orotracheally intubated and oxygen supplemented at a flow rate
of 15 L/min. Anesthesia was maintained for the period of nerve
harvesting by infusing a 5% guaifenesin solution (Guailaxin;
Fort Dodge Laboratories) containing detomidine (Domosedan;
Pfizer Animal Health, Exton, Pennsylvania, USA), 0.03 mg/mL plus
ketamine (2 mg/mL) to effect at a rate of 1 to 5 mL/kg per hour. In
each horse, skin was incised and subcutaneous and muscle tissues
were carefully dissected to harvest the 5 to 6 cm long specimens from
the LPN and MPN just proximal to the communicating branch in
each forelimb following a technique previously described (7). Nerve
specimens were gently stretched out on small pieces of wooden
tongue depressors, longitudinally sectioned in half with a sharp
blade and then submerged into the fixation solution (Karnovsky’s
Fixative) (8). Following tissue harvest, horses were euthanized with
a barbiturate overdose (Beuthanasia-D; Schering, Kenilworth, New
Jersey, USA).
Morphometric analysis
Fixed nerve specimens were embedded in plastic blocks and
transverse semithin (1 mm) sections were cut from each block.
Sections were photographed in 3 different areas selected randomly.
Depending on the diameter of the harvested nerve sample, 71 to
112 fibers were measured from each photograph using a computerized video-morphometric unit display image analysis system
for quantitative studies (Quantimet 400 analyzer system; Leica
Microsystem S.p.A., Milan, Italy). Diameter distributions were plotted, and the mean, median, and peak diameters were calculated. The
percentage of large diameter fibers as an assessment of motor fiber
composition were calculated based on data from the literature which
considered “large fibers” the MFs . 7 mm in diameter (7).
In-vivo study
Validation of sensory nerve conduction velocity recording
In preliminary experiments we measured SNCVs in the palmar
nerves of the equine forelimb in 2 anesthetized Standardbreds (a
3-year-old mare weighing 390 kg and a 4-year-old gelding weighing
490 kg) in which no CPNB catheters were placed and thus served as
sham controls. These experiments were conducted at the University
of Bari, Italy in compliance with the Italian Animal Welfare Act and
statutes of the School of Veterinary Medicine at the University of
Bari. Horses were placed under general anesthesia and then instrumented for subsequent SNCV recordings. Both the anesthesia and
SNCV recording protocols were as described below for animals in
which CPNB catheters had been placed.
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Continuous peripheral nerve block (CPNB) experiments
The core part of the in-vivo study was conducted in 5 horses after
approval of the Institutional Animal Care and Use Committee at the
University of Pennsylvania. The animals (1 Thoroughbred gelding,
3 Thoroughbred mares, 1 Warmblood gelding) were 3- to 9-years-old
and weighed 456 to 675 kg. They were judged systemically healthy
based on results of a physical examination and did not show lameness in their forelimbs.
CPNB catheter placement
On day 0 of the experiment, a lateral and a medial palmar CPNB
catheter were inserted in both front limbs of each horse using an
approach described previously in detail (6). In brief, after clipping
both front limbs and placement of an IV catheter in the left jugular vein horses initially received IV acepromazine 35 to 45 mg/kg
(PromAce; Fort Dodge Laboratories) followed 20 to 30 min later by
an IV bolus of medetomidine 4 to 6 mg/kg (Domitor; Pfizer Animal
Health) combined with butorphanol 15 to 20 mg/kg (Torbugesic;
Fort Dodge Laboratories). Sedation was maintained by a continuous
rate infusion (CRI) of medetomidine titrated to effect (usually at a
rate of 0.05 mg/kg per minute, but intermittently being as low as
0.005 mg/kg per minute or as high as 0.2 mg/kg per minute.
Once adequately sedated, the skin insertion sites for the
CPNB catheters were infiltrated with 3 mL of mepivacaine 2%
(Carbocaine-V, Company Division of Pfizer, New York, New York).
Subsequently, a Tuohy-Schliff needle [18 ga (1.3 mm I.D.), 8.9 cm,
B. Braun, Bethlehem, Pennsylvania, USA] was inserted through the
skin, advanced for 2.5 to 3.0 cm into the subcutis, then redirected
medially and cautiously pushed through the palmar fascia and further advanced distally until its tip reached a point distal to the communicating branch. Subsequently, a CPNB catheter [Perifix closed
tip, radiopaque polyamide catheter; 20 ga (0.80 mm I.D.), 104 cm,
0.26 mL filling volume; B. Braun] was threaded through the Tuohy
needle and advanced 2 to 3 cm beyond the needle tip. Thereafter,
the Tuohy needle was withdrawn and the CPNB catheter secured
in place by tunneling its free (proximal) ending under the skin and
fixing the resulting catheter loop to the skin. The free end of the
catheter was eventually capped and its skin insertion site covered
with a sterile gauze pad and bandaged for later connection to mini
infusion pumps.
Correct positioning of the distal portion of the CPNB catheters
in their target locations (parallel to the lateral and medial palmar
nerves and just distal to the communicating branch) was verified
by an initial bolus of SAL or diluted LA test solution through the
catheter under ultrasound control to enhance visualization of the
catheter tip. Images were recorded with a portable ultrasound (US)
scanner (My Lab 30Vet; Esaote Piemedical, Indianapolis, Indiana,
USA) operating with a 12 MHz linear probe.
All horses were observed 15 d following CPNB catheter removal
in order to monitor for any potential late complication.
Drug administration
The free endings of the CPNB catheters were connected to reusable
lightweight, battery-driven ambulatory infusion pumps (Ace Medical
Automed 3400; Curlin Medical, Huntington Beach, California, USA)
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by means of proprietary tubing (AM-360/370; Curlin Medical). The
pumps as well as the drug-containing bags and accompanying tubing were safely placed in horse boots (Ice Horse insulated leg wraps,
Mackinnon Ice Horse, San Diego, California, USA).
All horses received, based on random selection, a bolus of 3 mL
SAL containing 0.04% NaHCO3 and epinephrine 1:200 000 (SAL
group) in one front limb. In the other forelimb, they received 3 mL
of BUP (Bupivacaine HCl USP, Hospira, Lake Forest, Illinois, USA)
0.125% containing 0.04% NaHCO3 and epinephrine 1:200 000 (BUP
group). Delivered through each CPNB catheter, this was followed by
a constant rate infusion (CRI) of 2 mL/h of SAL and BUP solution,
respectively for 6 d, after which CPNB catheters were removed. Test
solutions were supplemented with epinephrine 1:200 000 (5 mg/mL
LA solution) to prolong the duration of local anesthetic action while
diminishing vasodilatatory effects (9,10). Given the strong acidic pH
of 0.125% BUP solution (pH 3.97) (11), 0.1 mL NaHCO3 8.4% was
added to each 20 mL of BUP solution, yielding a final pH of 7.32;
the NaHCO3 additive also helped to reduce the time to onset of the
local anesthetic effect as reported in the literature (12).
Hoof-withdrawal response evaluation
The antinociceptive efficacy of continuous low-dose BUP versus
SAL infusion was assessed. In all 5 horses the hoof-withdrawal
response (HWR), (lifting of the hoof from the ground in response to
noxious stimulation), was elicited by mechanical stimulation of the
skin a few centimeters above the median dorsal coronary band in
both forelimbs using a calibrated, spring-loaded pressure algometer
(Chatillon type-719 dynamometer; Chatillon Precision Instruments,
New York, New York, USA) and progressive increase in stimulus
intensity. The sequence of HWR testing in the forelimbs (SAL versus
BUP) was randomized. The skin above the dorsal median coronary
band belongs to an overlap zone supplied by both the lateral and
medial palmar nerves (13,14). The HWR test was performed daily
in the late morning by the same 2 examiners beginning 2 d prior to
CPNB catheter instrumentation (Day 22) and extending to day 21
after catheter placement (day 0 being the day of instrumentation;
day 21 the time of discharge of the horses back to their pastures;
see Table I). The evaluators were not blinded to treatment as the
effect of LA compared to SAL infusion proved to be too obvious
in preliminary experiments. The pressure (in kg) that evoked hoof
lifting was recorded as HWR threshold, with maximum mechanical
load limited to 6 kg to avoid persistent skin and deep tissue injury.
Each forelimb was tested only once because preliminary tests had
demonstrated that sensitivity towards mechanical stimulation in the
SAL forelimb was reliably greater upon second stimulation.
Sensory nerve conduction velocity (SNCV)
measurements
Anesthesia
In all 5 horses, in the late afternoon of day 6 post CPNB catheter placement an IV catheter (14 ga, 13.3 cm; Angiocath; Becton
Dickinson Infusion Therapy Systems) was placed under aseptic
conditions in the left jugular vein. Animals were sedated with IV
acepromazine (PromAce; Fort Dodge Laboratories), 20 mg/kg and
xylazine (Rompun; Bayer Corporation), 0.4 mg/kg and then placed
in a sling (Liftex sling; Liftex, Warminster, Pennsylvania, USA)
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before anesthesia was induced by IV administration of 5% guaifenesin (Guailaxin; Fort Dodge Laboratories), 40 mg/kg, diazepam
(Diazepam; Abbott Laboratories), 0.1 mg/kg, and IV ketamine
(Ketaset; Fort Dodge Laboratories), 2.0 mg/kg. Animals were
subsequently intubated using an orotracheal tube (ID 26 mm)
and placed on a padded large animal surgery table in right lateral
recumbency. Anesthesia was maintained with isoflurane (Isoflo;
Abbott Laboratories) in oxygen using a large animal anesthetic
circle system (North American Dräger, Telford, Pennsylvania, USA).
All animals were mechanically ventilated using a large animal respirator (Narkovet Large Animal Control Center, North American
Dräger). Horses were instrumented and anesthetic depth and
cardio-respiratory functions (heart rate, invasive blood pressures,
arterial blood gases) monitored. Crystalloid solutions were infused
intravenously at a rate of 5 to 10 mL/kg per hour. Hemodynamic
support also included IV infusion of 0.1 % dobutamine (Dobutrex;
Abbott Laboratories) at a rate of 0.5 to 5 mg/kg per minute, which
was discontinued when mean arterial blood pressure reached values
. 85 mmHg or heart rate increased to . 60 beats/min.
Following the removal of CPNB catheters legs were bandaged
and horses moved to a recovery stall to wake up from anesthesia.
Electro-diagnostic evaluation
Potential injury to the palmar nerves from perineural CPNB
catheter placement was determined by measuring SNCV under
general anesthesia in all 5 horses on day 6 prior to discontinuation
of SAL or BUP infusion and CPNB catheter removal, always beginning with the dependent forelimb to maintain randomization. The
method used is described in detail in the literature (14–18). In brief,
anesthetized horses were positioned on a surgery table in right
lateral recumbency. Ulnar and median nerves were selected for
recording impulse conduction because it was difficult to find a site
below the carpus far enough proximal to the skin insertion points
of the CPNB catheters to record from the palmar nerves themselves.
A portable electromyography (EMG) unit equipped with a nerve
stimulator and a digital signal averager (NCS/EMG system; Medelec
Synergy Notebook EMG, Viasys Health Care, Milan, Italy) was used,
and Teflon-insulated, sterile monopolar needle electrodes (Dantec
Dynamics, Viasys Health Care) were chosen for both stimulation
and recording. Stimulating and recording (exploring) electrodes
were placed percutaneously after localizing the corresponding
nerves by digital palpation (PNs) or ultrasonographically (ulnar
and median nerves) (19) with the previously mentioned US scanner
and probe. Nerve activities were registered for LPN and MPN in
both CPNB catheter-instrumented forelimbs, with the SAL-infused
limb serving as control. The active recording (exploring) electrodes
were positioned under US guidance as close as possible to the
ulnar and median nerves, respectively. The stimulation site for the
lateral digital palmar nerve was at the base of the fetlock, on the
abaxial surface of the lateral sesamoid bone; stimulation needles
were inserted parallel to each other with an inter-electrode distance
of 5 mm, anode being distal to the cathode. The optimal recording site for the ulnar nerve was found on the caudal aspect of the
antebrachium, approximately 10 to 12 cm proximal to the accessory
carpal bone, where the electrodes were inserted between the flexor
carpi ulnaris and ulnaris lateralis muscles and positioned under US
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control. The stimulation site for the medial digital palmar nerve was
located at the base of the fetlock, that is, on the abaxial surface of
the medial sesamoid bone. Needles were inserted parallel to each
other at 5 mm distance, anode being distal to cathode. The site for
recording from the median nerve was located on the medial side
of the mid antebrachium just caudal to the cephalic vein, and the
active recording needle was directed laterally through the distal 1/3
of the flexor carpi radialis muscle along the caudo-medial surface
of the radius. The reference electrode was placed linearly along the
course of the respective nerve approximately 2 to 3 cm apart from
the active recording (exploring) needle (20,21). A needle electrode
with SAL-soaked cotton around it served as the ground electrode,
and was placed midway between the stimulating and the recording electrodes for both nerve recorSingle stimuli of 0.2 ms duration
were generated at a frequency of 10 Hz with increasing intensity,
and individual waveforms were averaged. A threshold (minimum
voltage required to generate a triphasic waveform as depicted in
Figure 1) was determined each time, and a 23 threshold voltage was
then applied for SNCV determinations. Individual nerve recordings
were obtained by averaging 640 to 800 traces.
The distance between the stimulating (cathode) and recording
(exploring) electrode (distance B-C in Figure 1) was measured with
a flexible measure tape placed on the skin along the nerve’s course to
minimize error. Latencies were measured from the stimulus artifact
to the first positive deflection on the compound action potential,
and nerve conduction velocities (in m/s) were computed by the
NCS/EMG unit.
Skin temperature was measured with an infrared temperature
probe (T 7350 Infrared thermometer, Professional Equipment,
Hauppauge, New York, USA) kept at a distance of 15 cm, and velocities were corrected to 33°C by the factor of 2.15 m/s per °C (22).
In one horse a baseline electrophysiological recording in a SALinfused forelimb (medial digital palmar to median nerve) was
obtained for a second time and then a bolus of 3 mL of the 0.125%
BUP solution was administered over 4 min followed by a 2-mL/h
CRI of the same LA solution. Subsequently SNCV was repeatedly
recorded at intervals of 3 min (Figure 1B).
S t a t i s t i c a l a n a l ys i s
Results of the morphometric study are given as median and
mean 6 standard deviation (s). Statistical analysis of morphometric
data included a t-test to determine whether there were differences
in nerve fiber composition between LPN and MPN. Results of
the in-vivo experiments are presented as arithmetic means 6 s.
Significant change over time of HWR thresholds measured in each
forelimb were determined with the Friedman’s analysis of variance
by ranks followed by post-hoc pair-wise comparisons to day 0.
Significant differences between limbs at specific time points were
determined by the Kruskal–Wallis sign rank test. Sensory nerve
conduction velocities recorded in lateral and medial palmar nerves
in sham controls and CPNB-catheter instrumented horses were
analyzed for in-between nerve and in-between horse groups using
a two-sided t-test. A P , 0.05 was taken to indicate statistically
significant differences. All analyses were performed with WinStAT
(for Windows Excel).
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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.
Results
Morphometric study
Myelinated nerve fiber distributions in the LPN were bimodal in
all 3 horses with the median diameter amounting to 7.11 and 7.78 mm
for the lower and upper peak, respectively, and the mean diameter to
7.82 6 3.75 and 8.39 6 3.94 mm, respectively. The proportion of large
fibers (. 7 mm in diameter) ranged from 53.9% to 61.4%. Myelinated
nerve fiber distributions in the MPN were also bimodal, although in
1 horse both lower and upper peaks were . 7 mm. Median diameter
ranged from 8.25 to 9.13 mm, and the mean diameter from 8.26 6 3.92
to 9.30 6 3.64 mm; the proportion of large fibers (. 7 mm ID) ranged
from 63.4 to 79.4%. Lateral and medial PN specimens did not differ
significantly from each other in mean fiber diameters measured but
the proportion of large fibers was significantly higher in specimens
of the MPN (P , 0.05).
In vivo study
Continuous peripheral nerve block (CPNB) experiments
The CPNB catheters could be correctly placed along the LPN and
MPN in all 5 horses as US imaging demonstrated. All animals tolerated the CPNB catheters very well. The subsequent constant rate
2000;64:0–00
administration of SAL or BUP solutions over the 6-day period did
not cause apparent discomfort or difficulty for the horses to walk
around in their stalls or to lie down in sternal or lateral recumbency.
Daily US examinations indicated that during the study period
catheters did not always stay in their original position but moved
slightly up or down along the PNs or, as observed in 2 cases, formed
loops shifting the tip of the catheters a little away from the PNs.
However, this did not affect the efficacy of delivering either SAL or
BUP solutions to the perineural area. None of the catheters placed
in the forelimbs of the study horses slipped out during this period.
Following catheter removal mild skin rash and tissue swelling
occurred which was more prominent in forelimbs that had been BUP
infused. The degree of swelling differed from animal to animal but
was successfully treated with leg wraps and topical 1% silver sulfadiazine cream (Thermazene; Tyco Healthcare Group LP, Mansfield,
Massachusetts, USA) for a few days following catheter removal.
Hoof-withdrawal response evaluation
Prior to CPNB catheter instrumentation on day 0, a HWR could
be immediately elicited in all horses by applying mild pressure with
the algometer a few centimeters above the dorsal coronary band. In
the SAL-infused forelimb, the mechanical HWR threshold recorded
in the days prior to and following CPNB catheter placement did not
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309
differ significantly from values recorded on day 0 (Table I). However,
the mechanical HWR threshold decreased somewhat after CPNB
catheter removal and was significantly lower on day 8 compared to
day 0 (Table I). While the HWR threshold was not significantly different between the SAL- and BUP-infused limbs in the days prior to
instrumentation, a significant difference in the nocifensive response
to mechanical stimulation became obvious in the days following
CPNB catheter instrumentation and infusion of the LA solution
(Table I). From the second day of BUP infusion onwards (day 3) up
to day 6 no HWR could be evoked anymore with a pressure load of
up to 6 kg. After cessation of the LA infusion and removal of CPNB
catheters, the HWR threshold slowly returned to baseline over the
subsequent 5 d and by day 21, when the animals left the study, did
not differ anymore from the value recorded on day 0 or in the contralateral (control) limb (Table I).
Electro-diagnostic evaluation
We could obtain SNCV recordings in the 2 sham controls that
were not instrumented with CPNB catheters. The compound sensory action potentials recorded from the ulnar and median nerves,
respectively had the typical triphasic appearance (16), the first being
a positive deflection (downward deflection based on electrophysiological convention) (cf. Figure 1 A, B left hand panels). Pooling data
from both limbs in each horse, sensory nerve impulses were conducted from the LPN to the ulnar nerve at an average speed of 54.4 6
2.7 m/s, while from the MPN to the median nerve at a significantly
higher average speed of 65.4 6 4.8 m/s (n 5 4; P , 0.05; see Table II
for individual horses’ data).
Nerve impulse conduction along the PNs was also being
recorded in all SAL-infused forelimbs of the 5 study horses. The
average SNCVs were 57.7 6 3.9 m/s for the LPN to ulnar nerve
and 62.9 6 4.3 m/s for the MPN to median nerve (see Table II for
individual horses’ data). They were statistically neither significantly
different from corresponding values recorded in sham controls, nor
in-between nerves (LPN versus MPN; P . 0.05). When data from
sham controls and SAL-infused forelimbs in study horses were
pooled, the average SNCV measured in both the LPN and MPN was
60.1 6 5.6 m/s (n 5 18), while separated by nerve SNCV amounted
to 56.2 6 3.7 and 64.0 6 4.4 m/s in the LPN and MPN, respectively
(n 5 9; P , 0.05).
No waveforms and thus SNCV recordings could be obtained from
the LPN and MPN in forelimbs of the study horses that were infused
with BUP [Table II; Figure 1A (upper panel)].
When a 3-mL bolus of 0.125% BUP solution (containing 0.04%
NaHCO3 and epinephrine 1:200 000) was administered via the MPN
CPNB catheter followed by a 2-mL/h CRI of the same LA solution
in a previously SAL-infused forelimb, rapid sensory nerve impulse
conduction from the medial digital palmar nerve to the median nerve
started to decrease within 6 min and was completely abolished with
loss of a traceable waveform within 18 min (Figure 1B).
Discussion
The results of the present study support the idea that in horses
continuous perineural infusion along the PNs of a 0.125% BUP solution (supplemented with 0.04% NaHCO3 and epinephrine 1:200 000)
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Table I. Hoof withdrawal response (HWR) thresholds (kg) as
determined by mechanical stimulation of the skin few
centimeters above the median dorsal coronary band in
5 horses instrumented with CPNB catheters along the lateral
and medial palmar nerves
Day of Saline-infused
Bupivacaine
experiment
limb
0.125%-infused limb
Day22
1.95 6 1.46
1.40 6 0.89
Day21
1.50 6 1.46
1.90 6 1.52
Day 0
0.90 6 0.42
1.20 6 1.04
Day 1
2.14 6 2.34
5.48 6 1.39a,b
Day 2
1.50 6 1.97
. 6a,b
Day 3
1.80 6 1.40
. 6a,b
Day 4
1.82 6 2.43
. 6a,b
Day 5
0.80 6 0.27
. 6a,b
Day 6
1.10 6 0.55
. 6a,b
a
Day 7
0.80 6 0.27 4.54 6 2.32a,b
a
Day 8
0.50 6 0.00 4.16 6 2.71a,b
Day 21
0.60 6 0.22
0.80 6 0.67
HWR threshold determinations were made on 3 occasions
(days 22, 21, 0) prior to and on 9 occasions (days 1–21) following
catheter instrumentation and constant rate infusion of either saline
or bupivacaine 0.125% solutions containing 0.04% NaHCO 3 and
epinephrine 1:200 000 as indicated.
a Indicates differences between Day 0 and subsequent days
(P , 0.05).
b Indicates differences between saline and bupivacaine-infused limbs.
(P , 0.05).
provides effective antinociception for an extended time without
causing major side effects or nerve injury.
The choice and concentration of an LA solution to be used for
optimal conduction block efficacy as well as the rate of its perineural infusion necessary to maintain persistent inhibition of sensory
nerve conduction is — besides other factors — depend on the fiber
composition of the nerves at the site of the tip of the inserted CPNB
catheter (23,24), with un-myelinated fibers (UFs) representing mainly
C fibers. The potency of LAs depends on their physico-chemical
properties and concentration (25). For example, perineural BUP
0.025% depresses nerve impulse conduction in Aa, Ad, and C fibers
of the cat saphenous nerve by 28%, 48%, and 90%, respectively, while
BUP 0.05% inhibits conduction by 44%, 74%, and 96%, respectively
demonstrating a much weaker BUP effect on MFs (26). Thus, LA
efficacy might be predicted by the MF composition of the palmar
nerves at the site, where the LA solution is being administered during CPNB, especially when thin fibers (Ad fibers) that are associated
with sensory nerve impulse conduction predominate.
As reported in the literature, the PNs in the equine are principally
sensory nerves and, based on electron microscopy in the MPN, are
mostly composed of UFs (0.4 to 2.4 mm in diameter; UFs:MFs 5 4:1)
(27). In the LPN just proximal to the metacarpophalangeal joint
MFs, but not UFs, follow a bimodal distribution pattern in most
horses (7,27). The median diameter ranges from 5.18 to 9.76 mm and
the mean diameter from 6.20 6 2.27 mm to 9.30 6 3.33 mm for the
2000;64:0–00
Table II. Sensory nerve conduction velocity (SNCV) recorded for the lateral and medial palmar nerves in both forelimbs of
2 anesthetized control horses and 5 anesthetized study horses instrumented with CPNB catheters along the lateral and medial
palmar nerves
Control horses
CPNB catheter-instrumented study horses
Forelimb
Stimulating site
Recording site Horse 1 Horse 2 Horse 1 Horse 2 Horse 3 Horse 4 Horse 5
Right
Lateral digital nerve Ulnar nerve
54.3
54.3
Medial digital nerve Median nerve 70.6
59.2
Left
Lateral digital nerve Ulnar nerve
57.7
51.2
Medial digital nerve Median nerve 67.2
64.4
Saline-infused limb
Lateral digital nerve Ulnar nerve
53.4
59.4
63.6
56.1
56.1
Medial digital nerve Median nerve
62.5
59.1
58.3
66.9
67.5
Bupivacaine Lateral digital nerve Ulnar nerve
n.d.
n.d.
n.d.
n.d.
n.d.
0.125%-infused limb Medial digital nerve Median nerve
n.d.
n.d.
n.d.
n.d.
n.d.
SNCV measurements (m/s) were conducted under general anesthesia in 2 non-instrumented control horses and in 5 study horses, instrumented
in both forelimbs with continuous peripheral nerve block (CPNB) catheters along the palmar nerves. In study horses recordings were performed
on day 6 prior to discontinuation of saline (in 2 right and 3 left forelimbs) or bupivacaine solution infusion (in 3 right and 2 left forelimbs) and
CPNB catheter removal. Listed are the SNCV determinations for the palmar nerves of each horse’s forelimb; n.d. — no triphasic compound
action potential detected.
2 peaks, with larger fibers [. 7 mm in diameter (for example, Ab)]
representing 38.5% to 64.4% of all MFs (27). Since in the equine MPN
and LPN differ substantially in diameter, the former being almost
twice as thick as the latter (16), we were exploring the possibility
that the 2 nerves differ substantially in their distribution of small and
large MFs, which in turn could imply different sensitivity towards
the action of BUP or other LAs. Our morphometric data for the LPN
were in agreement with previous findings in horses (7,27). However,
we did not observe a significant difference in mean or median diameters of MFs between the MPN and LPN. Although the proportion
of larger MFs was significantly greater in the MPN than LPN, this
difference was overall small and likely not clinically relevant.
In order to avoid undesirable tissue swelling and minimize vasodilatation in LA-infused forelimbs that we had previously observed
in pilot experiments (5), we decided to lower the BUP concentration
from 0.5% to 0.125%. The solution was supplemented with epinephrine 1:200 000 (5 mg/mL LA solution) based on findings in rats
demonstrating a 7 times longer duration of action of epinephrinesupplemented levobupivacaine (9) and the common observation of
delayed drug absorption and enhanced duration and intensity of
most LAs used for peripheral nerve blockades (10, 28).
The results of the present study extend our previous experiences
with this CPNB technique in horses (5). The perineural catheters
were well-tolerated by all animals over the testing period of 6 d.
Furthermore, overall they migrated minimally and infusion of the
less concentrated BUP solution (unlike SAL infusion in the contralateral control limb) consistently abolished the nocifensive reaction
(HWR) to mechanical stimulation of tissues above the dorsal coronary
band, an area densely innervated by branches of both PNs (Table I).
In fact, a significant antinociceptive effect persisted for at least 2 d
following CPNB catheter removal, while in the SAL-infused control
limb a period of hypersensitivity to mechanical stimulation was noted
(Table I). The mild tissue swelling after CPNB catheter removal probably was associated with local tissue inflammation that temporarily
caused peripheral sensitization and a decreased HWR threshold.
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Standard sensory nerve conduction studies, as performed in these
experiments, reflect only activity in the largest and fastest fibers
while activity in UFs is not recorded (20). The SNCV recordings in
the PNs of the 2 sham controls produced reproducible data that were
very similar to recordings reported in the literature (14,15,17,22).
Furthermore, no significant differences were noted between SALinfused forelimbs and forelimbs of sham controls or literature
data, indicating that neither the presence of a CPNB catheter nor
epinephrine and NaHCO3 in the solvent caused axonal neuropathy.
Axon neuropathies and as a result slower nerve impulse conduction
typically affects thick MFs first (20).
When temperature-corrected SNCV values from sham and SALinfused control limbs were considered together, the average maximum velocity was 60 m/s. This value indicates not only that palmar
sensory nerve conduction function was normal in the tested animals
(17), but furthermore that recorded impulse activity was transmitted via fast-conducting sensory Ab fibers (30 to 120 m/s) within
the digital palmar nerves as also suggested in previous studies
(27,29).
When SNCV data collected in sham controls and SAL-infused
forelimbs were pooled, a 14% faster mean impulse conduction velocity was noted in the MPN versus LPN (64 versus 56 m/s). It appears
that this functional difference accords with the morphological finding of a slightly greater fraction of larger and hence faster conducting
MFs in the medial compared to lateral palmar nerve trunk.
In BUP-infused forelimbs none of the typical triphasic compound
action potentials could be recorded and hence SNCV not be determined (Table II, Figure 1), suggesting that the low BUP concentration and low infusion rate chosen were sufficient to effectively
block impulse conduction via the MFs that are less sensitive than
C fibers towards BUP (26,30). As shown in Figure 1B, the complete
local anesthetic effect (disappearance of the triphasic waveform of
the compound sensory action potential) occurred within 18 min,
which concurs with pharmacological data describing a 10 to 20 min
period for onset of peripheral nerve blockade with BUP 0.25% to
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311
0.75% (30). It may be concluded, therefore, that nociceptive signal
transmission was effectively suppressed under the present experimental conditions, albeit in our electro-diagnostic evaluation we did
not focus specifically on electrical activity in the slow conducting
nociceptive fibers.
The noxious stimuli in these experiments were rather brief and
may have activated a smaller or different group of afferents than
those that would be activated following tissue trauma and inflammation. It is not possible, therefore, to predict whether or not the
BUP concentration and infusion regimen used in this study would be
equally effective in clinical patients suffering from persistent inflammatory pain and/or tissue damage. In addition, discharge patterns of
nociceptive afferents typically change as a result of peripheral sensitization and recruitment of silent nociceptors (31,32), thus potentially
affecting the analgesic efficacy of a palmar CPNB with BUP.
The data herein show that our previously developed method
of continuous perineural block of the palmar nerves in the equine
can be successfully applied without causing major complications.
Perineural low-volume infusion of 0.125% BUP is associated with
less adverse effects than higher concentrated LA solutions, yet
promises to provide pronounced analgesia in horses suffering from
severe pain in their distal forelimbs.
Acknowledgments
This study was conducted with the generous support of the Morris
Animal Foundation (Grant D0EQ-024; P.I. B.D.) and intramural funds
of the Anesthesia Service of the Department of Clinical Studies at
New Bolton Center, School of Veterinary Medicine, University of
Pennsylvania.
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