Validation of mechanical, electrical and thermal nociceptive stimulation methods in horses

Antinociceptive and wound healing effects of a commercial formulation of lidocaine, bupivacaine, adrenaline and cetrimide applied topically to superficial skin wounds in horses

BACKGROUND

Post-traumatic distal limb wounds cause discomfort and heal gradually by second intention. The topical application of Tri-Solfen (lidocaine hydrochloride, bupivacaine hydrochloride, adrenaline acid tartrate and cetrimide [LBAC]) produces effective postsurgical cutaneous analgesia in lambs, calves and piglets; however, its effect on wounds in horses is unknown.

METHODS

The antinociceptive effect, measured by mechanical threshold (MT), and the wound healing impacts of LBAC compared with saline were investigated on surgically created 20 × 20 mm distal limb wounds in 10 horses. Treatment was applied once daily for 7 days following wounding on day 0. Mechanical thresholds were measured after treatment on days 1, 2 and 3. Healing was observed for 25 days.

RESULTS

The topical application of LBAC immediately following wounding and its reapplication 24 hours later increased the average MT on the first post-traumatic day by 3 Newtons. However, no antinociceptive benefit was observed on days 2 or 3. Treatment with LBAC did not adversely affect wound healing when compared with saline.

LIMITATIONS

Methodological differences preclude absolute MT comparisons between studies. The experimental design did not include a model of contaminated or naturally occurring wounds.

CONCLUSION

LBAC may provide an early antinociceptive benefit when applied to uncontaminated surgically created wounds without compromising healing.

Liposomal bupivacaine is both safe and effective when administered via local infiltration at surgical site and mesovarium for laparoscopic ovariectomy in mares

BACKGROUND

Liposomal local anaesthetic solutions may provide extended-duration analgesia postoperatively but have not been assessed following intra-peritoneal local infiltration in any species.

OBJECTIVES

To evaluate two doses of 1.33% liposomal bupivacaine (LB) versus 0.75% bupivacaine HCL (BHCl) for analgesia following laparoscopic ovariectomy in mares.

STUDY DESIGN

Prospective cohort study.

METHODS

Fifteen healthy Quarter Horse mares (age 2-20 years) with normal bilateral ovarian palpation and appearance were enrolled. Horses were restrained in standing stocks and administered an α-2 agonist, butorphanol, and flunixin meglumine, followed by a variable rate infusion of sedation with α-2 agonists. Bilateral paralumbar fossa ovariectomies were performed. Treatment with either 30 ml 0.75% BHCl followed by 20 or 40 ml LB 13.3% (LB20 and LB40) volume expanded with saline to 80 ml total (n = 6/group) or 80 ml BHCl alone (n = 3, BCHL) was infused around incision sites and each mesovarium (LB or BHCl) prior to ovariectomy. Horses were monitored for 72 h by physical examination, algometry, and pain scoring (composite pain scale by Bussieres et al., Horse Grimace Scale). Abdominocentesis with peritoneal fluid analysis was performed at 72 h.

RESULTS

Analgesia achieved with all treatment protocols allowed completion of ovariectomy procedures. Pressure algometry scores were lower in BHCl-treated horses versus both LB groups overall. Pain scores were improved with LB-treated horses in a dose-dependent fashion (Horse Grimace Scale scores LB40 < LB20 < BHCL; composite pain scale scores LB40 < BHCL, LB20 < BHCL, BHCL, and LB20 did not differ). Peritoneal fluid total protein was lower in LB40 versus LB20 and BHCL horses. No complications from LB administration were appreciated.

MAIN LIMITATIONS

Small patient sample size, lack of follow-up past 72 h or histopathology.

CONCLUSIONS

Analgesia duration was extended and pain scores improved postoperatively with LB versus BHCl in a dose-dependent fashion. Further clinical evaluation of extended-duration local anaesthetics in horses for improved postoperative pain management is warranted.

The Antinociceptive Effect of Magnesium Sulphate Administered in the Epidural Space in Standing Horses

To study the antinociceptive properties of epidural magnesium sulphate (MgSO₄) in standing horses Experimental, placebo-controlled, masked, cross-over A group of six healthy horses Through an epidural catheter, 1 mg kg ^-1 MgSO₄ (treatment Mg) diluted to a volume of 15 mL or the same volume of saline (treatment S) was administered over 15 minutes. Electrical, thermal and mechanical nociceptive thresholds were determined on the pelvic limb before and 20, 40, 60, 80, 100, 120, 140, 160 and 180 minutes after the start of the injection. Heart rate (HR) and respiratory frequency (f_R) were recorded every 10 minutes. Blood samples were collected before treatment and every 30 minutes throughout the study period. Data were assessed for normality using a Shapiro-Wilk test. A linear mixed model with horse as random effect and time, treatment and their interaction as fixed effects was used. Treatments were compared at 20, 60, 120 and 180 minutes using the Wilcoxon rank sum test stratified for horse (global α = 0.05, with Bonferroni correction α = 0.0125). Epidural MgSO₄ caused a significant increase in the electrical threshold (mA) (P = .0001), but no significant differences in thermal and mechanical nociceptive thresholds. During the injection of MgSO_4, two horses collapsed. One stood up within 20 minutes and was able to continue the study, the second one was excluded. A significant difference was found for HR at T180 (Mg 44 ± 23 beats minute^-1; S 32 ± 9 beats minute^-1) (P = .0090). Epidural administration of MgSO₄ caused an increase in the electrical threshold of the pelvic limbs of horses. Caution is warranted however, as with the current dose, 2 horses collapsed.

Intravenous magnesium sulphate in standing horses: effects on physiological parameters, plasma concentration of magnesium and nociceptive threshold tests

A bolus of 50 mg kg ^-1 MgSO₄ (treatment Mg) or the same volume of saline (treatment S) was infused over 15 minutes in 5 adult healthy horses. T0 was the end of the infusion. Physiological parameters were recorded throughout the study period. Measurements of electrical, thermal and mechanical nociceptive thresholds were performed at the pelvic limbs at baseline (before T0), and at specific timepoints. Blood samples were taken at fixed timepoints before, during and until 12 hours after the infusion. For statistical analysis, the 95% confidence intervals (CI's) for the differences in nociceptive thresholds between treatments were calculated. Physiological parameters were compared using a linear mixed model (global α = 0.05, with Bonferroni correction α = 0.0125). The concentrations of ions were also compared with baseline values at specific timepoints, using a linear mixed model. The Pearson's correlation coefficient was derived between the ion concentrations. The 95% CI's of thermal, mechanical and electrical thresholds were [-1; +2]°C, [0; +3] N and [-1; +1] mA (positive differences indicate higher thresholds for treatment Mg), respectively. Heart rate was significantly higher (p < 0.0001) and non-invasive systolic arterial pressure (p < 0.0001) and respiratory rate (p = 0.0002) significantly lower after treatment Mg compared to treatment S. Additionally, non-invasive systolic arterial pressure was significantly different at T45 (p < 0.001). Although mild changes in cardiovascular parameters and plasma concentrations were seen with intravenous administration of MgSO₄, no changes in nociceptive thresholds were detected in standing non-sedated horses.

Comparison of Threshold and Tolerance Nociceptive Withdrawal Reflexes in Horses

Simple Summary

Nociception is the physiological basis of the complex experience of pain. An established model for its quantification in equine studies is based on the nociceptive withdrawal reflex evoked by electrical stimulation of a sensory nerve. The reflex is recorded via electromyography and it is common to determine the threshold at which a nociceptive-specific reflex activity can be observed. In the present study, the classical methodology was expanded for a deeper understanding of the physiology of nociceptive reflexes in horses. First, for each individual horse, a threshold was determined as the minimal stimulation intensity able to evoke a nociceptive withdrawal reflex. Second, the stimulation intensity was stepwise increased up to tolerance, which was defined as the stimulus that is able to elicit the maximal tolerable behavioral reaction. The characteristics of the reflex activity on the electromyographic records were compared for threshold and tolerance stimulation intensities. At tolerance, the reflex became faster and wider than at threshold, indicating that either a spinal summation mechanism or the recruitment of faster sensory fibers occurs in response to high-intensity noxious stimuli. A novel endpoint (i.e., tolerance) can now be considered when applying the nociceptive withdrawal reflex model in equine studies.

Abstract

The nociceptive withdrawal reflex (NWR) is used to investigate nociception in horses. The NWR threshold is a classical model endpoint. The aims of this study were to determine NWR tolerance and to compare threshold and tolerance reflexes in horses. In 12 horses, the NWR was evoked through electrical stimulation of the digital nerve and recorded via electromyography from the deltoid. Behavioral reactions were scored from 0 to 5 (tolerance). First, the individual NWR threshold was defined, then stimulation intensity was increased to tolerance. The median NWR threshold was 7.0 mA, whereas NWR tolerance was 10.7 mA. Upon visual inspection of the records, two main reflex components R1 (median latency 44 ms) and R2 (median latency 81 ms) were identified at threshold. Increasing stimulation intensity to tolerance led to a significant increase in the amplitude and duration of R1 and R2, whereas their latency decreased. At tolerance, a single burst of early, high-amplitude reflex activity, with a median latency of 39 ms, was detected in 15 out of 23 stimulations (65%). The results of this study suggest that (1) it is feasible to determine NWR tolerance in horses and (2) high-intensity stimuli initiate ultrafast bursts of reflex activity, which is well known in practice and has now been quantified using the NWR model.

Duration of skin desensitisation following palmar digital nerve blocks with lidocaine, bupivacaine, mepivacaine and prilocaine

Objective comparative evidence of the time to onset and duration of effect provided by local anaesthetic (LA) agents for perineural blocks in the horse is lacking. Clear knowledge of these properties is required to guide clinically appropriate agent selection and aid interpretation of response to diagnostic blocks for lameness examinations. An interventional study, with complete, randomised crossover design was used to compare time to onset and duration of skin desensitisation provided by four LA agents applied to palmar digital nerve blocks in 12 horses. Effect at each time point was determined using a pressure gauge to measure the mechanical nociceptive threshold (MNT) over the heel bulbs. Complete desensitisation was defined when MNT was greater than four times the pre-block baseline. Onset and duration of complete desensitisation were recorded and compared across agents using a mixed linear model. When significant (P ≤ 0.05), post-hoc paired comparisons between agents were performed against a Tukey's corrected P ≤ 0.05. Onset of complete skin desensitisation for each agent was <5 min. Duration for lidocaine (mean 25 min; 95% confidence interval [CI] 9-42) was shorter than bupivacaine (53 min; 95% CI 39-65), which was shorter than both prilocaine (102 min; 95% CI 81-123) and mepivacaine (107 min; 95% CI 92-121), which were not different. Although onset of complete skin desensitisation was not different for the LA agents tested, duration varied from 25 min to nearly 2 h. Prilocaine and mepivacaine provided the most prolonged duration of effect, both exceeding bupivacaine.

Cardiorespiratory, Sedative and Antinociceptive Effects of a Medetomidine Constant Rate Infusion with Morphine, Ketamine or Both

Simple Summary

Standing surgery and diagnostic procedures in equine patients under deep sedation reduce the risk associated with general anesthesia. Sedation protocols must be safe, provide a good quality of sedation without producing cardiorespiratory depression and severe ataxia. The use of adrenergic alpha-2 receptors agonist in combination with opioids and/or ketamine can achieve an adequate sedation and provide sufficient analgesia for surgical procedures. Medetomidine and medetomidine with morphine in intravenous constant rate infusion have been evaluated for standing sedation but have not been compared directly. Although ketamine has been combined with other alpha-2 agonists successfully, it has not been evaluated in combination with medetomidine. The objective of this study was to compare four medetomidine-based protocols with the addition of morphine and/or ketamine, including cardiorespiratory, sedative and mechanical antinociceptive variables. All four protocols produced a similar degree of sedation and mechanical antinociception without clinically relevant impact on cardiorespiratory variables.

Abstract

Standing surgery under sedation reduces anesthetic-related mortality in horses. Medetomidine, alone and combined with morphine in a constant rate infusion (CRI), has been described for standing surgery but their cardiorespiratory, sedative and antinociceptive effects have never been compared. The addition of ketamine could improve analgesia in these procedures with minimal cardiorespiratory consequences. The objectives were to compare the cardiorespiratory effects, quality of sedation, antinociception and ataxia produced by administration of a medetomidine-based CRI with morphine, ketamine or both, in standing horses. A prospective, blind, randomized crossover, experimental design with six healthy adult horses was performed, in which four treatments were administered to all horses with at least two weeks of washout period: medetomidine (M); medetomidine and ketamine (MK); medetomidine and morphine (MMo); and medetomidine, morphine and ketamine (MMoK). Dosages were the same in all treatment groups: medetomidine at 5 µg/kg bwt followed by 5 µg/kg bwt/h, ketamine at 0.4 mg/kg/h and morphine at 50 µg/kg bwt, followed by morphine 30 µg/kg bwt/h. Drug infusions were maintained for 120 min. Cardiorespiratory variables, sedation degree and antinociceptive effects were evaluated during the procedure. All combinations produced similar sedation and antinociceptive effects and no clinically relevant alterations in cardiorespiratory variables occurred. Medetomidine CRI combined with morphine, ketamine or both are suitable and safe protocols for standing sedation in horses and the addition of morphine and/or ketamine did not cause any negative effect but no improving effect on sedation and antinociception was detected.

Heat hypersensitivity at a site proximal to a surgically induced osteochondral fracture in horses

OBJECTIVE

To evaluate thermal nociceptive thresholds (TNTs) before and after inducing a standardized radiocarpal bone osteochondral fracture (OCF) in horses.

STUDY DESIGN

Prospective, controlled, randomized, masked study.

ANIMALS

A group of 10 Thoroughbred fillies aged 2 years.

METHODS

Skin temperature and TNTs were measured on the skin over the triceps brachii muscle in both the thoracic limbs before (week 0) and weekly (weeks 1-8) after unilateral arthroscopic induction of a radiocarpal OCF (n = 4) or sham surgery (n = 6) followed by a standardized exercise programme. The contralateral, non-operated thoracic limb was used as a control within each horse. Percentage thermal excursion (%TE) defined as %TE = 100 ∗ (TNT - skin temperature)/(cut-off temperature - skin temperature) was calculated. Data were analysed with a mixed-effects model followed by Dunnett's and Tukey's tests for within and between-limbs comparisons, respectively; p < 0.05 was considered significant.

RESULTS

Skin temperature in the control limb of OCF horses was significantly higher at week 7 than at week 0 (p = 0.0125). At week 1, TNTs and %TE values in operated limbs of OCF horses were significantly reduced compared with their baseline values at week 0 (p ≤ 0.0153) and their values in contralateral control limbs (p ≤ 0.0024) and operated limbs of sham-operated horses (p ≤ 0.0162). At week 2, TNTs and %TE values in operated limbs of OCF horses remained significantly reduced compared with values in operated limbs of sham-operated horses (p ≤ 0.0248).

CONCLUSIONS AND CLINICAL RELEVANCE

Creation of an OCF in a radiocarpal bone induced transitory (<2 weeks) ipsilateral heat hypersensitivity proximal to the surgery site (skin over the triceps brachii muscle) in horses. Surgically induced OCF may cause somatosensory abnormalities consistent with secondary thermal hyperalgesia.

The effect of regional hypothermia on mechanical nociceptive thresholds in the equine distal forelimb

Regional hypothermia has shown promise as analgesic in horses when used to manage painful conditions of the distal limb such as laminitis. In this prospective study, the analgesic effects of regional hypothermia were assessed using mechanical nociceptive thresholds during distal limb cooling. The study population consisted of eight healthy adult Standardbred horses, selected from a teaching herd. A distal forelimb of each horse was cooled using water immersion at the following sequential target water temperatures: 34 °C, 20 °C, 10 °C, 5 °C, 1 °C, 5 °C, 10 °C, 20 °C. Limb surface temperature was measured after 30 min at each target water temperature and the mechanical force required to elicit a response (mechanical nociceptive threshold) was determined using a pneumatic actuator. Both forelimbs of each horse were tested one week apart. At skin surface temperatures above 7 °C, there was little association between skin surface temperature and the mechanical force required to elicit a response. As the skin surface temperature decreased below 7 °C, there was a rapid increase in the force required to elicit a response (P = 0.036). Skin surface temperatures of <7 °C required water temperatures below 2 °C. The results of this study suggest that hypothermia has potential to provide distal limb analgesia in horses at skin surface temperatures below 7 °C. Further evaluation of the technique is warranted.

Pressure Algometry for the Detection of Mechanical Nociceptive Thresholds in Horses

Simple Summary

It is difficult to measure pain in horses. As animals are not able to verbalize what they feel, we are left with trying to interpret the different signs that they display when they are in pain. Many of these signs are vague (e.g., not eating their food), but some are more readily identified if the animal moves away or lifts their leg when pressure is applied to a sensitive area. Pressure algometry is a tool used to detect responses to applied mechanical stimuli within painful and nonpainful tissues. Pressure algometry has been used in many different studies, but there is no consensus on how to synthesize this information to better diagnose and treat pain in horses. The purpose of this study was to summarize the results of these studies. Based on that review, we conclude that there is good evidence that pressure algometry is a reliable and objective method to measure pain responses. This information will help to improve the diagnosis and treatment of pain in horses.

Abstract

The clinical assessment of pain is subjective; therefore, variations exist between practitioners in their ability to identify and localize pain. Due to differing interpretations of the signs or severity of pain equine practitioners may assign varying levels of clinical significance and treatment options. There is a critical need to develop better tools to qualify and quantify pain in horses. Palpation is the most common method to detect local tenderness or sensitivity. To quantify this applied pressure, pressure algometry has been used to gradually apply pressure over specified landmarks until an avoidance response is noted, which is defined as the mechanical nociceptive threshold (MNT). Numerous studies have used pressure algometry in different applications to measure MNTs in horses. There is an acute need to establish normative values within different body regions and to develop standardized methods of testing MNTs to better guide practitioners in the diagnosis and treatment of pain. The aim of this systematic review was to summarize the evidence for the use of pressure algometry in horses. There is good evidence that pressure algometry is a repeatable, semi-objective method that can be used in a wide array of clinical and research applications to assess MNTs in horses.

Remote Controlled Nociceptive Threshold Testing Systems in Large Animals

Simple Summary

Measurement of the nociceptive threshold (NT) is widely used in the study of pain and its alleviation. This records the intensity of a stimulus that causes pain to the test subject. The end point of the test that indicates when the subject experiences pain, the NT, is a behavioural escape response. Detection of a reliable and repeatable response depends on the animal behaving normally throughout testing. Restraint and an unfamiliar environment may prevent the animal from displaying normal behaviour and impede acquisition of robust NTs. Remotely controlled testing enables NT data to be collected from unrestrained animals behaving normally. Development of a remote controlled system for measurement of thermal and mechanical NTs in a range of large animal species is described. Normal “baseline” thermal and mechanical NTs from untreated animals are reported. This information can be used to improve both the welfare of the animals under investigation and the quality of the data collected. Remote controlled systems are now in use worldwide in both the study of pain physiology and in developing new pharmaceutical and non-drug-based methods of pain relief.

Abstract

Nociceptive threshold (NT) testing is widely used for the study of pain and its alleviation. The end point is a normal behavioural response, which may be affected by restraint or unfamiliar surroundings, leading to erroneous data. Remotely controlled thermal and mechanical NT testing systems were developed to allow free movement during testing and were evaluated in cats, dogs, sheep, horses and camels. Thermal threshold (TT) testing incorporated a heater and temperature sensor held against the animal’s shaved skin. Mechanical threshold (MT) testing incorporated a pneumatic actuator attached to a limb containing a 1–2 mm radiused pin pushed against the skin. Both stimuli were driven from battery powered control units attached on the animal’s back, controlled remotely via infra-red radiation from a handheld component. Threshold reading was held automatically and displayed digitally on the unit. The system was failsafe with a safety cut-out at a preset temperature or force as appropriate. The animals accepted the equipment and behaved normally in their home environment, enabling recording of reproducible TT (38.5–49.8 °C) and MT (2.7–10.1 N); precise values depended on the species, the individual and the stimulus characteristics. Remote controlled NT threshold testing appears to be a viable refinement for pain research.

Post-operative pain behaviour associated with surgical castration in donkeys (Equus asinus)

Background

Recognising pain in donkeys is challenging because they are stoic.

Objectives

To identify the responses of donkeys before and after surgical pain.

Study design

Prospective, short‐term longitudinal pre‐ and post‐intervention observations.

Methods

Forty adult donkeys underwent surgical castration after sedation with intravenous (IV) xylazine, induction with guaiphenesin/thiopental IV and maintenance of anaesthesia with isoflurane and local anaesthetic blockade. Four hours after recovery from anaesthesia, flunixin meglumine 1.1 mg/kg, dipyrone 10 mg/kg and morphine 0.2 mg/kg IV were administered. Behavioural responses exhibited by the animals housed in individual stalls were recorded in four 30‐min videos: before castration (M0), and 3.5‐4.0 hours (M1), 5.5‐6.0 hours (M2) and 23.5‐24.0 hours after recovery from anaesthesia (M3). To exclude the influence of insects, the behaviour of six apparently pain‐free donkeys was compared with and without the presence of faeces and urine in the stall.

Results

When compared with presurgical baseline behaviours (M0), after surgery (M1) donkeys raised their pelvic limbs more (P = .003). When compared with M1, after analgesia (M2), the median frequencies of ear movements (44 vs 16; P < .001), head shaking (7 vs 1; P < .001), head turning (5 vs 0; P < .001) and lifting of the both limbs (7 vs 0; P = .008) decreased; feeding (0 vs 29; P < .001) and water intake (0 vs 0, range 0‐1 vs 0‐7; P = .05) increased. The dirty stall increased tail (53 vs 80; P = .03), head (16 vs 30; P = .03) and ear movements (50 vs 78; P = .04).

Main limitations

The dirty stall and presence of insects possibly contributed to the expression of behaviours unrelated to pain.

Conclusion

Lifting the pelvic limbs was the only specific pain behaviour after castration in donkeys. Analgesia restored appetite and water intake and reduced the frequency of head shaking and turning, ear movement and lifting the limbs. Tail, head and ear movements are unspecific responses related both to pain and a dirty stall, and are confounding factors when pain is assessed in donkeys in the presence of insects.

Quantitative sensory testing of the equine face

BACKGROUND

Quantitative sensory testing methods are now standard in the evaluation of sensory function in man, while few normal equine values have been reported.

OBJECTIVES

The aim of this experimental study was (a) to define the tactile sensory, mechanical nociceptive and thermal nociceptive thresholds of the equine face; (b) to assess the effect of age, sex, stimulation site and shaving; (c) to evaluate the reliability of the methods and (d) to provide reference facial quantitative sensory testing values.

STUDY DESIGN

Method description.

METHODS

Thirty-four healthy Warmblood horses were used in the study. Six (tactile sensory threshold) and five (mechanical nociceptive and thermal nociceptive thresholds) areas of the left side of the face with clear anatomical landmarks were evaluated. Ten horses had two (mechanical nociceptive threshold) or three (tactile sensory and thermal nociceptive thresholds) of these areas shaved for another study. A linear Mixed model was used for data analysis.

RESULTS

All thresholds increased with age (tactile sensory threshold: by 0.90 g/y (CI = [0.12 g; 0.36 g]) P = .001; mechanical nociceptive threshold: by 0.25 N/y (CI = [0.13-0.36 N]) P = .000; thermal nociceptive threshold: by 0.2°C/y (CI = [0.055-0.361]) P = .008). Sex had no effect on thresholds (tactile sensory threshold: P = .1; mechanical nociceptive threshold: P = .09; thermal nociceptive threshold: P = .2). Stimulation site affected tactile sensory and mechanical nociceptive thresholds (P = .001 and P = .008), but not thermal nociceptive threshold (P = .9). Shaving had no significant effect on any of the thresholds (tactile sensory threshold: P = .06; mechanical nociceptive threshold: P = .08; thermal nociceptive threshold: P = .09).

MAIN LIMITATIONS

Only the left side was investigated and measurements were obtained on a single occasion.

CONCLUSIONS

Handheld quantitative sensory testing does not require shaving or clipping to provide reliable measurements. Stimulation over the nostril (tactile sensory threshold), temporomandibular joint (mechanical nociceptive threshold) and supraorbital foramen (thermal nociceptive threshold) resulted in the most consistent thresholds.

Thermal, mechanical and electrical stimuli in antinociceptive studies in standing horses: an update

OBJECTIVE

To perform a literature review of the thermal and mechanical antinociceptive devices used in pharmacological studies in standing horses published after 2011 (2012-2019). To complete a full literature review about electrical stimulation used for evaluation in similar studies.

DATABASES USED

PubMed, Google Scholar and Web of Science.

CONCLUSIONS

A high level of standardization has been reached in antinociceptive studies in standing horses using thermal and mechanical stimuli in most recent years. Commercially available testing devices to deliver thermal, mechanical and electrical stimuli, with observation of aversive responses to these stimuli, are reliable, sensitive and specific. For electrical stimulus testing, there is evidence that the resistance between the electrodes should be measured and should not exceed 3 kΩ to guarantee consistent and reproducible stimuli. The specific analysis of electromyographic activity after an electrical stimulus provides more detailed information about the neurons stimulated.

Mechanical nociceptive thresholds in endurance horses

BACKGROUND

Alteration of limb sensitivity is forbidden in equine sports but difficult to enforce. We aimed to develop an objective field method to assess mechanical nociceptive threshold (MNT) in endurance horses.

METHODS

A remotely controlled pneumatic actuator (1 mm tip) was used to measure forelimb pastern MNT in 108 endurance horses.

RESULTS

Median (IQR) MNT at rest was 1.9 N (0.9-3.5). Icing had no significant effect on limb sensitivity. MNT measured at weekly intervals increased from week 1 (1.2 N (0.6-1.8)) to week 3 (1.9 N (1.2-2.8)) (P<0.05). In 17 horses without impaired sensitivity, MNT increased from 1.2 N (0.6-2.3) before to 2.4 N (1.2-5.2) after racing (P=0.0017). In desensitised horses, MNT after racing was higher (8 limbs-23.1 N (21.4 to >25)) than in horses without impaired sensitivity (42 limbs-2.2 N (1.2-4.3)) (P<0.0001). Desensitisation with mepivacaine increased MNT to above the safety cut-off (25 N) at 10 minutes; sensitivity return to baseline varied between individuals but was restored by 330 minutes. None of the horses became averse to the technique.

CONCLUSION

MNT was practical, non-traumatic, repeatable and well tolerated under field conditions in endurance horses. The technique differentiated postracing MNT in horses with normal sensitivity from those with impaired sensitivity.

Pharmacokinetic-pharmacodynamic modelling of the antinociceptive effect of a romifidine infusion in standing horses

OBJECTIVE

To evaluate the effect of a romifidine infusion on antinociception and sedation, and to investigate its relationship with plasma concentration.

STUDY DESIGN

Prospective, experimental, nonrandomized trial.

ANIMALS

A total of 10 healthy adult warmblood horses.

METHODS

Romifidine (loading dose: 0.08 mg kg^-1, infusion: 0.03 mg kg^-1 hour^-1) was administered intravenously over 120 minutes. Romifidine plasma concentrations were determined by capillary electrophoresis. Sedation quality and nociceptive thresholds were evaluated at regular time points before, during and after romifidine administration. The nociceptive withdrawal reflex was elicited by electrical stimulation at the thoracic limb using a dedicated threshold tracking algorithm and recorded by electromyography at the deltoid muscle. A pharmacokinetic-pharmacodynamic model was established and correlation between romifidine plasma concentration and main output variables tested.

RESULTS

A two compartmental model best described the romifidine pharmacokinetic profile. The nociceptive thresholds increased compared with baseline in all horses from 10 to 146 minutes after romifidine administration (p < 0.001). Peak effect reached 5.7 ± 2.3 times the baseline threshold (mean ± standard deviation). The effect/concentration relationship followed a counter-clockwise hysteresis loop. The mean plasma concentration was weakly correlated to nociceptive thresholds (p < 0.0071, r = 0.392). The sedative effects were significant until 160 minutes but variable, not correlated to plasma concentration (p = 0.067), and weakly correlated to nociceptive thresholds (p < 0.0001, r = 0.33).

CONCLUSIONS AND CLINICAL RELEVANCE

Romifidine elicited a marked antinociceptive effect. Romifidine-induced antinociception appeared with a delayed onset and lasted longer than sedation after discontinuing its administration.

Synovial butorphanol concentrations and mechanical nociceptive thresholds after intravenous regional limb perfusion in standing sedated horses

OBJECTIVE

To determine synovial butorphanol concentrations and mechanical nociceptive threshold (MNT) changes after butorphanol intravenous regional limb perfusion (IVRLP).

STUDY DESIGN

Experimental ANIMALS: Six adult horses.

METHODS

Cephalic IVRLP was performed with 10 mg butorphanol in sedated horses with a wide rubber tourniquet and a total volume of 30 mL. Radiocarpal synovial fluid and serum concentrations along with MNT were evaluated prior to and 0.5, 1, 2, 4, and 6 hours after IVRLP. Butorphanol concentrations were determined with liquid chromatography coupled to tandem mass spectrometry positive electrospray ionization.

RESULTS

Butorphanol concentrations reached mean (SD) peak concentrations of 9.47 ng/mL (±12.00) in synovial fluid and 3.89 ng/mL (3.29) in serum 30 minutes after IVRLP. Concentrations remained above baseline for 4 hours in synovial fluid (P ≤ .017) and for 2 hours in serum (P ≤ .016). The only difference in MNT was detected 1 hour after IVRLP, when MNT were higher in controls than in treated horses (P = .047).

CONCLUSION

Butorphanol IVRLP seemed well tolerated and resulted in measurable levels of butorphanol in the radiocarpal synovial fluid of five of six horses.

CLINICAL SIGNIFICANCE

Intravenous regional limb perfusion appears to be a viable alternative to administer butorphanol, but additional investigation is required to evaluate the dose and local concentrations required for analgesia.

Sedative and antinociceptive effects of different detomidine constant rate infusions, with or without methadone in standing horses

BACKGROUND

Standing surgery avoids the risks of general anaesthesia in horses.

OBJECTIVES

To assess sedation, antinociception and gastrointestinal motility in standing horses after a detomidine loading dose and 2-h constant rate intravenous (i.v.) infusion, with or without methadone.

STUDY DESIGN

Blinded, randomised, crossover with seven healthy adult cross-bred horses, three geldings and four females (404 ± 22 kg).

METHODS

Five i.v. treatments were administered to all horses with 1-week washout period: saline (SAL), detomidine low (2.5 μg/kg bwt + 6.25 μg/kg bwt/h) (DL) and high doses (5 μg/kg bwt + 12.5 μg/kg bwt/h) (DH) alone or combined with methadone (0.2 mg/kg bwt + 0.05 mg/kg bwt/h), (DLM) and (DHM), respectively. Height of head above the ground (HHAG), electrical (ET), thermal (TT) and mechanical (MT) nociceptive thresholds and gastrointestinal motility were evaluated at predetermined times between 5 and 240 min. A mixed effect model and Kruskal-Wallis test were used to analyse normally and non-normally distributed data, respectively.

RESULTS

Sedation (<50% basal HHAG) was achieved for the duration of the infusion, and for an additional 15 min in DH and DHM groups. Nociceptive thresholds were higher than baseline, to the greatest degree and the longest duration, with DHM (ET and TT for 135 min and MT for 150 min). After DH, TT was significantly higher than baseline from 30 to 120 min and MT from 15 to 135 min. After DLM, ET was increased at 90 min, TT at 30 min and MT for 120 min. Gastrointestinal motility was reduced for up to 135 min after DL, 150 min after DLM and 210 min after DH and DHM.

MAIN LIMITATIONS

Nociceptive thresholds are not equivalent to surgical stimuli.

CONCLUSION

Methadone with the highest detomidine dose (DHM) may provide sufficient sedation and analgesia for standing surgical procedures and warrants further investigation.

Characterisation of the in vivo interactions between detomidine and methadone in horses: Pharmacokinetic and pharmacodynamic modelling

BACKGROUND

Pharmacokinetic (PK)/pharmacodynamic (PD) modelling offers new insights to design protocols for sedation and analgesia in standing horses.

OBJECTIVES

To evaluate the parameters and interactions between detomidine and methadone when given alone or combined in standing horses.

STUDY DESIGN

Randomised, placebo-controlled, blinded, crossover.

METHODS

Eight adult healthy horses were given six treatments intravenously: saline (SAL); detomidine (5 μg/kg bwt; DET); methadone (0.2 mg/kg bwt; MET) alone or combined with detomidine (2.5 [MLD], 5 [MMD] or 10 [MHD] μg/kg bwt). Venous blood samples were obtained at predetermined times between 0 and 360 min after drug administration. Plasma detomidine and methadone were measured using a single, liquid/liquid extraction technique by liquid chromatography coupled with a triple quadrupole mass spectrometer (LC-MS/MS). Sequential PK/PD modelling compared rival models, with and without PK and PD interaction between drugs, to fit the PD data including height of the head above the ground (HHAG), a visual analogue scale for sedation (VAS), electrical (ET), thermal (TT) and mechanical (MT) nociceptive thresholds and gastrointestinal motility (GIM) [1].

RESULTS

Two and three compartment models best described the PK of detomidine and methadone, respectively. Detomidine decreased its own clearance as well as the clearance of methadone. The interaction of methadone on the effect of detomidine revealed an infra-additive (partial antagonism) effect for HHAG (α = -1.33), VAS (α = -0.98) and GIM (α = -1.05), a positive potentiation for ET (pot = 0.0041) and TT (pot = 0.133) and a synergistic to additive effect for MT (α = 0.78).

MAIN LIMITATIONS

This is a small experimental study.

CONCLUSIONS

Different PK/PD interactions were demonstrated for each PD parameter and could be modelled in vivo. The modelling of our data will allow us to simulate and predict the effect of constant rate infusions of both drugs for future investigations.

Evaluation of intravenous regional anaesthesia and four-point nerve block efficacy in the distal hind limb of dairy cows

Background

Intravenous regional anaesthesia (IVRA) and hindfoot four-point nerve block anaesthesia (NBA) are recommended for local anaesthesia (LA) in the distal limb of dairy cows. Two studies were conducted to compare the efficacy, time until onset and stress responses to IVRA and NBA in dairy cows. In the first cross-over designed study, eight healthy unsedated German Holstein cows, restrained in lateral recumbency (LR) on a surgical tipping table, were treated with IVRA and NBA using procaine 2% as a local anaesthetic. Distal limb desensitization was tested by electrical (e-), mechanical (m-) and thermal (t-) nociceptive stimulation 10 min before and 15 and 30 min after LA. Hormonal-metabolic (blood concentrations of cortisol, lactate, non-esterified fatty acids, and glucose) and cardio-respiratory (heart and respiratory rate, mean arterial blood pressure) stress responses to treatment were assessed at predetermined intervals. In the second study, six healthy, unsedated German Holstein cows in LR were treated (crossover design) with IVRA and NBA. Short-interval e-stimulation was measured by the time until complete distal limb desensitization.

Results

In the first study, four of eight cows responded to e-stimulation 15 min after IVRA, while none of the cows treated with NBA responded until the safety cut-off level was reached. E-stimulation revealed complete desensitization of the distal limb 30 min after LA in all cows. Half of the cows did not respond to m- and t-stimulation before LA, so no further evaluation was performed. Stress reactions to IVRA and NBA treatment were similar, but differences may have been masked by stress response to LR restraint. In the second study, complete desensitization was achieved 12.5 min after NBA, while one of the six cows still responded to e-stimulation 20 min after IVRA.

Conclusion

Hindfoot nerve block anaesthesia and intravenous regional anaesthesia induced complete desensitization of the distal hind limb in dairy cows. However, the anaesthesia onset after NBA was significantly faster than that of IVRA, which may be clinically relevant in the field, particularly when distal limb anaesthesia is required for major claw surgeries under time constraints.

Sedative and antinociceptive effects of different combinations of detomidine and methadone in standing horses

OBJECTIVE

To evaluate intravenous (IV) detomidine with methadone in horses to identify a combination which provides sedation and antinociception without adverse effects.

STUDY DESIGN

Randomized, placebo-controlled, blinded, crossover.

ANIMALS

A group of eight adult healthy horses aged (mean ± standard deviation) 7 ± 2 years and 372 ± 27 kg.

METHODS

A total of six treatments were administered IV: saline (SAL); detomidine (5 μg kg^-1; DET); methadone (0.2 mg kg^-1; MET) alone or combined with detomidine [2.5 (MLD), 5 (MMD) or 10 (MHD) μg kg^-1]. Thermal, mechanical and electrical nociceptive thresholds were measured, and sedation, head height above ground (HHAG), cardiopulmonary variables and intestinal motility were evaluated at 5, 15, 30, 45, 60, 75, 90, 120 and 180 minutes. Normal data were analyzed by mixed-model analysis of variance and non-normal by Kruskal-Wallis (p < 0.05).

RESULTS

Nociceptive thresholds in horses administered methadone with the higher doses of detomidine (MMD, MHD) were increased above baseline to a greater degree and for longer duration (MMD: 15-30 minutes, MHD: 30-60 minutes) than in horses administered low dose with methadone or detomidine alone (MLD, DET: 5-15 minutes). No increases in nociceptive thresholds were recorded in SAL or MET. Compared with baseline, HHAG was lower for 30 minutes in MMD and DET, and for 45 minutes in MHD. No significant sedation was observed in SAL, MET or MLD. Intestinal motility was reduced for 75 minutes in MHD and for 30 minutes in all other treatments.

CONCLUSIONS

Methadone (0.2 mg kg^-1) potentiated the antinociception produced by detomidine (5 μg kg^-1), with minimal sedative effects.

CLINICAL RELEVANCE

Detomidine (5 μg kg^-1) with methadone (0.2 mg kg^-1) produced antinociception without the adverse effects of higher doses of detomidine.

Antinociceptive effects of methadone combined with detomidine or acepromazine in horses

REASONS FOR PERFORMING STUDY

To investigate two protocols to provide antinociception in horses.

OBJECTIVES

To evaluate the antinociceptive effects of intravenous methadone combined with detomidine or acepromazine in adult horses.

STUDY DESIGN

Randomised, blinded, crossover study.

METHODS

Mechanical, thermal and electrical stimuli were applied to the dorsal left and right metacarpus and coronary band of the left thoracic limb, respectively. A thermal stimulus was applied caudal to the withers. The horses were treated with saline (C), a combination of methadone (0.2 mg/kg bwt) and detomidine (10 μg/kg bwt) (MD) or methadone (0.2 mg/kg bwt) and acepromazine (0.05 mg/kg bwt) (MA) at 1 week intervals. Nociceptive thresholds were measured before and at 15 min intervals until 150 min after treatment. Wilcoxon rank-sum and Wilcoxon signed rank tests were used to compare data between groups at each time point and over time within each group, followed by the Bonferroni method to adjust the P value.

RESULTS

The mechanical stimulus was the most sensitive test to differentiate the antinociceptive effects of the treatments. Mechanical thresholds were greater after MD than MA between 15 and 30 min and with both MD and MA these thresholds were greater than C from 15 to 60 min. Electrical and thermal limb thresholds were greater after MD than C at 15 and 45 min and at 15, 30, 45, 75 and 105 min, respectively. Thermal limb thresholds were greater with MA than C at 30 min. Thoracic thermal threshold in MD and MA were higher than C at 45, 75, 90 and 120 min and from 30 to 75 min, respectively.

CONCLUSIONS

Methadone and acepromazine produced less pronounced mechanical antinociception than MD.