Lidocaine in the horse: its pharmacological effects and their relationship to analytical findings

Abstract

Lidocaine is a local anaesthetic agent that is widely used in equine medicine. It is also an Association of Racing Commissioners International (ARCI) Class 2 foreign substance that may cause regulators to impose substantial penalties if residues are identified in post race urine samples. Therefore, an analytical/pharmacological database was developed for this drug. Using our abaxial sesamoid local anaesthetic model, the highest no-effect dose (HNED) for the local anaesthetic effect of lidocaine was determined to be 4 mg. Using enzyme-linked immunosorbent assay (ELISA) screening, administration of the HNED of lidocaine to eight horses yielded peak serum and urine concentrations of apparent lidocaine of 0.84 ng/mL at 30 min and 72.8 ng/mL at 60 min after injection, respectively. These concentrations of apparent lidocaine are readily detectable by routine ELISA screening tests (LIDOCAINE ELISA, Neogen, Lexington, KY). ELISA screening does not specifically identify lidocaine or its metabolites, which include 3-hydroxylidocaine, dimethylaniline, 4-hydroxydimethylaniline, monoethylglycinexylidine, 3-hydroxymonoethylglycinexylidine, and glycinexylidine. As 3-hydroxylidocaine is the major metabolite recovered from equine urine, it was synthesized, purified and characterized, and a quantitative mass spectrometric method was developed for 3-hydroxylidocaine as recovered from horse urine. Following subcutaneous (s.c.) injection of the HNED of lidocaine, the concentration of 3-hydroxylidocaine recovered from urine reached a peak of about 315 ng/mL at 1 h after administration. The mean pH of the 1 h post dosing urine samples was 7. 7, and there was no apparent effect of pH on the amount of 3-hydroxylidocaine recovered. Within the context of these experiments, the data suggests that recovery of less than 315 ng/mL of 3-hydroxylidocaine from a post race urine sample is unlikely to be associated with a recent local anaesthetic effect of lidocaine. Therefore these data may be of assistance to industry professionals in evaluating the significance of small concentrations of lidocaine or its metabolites in postrace urine samples. It should be noted that the quantitative data are based on analytical methods developed specifically for this study, and that methods used by other laboratories may yield different recoveries of urine 3-hydroxylidocaine.

Duration and Efficacy of Different Local Anesthetics on the Palmar Digital Nerve Block in Horses

“…In the group treated with lidocaine associated with epinephrine, moderate or total effect lasted 6 hours in most animals, reaching 9 hours in two animals, with a maximum effect at 60 and 90 minutes. Comparing the results with Harkins et al [9] and Spoormakers et al [10], similar duration of analgesia with 30, 40, and 200 mg of lidocaine per site was observed in this study, suggesting that the drug has one dose-dependent effect as of 30 mg. To use the lowest dose capable of producing analgesia, it should be noted that the dose of 30 mg of lidocaine was able to revert in 77% of the experimental lameness at 30 minutes, and perhaps a higher dose would be necessary to produce more consistent clinical outcome.…”

“…The concentration of ropivacaine was less than that used by Santos et al [17] and greater than that used by Harkins et al [7]. The dose and concentration of the drugs can influence the timing of beginning and ending of the anesthetic action, explaining the differences in time of action found by different authors [7][8][9][10]17].…”

“…As lidocaine provides analgesia for a short period, its use is most suitable in animals with only one source of pain [3]. Harkins et al [9] evaluated the skin sensitivity after exposure to a heat-projection lamp observed a significant anesthetic effect of PD nerves performed at the sesamoid bones with 10 and 40 mg of lidocaine, 15 minutes after administration. Analgesia persisted for 30 and 90 minutes, respectively.…”

“…The concentration of lidocaine and bupivacaine used was the same one used by Harkins et al [8,9] and Spoormakers et al [10]. The concentration of ropivacaine was less than that used by Santos et al [17] and greater than that used by Harkins et al [7].…”

“…Schumacher et al [6] observed inefficacy of ketamine as a local anesthetic agent when compared with lidocaine in abaxial nerve block in horses with chronic lameness evaluated with the aid of a body-mounted inertial sensor system. Differently from the studies mentioned previously, several authors have evaluated the efficacy and duration of local anesthetics using the heat-lamp model to observe the loss of cutaneous sensitivity [7][8][9][10].…”