Pharmacokinetics of antipyrine, acetaminophen and lidocaine in fed and fasted horses

Pharmacokinetics, clinical efficacy and safety of acetaminophen (paracetamol) in adult horses with naturally occurring chronic lameness

BACKGROUND

Acetaminophen is used clinically in horses with musculoskeletal pain; however, no studies have been performed in horses with chronic lameness.

OBJECTIVES

To determine the pharmacokinetics, safety and efficacy of chronic dosing of acetaminophen in horses with naturally occurring chronic lameness.

STUDY DESIGN

Longitudinal.

METHODS

Twelve adult horses with chronic lameness were treated with acetaminophen (30 mg/kg PO) every 12 h for 21 days. Plasma concentrations of acetaminophen were analysed on days 7 and 21 via LC-MS/MS and noncompartmental pharmacokinetic analysis. Lameness was evaluated by body-mounted inertial sensor (BMIS) and 10-point subjective lameness score on day 21 and compared to untreated baseline evaluation on day 35. Clinicopathological analysis (n = 12), hepatic biopsy (n = 6) and gastroscopy (n = 6) were evaluated on days -1 and 22.

RESULTS

Maximum plasma acetaminophen concentration (Cmax ) was 20.83 ± 10.25 μg/mL at time (Tmax ) 0.40 ± 0.22 h on day 7. The Cmax on day 21 was 17.33 ± 6.91 μg/mL with a Tmax of 0.67 ± 0.26 h. Subjective lameness scores significantly improved at 2 and 4 h post-treatment; Significant percent improvement was detected in PDmax for horses with hindlimb lameness at 1, 2 and 8 h post-treatment. There were no significant differences in gastroscopy or hepatic biopsy scores between days -1 and 22.

MAIN LIMITATIONS

Small sample size, multi-limb lameness of varying severity and aetiology, lack of intermediary lameness evaluation.

CONCLUSIONS

In horses with naturally occurring chronic lameness, acetaminophen at 30 mg/kg produced a transient improvement in subjective lameness and BMIS evaluation. Acetaminophen may not be effective as a monotherapy. Acetaminophen was safe following 21 days of 30 mg/kg PO every 12 h, with no evidence of clinically significant changes in clinicopathological analysis, hepatic biopsy or gastric ulceration scores.

Pharmacokinetics of acetaminophen after a single Oral administration of 20 or 40 mg/kg to 7-9 Day-old foals

Background

Acetaminophen is utilized in human infants for pain management and fever. Neonatal foals might benefit from administration of acetaminophen but effective and safe dosage regimens for neonatal foals remains to be determined.

Objective

The objective was to determine the plasma pharmacokinetics of acetaminophen following oral administration of a single dose of 20 mg/kg or 40 mg/kg to neonatal foals. A secondary objective was to evaluate any changes in hematology and biochemistry profiles.

Study design

Randomized study.

Methods

Eight clinically healthy 7-9-day old Quarter Horse foals (3 colts and 5 fillies) received a single oral dose of acetaminophen either 20 (n = 4) or 40 (n = 4) mg/kg. Hematology and biochemistry profiles were evaluated before and 7 days after drug administration. Blood samples were collected before and 8 times after acetaminophen administration for 48 h to quantify plasma acetaminophen concentrations. Plasma pharmacokinetic parameters were estimated using non- compartmental analysis.

Results

The median peak plasma concentrations (and range) occurred at 1.5 (0.5-2) hours, and 1.0 (1-2) hours for the 20 and 40 mg/kg doses. The maximum plasma concentration (and range) was 12 (7.9-17.4) μg/mL for the 20 mg/kg dose and 14 (11-18) μg/mL for 40 mg/kg dose. The median AUC_0-∞ ranged from 46 to 100 and 79 to 160 h*-μg/mL for the 20 and 40 mg/kg dose, respectively. Hematology and biochemistry profiles remained within normal limits.

Conclusion

Plasma disposition of acetaminophen after oral administration of 20 and 40 mg/kg to neonates is comparable to adult horses. However, safety and the optimal dosage regimen of acetaminophen for treating pain and or pyrexia in neonates in this age group remains to be determined.

The Clinical Pharmacology and Therapeutic Evaluation of Non-Steroidal Anti-Inflammatory Drugs in Adult Horses

This review firstly examines the underlying pathophysiology of pain and inflammation associated with orthopedic disease and endotoxemia. Then, it reviews the clinical pharmacology (pharmacokinetics and pharmacodynamics) of both conventional and non-conventional NSAIDs in the adult horse, and finally provides an overview of different modalities to evaluate the therapeutic efficacy of NSAIDs in research.

Pharmacokinetics and clinical efficacy of Acetaminophen (Paracetamol) in adult horses with mechanically induced lameness

BACKGROUND

Acetaminophen has been used clinically in horses alone or combined with traditional NSAIDs for treatment of musculoskeletal pain in horses.

OBJECTIVES

To determine the pharmacokinetics and efficacy of acetaminophen at two doses in horses with mechanically induced lameness compared to phenylbutazone or placebo control.

STUDY DESIGN

In vivo experiment.

METHODS

Nine healthy mares with mechanical lameness induced via a reversible sole pressure horseshoe model were treated with acetaminophen (20 mg/kg PO; A20), acetaminophen (30 mg/kg PO; A30), phenylbutazone (2.2 mg/kg, PO; PB) and oral placebo (C) in a randomised 4-way Latin square model. Plasma concentrations for A20 and A30 were analysed via LC-MS/MS and noncompartmental pharmacokinetic analysis. Heart rate and heart rate variability were measured using a portable telemetry. Lameness was scored by three blinded boarded equine surgeons using the AAEP and 10-point scales.

RESULTS

Mean maximum plasma concentration (C_max ) for A20 was 20.01 ug/mL within 0.66h (T_max ) after administration; The mean C_max for A30 was 30.02 ug/mL with a T_max of 0.43 h. Post-treatment heart rate for A30 was significantly lower than A20 at 1 and 7 h; lower than PB at 2, 3, 4.5, and 7 h; lower than C at 2, 3.5, 4.5, 6, 7, and 8 h. 10-point Lameness scores were significantly improved for A30 than C at 2 and 4 h post-treatment; PB was significantly improved than C at 8 h post treatment. There were no significant differences in lameness between A20, A30, and PB.

MAIN LIMITATIONS

Small sample size, lack of objective lameness measurement.

CONCLUSIONS

Acetaminophen at 30 mg/kg produced a more rapid improvement in lameness scores and heart rate compared to other treatments in this model. Further evaluation of the pharmacokinetics and safety of repeated oral dosing of acetaminophen at 30 mg/kg is needed to determine clinical utility.

Impact of malnourishment on the pharmacokinetics of acetaminophen and susceptibility to acetaminophen hepatotoxicity

BACKGROUND

Acetaminophen hepatotoxicity is thought to be primarily caused by formation of the specific reactive metabolite N-acetyl-para-benzo-quinone imine (NAPQI). Malnourished individuals are at increased risk of acetaminophen-related hepatotoxicity. We report a case of low acetaminophen clearance in a severely underweight young woman, and elaborate on the possible effects of malnutrition on the total clearance of acetaminophen as well as on the separate contributions of the different metabolic pathways.

CASE REPORT

An 18-year-old Caucasian woman weighing 43 kg with a history of eating disorder-related hospital admissions presented at the emergency department after having ingested 33 tablets of acetaminophen 500 mg two hours earlier. She then received intravenous N-acetylcysteine for 33 h. Nine hours after ingestion, the acetaminophen elimination half-life (t½) was estimated to be >100 h.

DISCUSSION

While decreased total acetaminophen clearance (twofold) due to malnutrition has been reported in literature, the extremely low clearance in this specific patient cannot be explained. Malnourished individuals generally have reduced antioxidant reserves, coinciding with a shift in metabolic routes toward oxidative metabolism. This may result in increased formation of NAPQI and reduced neutralizing capacity, thereby increasing the risk of acetaminophen-induced hepatotoxicity. Evidence for this observation can be found in animal and to a lesser extent in human studies.

Investigation of plasma concentrations of paracetamol, metacetamol, and orthocetamol in Japanese racehorses using liquid chromatography-electrospray ionisation-tandem mass spectrometry

Paracetamol is used widely as an over-the-counter analgesic and antipyretic medication for humans, but not for Japanese racehorses. Paracetamol can be an environmental substance, and is found together with its two isomers, metacetamol and orthocetamol, in equine urine. However, the sources and routes of paracetamol exposure remain unclear. To control the misuse of paracetamol, it is appropriate to establish residue limits for paracetamol to differentiate the administration of paracetamol from its environmental levels. In this study, we developed and validated a quantitative method for paracetamol, metacetamol, and orthocetamol in equine plasma using liquid chromatography-electrospray ionization-tandem mass spectrometry and applied it to postrace samples from 320 Japanese racehorses for approximately 1 year. In addition, we conducted feed analysis and related pharmacokinetics simulations to evaluate the contributions from exposure via feed. The hydrolyzed plasma concentrations of paracetamol, metacetamol, and orthocetamol ranged from 0.787 to 39.8 ng/mL (median 5.87 ng/mL), 0 to 2.13 ng/mL (0.347 ng/mL), and 1.98 to 82.8 ng/mL (16.6 ng/mL), respectively. Such low concentrations of paracetamol were deemed irrelevant to therapeutic effect. Based on statistical analysis, the preliminary Japanese residue limits of unhydrolyzed and hydrolyzed paracetamol were determined to be 70.5 ng/mL and 112 ng/mL, respectively, in plasma, at a risk factor of 1 in 10,000. Furthermore, we detected paracetamol and orthocetamol in feed samples. A pharmacokinetics simulation showed that the presence of orthocetamol is most probably related to daily feed rations. As for paracetamol, feed can be one of the sources and other possible sources require further investigation.

Is There Still a Place for Lidocaine in the (Postoperative) Management of Colics?

Intravenous lidocaine is widely used to prevent or treat postoperative ileus in horses. Clinical studies that support this approach are flawed and contradicted by others. Also, physical obstruction could be more important in causing postoperative reflux than postoperative ileus in the horse. The antiinflammatory properties of lidocaine and the role of inflammation from intestinal handling in the genesis of postoperative reflux are questionable. Because of cost and questionable efficacy of lidocaine, a well-designed clinical trial is required to support its continued use. However, lidocaine could be given to provide or enhance analgesia in selected cases with postoperative colic.

Pharmacokinetics and safety of repeated oral dosing of acetaminophen in adult horses

BACKGROUND

There are no published studies on the pharmacokinetics of acetaminophen at the dosage used clinically (20 mg/kg), nor has the safety of multiple doses in horses been investigated.

OBJECTIVE

Define the pharmacokinetic parameters of oral acetaminophen at 20 mg/kg in adult horses as a single dose, and twice daily for 14 days to assess the safety of multiple dosing.

STUDY DESIGN

Pharmacokinetic study, multiple dose safety study.

METHODS

Eight healthy Thoroughbred geldings were given acetaminophen (20 mg/kg; 500 mg tablets) orally as a single dose followed by doses every 12 h for 14 days. Serial blood samples were collected for determination of plasma acetaminophen concentrations using high performance liquid chromatography with ultraviolet detection. Serum biochemical analysis, gastroscopy and liver biopsy were examined during the safety study.

RESULTS

Following a single dose, mean maximum concentration (C_max ) was 16.61 μg/mL at 1.35 h (T_max ), and drug concentration was below the lower limit of detection in most horses by 24 h. Elimination half-life (T_1/2 ) was 2.78 h. No significant accumulation was noted following multiple doses. Average C_max of acetaminophen following multiple oral dosing was 15.85 μg/mL, with a T_max of 0.99 h and T_1/2 of 4 h. Serum activities of sorbitol dehydrogenase were significantly decreased and total bilirubin concentrations were significantly increased following the last dose. No statistically significant changes were noted in gastroscopy scores.

MAIN LIMITATIONS

Only one dose level (20 mg/kg) was studied, sample size was small and only a single breed and sex was used, with no pretreatment liver biopsies.

CONCLUSION

This study described the pharmacokinetics of acetaminophen following single and multiple 20 mg/kg oral doses in adult horses and demonstrated the safety of acetaminophen with multiple oral dosing over 14 days. The summary is available in Portuguese - see Supporting information.

Cardiac Therapeutics in Horses

Many cardiac therapeutics lack significant evidence of benefit in the horse, and in many cases their use is based on extrapolation of evidence from other species. In recent years there has been a push to develop a better understanding of both the pharmacodynamics and pharmacokinetics of these drugs. Recent data have described the use of antiarrhythmic agents including sotalol, flecainide, and amiodarone. Data about the use of ACE inhibitors in the management of congestive heart failure are encouraging and support their use in certain cases, wheras evidence for other medicines, such as pimobendan, remain speculative.

Pharmacokinetics of intravenous, subcutaneous, and topical administration of lidocaine hydrochloride and metabolites 3-hydroxylidocaine, monoethylglycinexylidide, and 4-hydroxylidocaine in horse

Intravenous (iv), subcutaneous (sq), and topical (tp) lidocaine was administered to six horses in a cross-over, randomized design study. Samples were collected for up to 72 hr. Compartmental models were used to investigate the pharmacokinetics of (LD) and its metabolites 3-hydroxylidocaine (3-OH), 4-hydroxylidocaine (4-OH), and monoethylglycinexylidide (MEGX). Metabolites 3-OH and 4-OH were present in conjugated forms, whereas LD and metabolite MEXG were present primarily in the un-conjugated form. Plasma concentrations of LD after iv administration (100 mg) were described by three-compartment model with an additional three compartments to describe the elimination of metabolites. Median (range) elimination micro-constants (K_e ) for LD, 3-OH, 4-OH, and MEXG were 4.12 (2.62-6.23), 1.25 (1.10-2.15), 1.79 (1.22-2.39), and 1.69 (1.03-1.99)/hr, respectively. Median (range) values of alpha (t_½α ), beta (t_½β ), and gamma (t_½γ ) half-lives were 0.08 (0.07-0.13), 0.57 (0.15-1.25), and 4.11 (0.52-7.36) hr. Plasma concentrations of LD after sq (200 mg) administration were described by absorption and two-compartment elimination model. The median (range) of the LD absorption half-life (t_½ab ) was 0.47 (0.29-0.61) hr. The K_e for LD, 3-OH, 4-OH, and MEXG was 3.91 (1.48-9.25), 1.00 (0.78-1.08), 1.76 (0.96-2.11), and 1.13 (0.69-1.33)/hr. The median (range) of t_½α and t_½β was 0.15 (0.06-0.27) and 3.04 (2.53-6.39) hr. Plasma concentrations of LD after tp (400 mg) application were described by one-compartment model with a t_½ab of 8.49 (5.16-11.80) hr. The K_e for LD, 3-OH, and MEXG was 0.24 (0.10-0.81), 0.41 (0.08-0.93), and 0.38 (0.26-1.14)/hr.

Successful use of lipid emulsion to resuscitate a foal after intravenous lidocaine induced cardiovascular collapse

BACKGROUND

Lipid emulsion has been reported to be effective for the treatment of local anaesthetic overdoses in rats, dogs and man.

OBJECTIVES

To describe the successful treatment of cardiovascular lidocaine toxicity in a foal with intravenous lipid administration.

STUDY DESIGN

Observational study: case report.

METHODS

An 8-month-old Arabian cross foal was anaesthetised for removal of the right alar fold and nasal plate. Anaesthesia was maintained with isoflurane in oxygen and lidocaine administered with a loading dose followed by a continuous rate infusion (CRI). The anaesthetic period was uneventful and 30 min before expected termination of the procedure lidocaine infusion was stopped. A sudden drop in mean arterial blood pressure was then observed. The ECG signal was lost, the end tidal CO₂ tension dropped from 40 to 10 mmHg, corneal reflex was absent and asystole diagnosed. Cardiopulmonary resuscitation manoeuvres were immediately initiated, but epinephrine and atropine were unsuccessfully administered. Lipid emulsion was administered and the heart rate and arterial blood pressure gradually returned to normal.

RESULTS

The foal recovered consciousness 3 h later, regained its sternal position, was responsive and 20 h later was able to stand up alone.

MAIN LIMITATIONS

It will be necessary to evaluate a greater number of cases to determine the effectiveness of lipids in foals intoxicated with lidocaine.

CONCLUSION

Intravenous lipid emulsion may be helpful in the treatment of potentially lethal cardiotoxicity attributable to lidocaine overdose in the foal.

Antinociceptive effects of three escalating dexmedetomidine and lignocaine constant rate infusions in conscious horses

Dexmedetomidine and lignocaine IV are used clinically to provide analgesia in horses. The aims of this study were to investigate the antinociceptive effects, plasma concentrations and sedative effects of 2, 4 and 6 µg/kg/h dexmedetomidine IV, with a bolus of 0.96 µg/kg preceding each continuous rate infusion (CRI), and 20, 40 and 60 µg/kg/min lignocaine IV, with a bolus of 550 µg/kg preceding each CRI, in 10 Swiss Warmblood horses. Electrically elicited nociceptive withdrawal reflexes were evaluated by deltoid muscle electromyography. Nociceptive threshold and tolerance were determined by electromyography and behaviour following single and repeated stimulation. Plasma concentrations of drugs were determined by liquid chromatography and mass spectrometry. Sedation was scored on a visual analogue scale. Dexmedetomidine increased nociceptive threshold to single and repeated stimulation for all CRIs, except at 2 µg/kg/h, where no increase in single stimulation nociceptive threshold was observed. Dexmedetomidine increased nociceptive tolerance to single and repeated stimulation at all CRIs. There was large individual variability in dexmedetomidine plasma concentrations and levels of sedation; the median plasma concentration providing antinociceptive effects to all recorded parameters was 0.15 ng/mL, with a range from <0.02 ng/mL (below the lower limit of quantification) to 0.25 ng/mL. Lignocaine increased nociceptive threshold and tolerance to single and repeated stimulation at CRIs of 40 and 60 µg/kg/min, corresponding to plasma lignocaine concentrations >600 ng/mL. Only nociceptive tolerance to repeated stimulation increased at 20 µg/kg/min lignocaine. Lignocaine at 40 µg/kg/min and dexmedetomidine at 4 µg/kg/h were the lowest CRIs resulting in consistent antinociception. Lignocaine did not induce significant sedation.

Partial intravenous anaesthesia in the horse: a review of intravenous agents used to supplement equine inhalation anaesthesia. Part 1: lidocaine and ketamine

OBJECTIVE

To review the literature with regard to the use of different intravenous agents as supplements to inhalational anaesthesia in horses. These drugs include lidocaine, ketamine, opioids and α2 -agonists. The Part 1 of this review will focus in the use of lidocaine and ketamine.

DATABASES USED

Pubmed & Web of Science. Search terms: horse, inhalant anaesthesia, balanced anaesthesia, partial intravenous anaesthesia, lidocaine, ketamine.

CONCLUSIONS

Different drugs and their combinations can be administered systemically in anaesthetized horses, with the aim of reducing the amount of the volatile agent whilst improving the recovery qualities and providing a multimodal analgesic approach. However, full studies as to whether these techniques improve cardiopulmonary status are not always available and potential disadvantages should also be considered.

A clinician's guide to factors affecting withdrawal times for equine therapeutic medications

Equine forensic science can now detect concentrations down to 25 femtograms/mL (parts per quadrillion, ppq) or less in blood and urine. As such, horsemen are increasingly at risk of inadvertent 'positives' due to therapeutic medication 'overages' or trace identifications of dietary or environmental substances. Reviewed here are the factors which determine detection times and 'withdrawal times' for substances administered to horses. Withdrawal times are affected by many factors, including dose, formulation, route and frequency of administration, bioavailability, plasma half-life, sensitivity of the analytical process, the testing matrix (plasma, urine, or other), and the environmental presence and/or persistence of administered substances. Of these factors only dose is known precisely. For any given administration, horse-to-horse differences in the volumes of distribution, systemic clearance, and terminal plasma elimination half-life of substances are major and totally uncontrollable factors driving horse-to-horse variability in withdrawal times. A further complication is that chemically stable medications administered to horses and eliminated in the urine inevitably become part of the environment of the horse. The presence of these substances in the equine environment is increasingly giving rise to trace identifications long after nominal administration of these substances has ceased. Because of the unknown and uncontrollable horse-to-horse variability in medication pharmacokinetics, any therapeutic medication administration to a horse by definition includes the possibility of an inadvertent medication overage. As such, the caveat that there are no guarantees in life most assuredly applies to advisories concerning equine therapeutic medication withdrawal times.

Species comparison of oral bioavailability, first-pass metabolism and pharmacokinetics of acetaminophen

Species differences in oral bioavailability, first-pass metabolism and pharmacokinetics of biopharmaceutics classification system (BCS) class I compound acetaminophen were studied. The absolute bioavailability was 42.2%, 39.0%, 44.5%, 75.5% and 91.0% in chickens, turkeys, dogs, pigs and horses, respectively. After hydrolysis of metabolites by beta-glucuronidase/sulfatase, apparent bioavailability increased significantly in all species (turkeys: 72.4%, dogs: 100.5%, pigs: 102.2%), except horses (91.6%). Mean metabolic ratios of [acetaminophen glucuronide]/[acetaminophen] between 0 and 1h were significantly higher after oral dosing in turkeys, dogs and pigs, revealing the role of first-pass metabolism in incomplete bioavailability. Evidence of species differences in acetaminophen metabolism is provided by differences in plasma clearance, which was inversely proportional to bioavailability. In conclusion, differences in BA appeared to originate predominantly from differences in first-pass metabolism, demonstrating that the BCS high permeability classification of acetaminophen is consistent across the mammalian species studied. In turkeys, however, incomplete absorption additionally seemed to contribute to the low BA.

Effect of a constant rate infusion of lidocaine on the quality of recovery from sevoflurane or isoflurane general anaesthesia in horses

REASONS FOR PERFORMING STUDY

Lidocaine constant rate infusions (CRIs) are common as an intraoperative adjunct to general anaesthesia, but their influence on quality of recovery has not been thoroughly determined.

OBJECTIVES

To determine the effects of an intraoperative i.v. CRI of lidocaine on the quality of recovery from isoflurane or sevoflurane anaesthesia in horses undergoing various surgical procedures, using a modified recovery score system.

HYPOTHESIS

The administration of intraoperative lidocaine CRI decreases the quality of recovery in horses.

METHODS

Lidocaine (2 mg/kg bwt bolus followed by 50 microg/kg bwt/min) or saline was administered for the duration of surgery or until 30 mins before the end of surgery under isoflurane (n = 27) and sevoflurane (n = 27).

RESULTS

Horses receiving lidocaine until the end of surgery had a significantly higher degree of ataxia and a tendency towards significance for a lower quality of recovery. There was no correlation between lidocaine plasma concentrations at recovery and the quality of recovery.

CONCLUSIONS

Intraoperative CRI of lidocaine affects the degree of ataxia and may decrease the quality of recovery.

POTENTIAL RELEVANCE

Discontinuing lidocaine CRI 30 mins before the end of surgery is recommended to reduce ataxia during the recovery period.

A clinical comparison of two anaesthetic protocols using lidocaine or medetomidine in horses

OBJECTIVE

To compare the effects of two balanced anaesthetic protocols on end-tidal isoflurane (Fe'ISO), cardiopulmonary performance and quality of recovery in horses.

DESIGN

Prospective blinded randomized clinical study.

ANIMALS

Sixty-nine client-owned horses, American Society of Anesthesiologists category I and II, undergoing elective surgery.

METHODS

The horses were premedicated with acepromazine (0.03 mg kg(-1)) IM 30-60 minutes before induction of anaesthesia and were randomly assigned to one of two treatments: in group L (37 horses) xylazine (1 mg kg(-1)) and in group M (31 horses) medetomidine (7 microg kg(-1)) was administered IV for sedation. Anaesthesia was induced 5 minutes later with ketamine (2.2 mg kg(-1)) and diazepam (0.02 mg kg(-1)) IV and maintained with isoflurane in oxygen/air (initial FIO2 0.40-0.50) and a constant rate infusion (CRI) of either lidocaine (2 mg kg(-1)/15 minutes loading dose followed by 50 microg kg(-1) minute(-1)) (group L) or medetomidine (3.5 microg kg(-1) hour(-1)) (group M). If horses showed movement or nystagmus, additional thiopental or ketamine was administered. Heart rate, mean arterial pressure (MAP), Fe'ISO and arterial blood gases were measured. Cardiac output was measured with the lithium dilution method in 10 (group L) and 11 (group M) horses every 45 minutes. Recovery was scored.

RESULTS

Heart rate and the cardiac index (CI) were significantly higher in group L with changes over time. In group M, MAP was significantly higher during the first 50 minutes. Group L needed more additional ketamine and thiopental to maintain a surgical plane of anaesthesia and Fe'ISO was significantly higher from 70 minutes. Recovery was longer in group M and of better quality. The significance level was set at p < 0.05.

CONCLUSIONS AND CLINICAL RELEVANCE

In group M, maintenance of stable anaesthetic depth was easier and lower Fe'ISO was required to maintain a surgical plane of anaesthesia. Recoveries were longer but of better quality. The CI was higher in group L but cardiovascular function was generally well maintained in both groups.

The disposition of lidocaine during a 12-hour intravenous infusion to postoperative horses

Lidocaine is administered as an intravenous infusion to horses for a variety of reasons, but no study has assessed plasma lidocaine concentrations during a 12-h infusion to horses. The purpose of this study was to evaluate the plasma concentrations and pharmacokinetics of lidocaine during a 12-h infusion to postoperative horses. A second purpose of the study was to evaluate the in vitro plasma protein binding of lidocaine in equine plasma. Lidocaine hydrochloride was administered as a loading dose, 1.3 mg/kg over 15 min, then by a constant rate IV infusion, 50 microg/kg/min to six postoperative horses. Lidocaine plasma concentrations were measured by a validated high-pressure liquid chromatography method. One horse experienced tremors and collapsed 5.5 h into the study. The range of plasma concentrations during the infusion was 1.21-3.13 microg/mL. Lidocaine plasma concentrations were significantly increased at 0.5, 4, 6, 8, 10 and 12 h compared with 1, 2 and 3 h. The in vitro protein binding of lidocaine in equine plasma at 2 microg/mL was 53.06+/-10.28% and decreased to 27.33+/-9.72% and 29.52+/-6.44% when in combination with ceftiofur or the combination of ceftiofur and flunixin, respectively. In conclusion, a lower lidocaine infusion rate may need to be administered to horses on long-term lidocaine infusions. The in vitro protein binding of lidocaine is moderate in equine plasma, but highly protein bound drugs may displace lidocaine increasing unbound concentrations and the risk of lidocaine toxicity.

Influence of gastrointestinal tract disease on pharmacokinetics of lidocaine after intravenous infusion in anesthetized horses

OBJECTIVE

To determine the disposition of lidocaine after IV infusion in anesthetized horses undergoing exploratory laparotomy because of gastrointestinal tract disease.

ANIMALS

11 horses (mean +/- SD, 10.3 +/- 7.4 years; 526 +/- 40 kg).

PROCEDURE

Lidocaine hydrochloride (loading infusion, 1.3 mg/kg during a 15-minute period [87.5 microg/kg/min]; maintenance infusion, 50 microg/kg/min for 60 to 90 minutes) was administered IV to dorsally recumbent anesthetized horses. Blood samples were collected before and at fixed time points during and after lidocaine infusion for analysis of serum drug concentrations by use of liquid chromatography-mass spectrometry. Serum lidocaine concentrations were evaluated by use of standard noncompartmental analysis. Selected cardiopulmonary variables, including heart rate (HR), mean arterial pressure (MAP), arterial pH, PaCO2, and PaO2, were recorded. Recovery quality was assessed and recorded.

RESULTS

Serum lidocaine concentrations paralleled administration, increasing rapidly with the initiation of the loading infusion and decreasing rapidly following discontinuation of the maintenance infusion. Mean +/- SD volume of distribution at steady state, total body clearance, and terminal half-life were 0.70 +/- 0.39 L/kg, 25 +/- 3 mL/kg/min, and 65 +/- 33 minutes, respectively. Cardiopulmonary variables were within reference ranges for horses anesthetized with inhalation anesthetics. Mean HR ranged from 36 +/- 1 beats/min to 43 +/- 9 beats/min, and mean MAP ranged from 74 +/- 18 mm Hg to 89 +/- 10 mm Hg. Recovery quality ranged from poor to excellent.

CONCLUSIONS AND CLINICAL RELEVANCE

Availability of pharmacokinetic data for horses with gastrointestinal tract disease will facilitate appropriate clinical dosing of lidocaine.

Influence of general anesthesia on pharmacokinetics of intravenous lidocaine infusion in horses

OBJECTIVE

To compare the disposition of lidocaine administered IV in awake and anesthetized horses.

ANIMALS

16 horses.

PROCEDURE

After instrumentation and collection of baseline data, lidocaine (loading infusion, 1.3 mg/kg administered during 15 minutes (87 microg/kg/min); constant rate infusion, 50 microg/kg/min) was administered IV to awake or anesthetized horses for a total of 105 minutes. Blood samples were collected at fixed times during the loading and maintenance infusion periods and after the infusion period for analysis of serum lidocaine concentrations by use of liquid chromatography with mass spectral detection. Selected cardiopulmonary parameters including heart rate (HR), mean arterial pressure (MAP), arterial pH, PaCO2, and PaO2 were also recorded at fixed time points during lidocaine administration. Serum lidocaine concentrations were evaluated by use of standard noncompartmental analysis.

RESULTS

Serum lidocaine concentrations were higher in anesthetized than awake horses at all time points during lidocaine administration. Serum lidocaine concentrations reached peak values during the loading infusion in both groups (1,849 +/- 385 ng/mL and 3,348 +/- 602 ng/mL in awake and anesthetized horses, respectively). Most lidocaine pharmacokinetic variables also differed between groups. Differences in cardiopulmonary variables were predictable; for example, HR and MAP were lower and PaO2 was higher in anesthetized than awake horses but within reference ranges reported for horses under similar conditions.

CONCLUSIONS AND CLINICAL RELEVANCE

Anesthesia has an influence on the disposition of lidocaine in horses, and a change in dosing during anesthesia should be considered.

The effect of food or water deprivation on paracetamol pharmacokinetics in calves

This study investigated the effect of food or water deprivation on the pharmacokinetics of paracetamol in 30 Holstein-Friesian preruminant calves (10 controls, 10 food withheld and 10 water-deprived) aged 24-25 days. Control calves were given paracetamol at 24-25 days and again at 28-29 days of age. In the food withheld and water-deprived calves paracetamol studies were performed before and after 4 days of food or water deprivation. In the control group there were no significant differences in pharmacokinetic parameters for paracetamol in 24-25 and 28-29-day-old calves. Witholding food for 4 days was associated with an increase in the mean residence time (MRT) of paracetamol (P < 0.01). When food was withheld total body clearance (ClB) of paracetamol was significantly decreased (P < 0.05). The volume of distribution (Vss) was not significantly altered. Similarly, water deprivation was associated with a significant increase in MRT and significant decrease in ClB of paracetamol (P < 0.01). The Vss was not significantly altered. Food or water deprivation also influenced the formation of major metabolites (glucuronide and sulphate) of paracetamol. It is concluded that food or water deprivation may impair the elimination drugs that undergo metabolism by UDP-glucuronyltransferase and sulphotransferase in cattle.

Effects of intravenous lidocaine overdose on cardiac electrical activity and blood pressure in the horse

This study aimed to identify blood serum lidocaine concentrations in the horse which resulted in clinical signs of intoxication, and to document the effects of toxic levels on the cardiovascular and cardiopulmonary systems. Nineteen clinically normal mature horses of mixed breed, age and sex were observed. Lidocaine administration was initiated in each subject with an i.v. loading dose of 1.5 mg/kg bwt and followed by continuous infusion of 0.3 mg/kg bwt/min until clinical signs of intoxication were observed. Intoxication was defined as the development of skeletal muscle tremors. Prior to administration of lidocaine, blood samples for lidocaine analysis, heart rate, mean arterial blood pressure, systolic blood pressure, diastolic blood pressure, respiratory rate and electrocardiographic (ECG) data were collected. After recording baseline data, repeat data were collected at 5 min intervals until signs of intoxication were observed. The range of serum lidocaine concentrations at which the clinical signs of intoxication were observed was 1.85-4.53 microg/ml (mean +/- s.d. 3.24 +/- 0.74 microg/ml). Statistically significant changes in P wave duration, P-R interval, R-R interval and Q-T interval were observed in comparison to control values, as a result of lidocaine administration. These changes in ECG values did not fall outside published normal values and were not clinically significant. Heart rate, blood pressures and respiratory rates were unchanged from control values. This study establishes toxic serum lidocaine levels in the horse, and demonstrates that there were no clinically significant cardiovascular effects with serum lidocaine concentrations less than those required to produce signs of toxicity.

The effect of short-term starvation or water deprivation on caffeine pharmacokinetics in calves

The aim of this study was to determine the effect of short-term starvation or water deprivation on the pharmacokinetics of caffeine in calves. The experiment was carried out on 30 Holstein-Friesian calves (10 calves in a control group, 10 calves in a 'starved' group and 10 calves in a 'water-deprived' group) aged 24-25 days. Control group calves were given caffeine at 24-25 and 28-29 days of age. In the experimental groups caffeine studies were performed before and after 4 days of starvation or water deprivation. In the control group no significant differences between the pharmacokinetic parameters of caffeine in 24-25 and 28-29 day-old calves were observed. Starvation for 4 days was associated with an increase in the mean residence time (MRT) of caffeine in each subject. The increase was statistically significant (P < 0.01). After starvation the total plasma clearance of caffeine decreased (about 20 per cent). The decrease was statistically significant (P < 0.05). The volume of caffeine distribution (V(ss)) was slightly but not significantly increased. Similarly, water deprivation was associated with significant increase in the mean residence time of caffeine in each subject. The total plasma clearance of caffeine decreased (about 30 per cent). This decrease was statistically significant (P < 0.01). The volume of caffeine distribution was slightly but not significantly decreased. Results obtained in this study indicate that short-term (for 4 days) starvation or water deprivation leads to a general inhibition of hepatic P450 enzymatic system and may impair the elimination of drugs that undergo metabolism by these enzymes.

Exercise-training-induced alterations in hepatic function in mares

The effects of exercise training on hepatic function in horses were determined by studying the plasma clearance of antipyrine (20 mg/kg iv) in adult mares that either underwent treadmill training for 5 wk (n = 7) or remained in box stalls for the same time period (n = 6). Training consisted of treadmill exercise at 60% (12 min/day) and 90% (3 min/day) of pretraining maximal oxygen consumption (V(O2)max) for 6 days/wk for 5 wk. V(O2)max and velocity to obtain a blood lactate concentration of 4 mmol/l were significantly increased (from 129 to 149 ml x min-1 x kg-1 and from 5.6 to 6.1 m/s, respectively) as a result of training. The plasma clearance and volume of distribution of antipyrine increased significantly in the trained group (from 5.5 to 6.4 ml x min-1 x kg-1 and from 813 to 881 ml/kg, respectively) and decreased significantly in the untrained group. Elimination half-lives did not change as a result of training or box rest. Increases in plasma antipyrine clearance were indicative of an increase in hepatic metabolism of antipyrine. Increases in the volume of distribution of antipyrine suggest that total body water increases as a result of exercise training.

Pharmacokinetics of sulfonamides and trimethoprim in the donkey (Equus asinus)

The body disposition of sulfadimidine (SDM), sulfadiazine (SDZ), sulfamethoxypyridazine (SMPZ) and a trimethoprim-sulfadimethoxine combination (TMP-SDMX) was investigated in the donkey. The four sulfonamides and TMP were injected intravenously at doses of 20 mg/kg (SDM, SDZ, SMPZ), 12.5 mg/kg (SDMX) and 2.5 mg/kg (TMP). The body clearance (ClB) of SDZ (1.70 +/- 0.14 ml/min/kg) was significantly higher than those of SDM (1.13 +/- 0.18 ml/min/kg), SMPZ (1.10 +/- 0.09 ml/min/kg) and SDMX (0.75 +/- 0.04 ml/min/kg). In contrast, the volume of distribution at steady state (Vss) was similar for the four sulfonamides (0.68 +/- 0.08 L/kg, 0.63 +/- 0.07 L/kg, 0.47 +/- 0.06 L/kg, and 0.46 +/- 0.05 L/kg for SDM, SDZ, SMPZ and SDMX, respectively). Both ClB and Vss were significantly higher for TMP (4.36 +/- 0.60 ml/min/kg and 2.71 +/- 0.86 L/kg) than for sulfonamides. Antipyrine ClB (3.49 +/- 0.35 ml/min/kg) and Vss (0.66 +/- 0.16 L/kg), determined in order to assess hepatic oxidative function and total body water volume, respectively, were either different from (ClB), or similar to (Vss), values calculated for sulfonamides. The results obtained were compared to those reported in horses.

Pharmacokinetics of lignocaine in Icelandic horses after infiltration anaesthesia

The pharmacokinetics of lignocaine was studied in four Icelandic horses after infiltration anaesthesia. A total of 240 mg of the drug was injected on either side of the left foreleg, over the medial and lateral branches of the palmar nerve. Blood samples were collected up to seven hours after injection and the concentrations of the drug in plasma were determined by gas chromatography/mass spectrometry. The results showed that lignocaine was rapidly absorbed. A mean maximum concentration of 232 ng/ml was observed after 20 minutes. In three of the horses the decline in the plasma concentration of the drug with time was best described by the sum of two exponential terms, but in one of the horses the decline was monoexponential. The mean half-life of the distribution phase (alpha) was 9.8 minutes and that of the elimination phase (beta) 48.4 minutes. In all the horses the plasma concentration was below the limit of detection (2 ng/ml) six hours after injection. Anaesthesia was tested in one horse and lasted for one hour.

Lidocaine elimination and monoethylglycinexylidide formation in the dehydrated camel

The elimination kinetics and the formation of the monoethylglycinexylidide (MEGX), a major metabolite of lidocaine, were studied in camels deprived of water for 14 days. The study was conducted on four camels in a crossover design. Lidocaine was administered intravenously at a dose of 1 mg/kg to adult female camels when water was given ad libitum (stage 1) and to the same camels after 14 days of dehydration. Blood samples were taken up to 6 h after dosing. Serum lidocaine and MEGX levels were analysed by polarization fluorescence immunoassay. The elimination profiles of lidocaine and the formation of the metabolite MEGX in the two phases of the study were essentially identical. No difference in any pharmacokinetic parameter was noticed between normally hydrated and water-deprived camels. It is thus concluded that dehydration does not affect the cytochrome P450 isozymes involved in degradation of lidocaine to MEGX nor does it affect the hepatic blood flow, which is a major determinant in the clearance of lidocaine. The very low clearance of lidocaine in the camel in comparison with other ruminant or monogastric mammals may be associated with the camel's ability to survive drought in the desert.

Pharmacokinetic values of drugs frequently used in performance horses

Tables of values of pharmacokinetic variables (volume of distribution, total body clearance, and plasma elimination half-life) of drugs frequently administered to performance horses are accompanied by explanatory notes. Drugs described include the nonsteroidal anti-inflammatory drugs, corticosteroids, anabolic steroids, central nervous system-modifying drugs, respiratory system drugs, diuretics, local anesthetics, and antibacterial drugs.

Effect of gonadal hormones on the plasma clearance and metabolite formation of antipyrine in the dwarf goat

The effect of gonadal hormones on the plasma elimination and urinary metabolite profile of antipyrine was studied in dwarf goats. Female goats were treated with testosterone and male goats were treated with 17 beta-oestradiol. Castrated males were treated with either testosterone or 17 beta-oestradiol. Antipyrine (25 mg/kg, i.v.) was given both before and after the hormonal treatments. The effects of the hormonal status on the plasma elimination of the parent compound were not consistent. This was possibly due to the fact that formation of the main metabolite of antipyrine in the goat, 4-hydroxy antipyrine (OHA), was not affected by sex or hormonal treatment. On the other hand, there were clear effects of hormonal status on urinary excretion of the three other metabolites. In females and castrated males testosterone suppressed the formation of norantipyrine (NORA), 3-hydroxymethylantipyrine (HMA) and 4,4'-dihydroxyantipyrine (DOHA). Intact males produced smaller amounts of these metabolites than females. It is concluded that distinct xenobiotic metabolizing pathways exist in the dwarf goat, which are influenced in their activity by gonadal hormones. This confirms previous findings in rats and mice. The possibility that sex hormones influence drug metabolism in food-producing animals could have consequences for veterinary therapeutics and public health. This study also demonstrates that, when using the antipyrine test for the assessment of hepatic drug metabolism, it is very important to include the determination of metabolites.

Lidocaine decay and hepatic extraction in the pig

Plasma lidocaine decay after injection was studied in five anaesthetized pigs and fitted to a two compartment open model. Derived pharmacokinetic parameters were employed to rapidly achieve plateau concentrations within 60 min of starting a two stage infusion of lidocaine hydrochloride. Hepatic extraction and clearance of lidocaine at steady state were determined in 10 pigs by transhepatic sampling and measurement of hepatic arterial and portal venous blood flow using perivascular ultrasonic flow probes placed at laparotomy. These data were compared with similar studies performed in man as well as the sheep, dog, monkey and cat. The lidocaine extraction ratio of 0.60 in the pig was found to be similar to that determined by others in man.

Plasma disappearance of creatinine as a renal function test in the dog

The serum concentration of creatinine at 120 minutes (SC120) after intravenous injection of 88 mg kg-1 of creatinine, the plasma half-life (t1/2) and the plasma clearance of creatinine (PCC) were evaluated as renal function tests in 30 healthy adult dogs and six adult dogs with known or suspected renal disease. The mean SC120 in the normal dog was 0.31 +/- 0.08 mmol litre-1 and in the clinical cases 0.71 +/- 0.19 mmol litre-1. The correlation coefficients between SC120 and renal creatinine clearance (RCC) for the normal dogs and the clinical cases were -0.76 and -0.69, respectively. At 120 minutes after injection, 95 per cent of normal dogs would be predicted to have a serum creatinine concentration below 0.46 mmol litre-1. The mean plasma t1/2 of creatinine for the normal dogs was 107.7 +/- 17.96 minutes, while the clinical cases had a wide range of values (148.8 to 620.1 minutes). Plasma t1/2 of creatinine was correlated with RCC for both the normal dogs and the clinical cases (r = -0.55, r = -0.91, respectively). The mean PCC for the normal dogs was 7.42 +/- 2.22 ml min-1 kg-1 (range 4.95 to 13.28 ml min-1 kg-1). There was a good correlation between RCC and PCC (r = 0.7). The PCC for the clinical cases ranged from 0.76 to 3.37 ml min-1 kg-1. The correlation between RCC and PCC was significant (r = 0.91). Thus SC120, t1/2 and PCC may be useful methods of assessing renal function in dogs with renal impairment insufficient to cause azotaemia.

Evaluation of hepatobiliary disorders in the horse

This article addresses clinical problems that present in equine liver disease. It also discusses the variety of laboratory tests available to the clinician that can differentiate the type and degree of liver dysfunction. This is followed by a more specific discussion regarding unique features of equine bilirubin and bile acid metabolism.