Plasma concentrations of lidocaine in dogs following lidocaine patch application

Transdermal absorption of lidocaine was determined by measuring plasma lidocaine concentrations following skin application of 5% lidocaine patches. Two lidocaine patches were placed on the ventral abdominal midline of seven dogs for 72 hours. Lidocaine was detectable in plasma 12 hours after patch application, and it reached steady-state concentrations between 24 and 48 hours. Plasma lidocaine levels decreased dramatically at 60 hours post-application. Low plasma lidocaine concentrations remained for 6 hours after patch removal. No clinically significant side effects were noted.     

Pharmacokinetics of lidocaine following the application of 5% lidocaine patches to cats

Lidocaine patches have been used to provide local analgesia in dogs and cats. We conducted this study to assess the systemic and local absorption of lidocaine from topical patches in cats. Eight 2-year-old cats received either intravenous lidocaine at 2 mg/kg or one 700 mg lidocaine patch placed on the lateral thorax for 72 h, in a cross-over randomized repeated measures design. Plasma was collected at specific times and the skin was biopsied at the time of patch removal for the quantitative analysis of lidocaine and its major metabolite, monoethylglycinexylidide (MEGX), by gas chromatography with mass spectrometry. Percent absorption time plots for systemic lidocaine appearance were constructed using the Loo-Riegelman method. Approximately, constant rate absorption was observed from 12-72 h after patch application at a mean +/- SD rate of 109 +/- 49 microg/kg/h, resulting in steady-state lidocaine plasma concentrations of 0.083 +/- 0.032 microg/mL and MEGX concentrations of 0.012 +/- 0.009 microg/mL. Overall bioavailability of transdermal lidocaine was 6.3 +/- 2.7%, and only 56 +/- 29% of the total lidocaine dose delivered by the patch reached systemic circulation. Skin lidocaine concentrations were much higher than plasma concentrations, at 211 +/- 113 microg/g in the thoracic skin beneath the patch and 2.2 +/- 0.6 microg/g in the contralateral thoracic skin without the patch. As both lidocaine and MEGX were recovered from contralateral skin, it is likely that lidocaine accumulated in the skin from low systemic concentrations of circulating lidocaine over the 72-h period of patch application. Plasma lidocaine concentrations remained well below systemically toxic concentrations, and no obvious clinical side effects were observed in any of the cats. The low systemic absorption rate coupled with high local lidocaine concentrations on the skin support the safe use of lidocaine patches in cats.     

Pharmacokinetics of a lidocaine patch 5% in dogs

Lidocaine is increasingly used in transdermal drug delivery systems for different pain conditions in human medicine whereby several pharmacokinetic studies have demonstrated minimal systemic absorption in men. In the present study, the pharmacokinetics of a lidocaine patch 5% was studied in six dogs. In the first experiment, one single lidocaine patch was applied for 12 h to the lateral side of the thorax after removing the hair either by clipping or by the application of a depilatory agent, according to a two-way crossover design. No potential adverse effects induced by the patches were observed in either group. In dogs with clipped hair, a mean peak plasma lidocaine concentration of 62.94 ng/ml was obtained after 10.67 h. In the depilatory group, a mean peak plasma concentration of 103.55 ng/ml was reached after 9.27 h. Significant differences in the AUC(0 --> infinity), C(max), k(a) and T(1/2a) were noticed between the two groups. No significant differences were found for the elimination parameters and for T(max). In the second experiment, the patches were applied for 60 h to the clipped skin in order to study the absorption kinetics after a prolonged application period. There, the mean peak lidocaine plasma concentration was 45.18 ng/ml achieved after 24 h and a final concentration of 29.37 ng/ml was obtained at 60 h. In conclusion, all dogs tolerated the transdermal lidocaine patch well. The results of this study suggest that there is an overall minimal absorption from the lidocaine patch. However, the application of a depilatory agent leads to a more rapid and increased absorption of lidocaine.     

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.     

Evaluation of Plasma Detectable Concentrations of Two Lidocaine Transdermal Formulations and Their Analgesic Effect in the Horse

Transdermal absorption of four lidocaine (L) patches (5%) was compared with the transdermal absorption of L cream (5%) to evaluate the pharmacokinetics of the two formulations applied on the same anatomic region under dressing in eight horses. The animals were also assessed for antinociceptive effect after removal of the patches and cream, using a psychophysical method visual analog scale, by pricking the patient's skin to test the response to pain. Horses were randomly assigned to four treatment groups: in groups I and II, four L patches were applied for a period of 24 hours with and without alcohol precleaning, respectively; in group III, 5% L cream was applied every 2 hours over a 24-hour period on the same anatomic site. Group IV was the control. No clinical side effects were noted with either formulation. L was detectable in plasma 6 to 24 hours after application of the patches, and the highest plasma concentrations were reached between 12 and 18 hours. The use of alcohol to preclean the skin appeared to reduce the transdermal drug absorption over time. After L cream application, the peak plasma drug concentration occurred at 24 hours. Nociception assessment after L patch or L cream application showed a decreased response when the L cream was used. The results of this study showed that there was minimal absorption from the L patches compared with the L cream in horses. Also, the L cream treatment reduced significantly the intensity of nociception quality as measured by the visual analog scale.     

Effects of 5% and 10% Guaifenesin Infusion on Equine Vascular Endothelium

Twelve horses of various breeds and either sex were anesthetized with xylazine and ketamine injected into a median or lateral thoracic vein. During anesthesia, with the horse in sternal recumbency, a 14-gauge, 8.9 cm catheter was inserted into each jugular vein by using aseptic technique. Guaifenesin in water (100 mg/kg or a maximum dose of 50 grams) was infused into one jugular vein and an equal volume of 0.9% saline solution was infused into the other jugular vein. Seven horses received 10% guaifenesin, and five horses received 5% guaifenesin. The catheters were removed before the horses recovered from anesthesia. The horses were euthanatized approximately 48 hours later, and the jugular veins were removed for histologic examination. Adherent thrombus material was observed in all veins exposed to 10% guaifenesin and in one vein exposed to 5% guaifenesin. No evidence of thrombus was observed in four veins infused with 5% guaifenesin or in those infused with saline solution. These findings are of particular significance with horses at increased risk for thrombosis or thrombophlebitis.     

Assessment of serum concentrations and sedative effects of fentanyl after transdermal administration at three dosages in healthy llamas

OBJECTIVE: To determine the serum concentrations and sedative effects of fentanyl after transdermal administration at 3 dosages in llamas. ANIMALS: 9 healthy adult female llamas (mean age, 8 +/- 3 years; mean weight, 150 +/- 18 kg). PROCEDURE: Llamas were allocated to 1 of 3 groups (3 llamas/group). Fentanyl patches (each providing transdermal delivery of 75 microg of fentanyl/h) were placed on shaved areas of the antebrachium of all llamas. In group 1, llamas were treated with 1 patch (anticipated fentanyl dosage, 75 microg/h). In group 2, llamas were treated with 2 patches (anticipated fentanyl dosage, 150 microg/h). In group 3, llamas were treated with 4 patches (anticipated fentanyl dosage, 300 microg/h). For each llama, the degree of sedation was assessed by use of a subjective scoring system and a blood sample was collected for determination of serum fentanyl concentration at 12, 24, 36, 48, 60, and 72 hours after patch placement. RESULTS: Following the placement of 4 patches, mean +/- SD serum fentanyl concentration in group 3 llamas reached 0.3 +/- 0.08 ng/mL within 12 hours. This concentration was sustained for 72 hours. In group 2, application of 2 patches provided inconsistent results; in group 1, application of 1 patch rarely provided measurable serum fentanyl concentrations. No llamas became sedated at any time. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that application of four 75 microg/h fentanyl patches provides consistent, sustained serum fentanyl concentrations without sedation in llamas. However, the serum concentration of fentanyl that provides analgesia in llamas is not known.     

Outcome of epidural catheterization for delivery of analgesics in horses: 43 cases (1998-2001)

OBJECTIVE: To determine reasons for epidural catheter placement among horses examined at a veterinary teaching hospital, efficacy of epidural administration of analgesics, duration of catheter placement, reasons for catheter removal, and complications encountered. DESIGN: Retrospective study. ANIMALS: 43 horses. PROCEDURE: Medical records were reviewed. RESULTS: A total of 50 epidural catheters were placed in the 43 horses. Underlying conditions included fractures, lacerations, septic arthritis, myositis, perineal injuries, and cellulitis. Horses ranged from 2 to 21 years old and weighed between 365 and 795 kg (803 and 1,749 lb). Median duration of catheter placement was 96 hours (range, 1.5 to 480 hours). The response to epidural drug administration was reported as positive in 34 horses and negative in 4. There was no apparent response in 2 horses, and response could not be determined in 3. Three temporary patient-related complications associated with epidural catheter administration were observed. Technical problems associated with the epidural catheters included dislodgement of the catheter itself (7 catheters) or of the adapter or filter (5), obstruction (5), and leakage (5). Twenty-two catheters were removed because of resolution of the underlying condition, and 10 were removed because of complications. For 6 catheters, the reason for catheter removal was not recorded. The remaining 12 catheters were in place when the horses were euthanatized. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that epidural catheterization can be used successfully for repeated epidural delivery of analgesics and anesthetics in horses with various clinical conditions. Complications associated with epidural catheters or epidural drug administration were infrequent and transient.     

Serial alterations in digital hemodynamics and endothelin-1 immunoreactivity, platelet-neutrophil aggregation, and concentrations of nitric oxide, insulin, and glucose in blood obtained from horses following carbohydrate overload

OBJECTIVE: To quantify changes in endothelium-derived factors and relate those changes to various aspects of digital hemodynamics during the prodromal stages of carbohydrate overload (CHO)-induced laminitis in horses. ANIMALS: 20 adult horses without abnormalities of the digit. PROCEDURES: Digital and jugular venous blood samples were collected at 1-hour intervals (for assessment of endothelin-1 [ET-1] immunoreactivity and measurement of glucose, insulin, and nitric oxide [NO] concentrations) or 4-hour intervals (CBC and platelet-neutrophil aggregate assessment) for 8 hours or 16 hours after induction of CHO-associated laminitis in horses treated with an ET-1 antagonist. Effects of treatment, collection site, and time and the random effects of horse on each variable were analyzed by use of a repeated-measures model. Where treatment and collection site had no significant effect, data were combined. RESULTS: Compared with baseline values, CHO resulted in changes in several variables, including a significant increase from baseline in digital blood ET-like immunoreactivity at 11 hours; digital blood ET-like immunoreactivity was significantly greater than that in jugular venous blood at 8, 9, 11, and 12 hours. Digital and jugular venous blood concentrations of glucose increased from baseline significantly at 3, 4, and 5 hours; insulin concentration increased significantly at 5 hours; and the number of platelet-neutrophil aggregates increased significantly at 12 hours. CONCLUSIONS AND CLINICAL RELEVANCE: In horses, concurrent increases in venous blood ET-1 immunoreactivity, insulin and glucose concentrations, and platelet-neutrophil aggregates support a role of endothelial dysfunction in the pathogenesis of CHO-induced laminitis.     

Evaluation of risk factors for development of catheter-associated jugular thrombophlebitis in horses: 50 cases (1993-1998)

OBJECTIVE: To evaluate risk factors associated with development of catheter-associated jugular thrombophlebitis in hospitalized horses. DESIGN: Retrospective case-control study. ANIMALS: 50 horses with thrombophlebitis and 100 control horses. PROCEDURE: Medical records from 1993 through 1998 were searched for horses with thrombophlebitis. Horses that were hospitalized for at least 5 days, had an i.v. catheter placed in a jugular vein (other than for solely anesthetic purposes), and had no evidence of thrombophlebitis during admission or hospitalization were chosen as controls. Signalment, history, clinicopathologic findings, primary illness, and treatment were obtained from the medical records. Data were analyzed by use of logistic regression to perform univariate and multivariate analyses. RESULTS: For a horse with endotoxemia, the odds of developing thrombophlebitis were 18 times those for a similar horse without endotoxemia. For a horse with salmonellosis, the odds of developing thrombophlebitis were 68 times those for a similar horse without salmonellosis. For a horse with hypoproteinemia, the odds of developing thrombophlebitis were almost 5 times those for a similar horse without hypoproteinemia. For a horse in the medicine section, the odds of developing thrombophlebitis were 16 times those for a similar horse in the surgery section. For a horse with large intestinal dise, the odds of developing thrombophlebitis were 4 times those for a similar horse without large intestinal disease. For a horse receiving antidiarrheal or antiulcerative medications, the odds of developing thrombophlebitis were 31 times those for a similar horse not receiving these medications. CONCLUSIONS AND CLINICAL RELEVANCE: Results indicated that patient factors, including large intestinal disease, hypoproteinemia, salmonellosis, and endotoxemia, were associated with development of catheter-associated thrombophlebitis in horses.     

Use of a jugular vein model for the instruction of equine intravenous catheter placement by third-and fourth-year veterinary students

Successful placement of an i.v. catheter in horses is a core competency required by the AVMA COE accreditation policy (section 7.11, #2,#3,#5). Therefore, we determined whether the use of a novel intravenous (i.v.) catheter placement training tool would improve confidence, time to successful placement and accuracy with which third- and fourth-year veterinary students acquire competence regarding this skill in adult horses. Students lacking earlier experience with i.v. catheter placement in horses were randomly assigned to either Group 1 (n = 15) or 2 (n = 10). Students with prior experience with the technique were assigned to Group 3 (n = 14). Group 1 students were instructed on i.v. catheter placement using the model and then assessed using a live horse. Group 2 students were instructed on i.v. catheter placement using a live horse and then assessed using a live horse. Group 3 students were assessed using a live horse. Assessments included number of placement attempts, number of redirects and time needed to achieve correct intraluminal placement. Student perceptions regarding the use of this model for the purpose of learning i.v. catheter placement in horses were evaluated using a questionnaire. Results of this study demonstrated that students receiving initial instruction with the model achieved better scores in terms of both time to accomplish the task and accuracy for catheter placement in a live horse when compared with students that received their first instruction on a live horse. The use of this i.v. catheter placement training tool therefore served to lessen the cost of training and to minimise animal discomfort during this component of veterinary student training.     

Short‐term anaesthesia with xylazine,diazepam/ketamine for castration in horses under field conditions: Use of intravenous lidocaine

Reasons for performing study: Lidocaine single boluses and/or constant rate infusions are commonly administered intraoperatively during inhalant anaesthesia to lower inhalant concentrations, promote or maintain gastrointestinal motility, and potentially supplement analgesia. The benefits of using lidocaine with injectable anaesthesia for field surgeries has not been fully explored to determine advantages and disadvantages of lidocaine as an anaesthetic and analgesic adjunct in these conditions and impact on recovery quality. Objectives: To evaluate the use of systemic lidocaine with a standard field injectable anaesthetic protocol related to the need for additional drug administration as well as overall recovery score and quality. Hypothesis: The administration of systemic lidocaine with xylazine‐diazepam/ketamine anaesthesia for castration in the field decreases the need for additional injectable doses required for maintenance, but prolong and potentially impact the overall recovery score and quality in horses. Methods: Thirty client‐owned horses underwent standard injectable anaesthesia for field castration. Fifteen horses received lidocaine 3 mg/kg bwt, i.v. as a single bolus, and 15 received saline equal volume. The horses were monitored for the need for additional injectable anaesthetics and scored for overall recovery and quality by a blinded anaesthetist. Results: There were no statistically significant differences in the overall recovery score and quality, or need for additional injectable anaesthetic between horses receiving lidocaine and those receiving saline. There was a significantly longer time for the horses to stand after induction in the lidocaine group (mean 30.7 min) vs. saline group (mean 22.5 min) (P<0.04). Conclusions: Lidocaine, 3 mg/kg bwt i.v., does not adversely affect recovery using injectable field regimes, but the overall recovery period was longer. Lidocaine does not appear to reduce the need for additional injectable administration during surgery. Potential relevance: Further research is warranted to define the benefit of systemic lidocaine with field anaesthesia in horses by exploring the ideal dose and plasma level of lidocaine with injectable anaesthesia.     

Evaluation of Indicators of Weight-Carrying Ability of Light Riding Horses

To answer the question of whether horse height, cannon bone circumference, and loin width can be used as indicators of weight-carrying ability in light horses, eight mature horses underwent a submaximal mounted standard exercise test under four conditions: carrying 15, 20, 25, or 30% of their body weight. Heart rate was monitored, plasma lactate concentration was determined in jugular blood samples pre-exercise, immediately post-exercise, and 10 minutes post-exercise, with serum creatine kinase activity determined at the same times as plasma lactate concentrations, with additional samples collected at 24 hours and 48 hours post-exercise. Muscle soreness and muscle tightness scores were determined using a subjective scoring system 24 hours before and 24 hours after exercise. Heart rates remained significantly higher when the horses carried 25 and 30% of their body weight. Plasma lactate concentrations immediately and 10 minutes after exercise differed when horses carried 30% of their body weight compared with 15, 20, and 25% weight carriage. Horses tended to have a greater change in muscle soreness and muscle tightness when carrying 25% of their body weight, and a significant change in soreness and tightness scores was found in horses carrying 30% of their body weight. Loin width and cannon bone circumference were found to be negatively correlated to the changes in muscle soreness and tightness scores. In conclusion, the data suggest that horses with wider loin and thicker cannon bone circumference became less sore when carrying heavier weight loads.     

The Use of Ultrasound to Assess the Thrombogenic Properties of Teflon and Polyurethane Catheters for Short-Term Use in Systemically Healthy Horses

The objective of this study was to establish an ultrasound scoring system to assess the degree of fibrin tag and clot formation associated with jugular catheters in horses. A second objective was to test the hypothesis that, in healthy horses undergoing short-term anesthesia, polyurethane (PU) catheters have a different ultrasound score compared with polytetrafluoroethylene (Teflon) catheters. Twenty-three systemically healthy horses requiring anesthesia were randomly assigned PU or Teflon catheters. The mean catheter dwell time was 5 hours. Ultrasound was used to examine the jugular vein before catheter placement, before catheter removal (mean, 4 hours 45 minutes), and approximately 24 hours after catheter insertion (mean, 24 hours 42 minutes). The repeatability of the ultrasound scoring system was assessed by producing a series of randomly ordered selected still images for independent evaluation by four radiologists on two separate occasions. The interobserver and intraobserver repeatability was determined using a weighted kappa statistic, with substantial intraobserver repeatability mean κ .77 (range, .59–.91) and moderate interobserver repeatability mean κ .53 (range, .33–.67). The proportion of positive bacteriologic cultures in the polyurethane (PU) and Teflon groups was compared using Fisher's exact test, with no significant difference (P > .99) between the two groups. The distribution of the scores from the PU and Teflon groups was compared using the Mann-Whitney test, with no evidence of a difference between groups at the time of catheter removal (P = .23) and 24 hours after catheter insertion (P = .20). Further studies with a larger sample of systemically healthy horses are required to substantiate this preliminary finding.     

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.     

Jugular thrombophlebitis resulting from an anaesthetic induction technique in the horse

Forty-eight hours after general anaesthesia was induced by glycerol guaiacolate and thiopentone sodium in 10 horses, the jugular veins were dissected out at post-mortem and examined visually and histologically. All veins showed a marked thrombophlebitis involving the media and intima at the site of injection. The influence of catheter composition, catheter placement and chemical composition on the occurrence of the thrombophlebitis is discussed. It is concluded that the thrombophlebitis was chemically induced.     

Transdermal fentanyl combined with nonsteroidal anti-inflammatory drugs for analgesia in horses

This study investigated the pharmcokinetics, efficacy, and safety of the fentanyl transdermal therapeutic system (TTS) in horses in which there was an inadequate analgesic response to nonsteroidal anti-inflammatory drugs (NSAIDs) alone. Nine horses with pain that was refractory to therapeutic doses of phenylbutazone (n = 3) or flunixin meglumine (n = 6) subsequently also received between 39 and 110 microg/kg of transdermal fentanyl. Blood samples were collected at 0, 1, 2, 3, 4, 5, 6, 12, 24, 36, 48, 60, and 72 hours after patch application, and a radioimmunoassay was used to determine serum fentanyl concentrations. Pharmacokinetic values were determined by noncompartmental analysis. Physical examination findings were recorded in all horses, and pain and lameness grading systems were used to assign scores to 8 and 6 horses, respectively. All horses tolerated the administration of fentanyl TTS, in that no clinically significant adverse effects attributable to fentanyl were observed. Use of the TTS resulted in variable serum concentrations of fentanyl, with a peak serum concentration of 2.2+/-1.1 ng/mL (mean+/-SD) and a time to peak serum concentration of 26+/-13 hours. After transdermal fentanyl administration, mean time to reach serum fentanyl concentrations consistent with analgesia in other species (1 ng/mL) was 14 hours. In addition, serum fentanyl concentrations of 1 ng/mL or greater were maintained in all but one horse for at least 18 hours. Pain scores were significantly decreased after fentanyl TTS and NSAID administration (P < .05), but lameness scores were not significantly different (P > .05). Overall, administration of fentanyl TTS had a favorable pharmacokinetic profile in horses with clinical pain, and the fentanyl TTS in combination with NSAIDs appeared to provide safe and effective analgesia in most of the horses with pain that was refractory to NSAID therapy alone.     

Serum Concentrations of Lidocaine and Its Metabolites MEGX and GX During and After Prolonged Intravenous Infusion of Lidocaine in Horses after Colic Surgery

Lidocaine is the most commonly used prokinetic after gastrointestinal surgery in horses. Cardiovascular status, hepatic function, and duration of therapy are the primary determinants of lidocaine metabolism, and these factors could affect equine patients after colic surgery. This study examined the systemic concentrations of lidocaine and its active metabolites monoethylglycinexylidide (MEGX) and glycinexylidide (GX), in horses that had undergone colic surgery and subsequently received prolonged postoperative lidocaine infusions. The mean lidocaine concentration increased over the course of treatment but did not exceed the therapeutic range. Concentrations of MEGX and GX increased progressively, and concentrations exceeding 1,000 ng/ml were observed frequently after 72 hours of infusion. None of the horses in the study developed severe signs of toxicity; however, the progressively increasing concentrations of lidocaine, MEGX, and GX are cause for concern in clinically ill patients receiving prolonged lidocaine therapy. The potential contribution of MEGX and GX should be considered when evaluating adverse reactions to prolonged lidocaine infusions.     

Transvenous electrical cardioversion of equine atrial fibrillation: technical considerations

Conventional treatment of equine atrial fibrillation (AF) involves administration of quinidine salts. Most uncomplicated cases respond to treatment, but pharmacologic cardioversion involves a range of adverse effects, and some horses are unable to tolerate medication. A study was undertaken to develop transvenous electrical cardioversion (TVEC) as an alternative treatment. Safety issues and catheter placement techniques with catheter-integrated cardioversion electrodes were investigated, and responses to shock application were evaluated. After the premortem catheterization of elective-euthanasia horses, no tissue abnormalities were detected at postmortem examination. To evaluate the response to the application of shocks and appropriate electrode positions, an electrical cardioversion of research horses in chronic AF was then attempted. After catheterization of the right atrium (RA) and pulmonary artery through the right jugular vein, horses were placed under general anesthesia. Biphasic, truncated exponential shock waves were delivered at incremental energies until cardioversion was achieved or until a maximum energy of 300 J was reached. Five treatment events were applied to 3 horses, with cardioversion achieved in one of the treatment events. No adverse effects of cardioversion attempts or general anesthesia were observed. The procedure was then applied to 8 client-owned horses, with cardioversion achieved in 7. No adverse responses to appropriately delivered shocks were observed. No antiarrhythmic medications were administered to any horse at any stage. Catheter design and placement technique evolved throughout the study, with combined ultrasonography and pressure guidance proving most effective in achieving appropriate electrode placement. Results suggest TVEC, as applied in the present study, is a safe, effective, and realistic therapeutic option for equine AF.     

Comparison of surgical and nonsurgical treatment of humeral fractures in horses: 22 cases (1980-1989).

Medical records of 22 horses with humeral fractures were reviewed. The horses were from 2 to 144 months old (mean, 25.8 +/- 37.3 months). Ten horses were treated with stall confinement, 3 were treated surgically, and 9 were euthanatized at the time of diagnosis. Seven of 10 horses treated nonsurgically (stall confinement) were able to be ridden 5 to 12 months after the diagnosis was made (mean, 7.5 +/- 2.6 months). One horse treated nonsurgically was euthanatized 6 months after diagnosis because of laminitis in the contralateral limb. Two horses treated nonsurgically were lost to follow-up evaluation. Two of the 3 horses treated surgically had fractures repaired with Rush pins. The fractured humerus of the third horse was repaired with lag screws. Of the 3 surgically treated horses, 1 was pasture sound 10 months after surgery, but developed varus deviation in the contralateral carpus 6 weeks after repair; 1 horse was euthanatized 2 weeks after surgery because of failure of the implant; and the other horse was sound for riding 10 months after surgery. On the basis of these findings, young horses with humeral fractures that are treated nonsurgically can become sound for riding.