Effect of feeding on the pharmacokinetics of oral minocycline in healthy adult horses.

Minocycline is commonly used to treat bacterial and rickettsial infections in adult horses but limited information exists regarding the impact of feeding on its oral bioavailability. This study's objective was to compare the pharmacokinetics of minocycline after administration of a single oral dose in horses with feed withheld and with feed provided at the time of drug administration. Six healthy adult horses were administered intravenous (2.2 mg/kg) and oral minocycline (4 mg/kg) with access to hay at the time of oral drug administration (fed) and with access to hay delayed for 2 hr after oral drug administration (fasted), with a 7-day washout between treatments. Plasma concentration versus time data was analyzed based on noncompartmental pharmacokinetics. Mean ± SD bioavailability (fasted: 38.6% ± 4.6; fed: 15.7% ± 2.3) and Cmax (fasted: 1.343 ± 0.418 µg/ml; fed: 0.281 ± 0.157 µg/ml) were greater in fasted horses compared to fed horses (p < .05 both). Median (range) Tmax (hr) in fasted horses was 2.0 (1.5-3.5) and in fed horses was 5.0 (1.0-8.0) and was not significantly different between groups. Overnight fasting and delaying feeding hay 2 hr after oral minocycline administration improve drug bioavailability and thus plasma concentrations.

Efficacy of intramuscular meperidine hydrochloride versus placebo in experimental foot lameness in horses.

REASONS FOR PERFORMING STUDY: There are no peer reviewed, blinded controlled studies regarding the skeletal analgesic efficacy of intramuscularly administered meperidine in horses. OBJECTIVES: Using an adjustable heart bar shoe model of equine foot pain, the objective of this study was to test the hypothesis that meperidine (pethidine) administered intramuscularly would prove more efficacious in alleviating lameness than a saline placebo. STUDY DESIGN: Crossover pharmacodynamic experiment. METHODS: Eight healthy adult Thoroughbred horses randomly underwent weekly i.m. treatments 1 h after lameness induction: saline placebo (1 ml/45 kg bwt) or meperidine hydrochloride (1 mg/kg bwt i.m.). Heart rate (HR) and lameness score (LS) responses were assessed by a blinded observer every 20 min for 5 h after lameness induction and then hourly through 12 h after treatment. Jugular venous blood samples were obtained at -1, 0, 0:05, 1, 2, 4, 6, 8, 10 and 12 h and were subsequently analysed for drug concentrations (lower limit of detection, 1 ng/ml). Repeated measures ANOVA and post hoc Tukey's test were used to identify analgesic effects at a significance level of P<0.05. RESULTS: Mean (± s.e.) HR were lower in meperidine trials at 2.3, 3.3 and 3.7 h post administration (P<0.05). Mean LS were lower in meperidine trials at 2.0, 2.3 and 3.3 h post administration (P<0.05). Mean plasma (meperidine) peaked at 227 ± 52 ng/ml at 1 h post administration and decreased to 2.7 ± 0.3 ng/ml at 12 h post administration. In 3 of 8 subjects, plasma (meperidine) was below the lower limit of detection at 12 h after administration. CONCLUSIONS: Intramuscular meperidine was more effective than the saline placebo but only for 2.0-3.7 h post administration compared with the 8-12 h durations of efficacy reported previously using this same model when horses were treated with nonsteroidal anti-inflammatory drugs (NSAIDs). Meperidine may be a suitable nonNSAID alternative analgesic for acute foot pain with efficacy lasting from 2-3 h after a single i.m. dose.

The effects of hyperglycemia and endotoxemia on coagulation parameters in healthy adult horses.

BACKGROUND: Hyperglycemia and endotoxemia have been associated with coagulation abnormalities in horses. Studies in humans suggest greater disturbances in coagulation with hyperglycemia and concurrent endotoxemia. OBJECTIVES: To compare coagulation parameters in horses administered with lipopolysaccharide (LPS) with and without concurrent hyperglycemia. ANIMALS: Twelve healthy adult horses. METHODS: Hyperglycemia (180-240 mg/dL) was maintained for 6 hours in 6 horses (GLU-LPS) using 140 mg/kg IV bolus of dextrose followed by a 20% dextrose constant rate infusion. A similar volume of saline was administered to an additional 6 horses (SAL-LPS). LPS (20 ng/kg) was administered to each horse. Fibrogen concentration, prothrombin time (PT), activated partial thromboplastin time (aPTT), thrombin antithrombin concentration (TAT), and thromboelastometry were measured at baseline and after 1, 1.5, 2, 2.5, 3, 4, 6, and 22 hours. Repeated measures analysis of variance was used to examine temporal changes. RESULTS: Increases in PT (P = .001) and TAT (P = .027) were observed in the GLU-LPS group. Changes in thromboelastometry parameters including increased clot formation time (In-TEM, P = .006; Ex-TEM, P = .002) and decreased alpha angle (Ex-TEM, P = .04) and maximal clot firmness (Ex-TEM, P = .014) were observed in the SAL-LPS group. Differences between SAL-LPS and GLU-LPS groups were limited to increased maximal clot firmness (Ex-TEM) at 3, 6, and 22 hours (P < .001) in the SAL-LPS group. CONCLUSIONS AND CLINICAL IMPORTANCE: Minor alterations in coagulation parameters identified for each group are most likely not clinically relevant. Observed differences between groups do not suggest that concurrent hyperglycemia and endotoxemia are associated with greater coagulation abnormalities in horses.

Dose titration of the clinical efficacy of intravenously administered flunixin meglumine in a reversible model of equine foot lameness.

REASONS FOR PERFORMING STUDY: There are no refereed controlled documentations of the skeletal analgesic efficacy of different dosages of flunixin meglumine (FM). OBJECTIVES: The objective of this experiment was to compare the efficacy of various dosages of FM with a negative control. The hypothesis was that higher doses would result in improved efficacy in a dose-dependent manner when tested in a reversible model of foot lameness. METHODS: Ten horses shod with adjustable heart bar shoes had weekly modified AAEP grade 4.0/5.0 lameness induced by tightening a set screw against the heart bar. Heart rate (HR) and lameness score (LS) were monitored by one double-blinded investigator at rest; every 20 min after lameness induction for 5 h and hourly for another 8 h. One hour after lameness induction, treatments were administered i.v. in a randomised order: negative control (isotonic saline: SAL) or FM at 0.55 (half-dose), 1.1 (single-dose) or 2.2 (double-dose) mg/kg bwt. Results were compared using RM ANOVA and Student-Newman-Keul's test with the level of significance set at P < 0.05. RESULTS: Compared to SAL, half-dose FM reduced HR at 2.33, 2.67, 4.0-8.0, and 10.0 h and LS at 1.33-12.0 h (P < 0.05). Single- and double-dose FM reduced HR from 0.67 to 12.0 h and LS from 1.0 to 12.0 h post administration (P < 0.05). Compared with half-dose FM, single- and double-dose LS were further decreased from 1.67 to 12.0 h post administration (P < 0.05). Mean peak and decaying plasma FM concentrations were different between dosages in a dose-dependent manner through 6 h post administration (P < 0.05). CONCLUSIONS: Flunixin meglumine administration affected dependent variables in a dose-dependent manner with half-dose FM clinically effective for a shorter period. Higher dosages did not perform differently from one another. POTENTIAL RELEVANCE: Practitioners must be aware that half-doses of FM are less efficacious than single doses but double doses are not more efficacious and yet are potentially more toxic.

The relationship between serum calcium concentration and outcome in horses with renal failure presented to referral hospitals.

BACKGROUND: Hypercalcemia is common in horses with renal failure, but it is not known whether it impacts prognosis. HYPOTHESIS/OBJECTIVES: The primary objective of this study was to determine whether hypercalcemia was associated with decreased likelihood of survival to discharge in horses with renal failure. Secondary objectives were to determine whether hypercalcemia was more common in acute (ARF) or chronic renal failure (CRF), whether feeding alfalfa was associated with hypercalcemia, and whether serum creatinine concentration was associated with survival. ANIMALS: Medical records of 63 horses presented to referral hospitals for renal failure were evaluated. Cases were classified as ARF or CRF based on historical and clinical findings. METHODS: The distribution of hypocalcemic, normocalcemic, and hypercalcemic cases in the ARF and CRF groups was determined. Mean serum calcium and creatinine concentrations for survivors and nonsurvivors, and for ARF and CRF cases, were compared. Mean serum calcium concentrations for cases fed alfalfa or not fed alfalfa were compared. RESULTS: Hypercalcemia was significantly more common in CRF than ARF cases. CRF cases fed alfalfa were significantly more likely to be hypercalcemic. There was no significant difference in serum calcium concentration between survivors and nonsurvivors. Serum creatinine concentration was significantly higher in nonsurvivors and in ARF cases. CONCLUSIONS AND CLINICAL IMPORTANCE: Horses with CRF are more likely to be hypercalcemic than horses with ARF. Hypercalcemia was not associated with outcome in renal failure cases in this study. Additional research on the impact of dietary calcium on long-term well-being in horses with CRF is warranted.

Phenylbutazone and flunixin meglumine used singly or in combination in experimental lameness in horses.

REASON FOR PERFORMING STUDY: Using an adjustable heart bar shoe model of foot pain, the objective of this study was to test the hypothesis that the combined use of phenylbutazone (PBZ) and flunixin meglumine (FM) would prove more efficacious in alleviating lameness than either drug alone. MATERIALS AND METHODS: One hour after induction of lameness at weekly intervals, 8 healthy adult Thoroughbred horses randomly underwent one of 4 i.v. treatments: saline (SAL) placebo (1 ml/45 kg bwt), PBZ (4.4 mg/kg bwt), FM (1.1 mg/kg bwt) or PBZ+FM (at the same dosages as given individually). Heart rate (HR) and lameness score (LS) responses were assessed in a blinded manner every 20 min for 5 h after lameness induction and then hourly for 12 h after treatment. Jugular venous blood samples were obtained at -1, 0, 0.05, 1, 2, 4, 6, 8, 10 and 12 h and subsequently analysed for drug concentrations. Repeated measures ANOVA and post hoc Tukey's test were used to identify analgesic effects at a significance level of P<0.05. RESULTS: Heart rate was lower in all nonsteroidal anti-inflammatory drug (NSAID)-treated trials from 2 h to 10 h post treatment (P<0.05). Analgesic effects of FM and PBZ+FM, as evidenced by decreases in HR, lasted for 12 h post treatment (P<0.05). Lameness score decreased earlier in PBZ and PBZ+FM trials than in FM trials (P<0.05) and the analgesic effect on LS lasted for 12 h post treatment for all NSAID trials (P<0.05). Peak PBZ plasma concentration was 73.7 ± 6.0 and 77.9 ± 5.5 µg/ml. Peak FM concentration was 12.0 ± 0.8 and 13.7 ± 1.0 µg/ml. CONCLUSIONS: It was concluded that the combination of PBZ+FM was not more effective than either PBZ or FM alone. These data do not support the hypothesis that the combination is more efficacious at these dosages than either drug alone in this model of acute foot pain.

Validation and comparison of two methods of measuring lactate in equine plasma.

REASONS FOR PERFORMING STUDY: Some methods of lactate (LA) measurement have not been validated appropriately for use in horses. OBJECTIVES: To validate 2 LA analysers (YSI 2300 Stat Plus and TDx Lactic Acid Assay) for use with equine plasma and to compare plasma [LA] determined by the 2 methods. METHODS: Both instruments were evaluated for linearity, parallelism, recovery and precision using serial dilutions of standard LA solutions and equine plasma and then comparing results with linear regression or paired t tests. Plasma [LA] results were compared in 275 blood samples collected from horses exercising at various intensities using Bland-Altman analysis. Level of significance was P < 0.05. RESULTS: YSI exhibited good linearity for both LA standards and equine plasma (P < 0.05) at 0-30 mmol/l. TDx had good linearity at 0-12 mmol/l (P < 0.05); with LA standard solutions >12 mmol/l and with equine plasma, linearity was decreased. YSI exhibited good parallelism between LA standards and equine plasma LA measurements throughout the 0-30 mmol/l range (P > 0.05). Parallelism was poor with TDx (P < 0.05). Mean ± s.d. % recovery was 101.7 ± 3.4% for YSI (acceptable) and 110.6 ± 8.4% for TDx (unacceptable). Within-run and mean between-run coefficients of variation (CV) for plasma samples tested from 3.3-29.5 mmol/l were 0.4-3.0% for YSI. CVs for samples tested from 2.8-8.0 mmol/l were 17.4-24.1% for TDx. In 275 plasma samples, [LA] ranged from 0.1-42.7 mmol/l and 0.3-50.6 mmol/l for the YSI and TDx methods, respectively. The difference in plasma [LA] determined by the 2 methods was -1.0 ± 3.2 mmol/l, documenting that the TDx overestimated the YSI results by a mean value of 1 mmol/l. CONCLUSIONS: It was concluded that the YSI method was a reliable method for measuring equine plasma [LA] from 0-30 mmol/l. The TDx method was found not to be suitable for use with equine plasma due to greater variability in measurements (high CV).

Efficacy of single-dose intravenous phenylbutazone and flunixin meglumine before, during and after exercise in an experimental reversible model of foot lameness in horses.

REASONS FOR PERFORMING STUDY: Objective blinded efficacy data during exercise are lacking on the use of single-dose i.v. nonsteroidal anti-inflammatory drugs (NSAIDs) before, during and after exercise. HYPOTHESIS: Single i.v. doses of either phenylbutazone (PBZ) or flunixin meglumine (FM) would prove more efficacious than negative saline control (SAL) before, during and after exercise in a reversible model of foot lameness. METHODS: Six Quarter Horse mares had lameness induced by tightening a set screw against a heart bar shoe 1 h prior to treatment. Randomised blinded treatments included PBZ (4.4 mg/kg bwt i.v.), FM (1.1 mg/kg bwt i.v.), and SAL (1 ml/45 kg i.v.). Heart rate and lameness score (LS) were recorded at rest; every 20 min after lameness induction for 5 h and at the end of 2 min treadmill workloads of 2 and 4 m/s. Heart rate was also recorded from 0.5-60 min post exercise. Results were compared using RM ANOVA and Student-Newman-Keul's test (HR) and Wilcoxon signed rank test (%ΔLS) with significance set at P < 0.05. RESULTS: Pre-exercise mean HR was decreased for both NSAIDs compared to SAL from 1:20-4 h post treatment (P < 0.05). Pre-exercise mean %ΔLS was decreased for PBZ (1:20-4 h) and FM (1-4 h) compared to SAL (P < 0.01). With exercise, there were no HR differences between treatments (P > 0.05), but mean %ΔLS was decreased for both NSAIDs compared to SAL (P < 0.01). Mean recovery HR was decreased for PBZ and FM from 1-60 min compared to SAL (P < 0.05). CONCLUSIONS: PBZ and FM demonstrated definitive clinical efficacy after single i.v. doses before, during and after exercise. Use of single i.v. doses during competition may mask lameness and may affect the ability of judges in determining the soundness of horses in competition.

Effects of single-dose intravenous phenylbutazone on experimentally induced, reversible lameness in the horse.

The objective was to test the hypothesis that phenylbutazone (PBZ) alleviates lameness in an adjustable heart bar shoe model of equine foot pain. Eight Quarter Horse mares underwent 4-weekly treatments randomly: 0.9% saline placebo (SAL: 1 mL/45 kg body weight i.v.) with no lameness; SAL with lameness; PBZ (4.4 mg/kg body weight i.v.) with no lameness; and PBZ with lameness. Blinded heart rate (HR) and lameness score (LS) were assessed every 20 min for 2 h and then hourly through 9 h. At 1 h SAL or PBZ was administered. Jugular venous samples were obtained at hours 0, 1, 2, 4, 6, and 8 and were evaluated for packed cell volume (PCV), cortisol, and drug concentrations. Repeated measures anova and t-tests were used to identify PBZ effects at a significance level of P<0.05. PBZ-treated LS was lower 2-8 h post-treatment, and HR was lower from 2 through 6 h post-treatment (P<0.05). Phenylbutazone did not change PCV and had minimal effect on cortisol. Mean plasma PBZ and oxyphenbutazone concentrations 7 h after treatment were 7.2-7.5 and 1.6-1.9 microg/mL, respectively. It was concluded that PBZ was efficacious in alleviating lameness in this model. Cortisol and PCV were not discriminating enough to distinguish between PBZ-treated and SAL-treated trials.

Effects of a pre-moistened multilayered breathable fabric in promoting heat loss during recovery after exercise under hot conditions.

REASONS FOR PERFORMING STUDY: Horses generate considerable internal heat burdens when exercising. Although common practice for a trainer or groom to place a wet blanket or towel on the dorsum of a hot horse post exercise, there are no data supporting the efficacy of this cooling method. OBJECTIVE: To test the hypothesis that a pre-moistened blanket designed with a multilayered breathable fabric would enhance heat loss in horses post exercise. METHODS: Eight treadmill-trained horses performed a standardised exercise test (SET) weekly for 3 weeks, with 3 different recovery treatments administered randomly. Pulmonary artery temperature (PAT) was measured via Swan-Ganz catheter. The SET consisted of 10 min at 3.7 m/sec, 3 min at 11.0 m/sec, 25 min at 3.7 m/sec and 20 min of recovery walking at 2.0 m/sec (58 min exercise and recovery under laboratory conditions of 35.0-40.6 degrees C and 27-49% RH). From 3-7 min during recovery, the treadmill was stopped and horses randomly received either: (a) no bath (negative control); (b) a bath consisting of 32 l of 1-4 degrees C water split into 3-4 cycles of bilateral water application (positive control) followed by water removal ('scraping'); or (c) application of a multilayered fabric blanket soaked in 16-19 degrees C water, wrung out, and placed over the dorsum and sides of the horse. PAT was compared using RM ANOVA with the Student Neuman-Keul's test used post hoc to discriminate between treatments at specific points in time. RESULTS: Mean PAT rose with each phase of exercise (P<0.001) and peaked at a mean of 40.2 +/- 0.2 degrees C. During recovery, the cold bath decreased HR and PAT for 9 min after walking resumed (P<0.001-P<0.05). The blanket did not decrease HR or PAT compared to negative control (P>0.05), and both were hotter than the cold bath treatment through 16 min of recovery (P<0.05). CONCLUSIONS: A specially-designed cooling blanket failed to reduce PAT when compared to negative control. Cold water bathing decreased HR and PAT but was not effective throughout all of recovery. POTENTIAL RELEVANCE: A specially-designed, pre-moistened multilayered breathable fabric failed to promote evaporative cooling compared to negative control. Cold water baths may need to be repeated throughout recovery to optimise their effect.

Forelimb skeletal scintigraphy responses in previously untrained Thoroughbreds undergoing initial treadmill training.

Scintigraphy has been used in numerous clinical settings to examine horses to determine the origin of lameness problems, but it has not been used previously to monitor prospectively the skeletal responses of a group of similarly-trained racehorses. Our hypothesis was that in naïve Thoroughbred (TB) racehorses, initial treadmill training induces increased radiopharmaceutical uptake in high-motion joints and in the dorsal third metacarpal bone (MC3). Eight previously-untrained TB racehorses underwent sequential skeletal scintigraphic examinations as they exercised daily for 9 weeks on an inclined treadmill. At the end of Weeks 0 (pre-training), 3 (trotting at 4.2 m/s and initial galloping), 6 (galloping at 7.5 m/s), and 9 (sprinting 600 m at 12.5 m/s), horses received 140 mCi 99m Technetium-methylene diphosphonate i.v. followed by a standard skeletal scintigraphic forelimb examination 2 h later. Views were graded for increased radiopharmaceutical uptake by 3 co-investigators who were blinded to horse identification, breed, sex, date, and clinical findings. Results were compared before and after training for each skeletal location using the Mann-Whitney Rank Sum Test with the level of significance set at P<0.05. Initial treadmill training resulted in increased radiopharmaceutical uptake in the carpus (P = 0.031), metacarpophalangeal joint (P = 0.021), proximal phalanx (P = 0.035), and distal phalanx (P = 0.003). Training did not affect dorsal MC3 radiopharmaceutical uptake (P>0.05).

Technical note: using calcium carbonate as an osmolar control treatment for acid-base studies in horses.

The efficacy of using calcium carbonate as an osmolar control treatment for acid-base studies in horses receiving alkalizing compounds was evaluated. Six mares were nasogastrically intubated with isomolar quantities of sodium or calcium as sodium bicarbonate or calcium carbonate or with water during three treatment periods. Doses of the carbonic acid salts were 500 mg/kg sodium bicarbonate mixed with 4 L of distilled water (positive control) and 595 mg/kg calcium carbonate mixed with 2 L of distilled water to yield isoosmolar treatments. Four liters of distilled water served as the negative control. Jugular venous blood samples were drawn before intubation and at hourly intervals for 6 h after intubation. The serum electrolytes Na+ and K+, blood pH, and HCO3- were determined. The sodium bicarbonate treatment increased blood pH and HCO3- (P < 0.01) above both the water and CaCO3 treatments. No differences (P > 0.05) were found between the water and CaCO3 treatments. These data indicate that calcium carbonate may serve as a suitable osmolar control treatment for studying the effects of treatments that affect acid-base status of horses.

Short- and long-term survival and prevalence of postoperative ileus after small intestinal surgery in the horse.

The records of 74 horses that recovered from anaesthesia after surgery for a small intestinal lesion from 1994 to 1999 were reviewed. Sixty-three horses (85%) had a strangulating lesion and 43 of these (68%) had a resection and anastomosis. Four of 11 horses (36%) without a strangulating lesion had a resection and anastomosis. Sixty-three horses (85%) survived to discharge, with a survival rate of 53/63 in horses with a strangulating lesion (84%) and 10/11 (91%) in others. For all lesions, short-term survival for all end-to-end anastomoses (91%; 21/23) and for no resection (92%; 23/25) were superior (P < 0.05) to survival for jejunocaecal anastomosis (76%; 19/25). Fourteen horses (19%) had a repeat abdominal surgery during hospitalisation; 9 of these (64%) survived short-term. Postoperative ileus developed in 7/70 horses (10%) after surgery for a problem other than proximal enteritis, and all had a strangulating lesion. Postoperative ileus (POI) was more likely after a jejunocaecostomy than after other procedures, and did not develop after a jejunojejunostomy. Survival > 7 months was 52/69 (75%) and for > 12 months was 39/57 (68%). The estimated prevalence of adhesions was 13%. Short-term survival was poorest in horses that had a jejunocaecostomy, but long-term survival was less affected by the anastomosis used. The sharpest decline in survival was during the first postoperative week and postoperative mortality then declined over time after surgery. A postoperative protocol that allowed early postoperative feeding was well tolerated. The results confirm that the overall prognosis after small intestinal surgery in horses is improved over earlier findings.

Equine respiratory pharmacology.

Differentiation of diseases of the equine respiratory tract is based on history, clinical signs, auscultation, endoscopy, imaging, and sampling of airway exudate. Upper respiratory therapies include surgical correction of airway obstructions; flushing of localized abscesses (strangles), guttural pouch disease, or sinusitis; and oral or parenteral antibiotic and anti-inflammatory therapy if deemed necessary. Pneumonia usually is treated with antimicrobials, anti-inflammatories, and bronchodilators. Pleural drainage is indicated if significant pleural effusion is present. The most commonly used therapies for early inflammatory and chronic allergic obstructive conditions include bronchodilators and anti-inflammatories. Acute respiratory distress, particularly acute pulmonary edema, is treated with diuretics (usually furosemide), intranasal oxygen, bronchodilators, corticosteroids, and alleviation of the underlying cause. Furosemide also had been used in North America as a race-day preventative for exercise-induced pulmonary hemorrhage (EIPH), but recent data have shown that furosemide may be a performance-enhancing agent itself.

Hemodynamic effects of ionized calcium in horses anesthetized with halothane or isoflurane.

OBJECTIVES: To evaluate the effects of halothane and isoflurane on cardiovascular function and serum total and ionized calcium concentrations in horses, and to determine whether administration of calcium gluconate would attenuate these effects. ANIMALS: 6 clinically normal adult Thoroughbreds. PROCEDURE: Catheters were inserted for measurement of arterial blood pressures, pulmonary arterial blood pressures, right ventricular pressure (for determination of myocardial contractility), right atrial pressure, and cardiac output and for collection of arterial blood samples. Anesthesia was then induced with xylazine hydrochloride and ketamine hydrochloride and maintained with halothane or isoflurane. An i.v. infusion of calcium gluconate was begun 75 minutes after anesthetic induction; dosage of calcium gluconate was 0.1 mg/kg of body weight/min for the first 15 minutes, 0.2 mg/kg/min for the next 15 minutes, and 0.4 mg/kg/min for an additional 15 minutes. Data were collected before, during, and after administration of calcium gluconate. RESULTS: Halothane and isoflurane decreased myocardial contractility, cardiac index, and mean arterial pressure, but halothane caused greater depression than isoflurane. Calcium gluconate attenuated the anesthetic-induced depression in cardiac index, stroke index, and maximal rate of increase in right ventricular pressure when horses were anesthetized with isoflurane. When horses were anesthetized with halothane, a higher dosage of calcium gluconate was required to attenuate the depression in stroke index and maximal rate of increase in right ventricular pressure; cardiac index was not changed with calcium administration. CONCLUSIONS AND CLINICAL RELEVANCE: I.v. administration of calcium gluconate may support myocardial function in horses anesthetized with isoflurane.

Necrotizing mycotic vasculitis with cerebral infarction caused by Aspergillus niger in a horse with acute typholocolitis.

An 18-year-old Morgan mare was presented to the Veterinary Medical Teaching Hospital, University of Illinois, with a 10-day history of watery diarrhea, depression, and dysphagia. On admission, the animal was severely dehydrated, depressed, and unable to swallow and had no clinical signs of diarrhea. The respiratory and heart rate and body temperature were within normal limits. Following fluid therapy, the mare developed severe watery diarrhea and continued to be depressed, incoordinated, and dysphagic. The animal died on the fourth day after admission and was sent to the Laboratories of Veterinary Diagnostic Medicine for necropsy. Gross postmortem findings were consistent with an acute cerebral infarction in the right cerebral hemisphere, an acute necrotizing typhlocolitis, multifocal petechial and ecchymotic hemorrhages, enlarged and congested pars intermedia of the pituitary gland, and marked bilateral adrenocortical hyperplasia with multifocal areas of necrosis and hemorrhage. Histologic evaluation of the affected brain demonstrated an area of coagulative necrosis of the gray matter, with hemorrhage, vasculitis, and thrombosis. There were many fungal hyphae 3.5-6.0 microm, pale basophilic, septate, and occasionally branching at 45 degrees present in the arterial walls and throughout the necrotic tissue. Immunohistochemical analysis revealed Aspergillus niger as the etiologic agent responsible for the mycotic vasculitis and infarction in the brain. Bacteria culture and immunohistochemical staining of the colon and cecum failed to demonstrate specific pathogens.

Techniques for evaluation of right ventricular relaxation rate in horses and effects of inhalant anesthetics with and without intravenous administration of calcium gluconate.

OBJECTIVES: To determine the most repeatable method for evaluating right ventricular relaxation rate in horses and to determine and compare effects of isoflurane or halothane with and without the added influence of intravenously administered calcium gluconate on right ventricular relaxation rates in horses. ANIMALS: 6 Thoroughbred horses from 2 to 4 years old. PROCEDURE: 6 models (2 for monoexponential decay with zero asymptote, 3 for monoexponential decay with variable asymptote, and 1 for biexponential decay) for determining right ventricular relaxation rate were assessed in conscious and anesthetized horses. The 2 methods yielding the most repeatable results then were used to determine right ventricular relaxation rates in horses anesthetized with isoflurane or halothane before, during, and after i.v. administration of calcium gluconate. Right ventricular pressure was measured, using a catheter-tip high-fidelity pressure transducer, and results were digitized at 500 Hz from minimum rate of change in ventricular pressure. RESULTS: 2 models that used monoexponential decay with zero asymptote repeatedly produced an estimate for relaxation rate and were used to analyze effects of anesthesia and calcium gluconate administration on relaxation rate. Isoflurane and halothane each prolonged right ventricular relaxation rate, with greater prolongation evident in halothane-anesthetized horses. Calcium gluconate attenuated the anesthesia-induced prolongation in right ventricular relaxation rate, with greater response obtained in isoflurane-anesthetized horses. CONCLUSIONS AND CLINICAL RELEVANCE: Right ventricular relaxation rate in horses is assessed best by use of a monoexponential decay model with zero asymptote and nonlinear regression. Intravenous administration of calcium gluconate to isoflurane-anesthetized horses best preserves myocardial relaxant function.

Parietal hernia of the small intestine into the epiploic foramen of a horse.

A 12-year old 573-kg (1,261-lb) Quarter Horse gelding was referred with colic of 12 hours' duration and with poor response to medical treatment. On the basis of physical and laboratory findings, a pelvic flexure impaction was suspected. The horse was treated medically. Because signs of mild abdominal pain persisted and the heart rate had increased, an exploratory celiotomy was performed 30 hours after signs of colic were first noticed. At surgery, the ileum was found partially entrapped within the epiploic foramen, in a left-to-right direction, to form a parietal hernia. The entrapped intestinal segment was reduced but not resected, and the horse recovered fully. In retrospect, the delay before surgery in this horse was tolerated because this was a parietal hernia and, therefore, did not cause complete ileal obstruction. This horse had an unusual form of small-intestinal strangulation in the epiploic foramen that might not cause sufficient obstruction initially to allow early detection.

Hemodynamic effects of thyroidectomy in sedentary horses.

OBJECTIVE: To investigate hemodynamic effects of thyroidectomy in horses at rest. ANIMALS: 6 healthy aged Quarter Horse mares. PROCEDURE: Horses were monitored for 5 months before and 4 weeks after thyroidectomy and for an additional 4 weeks after administration of thyroid hormone supplement (2.5 microg of thyroxine/kg of body weight, PO, q 12 h, and 0.6 microg of triiodothyronine/kg, PO, q 12 h). Responses to thyroid-stimulating hormone (TSH) were measured before and 4 weeks after thyroidectomy. Other variables monitored daily were resting rectal temperature (T), heart rate (HR), respiratory rate (RR), and body weight (BW). Monthly cardiac output (Q), blood volume (BV), plasma volume (PV), standard electrocardiographic measures, systolic and right ventricular blood pressure, and HR responses were determined after IV administration of isoproterenol and phenylephrine. Variables were analyzed by use of repeated-measures ANOVA. RESULTS: Complete thyroidectomy was confirmed by minimal response to TSH 4 weeks after surgery. Resting HR, RR, T, Q, and beta-adrenergic responsiveness to isoproterenol decreased significantly after thyroidectomy. Resting T, Q, and beta-adrenergic responsiveness increased after administration of supplement and was not significantly different from euthyroid values. Blood volume and PV increased significantly after thyroidectomy but did not return to euthyroid values despite administration of supplement. Response to phenylephrine was minimally different between treatments. CONCLUSIONS AND CLINICAL RELEVANCE: Thyroidectomy in horses caused decreased resting HR, RR, T, Q, and isoproterenol responsiveness and increased BV, PV, PQ interval, and QT interval corrected for HR. Some of these surgically induced changes appeared to be partially reversed by administration of thyroid hormone supplement.

Environmental stress and 3-day eventing: effects of altitude.

Three-day event horses are subject to various external environmental stresses including changes in ambient temperature, humidity, altitude, and test severity. Considerable research on the adverse effects of increased heat and humidity preceded the 1996 Olympic Summer Games in Atlanta, Georgia USA, but no research has been done previously on the effects of altitude on 3-day eventing. Physical and venous blood gas data were collected on horses (n = 24) competing in the High Prairie Preliminary (CCN*) and Intermediate (CCN**) 3-day events and Preliminary Horse Trials in Parker, Colorado (1900 m above sea level). Despite the increased altitude, only post exercise rectal temperature and pH were higher (P < 0.05) whereas heart rate (HR), [K+], and ionized calcium (ICa++) were lower (P < 0.05) in 3-day event horses compared to horse trial horses. All other variables (respiratory rate [RR], PCV, [Hb], PCO2, [tCO2], [HCO3-], BE, and [Na+]) were not different between groups (P > 0.05). When these preliminary horse trial horses in Colorado were compared to those previously studied at preliminary horse trials at sea level in Arizona, post exercise HR and RR were higher (P < 0.05) and pH, PCO2, [tCO2], [HCO3-], BE and [iCa++] were lower (P < 0.05) at altitude. These data show that increased altitude (1900 m above sea level) was more stressful for 3-day event horses, but did not result in the severe physiological changes and inability to complete prescribed exercise tests seen in previous studies with increased heat and humidity. It is clear from these and previous data that increased heat and humidity are the more important environmental stressors in 3-day eventing.