Retrospective study of the risk factors and prevalence of colic in horses after orthopaedic surgery

Evaluation of the effects of the opioid agonist morphine on gastrointestinal tract function in horses

OBJECTIVE

To evaluate the effects of morphine administration for 6 days on gastrointestinal tract function in healthy adult horses.

ANIMALS

5 horses.

PROCEDURES

Horses were randomly allocated into 2 groups in a crossover study. Horses in the treatment group received morphine sulfate at a dosage of 0.5 mg/kg, IV, every 12 hours for 6 days. Horses in the control group received saline (0.9% NaCl) solution at a dosage of 10 mL, IV, every 12 hours for 6 days. Variables assessed included defecation frequency, weight of feces produced, intestinal transit time (evaluated by use of barium-filled spheres and radiographic detection in feces), fecal moisture content, borborygmus score, and signs of CNS excitement and colic.

RESULTS

Administration of morphine resulted in gastrointestinal tract dysfunction for 6 hours after each injection. During those 6 hours, mean +/- SD defecation frequency decreased from 3.1 +/- 1 bowel movements in control horses to 0.9 +/- 0.5 bowel movements in treated horses, weight of feces decreased from 4.1 +/- 0.7 kg to 1.1 +/- 0.7 kg, fecal moisture content decreased from 76 +/- 2.7% to 73.5 +/- 2.9%, and borborygmus score decreased from 13.2 +/- 2.9 to 6.3 +/- 3.9. Mean gastrointestinal transit time was also increased, compared with transit times in control horses.

CONCLUSIONS AND CLINICAL RELEVANCE

Morphine administered at 0.5 mg/kg twice daily decreased propulsive motility and moisture content in the gastrointestinal tract lumen. These effects may predispose treated horses to development of ileus and constipation.

Cardiopulmonary effects of buprenorphine in horses

OBJECTIVE

To investigate the effects of buprenorphine on cardiopulmonary variables and on abdominal auscultation scores in horses.

ANIMALS

6 healthy adult horses.

PROCEDURES

Horses were restrained in stocks and allocated to 2 treatments in a randomized crossover design, with 1-week intervals between each treatment. Saline (0.9% NaCl) solution was administered IV as a control, whereas buprenorphine (10 mug/kg, IV) was administered to the experimental group. Cardiopulmonary data were collected for 120 minutes after buprenorphine or saline solution administration. Abdominal auscultation scores were monitored for 2 and 12 hours after drug administration in the control and experimental groups, respectively.

RESULTS

Following control treatment, horses remained calm while restrained in the stocks and no significant changes in cardiopulmonary variables were observed throughout the study. Buprenorphine administration caused excitatory phenomena (restlessness and head shaking). Heart rate, cardiac index, and arterial blood pressure were significantly increased after buprenorphine administration until the end of the observational period (120 minutes). Minimal changes were found in arterial blood gas tensions. Abdominal auscultation scores decreased significantly from baseline for 4 hours after buprenorphine administration.

CONCLUSIONS AND CLINICAL RELEVANCE

Buprenorphine induced excitement and hemodynamic stimulation with minimal changes in arterial blood gas tensions. These effects may impact the clinical use of buprenorphine in horses. Further studies are indicated to investigate the effects of buprenorphine on gastrointestinal motility and fecal output.

Pharmacokinetics of the opioid antagonist N-methylnaltrexone and evaluation of its effects on gastrointestinal tract function in horses treated or not treated with morphine

OBJECTIVE

To determine the pharmacokinetics and effects of the morphine antagonist N-methylnaltrexone (MNTX) on gastrointestinal tract function in horses when administered alone and in combination with morphine.

ANIMALS

5 healthy adult horses.

PROCEDURES

Horses were treated with MNTX (1 mg/kg, IV), and serial blood samples were collected for determination of drug pharmacokinetics. For evaluation of effects on the gastrointestinal tract when administered alone, MNTX was administered at a dosage of 0.75 mg/kg, IV, twice daily for 4 days. For evaluation of effects when administered concurrently with morphine, MNTX (0.75 mg/kg, IV, q 12 hours) and morphine (0.5 mg/kg, IV, q 12 hours) were administered for 6 days. Gastrointestinal variables evaluated were defecation frequency, weight of feces produced, fecal moisture content, intestinal transit time, and borborygmus scores.

RESULTS

The time-concentration data for MNTX disposition best fit a 2-compartment model with a steady-state volume of distribution of 244.6 +/- 21.8 mL/kg, t1/2 of 47.04 +/- 11.65 minutes, and clearance of 11.43 +/- 1.06 mL/min/kg. Adverse effects were not observed at doses <or= 1 mg/kg. Administration of MNTX increased daily fecal weight. When administered concurrently with morphine, MNTX partially prevented the effects of morphine on the gastrointestinal tract by increasing defecation frequency, fecal weight, fecal moisture content, and borborygmus score, and by preventing increases in intestinal transit time.

CONCLUSIONS AND CLINICAL RELEVANCE

Because MNTX does not cross the blood-brain barrier, administration of the drug should not alter the analgesic effects of opioids and may attenuate the adverse gastrointestinal effects associated with use of opioids in horses.

Morphine administration in horses anaesthetized for upper respiratory tract surgery

OBJECTIVE

To determine the effect of morphine administration on commonly monitored cardio-respiratory variables and recovery quality in horses undergoing anaesthesia and surgery.

STUDY DESIGN

Prospective, randomized clinical study.

ANIMALS

Thirty-eight thoroughbred horses, 32 geldings and six mares, 3-13 years old, weighing 411-600 kg.

MATERIALS AND METHODS

A standard anaesthetic technique was used. Twenty minutes after induction of anaesthesia horses received 0.1 mg kg(-1) (0.1 m) or 0.2 mg kg(-1) (0.2 m) morphine by intravenous injection. A control group did not receive morphine. Heart rate, respiratory rate (fr), mean arterial pressure (MAP) and blood gases were measured before morphine administration and every 10 minutes thereafter. Horses were positioned for 35 minutes in right lateral recumbency for tension palatoplasty by cautery and were then moved into dorsal recumbency for additional intraluminal surgery comprising one or more of aryepiglottic fold resection, sub-epiglottal mucosal resection, ventriculectomy and cordectomy. A subjective recovery score from 0 (worst) to 5 (best) was assigned by a single observer who was unaware of treatment group. Two-way repeated measures anova, one-way anova, Kruskal-Wallis test, Mann-Whitney test, Pearson and Spearman correlation coefficients, and chi-squared tests were used to analyse the data where appropriate.

RESULTS

Arterial partial pressure of oxygen (PaO(2)) decreased significantly over time and was significantly lower in horses that received morphine. One horse in the control group and two horses in each of the morphine groups had a PaO(2) <13 kPa. No other significant cardiopulmonary effects were detected. Recovery scores [median (range)] were higher in morphine recipients: 4 (2-5) in 0.1 m, 4 (3-5) in 0.2 m compared with 3 (2-4) in the control group.

CONCLUSIONS AND CLINICAL RELEVANCE

The lower PaO(2) in morphine recipients did not appear to be of clinical significance in healthy horses because the number of horses with a low PaO(2) was similar between groups. The quality of recovery was significantly better in morphine recipients. These results indicate that morphine may be considered for use in clinical cases although further work is required to assess the analgesic properties of the drug in this species.