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SUMMARY

We studied the temporal changes in respiratory mechanics associated with xylazine administration (1.1 mg/kg of body weight, iv) in standing horses (experiment 1), and determined the effects of head and neck position (experiment 2) and atropine administration (experiment 3) on the observed changes.

Thoroughbred geldings, 3 to 5 years old (5 in experiment 1, 4 in experiments 2 and 3) were used. Flow rates were obtained from a pneumotachograph and a differential transducer attached to a tight-fitting mask. Electronic integration of the flow signal gave tidal volume. Total pulmonary pressure (PL) was defined as the difference between esophageal pressure, measured with a balloon sealed to the end of a polyethylene catheter, and mask pressure. In experiment 3, a blunt cannula positioned in the dorsal third of the eighth or tenth intercostal space was used to estimate transpulmonary pressure. Lateral tracheal pressure was measured, using a polypropylene catheter inserted percutaneously in the mid-extrathoracic tracheal lumen. Upper and lower airway pressures were defined as the difference between mask pressure or transpulmonary pressure and lateral tracheal pressure, respectively.

Five observations were made: (1) There was a significant (P < 0.05) increase in PL from 10 to 40 minutes after administration of xylazine. (2) Although an overall agreement between head and neck position and PL was detected, the maximal PL value was not always obtained with lowest head and neck position. (3) Lower and upper airway resistance increased with low head carriage, with a greater increase in upper airway resistance resulting in a decrease in lower to total airway resistance ratio. (4) Increased airway resistance was reversed by elevating the head and neck. (5) Atropine did not prevent the increase in airway resistance during sedation with xylazine and had no effect on resistance with changes in head position.

Free access
in American Journal of Veterinary Research

SUMMARY

The partitioning of total pulmonary resistance (Rl) into upper airway resistance and lower airway resistance (Rl) was studied in 8 Thoroughbred geldings. In addition, the phase shift and amplitude distortion of 3 catheters used for pressure measurements in this study were evaluated under static and dynamic conditions.

Flow rate was obtained from a heated pneumotachograph attached to a tight-fitting mask placed over the nose. Electronic integration of the flow signal gave tidal volume. Transpulmonary pressure (Pl) was obtained from calculation of the difference between the esophageal balloon catheter pressure and mask pressure. Lateral tracheal pressure was measured from a polyethylene catheter placed percutaneously in the middle portion of the trachea. Lower airway pressure (Pl) was calculated as the difference between esophageal pressure and lateral tracheal pressure. Similarly, upper airway pressure was defined as the difference between lateral tracheal pressure and mask pressure. Pressures are reported as the difference between the maximal and the minimal pressures recorded during a respiratory cycle. Airway resistance was calculated, using the isovolume method, at 50% of tidal volume.

There were individual and group variations in Pl and Pl/Pl, although Pl accounted for more than 60% of Pl in all horses. In 6 horses, RI was more than 50% of Rl whereas in 2 horses, Rl was only 30 and 34% of Rl.

Amplitude distortion was minimal for the 3 catheters under static conditions in the in vitro study. Under dynamic conditions, amplitude distortion varied according to the catheter studied, the frequency, and the resistance of the system. There were no phase differences under static conditions at low frequency. However, phase discrepancy, which was variable through the cycle, was observed for some catheters at high frequency under static and dynamic conditions. It was concluded that, until measuring techniques are standardized in horses, variations in the partitioning of Rl are likely to be obtained between studies and between animals within studies.

Free access
in American Journal of Veterinary Research

Summary

The effects of 3 commonly used dosages (0.3, 0.5, and 1.1 mg/kg of body weight, iv) of xylazine on ventilatory function were evaluated in 6 Thoroughbred geldings. Altered respiratory patterns developed with all doses of xylazine, and horses had apneic periods lasting 7 to 70 seconds at the 1.1 mg/kg dosage. Respiratory rate, minute volume, and partial pressure of oxygen in arterial blood ( Pa o 2 ) decreased significantly (P < 0.001) with time after administration of xylazine, but significant differences were not detected among dosages. After an initial insignificant decrease at 1 minute after injection, tidal volume progressively increased and at 5 minutes after injection, tidal volume was significantly (P < 0.01) greater than values obtained before injection. Partial pressure of carbon dioxide in arterial blood ( Pa co 2 ) was insignificantly increased. After administration of xylazine at a dosage of 1.1 mg/ kg, the mean maximal decrease in Pa o 2 was 28.2 ± 8.7 mm of Hg and 22.2 ± 4.9 mm of Hg, measured with and without a respiratory mask, respectively. Similarly, the mean maximal increase in PaCO2 was 4.5 ± 2.3 mm of Hg and 4.2 ± 2.4 mm of Hg, measured with and without the respiratory mask, respectively. Significant interaction between use of mask and time was not detected, although the changes in PaO2 were slightly attenuated when horses were not masked. The temporal effects of xylazine on ventilatory function in horses should be considered in selecting a sedative when ventilation is inadequate or when pulmonary function testing is to be performed.

Free access
in American Journal of Veterinary Research

Summary

Effects of phenylbutazone (pbz) and furosemide (fur) on the respiratory tract of horses were evaluated, focusing on bronchial responsiveness. Four healthy Thoroughbreds were used and data were analyzed by use of a Latin square design. Histamine provocation tests (0.5, 1, 2, and 4 µg/kg/min, iv) were done: (1) without prior treatment with pbz or fur, (2) 30 minutes after administration of pbz (8 mg/kg, iv), (3) 1 hour after administration of fur (1 mg/kg, iv), and (4) after administration of pbz plus fur. Pulmonary function tests (dynamic compliance, resistance, respiratory frequency, and tidal volume) and heart rate were monitored throughout the experiments. Phenylbutazone did not influence basal pulmonary function test results, whereas fur caused a significant (P < 0.05) increase in dynamic compliance and decrease in resistance. Histamine infusion resulted in a dose-dependent decrease in dynamic compliance and a dose-dependent increase in resistance, respiratory frequency, and heart rate. Phenylbutazone administration significantly (P < 0.05) attenuated most of the changes induced by histamine, whereas fur had less protective action. Administration of pbz plus fur before administration of histamine was less effective than administration of pbz alone.

Free access
in American Journal of Veterinary Research

Abstract

Objective

To evaluate selected hemodynamic, respiratory, and behavioral responses to propofol in horses premedicated with xylazine or detomidine.

Design

Xylazine (0.5 and 1.0 mg/kg of body weight) was administered IV on different days to each of 6 horses prior to IV administration of propofol (2 mg/ kg). In a second group of 6 horses, detomidine (15 and 30 µg/kg) was similarly studied.

Animals

2 groups of 6 mature healthy horses.

Procedure

Rectal temperature, heart and respiratory rates, arterial blood gas tensions, and direct arterial blood pressures were recorded before and at fixed intervals after drug administration. Induction and recovery events were quantitatively and qualitatively assessed. Cardiopulmonary and behavioral data to follow were statistically analyzed (P< 0.05).

Results

Heart rate decreased in dose-dependent manner from a mean (± SD) of 39.5 ± 5.1 beats/min after xylazine and detomidine. Second-degree atrioventricular dissociation was commonly seen at the higher drug doses. After propofol administration, heart rate either transiently increased or was less depressed early in recumbency, compared with predrug values. Direct arterial blood pressures varied inconsistently from predrug values. Mean arterial carbon dioxide tension tended to increase after drug administration (significance variable) from predrug values of 42 to 46 mm of Hg in both drug groups. After xylazine or detomidine administration, arterial oxygen tension decreased significantly from predrug values of 97 to 103 mm of Hg. The magnitude and duration of decrease was dose-dependent and greatest during recumbency.

Behavioral responses to anesthetic induction were variable, but horses were uniformly calm and coordinated during recovery. Recumbency time increased in reponse to the higher dose of either premedicant drug. Mean (± SD) times to standing were 25.02 ± 4.42 and 35.57 ± 6.83 minutes for the low and high doses of xylazine, respectively and 41.04 ± 11.21 and 52.64 ± 14.67 minutes for the low and high doses of detomidine, respectively.

Conclusion

Neither xylazine nor detomidine prevented excitation associated with propofol injection in horses.

Clinical Relevance

Xylazine- or detomidine-propofol combinations likely will not replace common anesthetic induction techniques for horses. However, recovery characteristics associated with propofol encourage further study in horses. (Am J Vet Res 1996;57:512–516)

Free access
in American Journal of Veterinary Research

Abstract

Objective—To quantitate dose- and time-related anesthetic-sparing effects of xylazine hydrochloride (XYL) during isoflurane-induced anesthesia in horses and to characterize selected physiologic responses of anesthetized horses to administration of XYL.

Animals—6 healthy adult horses.

Procedure—Horses were anesthetized 2 times to determine the minimum alveolar concentration (MAC) of isoflurane in O2 and to characterize the anestheticsparing effect (MAC reduction) after IV administration of XYL (0.5 and 1 mg/kg of body weight, random order). Selected measures of cardiopulmonary function, blood glucose concentrations, and urinary output also were measured during the anesthetic studies.

Results—Isoflurane MAC (mean ± SEM) was reduced by 24.8 ± 0.5 and 34.2 ± 1.9% at 42 ± 7 and 67 ± 10 minutes, respectively, after administration of XYL at 0.5 and 1 mg/kg. Amount of MAC reduction by XYL was dose- and time-dependent. Overall, cardiovascular and respiratory values varied little among treatments. Administration of XYL increased blood glucose concentration; the magnitude of change was dose- and time-dependent. Urine volume increased but not significantly.

Conclusions and Clinical Relevance—Administration of XYL reduced the anesthetic requirement for isoflurane in horses. The magnitude of the decrease is dose- and time-dependent. Administration of XYL increases blood glucose concentration in anesthetized horses in a dose-related manner. (Am J Vet Res 2000;61:1225–1231)

Full access
in American Journal of Veterinary Research
in Journal of the American Veterinary Medical Association

Abstract

Objective

To characterize responses associated with two 1-hour total intravenous anesthesia techniques in horses.

Animals

6 mature, healthy mares.

Procedure

Each horse was anesthetized 3 times. Treatment order was determined by a series of Latin squares. After baseline measurements and instrumentation, horses were given xylazine (XYL) IV; anesthesia was induced 5 minutes later with 10% guaifenesin given IV, then either ketamine (KET) or propofol (PRO) was given IV. After anesthesia induction, each horse received an infusion of XYL and either KET or a low or high dose of PRO. Cardiopulmonary variables were measured at 20, 40, and 60 minutes after the start of the infusion; arterial blood samples were collected prior to each set of measurements, and blood gas tensions and plasma drug concentration were determined. A noxious stimulus was applied after each of the 3 sets of measurements.

Results

Differences in measured cardiopulmonary variables were significant among all treatments at different times. Most notable differences were between KET and high PRO. Times to regaining sternal and standing posture were shortest for KET, and differed significantly from values for low and high PRO. Purposeful responses were not observed for high PRO in horses after noxious stimulation. In contrast, 4 horses given KET responded at all time points and 1 horse given low PRO responded.

Conclusion

None of the infusion techniques were flawless, but results support continued efforts at technique refinement and selected clinical use. (Am J Vet Res 1998;59:1292–1298)

Free access
in American Journal of Veterinary Research

Summary

Twenty-seven horses (and 1 mule) with 32 histologically confirmed cutaneous tumors were studied to evaluate the effects of intratumoral injection of cisplatin initiated at the time of surgery. As a result of surgery, 9 of the wounds were closed primarily (5 sarcoids, 4 carcinomas) and 23 were left open to granulate (16 sarcoids, 6 carcinomas, 1 hamartoma). Chemotherapy consisted of 4 treatment sessions of intratumoral injection of cisplatin in purified sesame oil at 2-week intervals. The first treatment session was administered intraoperatively. A controlled-release formulation of cisplatin in sesame oil was used to limit drug egress from the injection site. Dosage was 1 mg of cisplatin/cm3 of tissue. The mean relapse-free interval was 41 ± 3.7 months. The estimates of overall relapse-free survival rates were 92 ± 5% at 1 year and 77 ± 11% at 4 years. Cisplatin-related local toxicosis was minimal and wound healing was not compromised. Intratumoral injection of cisplatin appears safe and effective when administered in the perioperative period for selected tumors in equidae.

Free access
in Journal of the American Veterinary Medical Association