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Objective: To evaluate cardiopulmonary effects of glycopyrrolate in horses anesthetized with halothane and xylazine., Animals: 6 horses., Procedure: Horses were allocated to 2 treatment groups in a randomized complete block design. Anesthesia was maintained in mechanically ventilated horses by administration of halothane (1% end-tidal concentration) combined with a constant-rate infusion of xylazine hydrochloride (1 mg/kg/h, i.v.). Hemodynamic variables were monitored after induction of anesthesia and for 120 minutes after administration of glycopyrrolate or saline (0.9% NaCl) solution. Glycopyrrolate (2.5 microg/kg, i.v.) was administered at 10-minute intervals until heart rate (HR) increased at least 30% above baseline or a maximum cumulative dose of 7.5 microg/kg had been injected. Recovery characteristics and intestinal auscultation scores were evaluated for 24 hours after the end of anesthesia., Results: Cumulative dose of glycopyrrolate administered to 5 horses was 5 microg/kg, whereas 1 horse received 7.5 microg/kg. The positive chronotropic effects of glycopyrrolate were accompanied by an increase in cardiac output, arterial blood pressure, and tissue oxygen delivery. Whereas HR increased by 53% above baseline values at 20 minutes after the last glycopyrrolate injection, cardiac output and mean arterial pressure increased by 38% and 31%, respectively. Glycopyrrolate administration was associated with impaction of the large colon in 1 horse and low intestinal auscultation scores lasting 24 hours in 3 horses., Conclusions and Clinical Relevance: The positive chronotropic effects of glycopyrrolate resulted in improvement of hemodynamic function in horses anesthetized with halothane and xylazine. However, prolonged intestinal stasis and colic may limit its use during anesthesia.

Objective: To evaluate the cardiorespiratory and intestinal effects of the muscarinic type-2 (M2) antagonist, methoctramine, in anesthetized horses., Animals: 6 horses., Procedure: Horses were allocated to 2 treatments in a randomized complete block design. Anesthesia was maintained with halothane (1% end-tidal concentration) combined with a constant-rate infusion of xylazine hydrochloride (1 mg/kg/h, i.v.) and mechanical ventilation. Hemodynamic variables were monitored after induction of anesthesia and for 120 minutes after administration of methoctramine or saline (0.9% NaCl) solution (control treatment). Methoctramine was given at 10-minute intervals (10 microg/kg, i.v.) until heart rate (HR) increased at least 30% above baseline values or until a maximum cumulative dose of 30 microg/kg had been administered. Recovery characteristics, intestinal auscultation scores, and intestinal transit determined by use of chromium oxide were assessed during the postanesthetic period., Results: Methoctramine was given at a total cumulative dose of 30 microg/kg to 4 horses, whereas 2 horses received 10 microg/kg. Administration of methoctramine resulted in increases in HR, cardiac output, arterial blood pressure, and tissue oxygen delivery. Intestinal auscultation scores and intestinal transit time (interval to first and last detection of chromium oxide in the feces) did not differ between treatment groups., Conclusions and Clinical Relevance: Methoctramine improved hemodynamic function in horses anesthetized by use of halothane and xylazine without causing a clinically detectable delay in the return to normal intestinal motility during the postanesthetic period. Because of their selective positive chronotropic effects, M2 antagonists may represent a safe alternative for treatment of horses with intraoperative bradycardia.

Treatment of bradycardia in horses has been historically ignored because of the motility depressant effects of nonselective antimuscarinics. This study evaluated the cardiopulmonary effects of a cardioselective (M2) muscarinic antagonist, methoctramine (MET), in anesthetized horses. In a previous in vitro study, we determined that supraphysiological doses of MET were necessary to inhibit acetylcholine-induced longitudinal jejunal smooth muscle contractions in this species. Six adult horses were allocated to two treatments in a randomized complete block design. Anesthesia was induced with xylazine/ketamine, and maintained with halothane (1% end-tidal) and a constant infusion of xylazine (1 mg kg-1 hour-1 ) under mechanical ventilation. Invasive hemodynamic variables were monitored at baseline (approximately 45 minutes after induction) and for 120 minutes after MET or saline (control) had been injected. MET was titrated at 10-minute intervals (10 µg kg-1 IV) until the heart rate (HR) increased at least 30% above the baseline, or a maximum cumulative dose of 30 µg kg-1 had been injected. A person blinded to the treatment evaluated recovery scores and monitored intestinal auscultation until 24 hours after the end of anesthesia using previously published methods. Cardiovascular parameters were analyzed by anova followed by a Dunnet's test, and nonparametrical data were analyzed by a Mann–Whitney U -test (p < 0.05). Values were mean ± SEM unless otherwise stated. MET significantly increased HR from baseline to 120 minutes post-injection (from 29 ± 1 to 36 ± 2 beats minute-1 at 20 minutes). Thermodilution cardiac output (CO) and mean arterial pressure (MAP) were increased from baseline to 75 minutes post-MET injection (from 13.9 ± 0.8 to 19.4 ± 2.0 L minute-1 for CO at 20 minutes, and from 82 ± 3 to 103 ± 5 mm Hg for MAP at 20 minutes). Recovery characteristics and bowel auscultation scores did not differ among the groups. The return to at least 75% of the maximum auscultation score occurred at 10 (8–18) hours [median (range)] for controls and at 9 (8–12) hours for MET. It was concluded that MET increased HR and improved hemodynamic function during halothane/xylazine anesthesia with no apparent effect on return to full-bowel motility, as assessed by auscultation. Accordingly, M2 muscarinic antagonists might be represented as a safer alternative to treat intraoperative bradycardia in horse. [ABSTRACT FROM AUTHOR]