Modification of cardiopulmonary and intestinal motility effects of xylazine with glycopyrrolate in horses.

Xylazine (XYL) administration in horses is accompanied by significant cardiovascular depression characterized by a 25-35% decrease in cardiac output (CO) which is likely to compromise tissue oxygen delivery (DO2), and usually vagally mediated bradycardia is an important cause of this reduced cardiovascular performance. To examine the possible benefit of preventing the bradycardiac response, 6 healthy horses were treated with intravenous (IV) saline (SAL) or 2.5 micrograms/kg glycopyrrolate (GLY) in a blinded, randomized, crossover trial. Fifteen minutes later, 1 mg/kg XYL was administered IV and systolic, diastolic and mean blood pressures (SBP, DBP, and MBP, respectively), central venous pressure (CVP), mean pulmonary artery pressure, heart rate (HR), CO, and arterial and mixed venous blood gases were measured at the following times: baseline, 2, 5, and 10 min post-SAL or GLY; and 2, 5, 10, 15, 30, 45 and 60 min post-XYL. Determination of cardiac index (CI), stroke index (SI), left ventricular work, systemic vascular resistance (SVR), DO2, oxygen uptake, and oxygen extraction ratio were made at the same time. Gastrointestinal (GI) motility was evaluated by four-quadrant auscultation for 24 h post-XYL. Statistical analysis of continuous variables was carried out using ANOVA for repeated measures and Wilcoxon's rank-sum test for non-parametric data. In GLY treated horses, HR, SBP, MBP, DBP, CI, DO2 and mixed venous oxygen tension were significantly higher up to 30 min after XYL (P < or = 0.02) while CVP and SI were significantly lower 2 and 5 min post-XYL, respectively. In both groups, GI motility as assessed by auscultation was virtually abolished for an hour, with a non-significant tendency for the decrease in motility to last longer in the GLY/XYL group. None of the treated horses developed abdominal discomfort. No significant difference was observed in the other variables. The study shows that 2.5 micrograms/kg GLY premedication reduces the cardiovascular depression caused by 1 mg/kg XYL, without adversely affecting GI motility.