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  • Author or Editor: Gwendolyn S. Light x
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Summary

We investigated the influence of parasympathetic tone on the arrhythmogenic dose of dobutamine in horses premedicated with xylazine, anesthetized with guaifenesin and thiamylal, and maintained on halothane in oxygen. Six horses were used in 12 randomized trials. In each trial, after end-tidal halothane concentration was stabilized at 1.1% (1.25 times minimum alveolar concentration [mac]) in oxygen, either saline solution (0.02 ml/kg of body weight) or atropine (0.04 mg/kg) was administered iv. Five minutes later, dobutamine infusion was started at dosage of 2.5 μg/kg/min, iv. The dobutamine infusion was continued for 10 minutes, or until 4 or more premature ventricular complexes occurred within 15 seconds, or sustained narrow-complex tachyarrhythmia clearly not sinus in nature occurred. If the criteria for termination were not met, dobutamine infusion was increased by 2.5 μg/kg/min, after the hemodynamic variables had returned to baseline. The horses were allowed to recover, and were rested for at least 1 week before the second trial. The arrhythmogenic dose of dobutamine was calculated by multiplying the infusion rate by the elapsed time into infusion when arrhythmia occurred. There was significant difference between the arrhythmogenic dose of dobutamine (add) in saline-treated horses (mean ± sem, add = 105.6 ± 16.3 μg/kg) and atropimzed horses (add = 36.2 ± 8.7 μg/kg). There were no differences in the prearrhythmia or immediate postarrhythmia ventricular heart rate (hr) or systolic (sap), diastolic (dap), or mean (map) arterial pressures between treated and control groups. The change in hemodynamic variables from prearrhythmia to immediate postarrhythmia formation was not different between the 2 groups. Ventricular beats were clearly evident in 8 of the 12 arrhythmias meeting the criteria for establishing the add. These results indicate that atropine may lower the arrhythmogenic threshold for dobutamine in halothane-anesthetized horses.

Free access
in American Journal of Veterinary Research

Summary

We investigated the influence of parasympathetic tone on the arrhythmogenicity of graded dobutamine infusions in horses anesthetized under clinical conditions. Six horses were used in 9 trials. Two consecutive series of graded dobutamine infusions were given iv; each continuous graded dobutamine infusion was administered for 20 minutes. The dobutamine infusion dosage (5, 10, 15, and 20 μg/kg of body weight/min) was increased at 5-minute intervals. Isovolumetric saline solution vehicle (v) or atropine (a; 0.04 mg/kg) was administered iv, or bilateral vagotomy (vg) was performed as a treatment before the second series of dobutamine infusions. Treatment was not administered prior to the first dobutamine infusion. Significant interaction between treatment and dosage of dobutamine infusion existed for differences from baseline for mean arterial pressure, systolic arterial pressure, diastolic arterial pressure, heart rate, and cardiac index at dosages of 5 and 10 μg of dobutamine/kg/min, given iv and for heart rate at dosage of 15 μg of dobutamine/kg/ min, given iv. Results for group-v horses were different from those for group-A and group-VG horses, but were not different between group-A and group-VG horses in all aforementioned cases, except for heart rate and cardiac index at dosage of 5 p.g of dobutamine/kg/min, given iv. Normal sinus rhythm, second-degree atrioventricular block, and bradyarrhythmias predominated during low dobutamine infusion rates during the first infusion series (nontreated horses) and in group-v horses during the second infusion series. Only tachyarrhythmias were observed during the second infusion series in the horses of the A and VG groups. The modulating influence of parasympathetic nervous system activity on hemodynamics and development of arrhythmia was conspicuous during low dobutamine infusion rates. Significant differences were not observed in hemodynamic responses to dobutamine, with respect to parasympathetic influence at high dobutamine infusion rates.

Free access
in American Journal of Veterinary Research