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Autonomic nervous system (ANS) activity can modify cardiovascular parameters in response to nociceptive stimuli or drugs in anesthetized animals. The aim of this study was to determine if a surgical nociceptive stimulus and morphine, ketamine, and dobutamine administration would modify ANS activity observed as a change in the mean parasympathetic tone activity (PTAm) in anesthetized horses. In 20 anesthetized horses, heart rate (HR), mean arterial pressure (MAP), and PTAm were monitored before and 1, 3, and 5 min after surgical incision, and before and 10 min after the administration of morphine (0.2 mg/kg IV). If nystagmus or spontaneous ventilation was observed, ketamine (0.5 mg/kg IV) was given, and the three variables were registered before and 3 and 5 min afterward. If MAP reached ≤ 62 mmHg, a dobutamine infusion was administered, and the three variables were recorded before and 5 min after starting/increasing the infusion (0.25 μg/kg/min IV every 5 min). The three variables were registered before and 1, 3, and 5 min after a PTAm decrease of ≥ 20%, HR increase of ≥ 10%, or MAP increase of ≥ 20%. The PTAm decreased 3 min after the administration of ketamine and 1 min after a PTA event. The surgical incision, dobutamine, and morphine did not modify PTAm. The absence of changes in ANS activity after the nociceptive stimulus and lack of correlation between PTAm and HR or MAP suggest that PTAm is a poor indicator of sympathetic activation under the study conditions. Ketamine seems to affect ANS activity by decreasing PTAm.

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