Effect of xylazine on the arrhythmogenic dose of epinephrine in thiamylal/halothane-anesthetized horses.

This study investigates the effect of xylazine, a sedative, on the dose of epinephrine required to induce heart arrhythmias in horses under anesthesia. It concludes that xylazine does not significantly alter this dose, and does not substantially change heart rate or blood pressure.

Objective of the Study

• The research aims to explore the influence of xylazine on the arrhythmogenic dose of epinephrine (ADE)—the lowest dose of epinephrine that can induce irregular heartbeats—in horses. The anesthetics used in the study were guaifenesin, thiamylal, and halothane.

Procedure Used in the Study

• The study involved 9 horses to which anesthesia was induced using a sequence of guaifenesin, thiamylal, and a minimal alveolar concentration of halothane.
• Baseline physiological parameters were established first. Electrocardiogram readings and facial artery pressure were constantly observed throughout the experiment.
• The researchers started an epinephrine infusion at an initial rate of 0.25 micrograms/kg/min for a maximum duration of 10 minutes. The ADE was established as the lowest epinephrine rate required to provoke at least four premature ventricular depolarizations within a 15-second window.
• Xylazine was administered after establishing the control ADE, to study its effect on the ADE.

Findings of the Study

• The results showed that administration of xylazine did not significantly affect the ADE, illustrating that the sedative doesn’t substantially influence the dose of epinephrine needed to induce heart arrhythmias.
• Xylazine led to a transient increase in blood pressure for about 8 minutes post-administration, but baseline systolic and diastolic arterial pressures and heart rate showed no significant difference from the control 15 minutes after xylazine administration.
• Blood pressure and heart rate considerably increased during both control and xylazine ADE determinations, but the pH, arterial oxygen pressure (PaO2), arterial carbon dioxide pressure (PaCO2), and base excess remained relatively stable across the study groups.
• Three horses experienced fibrillations (irregular heartbeats). One had atrial fibrillation, and two had ventricular fibrillation during the ADE determination phases of the experiment. However, the study doesn’t explicitly link these incidents to the use of xylazine.