Influence of yohimbine and tolazoline on the cardiovascular, respiratory, and sedative effects of xylazine in the horse.

This research investigated the effects of yohimbine and tolazoline on cardiovascular, respiratory, and sedative side effects induced by xylazine in horses. The study found that both yohimbine and tolazoline successfully countered the heart and central nervous system depression associated with xylazine use.

Research Procedure

• The experiment was conducted on four horses which were sedated with xylazine. Xylazine was administered as an intravenous bolus at a dose of 1.0 mg/kg, followed by a constant infusion at a rate of 12 micrograms/kg/minute.
• The vital parameters such as heart rate, respiratory rate, mean arterial pressure, arterial blood gases, and the chin-to-floor distance (a measure to quantify sedation level) were recorded throughout the experiment.
• After 60 minutes, the horses were treated with either yohimbine or tolazoline in increasing doses until reversal of sedation was achieved. This reversal was defined as the return of the chin-to-floor distance to baseline values.
• A control group where a saline bolus was administered instead of an antagonist drug was taken into account for comparison.

Research Findings

• The average dose of yohimbine administered was 0.12 +/- 0.02 (SEM) mg/kg. On the other hand, the average dose of tolazoline that was administered was 7.5 +/- 1.1 mg/kg.
• Tolazoline and yohimbine both countered the slow heart rate and A-V conduction disturbances that were observed with xylazine administration.
• ::No change in mean arterial pressure was recorded post yohimbine or xylazine administration. However, tolazoline led to mild systemic hypertension persistently, which means that it slightly increased blood pressure persistently.
• No clinically significant changes were noted in the respiratory rate or arterial blood gas values with the administration of xylazine, yohimbine, or tolazoline. This means these drugs do not impact breathing or the oxygen and carbon dioxide levels in the blood.
• The chin-to-floor distance, which directly measures the sedation level, significantly decreased with the administration of xylazine and increased markedly with either yohimbine or tolazoline treatment. This signifies that both yohimbine and tolazoline effectively reversed the sedative effects of xylazine.

Conclusion

The study concluded that both yohimbine and tolazoline successfully antagonize the cardiovascular and central nervous system depression caused by xylazine administration in horses. This implies that these drugs can be effectively used to counteract the side effects of xylazine sedation in veterinary practices.