Visceral analgesia: effects of xylazine, butorphanol, meperidine, and pentazocine in horses.

The visceral analgesic, cardiorespiratory, and behavioral effects induced by xylazine, butorphanol, meperidine, and pentazocine were determined in 9 adult horses with colic. Colic was produced by inflating a balloon in the horses' cecum. Heart rate, respiratory rate, mean arterial blood pressure, and cardiac output increased after cecal balloon inflation. Xylazine and butorphanol decreased the hemodynamic response to cecal balloon inflation. Meperidine and pentazocine had minimal effects on the cardiorespiratory changes induced by cecal balloon inflation. Xylazine produced the most pronounced visceral analgesia. The duration of visceral analgesia was longest with xylazine (approx 90 minutes) followed by butorphanol (approx 60 min) and then by meperidine and pentazocine (approx 30 to 35 min). Accurate assessment of the effects of visceral analgesics is dependent upon the use of objective tests to evaluate pain.