The pharmacodynamics of etorphine, and its combination with xylazine or azaperone in Boer goats (Capra hircus)

Abstract

The physiological effects of etorphine, and etorphine combined with xylazine and etorphine combined with azaperone on respiratory and cardiovascular function were determined in. Boer goats. The goats were habituated to the experimental procedures allowing the determination of respiratory and cardiovascular function while the animals stood quietly at rest. This resulted in the physiological changes induced by the three immobilizing drugs being measured and compared with those obtained prior to the administration of the immobilizing drugs. The: effectiveness of diprenorphine and atipamezole in antagonising the physiological changes induced by the immobilizing drug treatments was also determined. All three immobilizing drug treatments depressed respiratory function resulting in a decrease in PaO2and an increase in PaCO2. Etorphine caused limited changes to these blood gases as a result of decreases in respiratory minute volume and alveolar minute ventilation caused by a fall in respiratory rate. The administration of etorphine / azaperone did not decrease the efficiency of respiration more significantly than when etorphine was administered on its own. Etorphine injected in combination with xylazine resulted in a severe decrease in respiratory function. The decrease in PaO2 and the increase in PaCO2 were much greater than the changes to these two blood gases following the administration of either etorphine or etorphine in combination with azaperone. Compared to etorphine administered on its own, etorphine combined with xylazine caused more significant decreases in tidal volume and alveolar minute ventilation, and more significant elevations in both physiological shunt fraction and percentage dead space ventilation. The administration of etorphine, etorphine / xylazine and etorphine / azaperone caused three different sets of changes to cardiovascular function. The injection of etorphine resulted in significant increases in both total peripheral resistance and systemic mean arterial blood pressure, and a significant decease in cardiac output. The administration of etorphine / xylazine resulted in a rapid and significant decrease in the systemic mean arterial blood pressure, followed by a decrease in cardiac output. The peripheral resistance remained unchanged. Etorphine / azaperone caused a progressive decline in the total peripheral resistance. As the cardiac output did not change significantly, the systemic mean arterial blood pressure fell progressively. The administration of etorphine resulted in a gradual and limited decrease in the oxygen consumption index. Following the injection of etorphine / xylazine a rapid and significant decrease in the oxygen consumption index resulted, which was significantly lower, when compared to the goats immobilized with etorphine, at 5 and 35 minutes PDA. The injection of etorphine / azaperone resulted in a gradual decrease in the oxygen consumption index which reached a minimum value at 35 minutes PDA. At this time, the oxygen consumption index due to etorphine / xylazine was not significantly different from the value due to etorphine / azaperone. Diprenorphine effectively reversed the respiratory and cardiovascular effects due to etorphine. The physiological changes induced by the administration of etorphine / xylazine were partially and temporarily antagonised by the administration of diprenorphine, it was only following the injection of atipamezole that they return to the values measured in the goats prior to immobilization. Diprenorphine effectively reversed the respiratory depression induced by etorphine / azaperone, however a mild acidosis persisted until the end of the trial period. The cardiac output and systemic mean blood pressure improved dramatically following the injection of diprenorphine but there was no immediate change in total peripheral resistance.