respiratory effects of the two opioids within a species , but also the fact that if respiratory rate had been the only assessment for respiration in this species , the hypoxaemia would have been completely overlooked . This should be kept in mind in a practical field scenario and a subjective assessment of arterial blood colour and the use of pulse-oximetry may potentially be helpful in evaluating the full extent of respiratory depression experienced by the animals . 22
Cardiovascular function was also differently affected in the two species . Both opioids caused considerably higher heart rates in the impala ( with etorphine , mean heart rate was 98 beats / minutes and with thiafentanil , mean heart rate was 109 beats / minute ) compared to in the blesbok ( with etorphine , mean heart rate was 37 beats / minutes and with thiafentanil , mean heart rate was 43 beats / minute ). This was likely as a result of the impala ’ s sensitivity to opioids , resulting in classic reflex bradycardia in response to opioidinduced vasoconstriction and hypertension . Unlike the blesbok , when darted with etorphine the impala ’ s heart rate decreased over time , dropping from 133 beats / minute to 80 beats / minutes towards the end of monitoring .
Regardless of the opioid used , both species were hypertensive throughout monitoring with mean arterial pressures in excess of 130 mmHg . This was not surprising since opioids are known to have hypertensive effects . It does highlight the importance of considering the cardiovascular effects of the specific tranquilisers and / or sedatives to be added to opioidbased dart mixtures . Historically , azaperone has been the tranquiliser of choice for use with opioids in large herbivores that are prone to capture-induced hypertension . 23 This is because azaperone acts on alpha1-receptors in the periphery , causing vasodilation and as a result , mitigation of hypertension . Its inclusion in dart mixtures with etorphine or thiafentanil for use in impala and blesbok may be beneficial . For these species , care should be taken when including sedatives such as alpha2-agonists in opioid-based dart mixtures since the latter causes vasoconstriction in the vascular smooth muscle and a resultant initial increase in blood pressure which could potentially further exacerbate
24 , 25 the opioid-induced hypertension .
At the end of the monitoring period , all of the animals were reversed with intravenous injections of naltrexone at a dose of 20 mg and 10 mg naltrexone per mg etorphine and thiafentanil , respectively . Recovery in all of the animals was quick and uneventful irrespective of the opioid used , with animals getting up and walking away within a minute after naltrexone administration . Overall , the study was a good illustration of the varied responses to opioids in different species as well as the different physiological effects these opioids illicit . The effects of an opioid on the cardiovascular and respiratory response of a specific species should be carefully considered when selecting other medicines to be co-administered .
The results of this study have been published in the Open Access Journal of the South African Veterinary Association and can be accessed via the Wildlife Pharmaceuticals website at www . wildpharm . co . za . The full product information of etorphine and thiafentanil is also accessible to veterinarians and veterinary nurses that register on the Wildlife Pharmaceuticals website . Alternatively , Wildlife Pharmaceuticals can be contacted directly at vetsupplies @ wildpharm . co . za for more information .
References
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2 . Kock , M . D . & Burroughs , R . E . Chemical and physical restraint of wild animals . ( International Wildlife Veterinary Services ( Africa ), 2012 ).
3 . Stanley , T . H ., Mcjames , S ., Kimball , J ., Port , J . D . & Pace , L . Immobilization of elk with A-3080 . J . Wildl . Manage . 52 , 577 – 581 ( 1988 ).
4 . Rosenbaum , J . S ., Holford , N . H . G . & Sadée , W . Opiate receptor binding-effect relationship : Sufentanil and etorphine produce analgesia at the μ-site with low fractional receptor occupancy . Brain Res . 291 , 317 – 324 ( 1984 ).
5 . Kurowski , M ., Rosenbaum , J . S ., Perry , D . C . & Sadée , W . [ 3H ] - etorphine and [ 3H ] -diprenorphine receptor binding in vitro and in vivo : differential effect of Na + and guanylyl imidodiphosphate . Brain Res . 249 , 345 – 352 ( 1982 ).
6 . Riviere , J . E . & Papich , M . G . Veterinary Pharmacology & Therapeutics . ( Wiley-Blackwell , 2009 ).
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