 Reference Base  Arterial blood pressure and blood gas composition of whit... |
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| Location: |
World |
| Subject: |
Translocation - Immobilization |
| Species: |
All Rhino Species |
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Etorphine effects. Immobilization of black Diceros bicornis and white Ceratotherium simum rhinoceroses using etorphine in combination with various other drugs such as fentanyl is routinely carried out in southern African national parks. However, capture, confinement and translocation procedures are not without complications and, while these animals appear to be relatively resistant to peracute and acute capture stress, the incidence of post-capture mortality remains unacceptably high. The successful immobilization of rhinoceroses using etorphine is well described, yet its effects on cardiopulmonary function in these animals are not well described, despite the fact that the use of etorphine has been associated with adverse side effects such as hypertension, hypoxaemia and hypercapnia in the white rhinoceros.
The importance of investigating physiological responses to immobilizing drugs or drug combinations becomes evident if one considers the possibility that certain of these responses may contribute to postcapture morbidity and/ or mortality. The purpose of this study was to report on arterial blood pressure and blood gas composition in white rhinoceroses immobilized with etorphine/ fentanyl and etorphine/ azaperone mixtures.
Method
Animals
Adult white rhinoceroses (7 male and 5 female), immobilized in the Kruger National Park during the period June to September 1992 for translocation to other reserves, were used in this study. One group (n = 6) was darted with a mixture of 2,0 mg elorphine HCl (M99, HMC Manufacturing Chemists Ltd., Dundee, Scotland) and 30 mg fentanyl (Janssen Pharmaceutica, Beerse, Belgium). A second group (n = 6) received a mixture of 3,0 mg etorphine and 25 mg azaperone (Stresnil, Janssen Pharmaceutica, Beerse, Belgium). All immobilizations took place in the early morning by approaching the rhinoceroses in a helicopter and darting them from the air. The animals were not weighed but body masses were estimated at about 1600 kg.
Blood sampling and arterial pressure measurement
As soon as possible after recumbency, which in most cases was within 10 min of darting, a 20-gauge arterial catheter (Jelco, Critikon RSA, Johnson & Johnson (Pty) Ltd., RSA) was placed into an auricular artery. A sample of arterial blood from each animal was then collected anaerobically into heparinized 1-ml syringes and immediately placed on ice. These samples were used for subsequent analysis of oxygen and carbon dioxide content using a Radiometer PHM 71 analyser and BMS 3 MK2 blood microsystem.Re arterial catheter, still in position within the artery, was then attached to a pressure transducer using a 0,9% heparinized saline-filled pressure line. The pressure transducer was zeroed to atmospheric pressure and connected to a muldchannel oscillograph (Harvard Universal), which was cali- brated against a mercury manometer prior to recording. Mean arterial pressure was then monitored for 2-3 min during the period of recumbency.
Table 1 Mean arterial blood pressure and arterial P02 and PC02 of white rhinoceroses immobilized with etorphine/ fentanyl (n = 6) and etorphine/ azaperone (n = 6) mixtures
Etorphine/fentanyl Etorphine/azaperone
Arterial pressure (mm Hg) 183 ? 16 141 ? 24
Arterial P02 (mm Hg) 38 ? 6 57 ? 16
Arterial PC02 (mm Hg) 73 ? 16 68 ? 18
Results
Mean arterial pressure and arterial P02 and PC02 (mean and standard deviation), obtained as soon as possible after recumbency for groups of animals immobilized with etorphine/fentanyl and etorphine/azaperone mixtures, are presented in Table 1. Blood pressure values represent an average of several recordings for each animal obtained in the 2-3 min after the arterial catheter was in position within the vessel. In rhinoceroses receiving an etorphine/fentanyl mixture, mean arterial pressure ranged between 160 and 200 mm Hg, while in those animals given an etorphine/ azaperone mixture, the recorded range was lower (100-160 mm Hg). In the former group, arterial P02 was lower and PC02 higher than the respective values obtained in the latter group.
Discussion
Published data regarding arterial pressure measurements in etorphine-anaesthetized white rhinoceroses exist as case reports with observations on single, captive animals. In one study, mean arterial pressure was monitored during prolonged anaesthesia and ranged between 107 and 168 mm Hg (Heard et al. 1992). Another study reports a mean arterial pressure of 280 mm Hg 15 min after recumbency, which subsequently stabilized at 210 mm Hg (LeBlanc et al. 1987). These results, as well as the range of mean arterial pressures obtained for anaesthetized rhinoceroses in this study, are difficult to interpret in the absence of control blood pressure data obtained from conscious animals under resting conditions, although etorphine-induced hypertension is suspected. Increased blood pressure associated with etor- phine anaesthesia has also been documented in the horse and, although the causative agents are at present uncertain, accompanying increased sympathetic nervous system activity, peripheral vaso-constriction and hypoxia have been suggested as contributing factors.
Of interest in this study was the fact that rhinoceroses immobilized with etorphine/azaperone mixtures displayed lower blood pressures immediately after recumbency than those given etorphine/fentanyl mixtures. A higher dose of etorphine was used to anaesthetize animals in the former group since, in the absence of fentanyl, it was not certain whether a 2-mg etorphine dose would be sufficient for successful immobilization. Given that etorphine may be responsible for elevating blood pressure in animals anaesthesized with this drug, the lower blood pressure in the group receiving a higher dose of etorphine is surprising. It is possible that the azaperone itself is responsible for this effect although further research is required to substantiate such a suggestion. Azaperone has antagonistic peripheral C4 adrenergic receptor properties and was therefore considered suitable for use in this study. to overcome possible blood pressure elevation during immobilization.
Etorphine/fentanyl and etorphine/azaperone anaesthesia in rhinoceroses was accompanied by hypoxaemia and hypercapnia (Table 1). A similar, less severe change in arterial P02 (which improved with oxygen supplementation) was observed by Heard et al. (1992) in an etorphine-anaesthetized white rhinoceros. Hypoxaemia and hypereapnia are probably a direct result of etorphine-induced respiratory depression which is a recognized action of this drug in many species. The more favourable P02 observed in the etorphine/azaperone group in spite of the higher etorphine dosage used may be related to the absence of fentanyl which is also reported to suppress respiration when used alone (Harthoorn 1973). Although such a suggestion requires confirmation, the results obtained here support the recommendation by Heard et al. (1992) that the management of anaesthesia in etorphine-immobilized rhinoceroses should include oxygen supplementation.
Kock et al. (1990) suggested that certain physiological responses to capture in the initial period of management may predispose the animals to adverse effects of further stress, resulting in mortalities one week to two months after capture. It is also possible that increased blood pressure, hypoxaemia and hypercapnia, whether induced by drugs, stress and/or postural changes during immobilization, may contribute to the post-capture morbidity and/or mortality reported in rhinoceroses. This study was a preliminary investigation and the need for further research into the effects observed is imperative. The results obtained above suggest that the addition of azaperone to immobilization mixtures may alleviate possible blood pressure elevation during immobilization. Secondly, either oxygen supplementation or the administration of a respiratory stimulant (such as doxapram) as soon as possible after recumbency to improve blood gas status is indicated.
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