Physiological effects of high and low medetomidine doses in combination with zolazepam/tiletamine for the immobilisation and anaesthesia of spotted hyenas (Crocuta crocuta)

Abstract

Objective: To compare the physiological effects of high and low dose medetomidine in combination with zolazepam-tiletamine for the immobilisation and anaesthesia of the spotted hyenas (Crocuta crocuta). To identify a safe and effective medetomidine dose in combination with zolazepam-tiletamine in the immobilisation of the spotted hyenas (Crocuta crocuta). Study design: Prospective, blinded, randomised cross-over study Animals: Eight captive adult spotted hyenas, five males and three females, weighing a mean (± SD) of 65.2 (± 8.4) kg. Material and methods: All hyenas were immobilised on two separate occasions for at least 60 min with either a high (HD treatment – 0.04 mg kg-1) or low dose (LD treatment – 0.02 mg kg-1) of medetomidine in combination with zolazepam-tiletamine (1 mg kg-1) administered via IM dart. Physiological variables and anaesthetic plane were recorded at 5-min intervals. Cardiorespiratory variables were monitored using a multiparameter monitor, spirometry, ECG, high definition oscillometer correlated with pulse oximeter and auscultated heart and respiratory rates. Rectal temperature was measured using a digital portable rectal thermometer. Two arterial blood samples were collected for blood gas analysis. The first arterial blood gas sample was collected 20 min after darting. Supplementary oxygen was then provided at 2 L min-1. The second arterial blood gas sample was collected 50 min after darting. Once the 60-min anaesthetic period was completed, atipamezole was administered IM at five times the medetomidine dose and recovery was observed. Data are reported as mean ± SD; all variables were compared using two-way repeated measure ANOVA’s and paired sample t-tests with post-HOC tests. Significance was determined at p < 0.05. Results: All hyenas were safely and successfully immobilised with time to recumbency of 4.2 (± 2.1) min and 7 (± 5.5) min for the HD and LD treatments, respectively. A stable cardiorespiratory function was maintained throughout anaesthesia. Blood oxygen saturation did decrease significantly after darting, indicating a mild to moderate hypoxaemia (SaO2 range of 79-91%) possibly attributed to a ventilation-perfusion mismatch. End-tidal CO2, base excess, partial pressure of carbon dioxide and oxygen, bicarbonate and total carbon dioxide did significantly increase with oxygen supplementation, with pH decreasing slightly. Haemoglobin concentration and haematocrit significantly increased with the HD treatment, but remained within acceptable range for hyenas and other mammals. Glucose significantly increased slightly after drug administration. Ionised calcium increased slightly possibly with decreasing pH values. Heart rates were a mean of 42 and 40 beats min-1 for both HD and LD treatments, respectively. Both AV and SA blocks were observed, along with sustained hypertension during both treatments. MAP were a mean of 134.5 (± 25.7) mmHg and 123.5 (± 20.0) mmHg for HD and LD treatments, respectively. Cardiovascular effects did not differ between treatments. Vomition occurred during only one LD immobilisation (6%). Atipamezole, at five times the medetomidine dosage, successfully antagonised the effects of medetomidine in all hyenas. Time to standing was 26 min (± 15) and 26.4 min (± 23.1) for HD and LD treatments, respectively. Both treatments provided sufficient anaesthesia and calm recovery. Treatment HD provided more reliable and consistent results indicated by the small standard deviations for time to first signs of sedation, recumbency, handling, as well as time to sternal recumbency and standing during recovery. Conclusions and clinical relevance: Both low and high medetomidine doses, at 0.02 mg kg-1 and 0.04 mg kg-1 respectively, in combination with zolazepam-tiletamine at 1.0 mg kg-1 can provide adequate immobilisation and anaesthesia for routine field management and minor surgical procedures in the spotted hyena for at least 60-min duration. Minimal physiological differences were observed between the two treatments. Oxygen supplementation is advised with the use of this protocol in order to treat the hypoxaemia possibly caused by ventilation-perfusion mismatch. A decrease in heart rate was observed and was as low as 24 beats min-1, mean 41 beats min-1 with an overall mean MAP of 129 mmHg. Therefore, cautious monitoring is suggested. All cardiovascular effects were expected, and within acceptable clinical range based on observations noted in other wildlife and domestic species and possibly related to medetomidine use. An adverse effect observed was vomition which only occurred in 6% of the hyenas immobilised. All blood pH’s were <7.35, most likely related to the strict carnivorous diet of the spotted hyena as well as the mild hypercapnia experienced. Atipamezole was found to effectively antagonise the effects of medetomidine at five times the dosage of medetomidine administered. The one advantage of using the low dose medetomidine treatment is the reduction in cost, which can be an important consideration in developing countries where these free-ranging animals inhabit.