Investigation into the characteristics, triggers and mechanism of apnoea and bradycardia in the anaesthetized platypus (Ornithorhynchus anatinus)

J W Macgregor,C Holyoake,K S Warren,J H Connolly,I D Robertson,P A Fleming

doi:10.1093/conphys/cou053

Abstract

Health and conservation research on platypuses (Ornithorhynchus anatinus) may require anaesthesia to reduce stress and the risk of injury to both the animal and the researcher, as well as to facilitate examination and sample collection. Platypus anaesthesia can be difficult to manage, with reports of periods of apnoea and bradycardia described. This study investigated the conditions around sudden-onset apnoea and bradycardia in 163 field-anaesthetized platypuses as part of a health study. Anaesthesia was induced and maintained using isoflurane delivered in oxygen by face mask. Sudden-onset apnoea and bradycardia was observed in 19% of platypuses, occurring either at induction of anaesthesia, during recovery, or both. At induction, occurrence was more often recorded for adults (P = 0.19) and was correlated with low body temperature (P < 0.001), season (P = 0.06; greater incidence in summer) and longer pre-anaesthetic holding time (P = 0.16). At recovery, sudden-onset apnoea and bradycardia occurred only in platypuses that had been placed in dorsal recumbency as part of their examination, and correlated with poor body condition (P = 0.002), time in dorsal recumbency (P = 0.005), adults (P = 0.06), number of fieldworkers (P = 0.06) and females (P = 0.11). The sudden-onset apnoea and bradycardia we observed is likely to result from the irritant nature of isoflurane (stimulating the trigeminal nerve via nasal chemoreceptors). We propose that this mechanism is analogous to that of submersion of the face/nasal cavity in cold water during a natural dive response, but that the term 'nasopharyngeal response' would more appropriately describe the changes observed under isoflurane anaesthesia. Although we did not record any long-term adverse effects on platypuses that had undergone this response, the nasopharyngeal response could complicate the diagnosis of anaesthetic dose-dependent apnoea and bradycardia. Therefore, we suggest that these responses during anaesthesia of platypuses might be avoided by minimizing the stress around capture and handling, as well as reducing the time in dorsal recumbency.

Highlights

Anaesthesia is an important field technique for platypus health and conservation research, facilitating sample collection and measurements while minimizing stress for the animal and reducing the risk of envenomation for the researcher
Sudden-onset apnoeic/bradycardic events were never recorded during maintenance of anaesthesia, but were observed in 31 anaesthetized platypuses (19% of the 163 tested) during either induction (n = 9 individuals) or recovery (n = 17 individuals), or both (n = 5 individuals)
Transient apnoea and bradycardia were observed at induction or recovery in an additional 17 platypuses

Summary

Introduction

Anaesthesia is an important field technique for platypus health and conservation research, facilitating sample collection and measurements while minimizing stress for the animal and reducing the risk of envenomation for the researcher These changes have been reported to be self-resolving but to complicate the diagnosis of dose-d ependent apnoea and bradycardia, which continue to deteriorate unless the dose is reduced. Apnoea and profound bradycardia under anaesthesia have been reported in marine mammals and have been identified as the cause of anaesthetic deaths (McDonell, 1972; Gales and Burton, 1988; Phelan and Green, 1992). In both platypuses and marine mammals, these respiratory and cardiac changes under anaesthesia have been described as an inappropriate initiation of the dive response

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