31 Oct 2016
Elisabetta Mancinelli
Job Title
An axolotl (Ambystoma mexicanum) was successfully anaesthetised by exposure to increasing concentrations (up to 5mg/L) of alfaxalone in a water bath (McMillan and Leece, 2011).
In part one of this article (VT46.38), the general characteristics of alfaxalone – a synthetic neuroactive steroidal anaesthetic that achieves its central effects via interactions with gamma aminobutyric acid (GABA) receptors – were taken into consideration.
This drug became available in the UK for IV use in dogs and cats in 2007 and is combined in a complex with cyclodextrin to achieve water solubility (Alfaxan: Jurox). It is widely used off-label due to its reportedly wide therapeutic index, smooth induction and rapid recovery characteristics (Shepard et al, 2013). Several reports exist in the literature about its use in a variety of exotic species.
In part one the author reviewed some of these studies focusing on small mammals. In this article, research looking into reptiles and amphibians will be taken into consideration. Please refer to the literature provided for more in depth information about this drug and its use in specific species.
Alfaxalone/alfadolone was reported as a useful anaesthetic agent in reptile patients (Lawrence and Jackson, 1983) but, since then, several reports have been published of alfaxalone use in cyclodextrin in a variety of reptile species. Bertelsen and Sauer (2011) administered 10mg/kg, 20mg/kg and 30mg/kg of alfaxalone IM to 10 healthy juvenile green iguanas.
Results showed IM administration of alfaxalone was a simple, rapid and reliable means of achieving relatively brief sedation or anaesthesia in this species:
The authors noticed dose-dependent respiratory depression, which gradually improved. Intubation, ventilation and adequate monitoring are, therefore, necessary. Knotek et al (2013a) evaluated the effect of 5mg/kg alfaxalone given IV to 13 adult male green iguanas via the ventral caudal vein. This protocol did not induce significant apnoea or affect the cardiorespiratory functions, proving to be a suitable and safe form for short-term anaesthesia in green iguanas.
In varanids (monitor lizards, Varanidae) and agamids (dragon lizards, Pogona species), alfaxalone was used at a dose rate of 5mg/kg 30 minutes following administration of the opioid analgesic butorphanol. This combination resulted in the smooth recovery of reptiles. On the other hand, satisfactory anaesthesia was not achieved after administration of alfaxalone alone at a dose of 9mg/kg IV in a study on Australian lizard species, although, in most of them, except for blotched blue tongue lizards (Tiliqua nigrolutea), an endotracheal tube could be inserted (Scheelings et al, 2010). In all species, some response to painful stimuli was maintained, suggesting the need for supplemental analgesia.
Kischinovsky et al (2013) looked at several indicators of anaesthetic depth after administration of 10mg/kg and 20mg/kg IM in 10 female red ear sliders kept in cool (20°C) and warm (35°C) temperature. They found this protocol provided smooth, rapid induction and uneventful recovery. At 35°C either dosage provided short (5 to 10 minutes) and light sedation. At 20°C, 10mg/kg provided sedation suitable only for short, non-invasive procedures. A dose of 20mg/kg provided anaesthesia of approximately 20 minutes duration, appropriate for induction of inhalational anaesthesia or for brief surgical procedures with supplemental analgesia. Higher doses and a lower temperature led to longer induction time and deeper and more prolonged anaesthesia.
In another study, Shepard et al (2013) compared the pharmacodynamics of two different doses of alfaxalone administered IM to red-eared sliders at two ambient temperatures. This study showed alfaxalone given at 10mg/kg or 20mg/kg IM may be used in red-eared sliders to produce variable degrees of muscle relaxation and improve compliance to handling for diagnostic or medical procedures, such as radiography, physical exam and venipuncture. However, the turtles had dose-dependent and inconsistent responses to alfaxalone.
Lower ambient temperature augmented the behavioural effects of this drug. In fact, turtles given 10mg/kg were more relaxed and easier to handle in cold rather than warm conditions. Warm turtles were more relaxed and easier to handle when given 20mg/kg than those given 10mg/kg. Lower ambient temperatures potentiated muscle relaxation, sedation (compliance to handling) and the duration of action of this drug in turtles.
Recovery from the effects of alfaxalone was significantly longer in groups given the higher dose of alfaxalone, as well as groups kept in colder ambient temperature environments. The turtles with the longest recovery time (257 +/- 25 minutes) were those administered 20mg/kg and kept in the cold condition, while turtles with the shortest recovery time (126 +/-15 minutes) were those administered 10mg/kg in the warm condition. This information serves to alert practitioners of the likelihood of increased effects of this drug in chelonians living in environments below their preferred temperature range.
Hypothermia should never be intentionally used to enhance the anaesthetic effects of this drug. In a two-part study, Knotek (2014) evaluated the effects of alfaxalone at a dose rate of 5mg/kg administered to 10 adult female red-eared terrapins (Trachemys scripta elegans) into the subcarapacial sinus.
In the second part of the study, 50 chelonians (20 red-eared terrapins, 10 Hermann’s tortoises, 8 spur-thighed tortoises, 6 marginated tortoises and 6 Russian tortoises) were treated with 5mg/kg alfaxalone IV 25 to 35 minutes after administration of 1mg/kg meloxicam and 2mg/kg butorphanol IM. Selected clinical indicators were recorded and the protocol proved to be a suitable method of induction to inhalation anaesthesia in terrapins and tortoises characterised by rapid onset of activity, facilitation of tracheal tube insertion, a short period of deep pain sensation loss and a more protracted period equating to a surgical plane of anaesthesia, with full recovery (where inhalational agents were not used) approximately 30 minutes post-induction.
The results of the second part of this study showed the head, neck and leg withdrawal reflex was lost within 12 to 40 seconds, the endotracheal tube could be inserted within 15 to 60 seconds, and the time to deep pain sensation loss was 20 to 70 seconds.
Knotek et al (2011) administered alfaxalone at a dose of 5mg/kg IV in 30 healthy veiled chameleons. This dose enabled endotracheal intubation and proved to be a safe and reliable form of short-term anaesthesia, which did not affect basic physiologic functions. The loss of deep sensitivity persisted from 2 minutes to 5 to 10 minutes, while full activity was restored 20.30 ± 5.10 minutes after alfaxalone administration.
Knotek et al (2013b) also studied the effect of 5mg/kg administered via the ventral caudal vein to 300 lizards. A surgical plane of anaesthesia was achieved after 2 minutes and lasted for 7 minutes. Full activity was restored 15.10 +/- 5.10 minutes after the initial alfaxalone injection.
Hansen and Bertelsen (2013) aimed at characterising some of the clinical and physiological effects of IM alfaxalone alone at 10mg/kg and 20mg/kg, and in combination with medetomidine (10mg/kg alfaxalone and 0.1mg/kg medetomidine or 20mg/kg alfaxalone and 0.05mg/kg medetomidine) in the Horsfield’s tortoise (Agrionemys horsfieldii).
The effects were rapid in onset and dose-dependent with variable individual effects.
The addition of medetomidine resulted in deep sedation or anaesthesia with significantly increased total anaesthetic time. Its addition also significantly reduced peripheral nociceptive sensation compared to the protocols using alfaxalone alone. This is similar to cats and rats, in which alfaxalone has been shown to have either no or minimal analgesic effect. Side effects included significant respiratory depression and bradycardia, such as periods of transient cardiac arrest, which may limit the use of this combination for routine anaesthesia in tortoises.
The presence of the GABA receptor has been documented in amphibians and neurosteroids appear to act similarly in them as in mammals (Hollis et al, 2004; Orchinik et al, 1994). An axolotl (Ambystoma mexicanum) was successfully anaesthetised by exposure to increasing concentrations (up to 5mg/L) of alfaxalone in a water bath (McMillan and Leece, 2011). Anaesthesia was induced and maintained with alfaxalone (5mg/L solution at 2ml/minute to 3ml/minute) via continuous irrigation of the gills and skin (cutaneous) with additional drops of alfaxalone undiluted (10mg/ml) administered branchially every 30 seconds as required.
Branchial and gular respiratory movements persisted at what was considered an appropriate anaesthetic depth. Anaesthetic depth could be rapidly deepened by branchial irrigation of alfaxalone solutions and lightened by irrigation using fresh dechlorinated water. Anaesthesia lasted approximately one hour, additional analgesia was required and recovery was rapid (within 15 minutes).
Adami et al (2015) carried out a two-phase study aimed at identifying an alfaxalone concentration capable of producing induction of anaesthesia (defined as immobility with a head down position and loss of responsiveness to stimulation with a stick) in five oriental fire-bellied toads (Bombina orientalis). This dose was then used in an additional eight toads during the following phase.
Immersion was found to be a suitable anaesthetic technique for oriental fire-bellied toads at 200mg/L alfaxalone concentration, which produced anaesthetic induction in 10 out of 11 toads. Side effects, such as skin irritation, erythema and changes in cutaneous pigmentation, were not observed in any animal. The duration of anaesthesia was variable and ranged from 10 to 30 minutes after toad removal from the alfaxalone bath, and surgical depth of anaesthesia was never achieved, meaning this protocol is only suitable for toads undergoing non-invasive short procedures and, at the dose used in this study, failed to inhibit the nociceptive response in the toads.
Adami et al (2016a) also determined a dexmedetomidine concentration of 0.3mg/100ml added to a bath containing 20mg/100ml alfaxalone resulted in surgical anaesthesia in two out of eight toads, and induction of anaesthesia was achieved in 50 per cent of the animals, as compared with 100 per cent of the toads in which only the alfaxalone bath was used.
The combination provided some analgesia in oriental fire-bellied toads, but failed to potentiate the level of unconsciousness, decreased the hypnotic effects produced by alfaxalone – making the anaesthesia less effective – and appeared to lighten the depth of anaesthesia.
The authors concluded this limitation renders the combination unsuitable for anaesthetising oriental fire-bellied toads for invasive procedures. In a study, Adami et al (2016b) established the effective butorphanol and morphine concentrations to be added to alfaxalone for immersion anaesthesia, and compared the anaesthetic and antinociceptive effects of the two drug mixtures (alfaxalone/butorphanol and alfaxalone/morphine), in B orientalis toads.
The two treatments were found to be comparable in terms of onset, duration of anaesthesia and occurrence of undesired effects. Results showed, at the investigated concentrations and in combination with alfaxalone by immersion, morphine provided better antinociception than butorphanol in oriental fire-bellied toads.
Elisabetta Mancinelli
Job Title