Page 28 - VETcpd - Vol 5 - Issue 3

VETcpd - Exotics

Elisabetta Mancinelli DVM CertZooMed ECZM Dipl. (Small Mammal Medicine and Surgery) MRCVSEuropean Veterinary Specialist in Zoological Medicine (Small Mammal Medicine and Surgery)

Elisabetta graduated from the University of Naples “Federico II”, Italy, in 2002.After a small animal focused internship, she has worked solely with exotics since 2003. She undertook an externship in France, mainly based on reptile medicine and completed an externship program at the “Angell Animal Memorial Hospital” in Boston (USA) focusing on exotic animal medicine and surgery.In 2007 she moved to the UK where she initially worked in private practice and wildlife charities. In 2009, Elisabetta started the first European College of Zoological Medicine (ECZM) Residency in Small Mammal Medicine, which she completed at The Royal (Dick) School of Veterinary Studies, Edinburgh. Elisabetta holds the RCVS Certificate in Zoological Medicine. In 2014, she obtained the ECZM Diploma, Specialty “Small Mammal”. Elisabetta is also an RCVS Recognised Specialist in Zoo and Wildlife Medicine.E-mail: referrals@highcroftvet.co.uk

As a prey species, rabbits are masters at disguising signs of disease and therefore often present at an advanced stage of illness. The reported overall peri-anaesthetic mortality rate of healthy pet rabbits is much higher than in dogs and cats and anaesthetising rabbits certainly poses several challenges. Careful pre-anaesthetic evaluation of the patient is essential and should include a detailed history and thorough physical examination. A minimum laboratory database is also important as it provides useful information on the general health status of the patient. Factors, such as the presence of underlying disease, clinician experience, drug dosages, analgesia and post-anaesthetic care, as well as the differences in anatomy and physiology to the more commonly encountered species, should also be considered to reduce risk and mortality rate. Elective anaesthesia must always be carefully planned. Patient monitoring during an anaesthetic is critical and an assistant must be available to monitor anaesthetic depth, response to painful stimuli, and vital parameters. The principles of anaesthetic monitoring are no different to those used in dogs and cats and the availability of more advanced equipment greatly enhances the chances of positive results.

Key words: Rabbit, Anaesthesia, Pre-anaesthetic evaluation, Intubation, Monitoring, Recovery

The challenges of

Rabbit Anaesthesia

Introduction Anaesthetising rabbits is becoming routine in our day-to-day work to perform a variety of procedures. However, Brodbelt et al. (2008) reported an overall peri-anaesthetic mortality rate of healthy pet rabbits of 1.39%. This equates to one in 72 rabbits dying within 48 hours of anaestheisa and lends credence to the general notion that rabbits are high risk anaesthetic candidates. When data from healthy animals were assessed separately, the reported mortality rate was one in 137 rabbits. These values are much higher than those found in dogs (1/1840) and cats (1/893), but represent an improvement compared to data reported in an earlier 1990 study where the mortality rate was one in 28 rabbits (Clarke et al. 1990). There are many factors that influence the safety of rabbit anaesthesia and these are discussed below. Inappropriate extrapolations of dosages and/or techniques may prove fatal in some cases. Drug doses taken from studies or textbooks based on laboratory animals should be considered carefully as they are often much higher than those recommended in clinical settings. In the past, the lack of validated research meant that clinicians had to rely mainly on their empirical knowledge and personal experience. However, the increasing interest in rabbit medicine and surgery has also led to an increase in clinical research, with resultant improved

standards of care. Recent advances in techniques and the availability of more sophisticated equipment as well as drugs with better safety margins have also resulted in an improved quality of rabbit anaesthesia. Whilst anaesthetising rabbits certainly poses several challenges, the risks can be minimised to decrease the rate of mortality and increase the chance of a positive outcome.

Pre-anaesthetic considerationsClinical assessmentAs a prey species, rabbits are masters at concealing signs of disease (Longley 2008a). They tend to hide signs of pain, meaning that they are often presented in a stage of illness that is more advanced than is initially evident. Furthermore, many factors result in stress and catecholamine release, whilst sympathetic nervous system stimulation may result in reduced gastrointestinal motility and gastric ulceration increasing the risk of complications (Longley 2008a). A careful pre-anaesthetic evaluation is therefore essential to identify animals with systemic disease (e.g. subclinical respiratory disease, dental disease, gastrointestinal disorders leading to dehydration and anorexia) for which a higher anaesthetic risk may be anticipated. A detailed history including information regarding husbandry, diet,

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Figure 1: Adequate preparation for a specific procedure is critical as it helps to minimise the length of anaesthesia. Dosages of all drugs required, including emergency drugs, should be calculated (with 1:9 dilution if required), recorded on the anaesthetic sheet and drawn up. All necessary equipment (e.g. Doppler sphygmomanometer, pulse-oximeter, capnograph, mechanical ventilator, anaesthetic machine and gas anaesthetic to be used, heat support device, rectal thermometer or probe, endotracheal intubation equipment) should be assembled and checked. An appropriate, detailed emergency plan should be established.

Figure 2: Stabilisation of sick patients before they undergo an anaesthetic is imperative. This may include oxygen administration, correction of hypothermia, fluid therapy, assisted feeding in anorexic patients, antibiotic administration if indicated by a significant bacterial infection and adequate pain relief. In the picture shown, a syringe driver is used for continuous intravenous fluid therapy. This device is calibrated in ml/h and is capable of delivering low infusion volumes (fluid rates between 0.2 ml and 16.5 ml per hour), which make it ideal for continuous rate infusions (CRI). The author often uses CRI during anaesthesia and recovery to provide fluids and analgesia. Intermittent IV boluses may also be used.

and medical history should be obtained. A complete physical examination should also be conducted and a minimum laboratory database, including packed cell volume (PCV), total protein (TP), albumin, blood urea nitrogen (BUN), creatinine and glucose will also provide useful preliminary information on the health status of the patient. A more advanced assessment should be carried out, where indicated. More often in practice, investigations are performed only if clinical examination or history suggest underlying diseases. As mucous membrane colour is often unreliable, conscious capnography and pulse-oximetry may prove promising for the detection of hypercapnia and hypoxia. Capnography has been reported to help identify patients with respiratory compromise as elevated carbon dioxide levels (ETCO

2 – end tidal carbon dioxide

levels) correlated with PaCO2 (partial

pressure of arterial carbon dioxide) levels in one study (Evans 1977). Elevated ETCO

2 would suggest pneumonia even

in patients with an apparently normal respiratory pattern and no audible abnormalities on thoracic auscultation (Chitty 2008; Grint 2013). However, according to Eatwell et al (2013), capnography readings are not a reliable guide to ventilation and oxygenation in the conscious rabbit as hyperventilation and breath holding are common in this species. Nevertheless, the authors of this paper advise that for hyperventilation to be readily identified, both the value of the ETCO

2 reading should be quantified

and visual interpretation of the waveform performed as, in this case, the carbon dioxide level never returns to baseline. The aforementioned study also assessed the feasibility and value of arterial blood gas analysis concluding that it should be considered as a superior tool for evaluation of acid-base status, electrolytes, ventilation and oxygenation in domestic rabbits (Eatwell et al. 2013). Its use in the peri-anaesthetic period should therefore be re-considered and valued more.

Stabilisation prior to anaesthesia Several factors may complicate an anaesthetic procedure, and their impact should always be taken into consideration. The presence of underlying disease, inappropriate husbandry and diet, lack of confidence and expertise of staff involved, inadequate planning, anatomical and physiological differences to more commonly anaesthetised species, inappropriate extrapolation of

drug dosages, inadequate analgesia and insufficient post-anaesthetic care may contribute to increased anaesthetic risks. Furthermore, minimising the anaesthesia time is crucial and can be achieved by thoroughly preparing for the procedure, including gathering all drugs and equipment required and establishing an appropriate emergency plan (Figure 1).

Patients with pre-existing disease should be stabilised before undergoing anaesthesia. This might include fluid therapy to correct dehydration, assisted feeding in anorexic patients, appropriate antibiotic therapy if indicated by the presence of a significant bacterial infection and adequate analgesia (Grint 2013) (Figure 2). The American Society of Anaesthesiologists (ASA) provides a classification system which is worth reviewing for rabbits too as a guide to whether a patient is fit for an anaesthetic (grade 1-2) or if higher risks may

be anticipated (grade>2). The peri-anaesthetic death risk increases for higher ASA grades. Rabbits identified as grades 1-2 were reported to have a 0.73% risk of anaesthetic related death, whilst for those in grades 3-5 the risk increased to 7.35% (Brodbelt 2006). In reality, most rabbits undergoing anaesthesia should be considered to be grade 2-4 due to their ability to mask illness (Eatwell 2014). A thorough pre-anaesthetic examination may reveal subtle signs that might indicate further diagnostics or postponement of anaesthesia in some cases. The clinical evaluation also determines baseline vital parameters (e.g. heart rate, respiratory rate, body temperature) which represent the expected normality and reference points specific for that individual. This may be particularly significant in this species as the reported reference ranges for physiologic parameters in rabbits are quite broad (Table 1). Since compensatory

Parameter rePorted range

Heart rate 150-300 beats/minute

respiratory rate 30-60 breaths/minute

Body temperature 38.5° - 40.0°

adult body weight (breed dependent) 1-6 kg

Systolic blood pressure (anaesthetised) 92-135 mmHg

table 1: Physiological reference ranges reported for rabbits. modified from meredith (2014) and eatwell (2013).