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EQUINE VETERINARY EDUCATION / AE / MAY 2015
Clinical Commentary
Mitigating the risk of airway obstruction during recovery from anaesthesia: The way is far from clear
J. M. Senior Philip Leverhulme Equine Hospital, University of Liverpool, School of Veterinary Science, Leahurst Campus, Neston, South Wirral, UK. Corresponding author email:
j.m.senior@
liverpool.ac.uk
Summary All equine anaesthetists should take steps to mitigate the risk of anaesthetic-related morbidities and mortalities where they can, and whilst some interventions against some of these are widely practiced, the number of approaches adopted by equine anaesthetists to mitigate the risk of airway obstruction suggests not only that the causes and severity of airway obstruction vary, but also that evidence for the superiority of any approach over another is limited. The limited evidence available suggests that the prevalence of severe airway obstruction in both the general and upper respiratory tract surgical equine populations is likely to be low, and that when cases do occur, rapid recognition and intervention often results in successful treatment. The above evidence considered with the understanding that pre-emptively placing endotracheal or nasotracheal tubes in situ for recovery may only delay the recognition of some causes (e.g. laryngeal) of airway obstruction, alongside an awareness that this approach is not a benign risk-free intervention, may lead some anaesthetists to re-evaluate the perceived risk of respiratory obstruction and whether their chosen intervention will reduce that risk, potentially exacerbate it or introduce a further risk.
Commentary
The case report by Potter et al. (2015) in this issue of EVE, highlights a potential complication of recovering horses with in situ endotracheal/nasotracheal tubes (ET/NTTs): aspiration of the tube. Like all good case reports, it not only notifies readers and discusses the background surrounding an interesting or unusual case, but also encourages reflection and review of our clinical practice, which in this brief commentary, is the use of in situ ET/NTTs to mitigate the risk of airway obstruction in horses.
Some degree of airway obstruction is common in horses
after anaesthesia, and can be described and defined in many ways, e.g. stertor/stridor, incomplete/complete, mild/ moderate/severe. The lack of precise definitions not only hinders interpretation and comparison of studies but can also confuse clinical debate. This commentary will focus mainly on airway obstruction that could reasonably be presumed to be a danger to the horse’s life, i.e. necessitates intervention. All equine anaesthetists should take steps to mitigate the
risk of anaesthetic-related morbidities and mortalities where they can, and whilst some interventions against certain morbidities and mortalities are widely practised, the number of approaches adopted by equine anaesthetists to mitigate the risk of airway obstruction suggests not only that the causes and severity of airway obstruction vary, but also that evidence for the superiority of any approach over another is limited.
© 2015 EVJ Ltd In the case of in situ ET/NTTs to mitigate the risk of airway
obstruction, whilst many texts (e.g. Taylor and Clarke 2007; Wagner 2008; Bednarski 2009) mention recovering horses with in situ ET/NTTs to maintain an airway, particularly when airway obstruction is noticed after extubation or deemed to be likely, only one (Ahern and Parente 2009) specifically recommends doing so in certain cases (after surgery to correct laryngeal hemiplegia). It is also worth noting that whilst many horses tolerate endotracheal tubes in their airway, not all do (Bettschart-Wolfensberger 2012), and, as noted by Potter et al. (2015) such an approach itself carries risks of airway obstruction through kinking, occlusion and also aspiration of the tube itself. Airway obstruction occurs most often during the recovery
period and is either as a result of the horse becoming positioned such that its airway (or endotracheal tube) becomes occluded or because of an anatomical or functional obstruction of the airway such as oedema, swelling and blood clots after surgery (Wagner 2008), displaced soft palate (Bednarski 2009) or laryngeal paralysis (Southwood et al. 2003). As stated above, some degree of airway obstruction is common after anaesthesia in horses but how often does severe airway obstruction necessitating interventions such as placing endotracheal tubes or performing tracheostomies occur? Equine anaesthesia still carries a fatality rate of around 1%
after emergency cases are excluded (Senior 2013). Although ‘respiratory causes’, when reported, contribute to 3.7–25% of fatalities (Mee et al. 1998; Johnston et al. 2002, 2004); ‘respiratory causes’ is a broad definition; indeed, the 2 cases detailed by Mee et al. (1998) stated that death was caused by respiratory arrest occurring intraoperatively in one case and 8 h after recovery in the other. There is limited reporting of the prevalence of respiratory
obstruction during recovery from equine anaesthesia in general horse populations. In our own hospital, we retrospectively reported respiratory obstruction in recovery resulting in pulmonary oedema in 3 out of 1704 (∼0.18%) anaesthetics (Senior 2005) and, in a prospective multicentre study into anaesthetic-related morbidities, respiratory obstruction necessitating intervention, such as tracheostomy or nasotracheal intubation was reported in 3 out of 861 (∼0.3%) anaesthetics (Senior et al. 2007). A retrospective report (Thomas et al. 1987) stated a prevalence of ‘post anaesthetic respiratory obstruction’ occurring in 1.4% of anaesthetics in their own hospital population and in 0–0.23% anaesthetics of hospital populations from 11 other veterinary school respondents to a questionnaire. Thomas et al. (1987) did not detail what they defined as respiratory obstruction (e.g. severity/requiring intervention), but they did report that of the
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