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responding veterinary schools, those that reported the lowest prevalence of ‘post anaesthetic respiratory obstruction’ were those institutions that routinely recovered horses with ET/NTTs tubes in place. Whilst some of the causes of airway obstruction (e.g. poor positioning of the head/neck during recovery, displaced soft palate, bilateral laryngeal paralysis) can occur after any recovery, it seems common sense that the risk of airway obstruction due to swelling and blood clots would be more likely after upper respiratory tract (URT) surgery. However, the evidence to support this assumption is conflicting. In one report of outcomes after ‘tie-forward’ surgeries in racehorses only one out of 116 cases (∼0.9%) had severe dyspnoea requiring intervention during recovery (Woodie et al. 2005). An early study reported 4 out of 21 (19%) draught horses had airway obstruction necessitating tracheostomies after laryngoplasty surgery to correct laryngeal hemiplegia (Bohanon et al. 1990), yet subsequent reports in larger populations showed a far lower prevalence of respiratory obstruction after similar surgeries in draught breeds (0 out of 104 surgeries, 0%) (Kraus et al. 2003), mixed breeds (0 out of 200 cases, 0%) (Dixon et al. 2003) and Standardbreds/ Thoroughbreds (1 out of 230 surgeries, ∼0.4%) (Hawkins et al. 1997). It would seem that the prevalence of airway obstruction, severe enough to necessitate replacing an ET/NTT or performing a tracheostomy is very low, even after URT surgeries. How often does severe airway obstruction during recovery
result in a fatality? Airway obstruction during recovery from anaesthesia sufficient to cause impairment of gas exchange and/or negative pressure pulmonary oedema may certainly be rapidly fatal (Kollias-Baker et al. 1993) and is an emergency. However, most case reports of severe respiratory obstruction in recovery have shown that it can be survivable, often with no observable long-term sequelae following rapid recognition and intervention (e.g. Abrahamsen et al. 1990; Ball and Trim 1996; Dixon et al. 1993; Tute et al. 1996; Borer 2005; Kaartinen et al. 2010). Another important feature of severe airway obstruction after anaesthesia in horses, especially where the larynx is involved, is that it can occur both immediately or at any time, even hours, after extubation (including after removal of in situ ET/NTTs for recovery) (Abrahamsen et al. 1990; Dixon et al. 1993; Southwood et al. 2003); therefore, the use of in situ ET/NTTs does not eliminate the risk of airway obstruction. It is for that reason that many authors recommend performing a tracheostomy to secure an airway for any length of time in cases of severe airway obstruction (Dixon 1988; Southwood et al. 2003; Wagner 2008; Bednarski 2009). As Potter et al. (2015) state; there are no published, prospective, multicentre randomised clinical trials to support (or not) the use of ET/NTTs to prevent upper airway obstruction and due to the low prevalence of severe airway obstruction such studies are unlikely ever to appear. However, the limited evidence available suggests that the prevalence of severe airway obstruction in both the general and URT surgical equine populations is likely to be very 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 ET/NTTs 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. In my own practice, after witnessing horses occlude
airways by kinking tubes in recovery, 2 horses aspirate nasotracheal tubes during recovery and one horse occlude its airway due to bilateral laryngeal paralysis after removal of an in situ ET tube after the horse had stood, I re-appraised the merit of pre-emptive in situ ET/NTTs in preventing airway obstruction, as these incidents seemed to be occurring with greater frequency than other causes of severe airway obstruction. As a result of the appraisal of the evidence alongside my own experience, I no longer routinely pre-emptively place ET/NTTs for recovery but have emergency tracheostomy kits ready by our recovery boxes. I extubate whilst the horse is in lateral recumbency as the swallow reflex returns (unless concerned about gastric reflux/blood etc. in the nasopharynx); if dyspnoea is detected at this stage, interventions such as replacing ET/NTTs or performing an emergency tracheostomy is more likely to be easier, safe and successful (Bettschart-Wolfensberger 2012).
Author’s declaration of interests No conflicts of interest have been declared.
References
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Bednarski, R. (2009) Tracheal and nasal intubation. In: Equine Anaesthesia, 2nd edn., Eds: W. Muir and J. Hubbell, Saunders Elsevier, St Louis. pp 277-287.
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