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Evaluating equine anaesthetic risk

02 July 2022
19 mins read
Volume 6 · Issue 4
Figure 2. Horses undergoing emergency exploratory laparotomy may have concurrent systemic compromise and, in some cases, may be suffering from muscle fatigue or exhaustion.
Figure 2. Horses undergoing emergency exploratory laparotomy may have concurrent systemic compromise and, in some cases, may be suffering from muscle fatigue or exhaustion.

Abstract

Currently, evaluation of equine anaesthetic risk is largely based on the American Society of Anaesthesiologists physical status classification system. However, a recent survey of experienced equine anaesthetists indicated that anaesthetic risk could also be based on patient factors and factors associated with the intended procedure including positioning, anticipated duration and ability or desire to assist recovery. Furthermore, additional anaesthesia-related risk factors have been identified through clinical research, which may warrant consideration when assessing individual patient risk.

Anaesthesia of the horse is more perilous than for any other common domestic species (Hubbell et al, 2022), making the high risk of mortality associated with general anaesthesia in horses one of the biggest concerns for equine practitioners and veterinary anaesthetists (Gozalo-Marcilla et al, 2021). An overall mortality rate of 1.9% and a mortality rate of 0.9% in healthy horses was reported by the authors of the Confidential Enquiry into Peri-anaesthetic Equine Fatality (CEPEF-2) study 20 years ago (Johnston et al, 2002). More recently, preliminary findings of the CEPEF-4 study, which is currently well underway, reported an improvement in these figures, but nevertheless, an overall mortality rate of 1.0% remains (Gozalo-Marcilla et al, 2021). The preliminary findings of this study detected a mortality rate of 0.6% in healthy horses undergoing procedures other than exploratory laparotomy for colic (Gozalo-Marcilla et al, 2021), which is still several times higher than in the equivalent cohort of dogs and cats (Brodbelt et al, 2008; Bille et al, 2012; Itami et al, 2017). The increased risk involved in anaesthetising horses has long been established, but there is evidence to indicate that the timing of anaesthesia-related complications has altered over the years. Intra-operative cardiac arrest or cardiovascular collapse accounted for 33% of deaths and fractures, and myopathy during recovery accounted for 32% in CEPEF-2. More recently, it has been reported that the vast majority of fatalities (81–100 %) occur in recovery (Dugdale et al, 2016; Laurenza et al, 2020; Nicolaisen et al, 2022). Reasons for this shift may include changes in the drugs used for induction of anaesthesia and the inhalants used (sevoflurane or isoflurane instead of halothane) (Hubbell et al, 2022); improvements in monitoring equipment and abilities; advancements in anaesthetist training and provision of dedicated case anaesthetists. The safe recovery of horses after general anaesthesia remains a challenge and the flighty nature of horses has been identified to have the greatest influence on recovery quality, and may well limit further risk reduction (Dugdale et al, 2016). This may have implications for equine anaesthetic safety as a whole as since it has been established that the recovery period is the most crucial, anaesthetic-related risk factors are more likely to be similar to those related to quality of recovery in the most recent epidemiology studies (Loomes and Louro, 2021). While some of the mortality risk factors are outside of the control of the anaesthetist, it is also apparent that improvements in recovery safety are required. Understanding the factors that influence recovery quality and overall anaesthetic-related mortality may help the anaesthetist to assess individual risk and tailor anaesthetic management accordingly to maximise patient safety.

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