5 Feb 2025
Deborah James
Job Title
Image: © TechArt / Adobe Stock
In small animal veterinary practice, general anaesthesia (GA) is undertaken on a daily basis for a variety of reasons from non-invasive procedures to emergency procedures.
However, every anaesthetic carries with it some degree of risk dependent on age and breed of the patient, co-morbidities, type of procedure and the condition of the patient at the time of the surgery. With the risk of mortality in humans at approximately 3 in every 100,000 (Shoop-Worrall et al, 2022), I will evaluate the current available evidence in the veterinary world and future predictions.
In 1951, Albrecht and Blakely conducted the first evidence-based study into anaesthesia mortality in canine and feline patients. The period that anaesthetic death was defined in this study was the time from induction until the patient had fully recovered in a conscious state (Albrecht and Blakely, 1951). They found that there was a mortality rate of 0.26% in dogs, 0.36% in cats and 5% in other species, which they clarified as rabbits and monkeys (Albrecht and Blakely, 1951).
Following this, Colorado State University conducted its own study between 1955-1957 into anaesthetic mortality and reported a higher mortality rate – 1.08% in dogs and 1.79% in cats (Lumb and Jones, 1973). They did clarify that the majority of these patients were anaesthetised and monitored via students, poor patient health and condition and the complex procedures being undertaken (Lumb and Jones, 1973).
Between 1979 and 1981, mortality rates at Colorado State University decreased to 0.43% in dogs and 0.26% in cats when they repeated the study; the decrease in mortality has been associated with improvements in their anaesthetic protocol and in particular using safer drugs and techniques, as well as better supervision of the students when monitoring these patients (Lumb and Jones, 1984).
In 1990, more than 150 veterinary surgeons representing 53 small animal veterinary practices collaborated on a study regarding anaesthetic emergencies (Clarke and Hall, 1990). This study was the first of its kind in this field – the first major, multi-centric study that documented more than 41,881 anaesthetics across the UK (Clarke and Hall, 1990).
An estimated 1 in 679 healthy dogs and cats died as a result of anaesthesia (0.23%) and the mortality rate in dogs and cats with pre-existing conditions was 1 in 31 (Clarke and Hall, 1990). In addition to this, they noted that complications associated with endotracheal intubation was the most common cause of death in cats and that no evidence existed, except in Pekingese, that certain breeds were more likely to die under anaesthesia than others (Clarke and Hall, 1990). Furthermore, anaesthetics that involved the use of xylazine were associated with a remarkably high mortality rate (Clarke and Hall, 1990).
Dodman and Lamb (1992) conducted an anaesthesia survey across 88 veterinary practices focusing on anaesthesia mortality and found an annual anaesthetic mortality rate of 0.11% in dogs and 0.06% in cats – it should be noted that these lower numbers may be as a result of only 39 surveys were completed, so the data is incomplete.
In 1993, a study across 66 veterinary practices assessed anaesthetic mortality in 8,087 dogs and found that while the incidence of complications was 2.1%, death occurred in 0.11% of cases (Dyson et al, 1998). These deaths were associated with the use of xylazine, ketamine, acepromazine or isoflurane, as well as minimal heart rate monitoring, mask inductions, orphan anaesthesia (where no anaesthetist is present) and finally the American Society of Anaesthesiologists (ASA) classification of the patients (Dyson et al, 1998).
Around the same time, a prospective study was carried out in 942 dogs undergoing anaesthesia, which found serious complications occurring in 79 of them and a mortality rate of 1.49% (Hosgood and Scholl, 1998). Their results indicated that older patients or those with an ASA grade above three were at significantly greater risk of death at 1.89% (Hosgood and Scholl, 1998).
Gaynor et al (1999) conducted a study the following year in 2,556 dogs and 683 cats undergoing anaesthesia. The percentage of anaesthetic complications arising in these patients were 12% in dogs and 10.5% in cats, while deaths occurred in 0.43% of dogs and 0.43% of cats (Gaynor et al, 1999).
In more recent years, further investigations have been conducted investigating anaesthetic mortality in small animal veterinary practice. Between 2002-2004, a confidential enquiry into small animal fatalities was conducted in 98,036 dogs, 79,178 cats and 8,209 rabbits across 117 veterinary practices (Brodbelt et al, 2008).
This paper was the first of its kind with a large participant number in all categories, playing a crucial role in the comprehension of small animal fatalities associated with anaesthesia and sedations. An overall anaesthetic mortality rate was 0.17% in dogs, 0.24% in cats and 1.39% in rabbits, with risks in healthy animals at 0.05% in dogs, 0.11% in cats and 0.73% in rabbits. Patients who were sick had much higher risks associated – 1.33% in dogs, 1.4% in cats and 7.37% in rabbits (Brodbelt et al, 2008). An interesting point to note from this study is that postoperative deaths in patients accounted for 47% of deaths in canine patients, 61% of deaths in feline patients and 64% in rabbit patients (Brodbelt et al, 2008).
Following on from this mammoth undertaking, Bille et al (2012) conducted an initial study between 2008-2010 with a follow-up study being conducted in 2010-2011. They conducted this follow-up study as a result of evidence-based medicine recommendations regarding the mortality rate in anaesthesia in small animal patients after the results of their original study. In the initial study conducted in 3,546 animals, no patient with an ASA grade 1 died, and only 1 dog with an ASA grade of 2 died further enhancing the necessity these classifications have in everyday practice (Bille et al, 2012).
The mortality rate for ASA grades 3, 4 and 5 were 2.9%, 7.58% and 17.33% respectively (Bille et al, 2012). An overall mortality rate was 1.35% in healthy patients and 4.77% in sick patients – those with ASA grade 3 and above – (Bille et al, 2012). In their follow-up study, they repeated the work in 6,231 animals undergoing anaesthesia and noted that after implementing improvements and recommendations their overall mortality rate decreased to 0.8% in healthy patients and 2.2% in sick patients, representing a significant decrease in mortality (Bille et al, 2014).
Between 2007-2008, Gil and Redondo (2013) conducted a study into anaesthetic deaths in dogs in order to identify any risk factors. A total of 39 veterinary practices took part, with 2,012 dogs included – 26 of which died (Gil and Redondo, 2013). Their mortality rate overall was 1.29%, with intraoperative deaths occurring in 23% of cases and postoperative deaths occurring in 77% of cases, with 20 of the 26 deaths happening in the postoperative period (Gil and Redondo, 2013).
A key finding was that patients given opioids in addition to NSAIDs were associated with a decreased risk of mortality and, therefore, may provide a protective mechanism in our patients (Gil and Redondo, 2013). However, vets in this study were incentivised to take part after concerns that their workload would increase, so an element of bias is present.
Moreover, Itami et al (2017) explored the risk factors associated with anaesthetic mortality. Out of 4,323 dogs across 18 hospitals, 0.65% died with 75% of those occurring in patients with pre-existing comorbidities (Itami et al, 2017). Initial findings found that preoperative serum glucose levels of below 77mg/dL, white cell counts of more than 15,200ml, ASA grade 3 to 5 and intraoperative hypoxaemia and tachycardia were the key risk factors associated with mortality (Itami et al, 2017).
Matthews et al (2017) identified that underweight dogs had 15 times the increased odds of fatality during anaesthesia than those of an ideal body condition score. They also noted that dogs that did not have pre-anaesthetic checks or those with a haematocrit outside the normal reference range were at 5.5 times increased risk of death (Matthews et al, 2017).
In 2022, 157,318 dogs underwent anaesthetics at more than 300 veterinary practices (Shoop-Worrall et al, 2022). Of these patients, 89,852 underwent neutering, with eight fatalities (0.009%) and the study had an overall fatality of between 0.1% to 0.14% (Shoop-Worrall et al, 2022). Older patients, those with a higher ASA grade and urgent, non-routine procedures had the highest mortality rates, while Rottweilers and West Highland white terriers had a higher rate of death and cocker spaniels had the lowest rate of death (Shoop-Worrall et al, 2022).
Redondo et al (2023) carried out a prospective cohort study investigating anaesthesia-related deaths. A total of 55,022 dogs from 405 veterinary practices were part of the study cohort – the anaesthetic mortality rate was 0.69% with 81% of deaths occurring during the postoperative period (Redondo et al, 2023). Short procedures, urgent procedures, unscheduled but routine procedures as well as age and obesity were associated with higher mortality rates, while some analgesics and the use of local anaesthesia were associated with a decrease in the risk of mortality in patients (Redondo et al, 2023).
Over the years, mortality associated with anaesthesia in our patients has been an ongoing learning curve with mixed results and conclusions.
Primarily, the application of evidence-based medicine may aid veterinary practitioners in effectively decreasing the mortality rates that are seen in practice. Preoperative patient evaluations to include ASA classification of every patient (whether this be pre-operative checks or pre-anaesthetic bloods), stabilisation of any clinical conditions the patient presents with or has ongoing, and the identification of further risk factors are all ways in which veterinary practices can aid in reducing the mortality rates in their patients. Finally, greater care must be administered to our patients in the postoperative period, as this is where the majority of deaths occur and this is where the greatest changes need to be implemented.
In terms of future predictions, potentially with evidence-based practice being used daily by veterinary clinics and hospitals, there should be a decrease in the mortality rates. However, there may be an increase initially, with the potential for reporting figures to become more prominent for the benefit of others so more data will be available to evaluate. In addition, protocols and risk mitigation measures need to be trialled and implemented, while every practice is different and this will take time to instigate and instil in the profession.
The author wants to say a massive thank you to Courtney Scales for her constant support and belief throughout this whole procedure. She wants to thank Lacey Pitcher for giving her the opportunity and Lou Northway for introducing her to these people.
Finally, she would like to thank her wonderful fiancé. She couldn’t do any of this without him and his unwavering love and support. He is her absolute rock in this ever-changing world and profession.
