The foundation of veterinary anaesthesia is a veterinary nurse or technician who is a safe anaesthetist. A steady evolution of knowledge in the field of veterinary anaesthesia has made anaesthesia much more than merely using drugs to provide immobilisation. Increased knowledge of anatomy, physiology, and pharmacology; animals living longer lives; improved understanding of disease treatments; and performing more complex surgical procedures have all lead to improved methods of anaesthetic delivery for animals. These improvements in veterinary anaesthesia, along with the development of new drugs, equipment and techniques has caused an exponential growth in the knowledge and skills required to provide the best care to animals under anaesthesia.
This paper will revise the risk of morbidity and mortality in veterinary anaesthesia as well as review some of the more recent scientific literature on checklists, low-stress handling, fasting and pre-anaesthetic laboratory testing.
Morbidity and mortality in anaesthesia
Although this paper focuses on pre-anaesthesia preparation, it is important to understand factors that affect complications related to anaesthesia. Several studies have been published determining the risk factors in anaesthesia, the number of deaths that occur in anaesthetised patients, the risk of anaesthetic death based on the American Society of Anesthesiologists (ASA) physical status classification system (2020) and the time during anaesthesia where an animal is most likely to die.
The Confidential Enquiry into Perioperative Small Animal Fatalities (CEPSAF) was a large multicentre small animal practice-based study undertaken in the UK between 2002 and 2004. Of the 79 178 cats in the study the risk of death in cats undergoing sedation and/or anaesthesia ranged from 0.11% (ASA I–II) to 1.4% (ASA III–V). Overall, the incidence of death across all animals in this study was 0.24% (1 in 418) (Brodbelt et al, 2008).
The timing of death is significant because it helps the veterinary technician or nurse focus on the most likely time that complications may occur, which enables personnel and equipment to be targeted to the most ‘at risk’ time for anaesthetic death (Table 1). Most anaesthetic deaths in cats occurred within the first 3 hours after surgery. Cardiovascular and respiratory system causes accounted for 66% of deaths in the CEPSAF study (Brodbelt et al, 2008). Factors found to increase the risk of anaesthesia in cats were age, urgency of procedure, major procedures, ASA classification status, extremes of weight, tracheal intubation and intravenous fluid administration (Brodbelt, 2009; Brodbelt, 2010; Brodbelt et al, 2008).
Table 1. Timing of deaths relate to anaesthesia in dogs and cats
| Time of death | Cat | Dog |
|---|---|---|
| After premedication | 1% | 1% |
| Induction of anaesthesia | 8% | 6% |
| Maintenance of anaesthesia | 30% | 46% |
| 0–3 hours postoperative | 38% | 20% |
| 3–6 hours postoperative | 5% | 7% |
| 6–12 hours postoperative | 4% | 8% |
| 12–24 hours postoperative | 7% | 9% |
| 24–48 hours postoperative | 6% | 2% |
| Unknown | 1% | |
| Postoperative death 0–48 hours | 61% | 46% |
In the CEPSAF study, the risk of death in dogs under-going sedation and general anaesthesia was lower than for cats. The survey included 98 036 dogs and the risk of death ranged from 0.05% (ASA I–II) to 1.33% (ASA III–V). Overall, the incidence of death of dogs in this study was 0.17% or 1 in 601. Factors found to increase the risk of anaesthesia in dogs were age, small breed, brachycephalic breed, ASA classification status, major and urgent procedures. This study also determined that a high-risk time for death was postoperatively (Table 1).
The study suggests that the lower rate of mortality during the maintenance phase of anaesthesia compared with the postoperative period may indicate improved standards in maintenance and monitoring during anaesthesia. Most postoperative deaths occurred 0–3 hours after anaesthesia, suggesting that increased vigilance and greater care in the postoperative period may be warranted (Brodbelt et al, 2008; Brodbelt, 2009).
Checklists
The incidence of anaesthesia safety incidents before and after checklist implementation has been studied (Hofmeister et al, 2014). A significant reduction in medical errors was evident. Complications that decreased in incidence included: incorrect drug administrations (or route), closed adjustable pressure limiting (APL) valve, oesophageal intubation and incorrect endotracheal tube usage. A large degree of human morbidity and mortality is also believed to have been reduced by implementing pre-surgical/pre-anaesthetic checklists.
There are many studies investigating their use in veterinary medicine. The frequency and severity of postoperative complications in cats and dogs was significantly decreased by implementing safety checklists designed by the World Health Organisation (WHO) (Bergström et al, 2016). Complications that were decreased in incidence after implementation of the checklists included: re-entry into theatre postoperatively; surgical site infections; wound healing; circulatory, organ and nervous system dysfunctions; systemic inflammatory respiratory syndrome/multiple organ dysfunction syndrome and death. The Australian Veterinary Association's (AVA's) checklist and recommended procedures are shown in Figure 1 (Association of Veterinary Anaesthetists, 2015).
Cray et al (2018) investigated the implementation on both peri- and postoperative complications and found a reduction in complication incidence in cats and dogs. By odds ratio, the occurrence of complication was decreased by 40% when checklists were used. Similar to the study by Bergström et al (2016), decreases in complications associated with cardiovascular arrest, respiratory, nervous, gastrointestinal, systemic inflammatory respiratory syndrome/multiple organ dysfunction syndrome, coagulation and surgical re-entry and healing were evident.
Medical errors that result in morbidity and mortality in human and veterinary anaesthesia are preventable and responsibility lies with actions conducted by the anaesthetist (McMillan, 2014). The WHO's checklist is used by human and veterinary anaesthetists before induction of anaesthesia, before incision and before recovery, and has resulted in significant reductions in large-scale complications. The use of these checklists allows for safe anaesthetic practices and supports positive patient outcomes.
Low-stress handling and behaviour-modifying therapeutics
Psychogenic stress occurs in patients when they are separated from their owners and are in the veterinary clinic. Physiological changes such as oxidative stress and cortisol elevations in response to stress responses can result in delayed postoperative wound healing and the development of systemic disease processes. Dogs separated from their owners for 12 hours before their scheduled surgery showed significant increases in heart and respiratory rate, cortisol and oxidative index values when compared with their baseline values (Juodzente et al, 2018). General anaesthesia and surgery have been documented to increase oxidative stress, thus admitting an animal the day before the procedure may enhance these processes (Box 1).
Feline pre-anaesthetic and induction requirements are reduced when exposed to a low-stress transport and handling protocol. Cats that are transported to the clinic in an enzymatically cleaned carrier and F3 pheromone prepped, stable position of carrier in the car and attending a catfriendly consultation room required significantly less time to achieve sedation and required a lower dose of propofol for induction than control cats (Argüelles et al, 2021).
The authors emphasised that high basal anxiety in the human may cause an increase in intraoperative anaesthetic requirements and suggested that the use of low-stress handling protocols in conjunction with pre-visit pharmaceuticals in feline patients may improve welfare (Argüelles et al, 2021). Reduced feline fear aggressive behaviour after administering gabapentin has been widely documented. A single oral dose of 17–36 mg/kg of gabapentin 2 hours before a veterinary visit significantly decreased fear symptoms and enabled better compliance during physical examinations for animals with an existing history of stress during visits, when compared with control cats (Kruszka et al, 2021).
Many studies on alleviating stress have also been conducted in dogs. Trazodone is used in human medicine as an anxiolytic and antidepressant, and it has been suggested that it elicits similar effects in dogs. Gilbert-Gregory et al (2016) found that hospitalised dogs administered an oral trazodone dose of 3.5 mg/kg every 12 hours, up to 10–12 mg/kg or at an increased frequency of every 8 hours, depending on the desired calming and anxiolytic effect, led to significant reductions in stress-related behaviours. These included lip licking, panting, vocalisation, whale eye position, frenetic and freeze. The use of trazodone in dogs also reduces the minimum alveolar requirements of isoflurane by 17% (Hoffman et al, 2018).
Oromucosal dexmedetomidine gel in dogs has also been documented to significantly reduce fear and anxiety during veterinary visits. Administering 0.1 mg/g oromucosal gel at a dose of 125 μg/m2 or 250 μg/m2, 45–60 minutes before arrival, decreased stress behaviours including body posture and also increased the likelihood of achieving a positive treatment outcome when compared with control dogs (Korpivaara et al, 2021).
There is growing attention to the use of anxiolytic medication in dogs and cats with behavioural or emotional disorders. This article states only a few options available. It is important that veterinary nurses are able to identify and be knowledgeable about anxiolytic medications and their therapeutic actions, as well as possible interactions with pre- and peri-anaesthetic agents. There are gaps in the existing veterinary literature, so some information is based on human studies.
Major classes of anxiolytic/antidepressant therapy produce their activity via actions on serotonergic, norepinephrinergic, histaminergic and neurotransmitter enzyme pathways. Anxiolytics in combination with therapeutics that also have actions on the serotonergic pathways can induce excessive serotonin availability. Opiates have been documented to cause serotonin syndrome in animals. This condition is characterised by neurological changes including seizures and mentation, neuromuscular excitation and hyperthermia, cardiac conduction disturbances and gastrointestinal upset. Another therapeutic in veterinary medicine is fluoxetine, a selective serotonin re-uptake inhibitor. Therapeutics with similar actions such as mirtazapine, chlorpheniramine and trazodone have also been documented to cause this toxicity when combined with anaesthetic agents.
Box 1.Considerations for safe anaesthetic practiceTransportation to clinic
- Using low-stress strategies involving travel and clinical setting +/- anxiolytic medication
- If scheduled to have surgery: +/- proton pump inhibitor protocol +/- anxiolytic medication
Veterinary treatment: conducted in a low-stress environment
- Thorough patient history and physical examination. + investigate anomalies
- Pre-general laboratory work-up
- ASA classification status assessment and complication planning + Consideration for potential drug concurrency interactions
- Checking patient-specific fasting approach + gastric-oesophageal reflux risk assessment and associated therapeutics administration planning
- Ensure all components of the AVA checklist are completed
There are many potential interactions/complications that may be encountered in animals on concurrent anxiolytic/antidepressant medication that undergo anaesthesia, many of which have been documented in the human medical literature. One example is the tricyclic antidepressant, clomipramine (also used in both dogs and cats), which has structural similarity to local anaesthetics. The blockade of sodium channels affects cardiac conduction under general anaesthesia. Anticholinergic and alpha-1 antagonist actions have also been reported. These effects may predispose patients to systemic hypoperfusion of vital organs. Therefore, the use of sympathomimetic treatment during anaesthesia with concurrent pharmacological behaviour modulation therapeutics can have unpredictable effects. Planning appropriate interventions for peri-anaesthetic complications is advised.
Fasting and associated therapeutics
The concept of prolonged fasting regimens in both human and veterinary medicine is not largely supported by evidence. In fact, human studies have supported a decrease in stress responses to anaesthesia and surgery from the catabolic effects of fasting. It is reported that humans do not currently fast for more than 6 hours before a general anaesthetic.
The incidence of gastro-oesophageal reflux contributes to morbidity and mortality in multiple animal species, and plays a role in the pathogenesis of aspiration pneumonia, oesophagitis and oesophageal stricture formation. The lower oesophageal sphincter relaxes after therapeutic agents are administered, predisposing an animal's gastric contents to flow past the sphincter point.
Tsompanidou et al (2022) investigated three canine patient fasting regimens and the frequency of gastro-oesophageal reflux, and found that incidence is not correlated to the timeframe of fasting, and that reducing the amount of food preoperatively was associated with more acidic reflux. Shorter preoperative fasting periods may be advantageous to avoid changes in metabolic status and keep more substrate for anaesthetic-surgical stress responses.
The predisposing factors for gastro-oesophageal reflux and regurgitation were investigated (Viskjer and Sjöström, 2017). The authors found that age, dorsal recumbency and fasting times were of significance. These authors also found that dogs fed half-resting energy requirement (RER) canned food 3 hours before anaesthesia had significantly less acidic reflux than was seen with 18 hours fasting time. However, incidence of reflux and regurgitation was greater with the shorter fasting time. Different breeds will also play a role, for example, one-third of brachycephalic breed dogs may regurgitate during general anaesthesia (Viskjer and Sjöström, 2017).
Dogs fasted for 10 hours had greater gastric content acidity when compared with those fasted for 3 hours (Savvas et al, 2009). In addition, dogs fed canned food at half-RER in the 3-hour group did not have significantly increased gastric volume when compared with those given other food types, and those fed low-fat canned food had more acidic gastric content than those fed other canned food types. Feeding canned food may be of benefit to maintain normal energy substrate without increasing gastric volume or incidence of gastro-oesophageal reflux, and findings suggest that prolonged fasting time may not provide smaller gastric volumes or lower gastro-oesophageal reflux incidence.
The use of omeprazole and its effect on gastric pH and incidence of gastro-oesophageal reflux in cats and dogs has been investigated (Lotti et al, 2021), and it was found that in dogs 1 mg/kg oral omeprazole given twice in the 24-hour period (evening before and at 3 hours) before anaesthesia significantly reduced acidity of gastric pH, and thus the incidence of strong pH reflux events. Dosing once the day before anaesthesia did not give a significant reduction in acidity when compared with the control group.
Similar results are identified in feline patients. Gastric pH was significantly higher in cats that had two doses of oral omeprazole at 1.45–2.20 mg/kg, 18–24 hours and then 4 hours prior to the induction of general anaesthesia when compared with felines that did not (Garcia et al, 2017). These cats were fasted for 12 hours. The authors also found that 33.3% of the total study population had gastroesophageal reflux. However, the prevalence of reflux was similar to that documented in canine patients during general anaesthesia. By odds ratio, cats that did not receive omeprazole had 2.75 times the likelihood of a reflux event.
Maropitant use before general anaesthesia has supported multiple benefits in cats and dogs, including reductions of the following: opiate and alpha-2-adrenergic agonist induced nausea and vomiting, total opiate and inhalational requirements and visceral nociception (Hay Kraus, 2017). Smoother recovery transitions and faster return to eating postoperatively has also been documented with its use (Hay Kraus, 2017).
Considering an animal's age with fasting times is important. For example, neonatal and paediatric patients have an increased risk and incidence of regurgitation because of their under developed lower oesophageal sphincter tone (maturity at 5 weeks of age). Glycogen stores are low and depletion can result in hypoglycaemia, so suckling and paediatric patients should have either no fasting or not longer than 2-hour fasting periods, respectively (Duke-Novakovski et al, 2016).
Pre-anaesthetic laboratory work
It has been investigated whether pre-anaesthetic clinical examination influences anaesthetic/sedation and analgesic modality management protocols in dogs presenting for diagnostic or surgical procedures (Louro et al, 2021). Clinical anaesthetists designed a proposed anaesthetic and analgesic protocol after completing a questionnaire to familiarise themselves with clinical history and any relevant preliminary laboratory/diagnostic workup information, before physically examining animals. After physical examination, 23.3% and 8% of cases had altered anaesthetic protocols and ASA classification statuses, respectively. Of the 23.3%, the following percentile changes were made to protocols: 93.4% premedication phase, 65.9% therapeutic dose adjustments of both anaesthetic and analgesic agents, 52.7% type of agent used, 16.7% of dogs with audible cardiac murmur.
Alef et al (2008) found that pre-anaesthetic haematological and biochemical screening in dogs is unlikely to yield significant changes in anaesthetic protocols if no anomalies are detected in patient history and physical assessment, or in younger healthy patients. On retrieval of laboratory results, only 8% of patients had a change in ASA classification, 0.8% had surgery postponed, 1.5% had addition of pre-anaesthetic therapy and 0.2% had anaesthetic protocols changed. 84.1% of dogs were deemed by anaesthetists to not require laboratory workup. The majority of the dogs in the study population (63.9%) were ASA I. Dogs classified as ASA II or more correlated with anaesthetists advocating to conduct blood workup. Findings also supported a higher correlation between adverse incidents in dogs with abnormal blood results than those with normal blood results.
Another study reviewed an older population of dogs (mean age 9.64 years of age) and cats (mean age 11.65 years of age) presenting to a UK practice and the changes to anaesthetic management made after bloodwork results (Davies and Kawaguchi, 2014) reviewed. In dogs: 21.5% did not proceed to anaesthesia, 8.23% concern in abnormality was documented and 4% had anaesthetic protocol changes made. In cats, 21.5% did not proceed, while in 15.77% concern was documented, and in 9% protocols were changed. These results support the value of pre-anaesthesia general bloodwork.
In a study in dogs and cats greater than 8 years of age, anaesthetists were able to predict abnormal blood results in 64% based on history and physical examination, and the authors emphasise that mortality risk increases with a patient's age, but the relationship between pre-anaesthetic bloodwork and comorbidity is still an existing evidence gap in veterinary medicine (Díaz et al, 2021).
In another study, Joubert (2007) found that in a geriatric dog population of 101 animals, 30 new diagnoses were made on retrieval of blood results, and common diseases were of neoplastic, renal and endocrine origin. Of the 30 that had blood results, 13 did not proceed to general anaesthesia, with others having amendments to their original anaesthetic plans.
These results support the value of pre-anaesthetic blood-work in the ability to detect early-stage disease processes and allowing for stabilisation at earlier stages. Authors emphasised that in human medicine specific testing is conducted relevant to anaesthetic and surgical management, and geriatric screening includes specific cardiac, pulmonary, hepatic and renal testing.
Conclusions
This article reviews some of the recent literature on the benefits of preparation specific to each patient before anaesthesia including checklists, bloodwork, fasting and therapeutic interventions that may be of benefit to the patient before anaesthesia. Inquisitive minds, as well as attention to detail, are two traits that will well serve a veterinary nurse or technician performing anaesthesia. Learning from successes and failures and pursuing continued education will ensure that each technician or nurse continues to improve their skills and knowledge in this important field of veterinary science.
KEY POINTS
- Morbidity and mortality associated with anaesthesia can be decreased in incidence and severity by appropriate animal-specific anaesthetic planning and intervention by veterinary nurses and technicians.
- Conducting a thorough patient history and physical examination can identify abnormalities that may indicate an animal's anaesthetic risk and guide diagnostic workup.
- Allowing for a low-stress transition to general anaesthesia and the use of evidence-based therapeutics that can prevent complications enhances the safety of anaesthetic practices.