Hypotension is the most common reported general anaesthetic complication in dogs (Gaynor et al., 1999), with reported incidences varying from 7 to 38 percent (Gaynor et al., 1999; Redondo et al., 2007).
Hypotension is potentially life-threatening and, if left untreated, can result in injury to the central nervous system, heart and kidneys, and possibly serious morbidity and mortality (Quinn, 2024). Despite this, there are limited data on the risk factors for hypotension during general anaesthesia (Miller et al., 2023).
What is known so far?
During anaesthesia, hypotension can be caused by the cardiovascular side effects of anaesthetic and sedative drugs, or other contributing factors such as extremes of age, dehydration and intraoperative blood loss (Quinn, 2024).
A strong negative association between body mass and the probability of hypotension under general anaesthesia has been found in previous studies
A strong negative association between body mass and the probability of hypotension under general anaesthesia has been found in previous studies in female dogs undergoing neutering (Martin-Flores et al., 2019) and dogs undergoing thoracolumbar hemilaminectomy (Bruniges and Rioja, 2019). However, other studies have not found any association between body mass and hypotension (Lizuka et al., 2013).
Given the common occurrence of hypotension during anaesthesia, identifying underlying risk factors could facilitate improvements in pre-anaesthetic preparation, preventative strategies and peri-anaesthetic monitoring to reduce risk to the patient.
What did the latest study assess?
A 2023 study by Dr Lucy Miller and colleagues at the Royal (Dick) School of Veterinary Studies, published in the Journal of Small Animal Practice, has assessed whether the previously described association between body mass and hypotension is found in a larger, more diverse canine population (Miller et al., 2023). It also identified key risk factors for hypotension in dogs.
Medical records of dogs undergoing general anaesthesia at the University of Edinburgh’s Hospital for Small Animals between December 2018 and June 2020 were retrospectively searched and relevant anaesthetic data retrieved. Associations between possible explanatory variables and hypotension were assessed using multivariable generalised linear models. A total of 1,789 cases of general anaesthesia were included in the study (Miller et al., 2023).
What were the findings?
Of the cases studied, one third (32 percent) experienced at least one episode of hypotension during general anaesthesia. Median time to the first hypotensive episode was 31 (0 to 265) minutes post-anaesthetic induction, highlighting that hypotension is not just a problem of prolonged anaesthesia. Hypotension risk did not increase significantly with duration of anaesthesia.
Dogs with a lower body mass are at a higher risk of experiencing hypotension during general anaesthesia
The frequency, duration and severity of hypotensive episodes were not evaluated (Miller et al., 2023).
The study found an independent association between body mass and hypotension, indicating that dogs with a lower body mass are at a higher risk of experiencing hypotension during general anaesthesia. An increase in body mass was significantly associated with decreasing odds of hypotension. While this relationship has been previously reported in smaller studies, this is the first time it has been found to persist in a larger, more diverse canine population undergoing a wider range of procedures (Miller et al., 2023). These findings suggest that the association between body mass and hypotension is independent of body temperature, heart rate and procedure type (surgical or diagnostic), but instead may reflect difficulties in accurate blood pressure measurement in dogs of lower body mass.
Brachycephalic breeds had significantly greater odds of developing hypotension – an association that has not been previously reported, although brachycephaly is known to be a risk factor for peri-anaesthetic complications. An American Society of Anesthesiologists (ASA) physical status classification greater than three and undergoing a surgical (versus diagnostic) procedure were also associated with increased odds of hypotension (Miller et al., 2023).
The association between body mass and hypotension is independent of body temperature, heart rate and procedure type, but instead may reflect difficulties in accurate blood pressure measurement in dogs of lower body mass
Including an alpha-2 agonist in the pre-anaesthetic medication protocol almost halved the odds of hypotension. Increased body temperature (per 1°C) during general anaesthesia also decreased the odds (Miller et al., 2023).
The effect of hypotension during anaesthesia on mortality rates in dogs is unknown, although previous research has found that smaller dogs (weighing under 5kg) are at increased risk of anaesthetic-related death (Brodbelt et al., 2008).
Limitations of the study
The study was limited by the retrospective nature of the data collection and analysis. As normotension was not confirmed in all cases before the induction of anaesthesia, it is possible that some dogs were already hypotensive at the time of induction. Furthermore, blood pressure measurements may have been less accurate, particularly in cases of severe hypotension, as the majority were measured by non-invasive oscillometry, rather than direct invasive monitoring, and the blood pressure measurement site was not standardised or recorded (Miller et al., 2023).
Take-home message
Hypotension is a common complication during anaesthesia in dogs, and dogs with lower body mass and brachycephalic breeds are at higher risk. Understanding the factors associated with hypotension should support veterinary teams in reducing the risk and duration of hypotension during canine surgery. The findings of the study highlight the need for accurate, close monitoring of blood pressure and adequate preparation for intervention or treatment for hypotension during surgical or diagnostic procedures. This is particularly critical when treating small or brachycephalic dogs, which are at higher risk of hypotension (Miller et al., 2023).
