By Dr. Lydia Love DVM Diplomate American College of Veterinary Anesthesia and Analgesia
We all know that general anesthesia carries inherent risk. In order to bring about unconsciousness, immobility, and amnesia, general anesthetics also depress basic homeostatic functions, including the ability to maintain ventilation, vascular tone, oxygenation, cardiac output, and tissue oxygen delivery. Healthy patients can generally compensate for these physiologic derangements to some extent and recover from them without negative effects. However, deaths related to - or directly caused by - anesthesia do occur.
In this post, we will look at small animal veterinary anesthesia mortality rates over time, compare them to human anesthesia mortality rates, and talk a little bit about how we can improve outcomes. (We will delve into equine anesthetic mortality in a later post!)
As a caveat, it is important to understand that assessing the contribution of anesthetic care to perioperative mortality can be difficult. The definition of what constitutes an anesthetic death can vary wildly amongst researchers, ranging from strictly “intra-operative cardiac arrest” to “death up to 7 days later in which anesthesia could not be excluded”. In some studies, anesthesia may have contributed to death but there are also complicating patient and procedural factors that may not have been entirely explicated. In addition, most veterinary studies are weakened by the fact that they are retrospective in nature, but a few large prospective trials are available. Finally, it is also hard to make direct comparisons between studies due differences in patient populations and clinical practice over time.
We do however have some interesting data to look it. The table below presents a selection of veterinary anesthesia mortality in dogs and cats over the past 7 decades. These figures represent the entire range of anesthetized patients (ASA 1 – 5 [more on ASA Physical Status in a later post!]). Note that referral institutions generally have a sicker patient populations, which may lead to worse outcomes.
Table 1: Published Anesthesia Mortality Rates in Dogs and Cats
The UK study from the first part of the new millennium is by far the best data we have to date. It is a prospective study with 98,000 dogs, 79,000 cats, and a variety of birds and pockets pets. Over 100 facilities took part, ranging from general practices to tertiary referral institutions. It really is an amazing piece of work. However, that study is 15 years old now and much has changed about how we approach anesthesia and perianesthetic care.
The Banfield study is the most recent study available from North America or Europe. It is retrospective search of their electronic database. These patients were, for the most part, fairly healthy but if you look at the overall trend from the middle of the last century to today, it seems that mortality in small animal anesthesia has fallen.
What about anesthesia-related death rates in humans?
Table 2: Published Anesthesia Mortality Rates in Humans
It’s very different from the veterinary anesthesia data, isn’t it? And if we consider the fact that much more invasive procedures are being done on much sicker patients, the difference can seem even bigger. It is particularly interesting to look at the reported differences from the early 1990s to the present, during which time an apparent 10-fold reduction in human mortality from anesthesia has occurred. By the beginning of this century, only about 34 people a year in the US died solely due to anesthesia.
While keeping in mind that direct comparison of studies and patient populations is extremely difficult, it makes one wonder what changed in the last 2 decades to improve human anesthesia safety?
More importantly for us, what is different about anesthesia in dogs and cats that it seems so much more risky than anesthesia in humans?
I am of the opinion that both of these questions can be answered by focusing on the monitoring and supportive care provided in the perianesthetic time period. Pulse oximetry first became routinely available in the 1980s and, in 1994, pulse oximetry and capnography became standard monitoring equipment for human anesthetic events. The risk of respiratory complications and related deaths plummeted. Further advances in training, monitoring, and supportive care have led to the extremely safe nature of anesthesia for humans today.
As for why small animal veterinary patients appear to have a higher anesthetic risk than humans, it isn’t that their physiology is so drastically different. There are some unique challenges in canine and feline anesthesia, including, importantly, the patient’s inability to report pre-existing illness, but generally speaking, mammalian physiology is mammalian physiology. And it isn’t the drugs used that are so different: both human and veterinary anesthetists use isoflurane, sevoflurane, propofol, ketamine, alpha-2 agonists, benzodiazepines, and full mu agonist opioids.
What is considerably different between human anesthesia and veterinary anesthesia is the monitoring and supportive care provided during anesthetic events. Anesthesia disrupts homeostasis and it is this insult to normal physiology that make heavy sedation and general anesthesia risky. Monitoring for, preventing, and managing physiologic derangements can mitigate this risk. There is a very wide range in veterinary medicine as to who monitors the anesthetized patient (if anyone!!), how anesthetists are trained, and what objective monitors are available.
Certainly, the ethical and economic environments in human versus veterinary anesthesia are entirely different but I think we can improve outcomes in veterinary anesthesia by focusing our available resources on improving the monitoring and supportive care of dogs and cats. Here are a few ways that can be implemented fairly quickly:
Treat each animal as an individual: Look at the patient and the procedure and select your drug protocol and perianesthetic care plan accordingly.
Have a dedicated, highly trained individual monitoring the patient from the time of premedication until fully recovered.
Take baseline vital signs including temperature, HR, and respiratory rate prior to drug administration.
Always place an IVC, supply supplemental oxygen, and, for most patients, administer IV fluids.
Pre-oxygenate patients and attach monitors such as an ECG, a reflectance pulse oximeter, and oscillometric or Doppler blood pressure probe prior to induction. Induction is a big cardiorespiratory hit and lots of things can go wrong.
Add capnography to your monitoring tool kit: Capnography can tell you about ventilation, cardiac output, breathing circuit integrity, and cellular metabolism. It is an early warning sign for things that can lead to hypoxemia and can help you in the differential diagnosis of hypoxemia when it occurs.
Continue to subjectively and objectively monitor the patient once extubated. Some patients will benefit from continued oxygen and fluid therapy.
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