Anesthetic Risk in Humans

Let’s start with anesthetic risk in general. Back in the 1970s, the chance of a person dying under anesthesia was as high as 1 in 10,000. With remarkable advancements in specialized equipment, medications, and knowledge of how the body works, anesthetic mortality rates have declined to as low as 1 in 100,000 (0.001%) (Medellin 2023). This number is only a rough estimate. Because of the enormous variety of procedures performed today, and because anesthesia mortality is so rare, it is difficult for researchers to accurately study and quantify it.

Anesthetic Risk in Animals

What about animals? The numbers are relatively higher, but the risk is still very low. In dogs, one of the most recent, and lowest, published canine anesthetic mortality rates is 0.05%. That number is slightly higher in cats, with reported rates from 0.11% in the United States to 0.63% worldwide (NS Matthews et al. 2017) (Redondo JI et al. 2024). Researchers have some identified risk factors that may increase these rates (NS Matthews et al. 2017):

• Not having a physical exam recorded in the medical record before anesthesia takes place

• Undergoing an emergency procedure instead of an elective one

• Failure to use basic anesthetic monitoring equipment (pulse oximeter)

• Being underweight or small (less than 10 pounds)

• Yes, age

Age and Anesthetic Risk

An older study found that dogs greater than 12 years old have seven times the risk of anesthetic death compared to dogs 0.5-12 years old (Broadbelt et al. 2008). Cats great than 12 years old have two times the risk of anesthetic death compared to cats aged 0.5-5 years. Why?

• Older animals may not clear anesthetic drugs from their systems as effectively as younger ones. Reduced blood flow to the liver, slower metabolism, and decreased muscle mass all contribute to this heightened sensitivity to anesthetic drugs.

• Negative side effects may be more pronounced in older animals.

• Older animals may have weaker muscles and reflexes that protect the airway. This leads to a heightened risk for aspiration pneumonia, a serious complication caused by the inadvertent inhalation of gastric contents.

• Older animals are more likely to develop diseases that make the delivery of safe anesthesia more complicated.

What Really Matters When Assessing Anesthetic Risk

Scientific data suggest that age should be a factor in considering anesthetic risk. But that doesn’t tell the whole story, and it isn’t very helpful unless we think a little bit harder about what exactly is happening in older pets and how we can successfully treat them:

• Functional kidneys, liver, heart, and lungs are important for a successful anesthetic.

• Older animals are more likely to have dysfunctional kidneys, livers, and hearts, as well as decreased lung capacity.

• We need to assess kidney, liver, heart, and lung function prior to anesthesia, and this is especially the case for older pets.

How Do We Assess Health and Organ Function in Pets?

• Physical exam. A preanesthetic physical exam includes evaluation of mucous membrane color, pulse quality, and heart and lung sounds with a stethoscope. An abnormality detected on exam may warrant further testing, as described below.

• Blood and urine tests. We can get an idea of whether or not important organs are functioning adequately with a variety of blood tests. Values such as creatinine, blood urea nitrogen (BUN), urine specific gravity (USG), albumin, and alanine aminotransferase (ALT) can tell us if there is reduced kidney and liver function. It is important to note that occasionally, disease may be “subclinical,” meaning that it is there, but these tests fail to detect it. No single test is perfect, and so oftentimes multiple tests are recommended in order to obtain a better picture of health status.

• Imaging. X-rays of the chest can help us determine if the heart and lungs are able to handle general anesthesia. An ultrasound of the abdomen can evaluate the appearance of the liver and kidneys for evidence of shrinkage or degeneration. With these modalities, a board-certified radiologist sees subtle details that may change how we assess anesthetic risk and management for a pet. An echocardiogram performed by a board-certified cardiologist can look at the size of the chambers of the heart and measure if it is strong enough to handle general anesthesia.

What If Organ Function Isn’t Optimal?

If the above testing indicates disease, this means a pet has a higher ASA status. Does this mean the pet cannot undergo general anesthesia? Nope.

Just like in human medicine, there are board-certified anesthesiologists who are trained to anesthetize pets with complicated coexisting diseases. After veterinary school and a rotating internship program, veterinary anesthesiologists undergo a rigorous three-year residency program and must pass a certifying examination to become board-certified.

The Benefits of a Veterinary Anesthesiologist

As a result of their advanced training, veterinary anesthesiologists:

• Have extensive experience managing anesthesia for higher risk pets, including older pets.

• Understand how the body responds to anesthesia, especially when it comes to the organs most affected by anesthesia.

• Know the ins and outs of all anesthetic drugs, allowing them to select the most appropriate medications and doses necessary to deliver safe anesthesia that can be tailored to each individual patient.

• Perform special techniques that help reduce the amount of anesthesia required to keep a pet asleep and comfortable during the procedure. In fact, one of these techniques, locoregional anesthesia, has been shown to reduce the risk of anesthetic mortality in cats (NS Matthews et al. 2017)

• Have access to a wide variety of medications and advanced equipment, allowing them to provide the highest quality care possible.

Where Can I Find a Board-Certified Veterinary Anesthesiologist?

AESC employs a full-time veterinary anesthesiologist. In addition to having more complex medications and specialized monitoring equipment, AESC also has a multitude of specialists who are experts in their field. The anesthesiologist collaborates and consults with the other specialists for any pet undergoing anesthesia. For higher-risk pets, including – yes, older ones – AESC has the knowledge and tools to ensure they are as safe as possible.

References

1. Medellin S and Sessler DI. Preventable anesthetic deaths are not the major perioperative problem. Lancet Reg Health West Pac. 2023 Jun 10;37:100821.

2. NS Matthews et al. Factors associated with anesthetic-related death in dogs and cats in primary care veterinary hospitals J Am Vet Med Assoc. 2017 Mar 15;250(6):655-665.

3. Redondo JI et al. Anaesthetic mortality in cats: A worldwide analysis and risk assessment. Vet Rec. 2024 Jul 6;195(1):e4147.

4. Broadbelt et al. Results of the confidential enquiry into perioperative small animal fatalities regarding risk factors for anesthetic death in dogs. J Am Vet Med Assoc. 2008 Oct 1;233(7):1096-104.