Can a Veterinary Patient Be Too Old for Anesthesia?

In discussing risk for anesthesia-associated adverse events with clients, patient age is a common concern. However, similar to human gerontologists, veterinarians should consider the effects of age, not the age itself. Knowledge of anesthesia concerns specific to aged patients is a critical component of everyday practice. Improved preventive care and support of aging patients in veterinary medicine has led to longer lifespans; the percentage of senior/geriatric dogs and cats in veterinary practices is estimated at 44% of the total patient population.¹ Patients of all ages, including senior/geriatric patients, are likely to require anesthesia for emergency procedures (e.g., gastrointestinal foreign body surgery, fracture repair) and/or routine procedures that are necessary to maintain a good quality of life (e.g., dental prophylaxis/surgery, removal of ulcerated masses).

However, not all aged patients are at equal risk for anesthesia-associated adverse events, and the determination of age-associated physiologic changes is exceedingly more relevant than actual patient age. Human gerontology studies emphasize that frailty, defined as reduced physiologic reserve and increased vulnerability to stressors, better predicts perioperative morbidity and mortality than age alone.^2-4 In human medicine, to differentiate between age and frailty, the term “senior” generally means number of years aged, whereas the term “geriatric” means an aged patient with age-associated health concerns. However, in veterinary medicine, these terms are not generally differentiated, and this article uses the terms “aged/frail” or “aged/healthy.”

This article answers some of the most frequently asked questions general practice veterinarians have regarding the use of anesthesia in aged small animal patients.

What is the age cutoff for anesthetizing patients? Is it “senior at 7”?

There is no chronological age limit for anesthesia. Age is not a disease, and there is no basis for the phrase “senior at 7” (i.e., a patient is considered senior when it reaches 7 years of age). Definitions of the term “senior” in dogs vary due to the wide range of breeds and body sizes, meaning that all dogs do not age at the same rate; an exact senior age is not specified, but it is considered to be the last 20% to 25% of the animal’s normal expected lifespan.⁵ Cats are more uniform in size and breed, and the American Association of Feline Practitioners has defined senior cats as older than 10 years of age.⁶ However, as with humans, dogs and cats are individuals and age at different rates.

Do the physical and physiologic changes of age affect anesthesia plans?

Yes, and the risk for anesthesia-associated adverse events is determined by physiologic age-associated changes rather than the number of years that the patient has lived. Patients of the same chronological age are not necessarily the same physiologic or biological age. For example, an 8-year-old Labrador retriever may be very mobile and active, cognitively sound, and physiologically healthy, whereas another Labrador retriever of the same age may exhibit impaired mobility, cognitive decline, and negative physiologic effects of aging. The anesthesia plans would differ between these 2 patients. Similarly, the plans for 2 younger dogs or cats, one with an American Society of Anesthesiologists (ASA) score of 1 and the other with an ASA score of 4, would differ.⁷ Risk factors are based on health, not age, and the cumulation of age-associated health changes is often called “frailty.”²

Exactly how does frailty affect patients?

Frailty describes the physical and physiologic changes that limit organ reserve, which means that in a frail patient, physiologic responses to anesthesia-associated stressors, such as hypotension and hypoventilation, would be blunted. As an example, if hypotension is noted in an aged/healthy patient at an appropriate plane of anesthesia, physiologic changes such as increased heart rate and vasoconstriction often occur in response to the blood pressure change and may alleviate the hypotension without anesthetist intervention. In an aged/frail patient with blunted organ reserve, the physiologic response to hypotension may not occur, and blood pressure support may require rapid anesthetist intervention. The same would be true of hypercarbia, which would initiate physiologic changes to increase alveolar ventilation in an aged/healthy patient; however, the anesthetist would likely need to breathe for the aged/frail patient.

Are frailty scores available for veterinary patients, and should we be using them?

Yes, some veterinary frailty scores are still in development; however, clinical tools and a robust body of research information are also currently available for use.^8-20 Because of the increasing number of aged (senior/geriatric) patients, frailty scores should be considered when assessing these patients in general, not just for anesthesia. The scoring systems, or scales, may seem a bit daunting due to the frequently large number of questions, but much of the information included in the frailty scores is the same information as that assessed for a routine preanesthesia (or any other) examination. Included are physical examination findings, health history, blood/serum analyses, and specific tests (e.g., electrocardiography). Also included are findings more commonly seen in aged patients (e.g., decreased mobility, changes in hearing and vision, cognitive decline). Thus, frailty scores/scales are very robust evaluations that enable appropriate ASA status assignment.

What organ systems can be affected by age-associated changes?

Any organ system can be affected by the aging process, resulting in diminished reserve of that system (TABLE 1). The systems for which aging changes are most concerning for the anesthetist are the central nervous, cardiovascular, respiratory, renal, and hepatic systems, which may respond poorly or inappropriately to anesthetic changes. The patient may have arrhythmias, hypotension, hypercarbia, or hypoxia or may be overly sensitive to drugs and slow to eliminate drugs. Close monitoring and rapid responses are essential for aged/frail patients.

Which drugs are safest for aged/frail patients?

The answer depends on underlying age changes or disease (TABLE 2); however, for aged/frail patients, the safest choices are often fast onset/offset drugs that can easily be titrated to effect and quickly eliminated and/or drugs that have reversible effects. No drugs are contraindicated due to patient age, but underlying disease processes will affect choices. For example, α₂ agonists might be avoided for patients with dilated cardiomyopathy and NSAIDs might be avoided for patients with International Renal Interest Society stage 3 chronic kidney disease. Acepromazine might be avoided in patients with moderate-to-severe hepatic disease as hepatic elimination of the drug is required for cessation of drug effects.

When determining an anesthesia protocol, drug dose is often more important than actual drug choice. Frail patients may require little to no sedation or may be adequately sedated with opioids alone. Induction drug dosages should be lower than those required for aged/healthy patients, and the drug should be administered more slowly than to aged/healthy patients because circulation is often impaired, meaning that the time to reach the brain is longer. To remember not to overdose the patient by actual dose or rate of administration, use the phrase “go low and slow” when inducing anesthesia in frail patients. Anesthesia should be maintained with the lowest dose of inhalant possible because the cardiovascular and respiratory depressant effects of inhalants can be quite profound in aged/frail patients. The dose of inhalants can be decreased by using robust analgesia, especially local/regional blocks or longer-acting opioids for premedications, or continuous rate infusions during the procedure.

• At home: Before the patient is brought to the office, consider having the client administer low-dose anxiolytics (e.g., trazadone, gabapentin) and potentially antiemetics (e.g., maropitant). For patients with preexisting pain, analgesics should be started. Be cautious with use of NSAIDs as the risk for hypotension and possible renal damage can be greater in the frail patient.
• Premedication: For some patients, premedicate for sedation/analgesia with opioids alone or add a low dose of α₂ agonists (reversible) for those with no heart disease. If α₂ agonists are contraindicated, a low dose of alfaxalone could be added to the intramuscular opioid. However, the current concentration of alfaxalone limits the intramuscular route of administration to cats and small dogs. Adequate sedation or calming is critical—do not wrestle with the patient. The physiologic stress of being excessively restrained is highly dangerous for aged/frail patients due to their low physiologic reserve.
• Induction: Use a benzodiazepine/propofol “sandwich” (propofol-benzodiazepine-propofol).
• Maintenance: Use low-dose inhalants plus analgesics.
• Recovery: Tailor care to the individual patient.

What do you say to clients who are concerned about anesthetizing their aged pets?

Honesty is crucial. The client should understand that anesthesia-associated physiologic changes can negatively affect aged pets and that the effect may be magnified if their pet is frail. Concerns should not be dismissed with phrases such as, “Don’t worry; nothing bad will happen.” Instead, reassure the client that a thorough assessment of their pet’s needs will be determined with diagnostics indicated by the individual pet and that a plan tailored to their pet will be developed. Assure them that a team member will be dedicated to their pet’s care, and let them meet that team member if possible.

What do we need to know to safely anesthetize frail patients?

The quick answer is do not use chronological age to determine the risk for anesthesia-associated adverse events. Thoroughly assess the patient, use the information in the frailty score, and assign an ASA status, as with patients of any age (TABLE 3). Control preexisting pain. Build physiologic support into the anesthesia plan, assign the patient to a dedicated veterinary nurse with anesthesia experience, diligently monitor the patient’s physiologic status throughout all stages of anesthesia, and quickly correct any negative physiologic changes.

Summary

Advanced patient chronological age is not a contraindication for anesthesia; however, age-associated physiologic changes that increase frailty (i.e., decreased physiologic reserve) can affect the anesthetic protocol and require increased monitoring and support. The frailty score should be incorporated into the ASA status determination. The keys for safe anesthesia include not only a thorough evaluation of patient health and stabilization of preexisting conditions (e.g., cardiac disease, chronic pain) but also a veterinary nurse dedicated to the patient and an anesthesia team that is prepared to recognize and correct any adverse intra-anesthesia physiologic effects.