Sheilah A. Robertson1 and Kirk A. Muñoz2 1 Lap of Love Veterinary Hospice Inc., Lutz, Florida, USA 2 Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University, Columbus, Ohio, USA A large study that included 8.9 million dogs and 2.4 million cats reported that life expectancy for both species increased between 2013 and 2019 [1]. Clinicians are anesthetizing more older pets and it is important to understand the specific changes that occur with age, and how these impact perioperative care and clinical outcomes. Biological aging can be defined as “the progressive accumulation of changes with time associated with or responsible for the ever‐increasing susceptibility to disease and death” [2,3]. The American Animal Hospital Association (AAHA) has proposed the following life stages in dogs: puppy, young adult, mature adult, senior, and end of life [4]. The Association of American Feline Practitioners (AAFP) and AAHA, place cats into five life stages: kitten, young adult, mature adult, senior, and end of life [5]. These stages are somewhat arbitrary, especially in dogs, because they are based on chronological age and not biological age. Both organizations have recently published senior care guidelines, which delve deeper into defining senior and geriatric patients [6,7]; details are shown in Table 69.1. In dogs, the term senior is used to describe the last 25% of estimated lifespan through to the end of life [4]. Because the lifespan of dogs varies greatly based on breed and size, it has been difficult to accurately allocate dogs into this life stage [8]. Life expectancy tables based on size, sex, and body condition confirm that life expectancy is decreased for the largest sizes of dogs compared to the smallest and provide life expectancy data for toy, small, medium, large, and giant dogs [1]. These life expectancy tables allow for the correct allocation of any size of dog to the life stage of “senior.” The term geriatric is a statement of health status and not a specific age [3,6]. McKenzie et al. have proposed the concept of the canine geriatric syndrome, which includes physical, behavioral, metabolic, and functional changes, frailty, clinical disease, and quality of life to facilitate the recognition of aging as a risk factor for negative health outcomes [3]. Anesthetic management encompasses the pre‐, intra‐, and postoperative period. Successful anesthetic management of older patients requires careful planning and includes an evaluation of comorbidities, functional status, frailty, nutrition, and a review of current medications. Older dogs and cats may have a variety of overt diseases (single or multiple) such as cardiac, renal, and hepatic disease. These disease processes require specific anesthetic management, which is covered elsewhere in this book. Access to a drug interaction checker (e.g., Plumb’s drug interaction checker: https://plumbs.com/features/drug‐interaction‐checker/) is extremely helpful due to the number of drugs some older patients may have been prescribed. Patient care does not stop in the immediate recovery period; arrangements must be in place for postoperative care, which includes a period of time after discharge. The ideal approach to the older patient involves a multidisciplinary team comprised of anesthesiologists, surgeons, rehabilitation clinicians, internists, and nutritionists. Table 69.1 Proposed life stages and definitions in dogs and cats derived from the AAHA and AAFP life stages and senior care guidelines [4–7]. Table 69.2 A quick reference guide to age‐related physiologic changes and their impact on anesthesia. GFR, glomerular filtration rate; RBF, renal blood flow. The physical and functional changes associated with aging in dogs and cats are well described [8–14]. Veterinary geroscience is an active area of research with a goal of developing therapies to mitigate age‐related dysfunction [3,15–17]. With respect to anesthesia in older animals, the key factor is understanding the impact of decreased functional or physiologic reserve in major body systems. In people, functional reserve in vital organs is reported to decrease by 1% every year after the age of 40 [18]. “Robustness” and “resilience” are terms that enhance the understanding of the aging process [19]. McKenzie encourages the use of “robustness” (the ability to resist deviation from an original or optimal state) and “resilience” (the ability to return to this state after deviations induced by external stressors) when assessing aging dogs [3,15]. These terms are easy to understand, and most clinicians know when they see an animal that is neither robust nor resilient and this may prompt a change in the anesthetic plan for these patients. The following section and Table 69.2, describe and summarize the age‐related changes in major body systems and their potential impact on anesthetic management; many detailed studies are available in humans, and it is assumed that similar changes occur in all mammalian species. The reader is referred to an excellent open‐access clinical review by Lim and Lee [20]. Similar to humans, cardiac reserve is decreased with increasing age in dogs. Cardiac output (CO) is decreased by up to 30% in older dogs, maximum heart rate and oxygen consumption in response to exercise are reduced, ventricular contraction is reduced, and myocardial stiffness is increased [21–23]. In aging humans, blood pressure is higher and CO lower than in young populations due to changes in connective tissues resulting in stiffness of blood vessels and the myocardium [24]. Age‐related changes in blood pressure have been reported in both dogs and cats [8,10,13,14,25]. Clinical implications Preanesthetic measurement of blood pressure is recommended as a reasonable step in older dogs and cats [26] so that changes from normal values can be detected and managed appropriately during anesthesia. Cats may have anxiety and stress‐related elevations in blood pressure but these can be partially mitigated by using feline‐friendly handling techniques [27]. A decrease in CO will result in an increase in the injection site to brain circulation time; therefore, it is essential that intravenous anesthetic drugs are given slowly, allowing sufficient time for drugs to reach the brain and exert an effect before additional drugs are administered. Reduced cardiac reserve leaves older patients less able to respond to acute changes in blood volume (loss or overload) and hypotension compared to younger animals. Intravenous fluids should be administered to meet pre‐existing needs and ongoing losses. Close cardiovascular monitoring is required, and inotropes and vasopressors should be available. Animals with a rightward shift in autoregulation due to chronic hypertension should be maintained within ~30% of their presenting blood pressure to ensure that perfusion to vital organs is maintained throughout the anesthetic event. Chest wall compliance decreases with age while lung compliance increases, increasing functional residual capacity (FRC) and the work of breathing [28]. Closing capacity also increases but faster than FRC and may eventually equal FRC, such that at normal tidal volume, areas of low or zero ventilation to perfusion ratios may exist. The surface area for gas exchange decreases; there is a reduction in diffusion capacity across the capillary–alveolar membrane; and respiratory muscle strength weakens [28]. Elderly humans (aged 65–79 years) have an approximately 50% reduction in ventilatory response to hypoxia and hypercapnia compared to young subjects [29]. Older humans have decreased pharyngeal muscle tone and less effective upper airway reflexes. This is likely true in dogs based on their incidence of aspiration pneumonia (see later). Laryngeal paralysis (a component of geriatric onset laryngeal paralysis and polyneuropathy [GOLPP]), a disease of older dogs, puts them at risk of aspiration and airway obstruction. Low‐grade chronic inflammation (“inflamm‐aging”) has been reported in humans and is associated with an increased incidence of morbidity and mortality [30]. This is also likely to affect our aging veterinary patients. Using sterile endotracheal tubes, appropriate inflation of the endotracheal tube cuff, and suctioning of the oral cavity prior to extubation are some measures that can be used to help protect these animals. In awake elderly patients, PaCO2 and PaO2 are usually well maintained, but when challenged by sedation and anesthesia involving respiratory depressant drugs, the lack of pulmonary reserve, decreased respiratory drive, and poor muscle strength render these patients at risk of hypoxemia and hypercapnia [31]. Older dogs and cats should be closely monitored perioperatively using a pulse oximeter, and capnography intraoperatively to determine if ventilation is appropriate. Respiratory support using manual or mechanical ventilation may be needed, especially when patients are in dorsal recumbency. Preanesthetic administration of antinausea and antiemetic medications is advised [32]. Oxygen supplementation prior to induction of general anesthesia and in the postoperative period until the animal is able to maintain normal oxyhemoglobin saturation on room air is recommended [33]. There are three distinct clinical problems associated with cognition in the postoperative period. Emergence delirium refers to restlessness in the immediate postoperative period and can occur in any age group, is usually short‐lived, can be managed with sedation, and has no long‐term effects. Postoperative delirium is defined as a “short‐term and transient” issue and is characterized by changes in consciousness and cognition, disorientation, anxiety, and fear which may fluctuate [34]. A diagnosis of frailty before surgery is associated with an increased risk of postoperative delirium in human patients [35]. Postoperative delirium can occur in any age group, but elderly patients are at the highest risk. It appears between the second and seventh day following anesthesia, and the pathophysiology is poorly understood [34]. This syndrome likely occurs in animals but may go undetected because animals do not need to function at a high level of cognition, and we are unlikely to monitor for this in the extended postoperative period. However, there are enough reports by owners of their pets being “different” after anesthesia that this warrants further investigation. Interventions including frequent orientation, noise reduction, early mobilization, and attention to hydration are effective at reducing postoperative delirium in humans, all of which can be applied in veterinary medicine [34]. Postoperative cognitive dysfunction (POCD) refers to longer‐term changes in cognition after surgery. It occurs in up to 12% of all patients and is usually self‐limiting [34,36]. The elderly and patients with pre‐existing cognitive impairment before surgery may show a dramatic decline after surgery [36]. No specific anesthetic drug or technique has been shown to cause or prevent POCD and the mechanism that triggers it is unknown [36]; however, some authors suggest that benzodiazepines should be avoided in elderly patients at high risk of developing POCD [20]. The majority of studies comparing POCD after general versus regional anesthesia fail to show a difference, suggesting that its etiology is complex and multifactorial [37]. Cognitive dysfunction (CD) is documented in dogs and cats and may affect up to 35% of aging dogs and more than 50% of cats over the age of fifteen [38–42]. Canine and feline CD are analogs of human Alzheimer’s disease; therefore, postoperative delirium and worsening of cognition are likely to occur. Reports of cognitive decline after anesthesia in older dogs and cats are anecdotal but some clinicians state that owners should be informed of this possibility and that it is more evident in animals that have cognitive impairment prior to anesthesia [43]. Confusion, anxiety, disturbed sleep/wake cycle, and decreased interaction with owners are all common clinical signs of canine CD and these can be exacerbated by a hospital stay. Diagnosis of CD is not straightforward but older animals should at least be screened for CD prior to anesthesia so that clinicians can prepare for the recovery period; assessment tools are available for dogs and cats [6,44]. Failing vision and hearing in older patients may add to confusion in the postoperative period. Changes in environment are poorly tolerated by older animals, especially those with CD. Whenever possible these patients should be scheduled as outpatients; when creating the day’s operating list, assign them the first slot so they can be cared for when the hospital is fully staffed and discharged at the end of the day. There is a gradual decline in renal function with increasing age; however, a significant loss of functional nephrons must occur before this is detectable with routine testing. The dangers of reduced renal reserve related to anesthesia are real. Perioperative dehydration, hypovolemia, and hypotension are not well tolerated in older patients, and animals with reduced renal reserve may suffer acute kidney injury. Many older dogs and cats are prescribed non‐steroidal anti‐inflammatory drugs (NSAIDs) for maladaptive pain states putting them at a higher risk of renal insult. Cyclo‐oxygenase‐1 and 2 are constitutively expressed in the kidneys and the products of these pathways include prostaglandins (PGs) and thromboxanes, which are involved in the regulation of glomerular filtration rate (GFR) [45]. In the face of decreased renal perfusion (e.g., decreased circulating volume), renal PGs play a vital compensatory role, including vasodilation to enhance renal blood flow and GFR [46]. NSAIDs block the ability of the kidney to autoregulate. Measurement of GFR is not a routine clinical test, though it may be reduced in patients with normal serum creatinine values [47]. Not knowing the GFR in older animals makes dosage adjustments of renally excreted drugs difficult. Gabapentin is a renally excreted drug and marked differences in serum concentrations were reported in normal cats compared to those with IRIS stage 2 and 3 chronic kidney disease (CKD) [48]. In the same study, serum gabapentin concentrations were correlated with serum creatinine and symmetric dimethylarginine, but it is not known if serum drug concentrations are correlated with GFR. CKD is common in older pets, especially cats [6]. Perioperative management of patients with CKD is discussed in Chapter 43. Maintaining euvolemia and renal perfusion pressure during anesthesia is essential when renal reserves are reduced, therefore fluid therapy, blood pressure monitoring, and treatment of hypotension are primary considerations in this population. Renally excreted drugs should be used with caution in older patients. In humans, liver mass and blood flow decrease with age [49,50]. Drugs that are processed via phase I reactions (involving the cytochrome P450 system) are “flow limited” and are likely to be cleared more slowly in older patients. Phase II elimination involves conjugation and drugs dependent on this pathway are less affected by age [50]. This is a simplistic approach and drug elimination is dependent on multiple factors; further details are available in Chapter 19. Liver enzymes do not reflect hepatic function, and hepatic function tests may be normal in older animals, yet their ability to clear drugs may be compromised. Due to the difficulty in predicting how an aging patient will process a specific drug, it is recommended that the anesthetist uses drugs that can be titrated to effect, have a high therapeutic index, and are reversible. Immunosenescence, which is defined as changes in the immune system associated with age, has received a lot of attention in humans, with increased vulnerability to infection and decreased immune response to vaccinations being recognized as contributing to mortality [51]. There is some, albeit limited, evidence that age‐related changes in immune function occur in dogs and cats [15]. Age‐related changes in the canine immune system are summarized by Bellows [10]. Older animals should be considered at higher risk than young patients for developing perioperative infections. Urinary retention is a risk factor for infection in older humans but the use of indwelling urinary catheters, which are not without risk, must not be used as a substitute for good nursing care [20]. Animals should have their bladders expressed at the end of the procedure and provided opportunities to void voluntarily in the postoperative period – this requires assisting large dogs to stand and providing suitable litterboxes to cats. If urinary catheters are used, they must be placed in a sterile fashion and properly maintained. With increasing age, there is loss of muscle mass and an increase in adipose tissue in humans, dogs, and cats [15
69
Aging Patients
Introduction
Life stage
Definition
Dog
Puppy
Birth to cessation of rapid growth (approximately 6 to 9 months, varying with breed and size)
Young adult
Cessation of rapid growth to completion of physical and social maturation; 3 to 4 years in most dogs
Mature adult
Completion of physical and social maturation until the last 25% of estimated lifespan (breed and size dependent)
Senior
The last 25% of estimated lifespan through end of life
The term “senior” is used to describe the older, aging pet. Because of the variability in aging based on breed and size, the chronological age of a senior will vary. Additional descriptive terms such as “frailty” are used to describe individuals
End of life
Terminal stage (depends on specific pathologies)
Cat
Kitten
Birth to 1 year
Young adult
1–6 years
Mature adult
7–10 years
Senior
> 10 years; some cats of specific breeds and genetic predispositions may be considered senior at an earlier chronological age
End of life
Variable
Geriatric
The term geriatric is a statement of health status and not a specific age
System
Age‐related changes
Impact on anesthesia
Cardiovascular
Reduced cardiac reserve, blood volume, blood pressure, and cardiac output
Reduced ability to compensate for cardiovascular changes
Slower onset of intravenous general anesthesia, resulting in increased risk of anesthetic overdose
Inotropic and blood pressure support may be needed
Pulmonary/respiratory
Loss of lung elasticity and decrease in chest wall compliance
Increase in functional residual capacity, with closing capacity increasing at a comparatively faster rate
Increased ventilation–perfusion mismatching
Risk of hypoxemia and hypercapnia
Respiratory support may be required
Neurologic
Reduction in brain mass and decreased neurotransmitters
Reduced cerebral blood flow
Altered central thermoregulation
Decreased anesthetic requirements
Increased risk of hypothermia
Renal
Decreased renal function secondary to a reduction in GFR, RBF, and renal mass
Less tolerant of fluid losses, allow water to be consumed until the time of premedication
Hepatic/metabolic
Reduced hepatic mass and perfusion
Variable and unpredictable effects on drugs that undergo hepatic metabolism
Immunologic
Immunosenescence
May be more prone to infection
Adhere to aseptic techniques
Physiologic changes related to age
Cardiovascular changes
Pulmonary changes
Clinical implications
Neurocognitive and behavioral assessment, postoperative delirium, and cognitive impairment
Renal changes
Clinical implications
Hepatic changes
Clinical implications
Immune function
Clinical implications
Changes in body composition
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