Anderson Fávaro da Cunha Department of Specialty Medicine, College of Veterinary Medicine, Midwestern University, Glendale, AZ, 85308, USA Age is not a disease; however, it is an important independent risk factor of morbidity and mortality [1, 2]. Age can be used as a predictor of perioperative outcome for anesthetized patients, as the odds for perioperative mortality in small‐animal anesthesia are age‐dependent [3, 4]. Morbidity and mortality studies suggest that a significant increase in mortality rate is observed in senior and geriatric patients, especially if they have comorbidities of the pulmonary, cardiac, renal, hepatic, and endocrine systems, alone or in combination [2, 5]. Even though age is an important variable considered during the preanesthetic evaluation, the deep understanding of the patient’s physical status is even more important while formulating an anesthetic plan. This chapter critically reviews the most common physiological and pharmacological changes in the senior and geriatric dog and cat, and their differing responses to sedation and general anesthesia. Defining the term “geriatric” is challenging in veterinary medicine. The patient’s species, size, and breed are important variables used for the classification [6–8]. Accordingly, using the American Animal Hospital Association (AAHA) guidelines, the dog [6] and cat [7] senior and geriatric stages are defined in Tables 20.1 and 20.2, respectively. It is important to note that the physiological status of older patients is more relevant than their chronologic age [9, 10]. Geriatric patients can be further subclassified into different categories based on their physiological status (Table 20.3) [9]. As age progresses, respiratory function deteriorates [11–13]. The common weakening of the respiratory muscles and loss of elastic tissue observed in senior and geriatric patients, coupled with pulmonary fibrosis, lead to a decrease in chest wall compliance and elastic recoil of the lungs [14]. This is associated with a decrease in vital lung capacity, leading to a decreased functional residual capacity (FRC). These physiological changes predispose to atelectasis in senior and geriatric patients when under anesthesia or prolonged recumbency. Aged patients may also present with increased airway resistance, decreased pulmonary diffusion capacity, and decreased capillary blood volume, all reducing expiratory efficiency and impairing gas exchange [13, 14]. Older patients also are increasingly susceptible to respiratory infections [14]. Altogether, these result in an increased susceptibility to perianesthetic hypoxemia, and oxygen supplementation is recommended. These patients should receive 100% oxygen for a minimum of 3 min prior to anesthetic induction via face mask (oxygen flows >150 ml kg−1 min−1). Intra‐anesthetic and postanesthetic oxygen support should be provided and dictated by pulse oximetry, or, in cases where hypoxemia is present or expected, arterial blood gas analysis. Oxygenation should be maintained with the goal of arterial (PaO2) oxygen >80 mmHg or hemoglobin saturation of oxygen on the pulse oximeter (SpO2) >95%; higher PaO2 will be expected if 100% O2 is delivered via endotracheal tube, as dictated by the alveolar gas equation (see Chapter 2). During anesthesia, intermittent positive pressure ventilation should be provided as necessary and dictated by capnography and blood gas analysis with the goal to maintain normal ventilation (end‐tidal [ETCO2] and arterial [PaCO2] carbon dioxide between 35 and 45 mmHg). Table 20.1 Definition of canine life stages [6]. Table 20.2 Definition of feline life stages [7]. Table 20.3 Subclassification of the geriatric population [9, 11]. For the geriatric patient, cardiovascular diseases are a significant concern [15]. The aging process is correlated with common multifactorial intrinsic physiologic changes in the cardiovascular system, including decreased baroreceptor activity and limited ability to adapt to hypotension, all leading to a reduced cardiac reserve [16]. Most of the common changes in the geriatric heart are primarily related to varying degrees of myocardial fibrosis, valvular fibrocalcification, and ventricular thickening [17]. The cardiac conduction system can also be compromised, leading to possible cardiac arrhythmias [17]. Therefore, drugs with negative inotropic or arrhythmogenic effects should be avoided or used with extreme caution. It has been estimated that approximately 10% of dogs older than 5 years of age have mitral insufficiency and approximately 25% of dogs between 9 and 12 years of age have evidence of cardiac disease [14]. The risk increases to approximately 33% in dogs older than 13 years of age [14, 16]. The most common canine cardiovascular abnormalities are valvular heart diseases, dilated cardiomyopathy, pericardial diseases, arrhythmias, and systemic hypertension [3, 16, 18, 19]. In older cats, hypertrophic cardiomyopathy is the most common disease. These abnormalities can increase the odds of morbidity and mortality in the geriatric patient. History of exercise intolerance, arrhythmias, cyanosis, abnormal pulse quality, cardiac murmurs, and/or syncope indicates a need for a more extensive preanesthetic cardiac evaluation. Thus, in addition to a thorough history and physical exam, a complete geriatric cardiac profile includes thoracic radiography, electrocardiogram (ECG), blood work (complete blood count (CBC) and serum chemistry/electrolyte analysis), and echocardiography. The most common cardiovascular side effect associated with sedation and general anesthesia is hypotension [20, 21]. It is essential for the anesthetist to ensure adequate blood pressure, euvolemia, venous return, and stroke volume to maintain normal tissue oxygen delivery [21]. Fluid rates should be based on individual need, hydration, and physical status. Often, older animals have compromised cardiac reserve and, therefore, fluid overload leading to congestive heart failure and/or pulmonary edema can easily occur. Total body water also decreases in the geriatric patient. Therefore, beat‐to‐beat cardiovascular monitoring with a dedicated anesthetist is imperative to quickly detect cardiovascular changes that could lead to anesthesia‐related complications. If cardiovascular instability is expected, arterial catheterization and invasive blood pressure measurement are recommended. Treatment of possible complications such as hypotension, cardiac arrhythmias, or fluid overload should be provided as needed (see Chapter 1). With age, hepatic function deteriorates. During the geriatric phase of life, dogs and cats commonly experience decreased liver mass and hepatic blood flow secondary to reduced cardiac output, decreased microsomal enzyme activity, and generalized metabolic activity reduction [22]. Decreasing hepatic function may lead to hypoproteinemia, hypoglycemia, hypothermia, and coagulopathies. For all geriatric patients, liver function analysis and coagulation status are prudent prior to anesthesia or sedation, especially when highly metabolized drugs are to be used. Hypotension should be avoided, since it further decreases hepatic blood flow, exacerbating the possible ischemic hepatic damage that is associated with advanced age. The aged patient may have compromised cognitive, sensory, motor, and autonomic functions [23]. Aging may be correlated with decreasing anesthetic requirements (inhalants, benzodiazepines, opioids, barbiturates) due to a decrease in brain size, loss of neurons, increase in cerebrospinal fluid volume, depletion of neurotransmitters (dopamine, norepinephrine, tyrosine, and serotonin), [24–26] decrease in cerebral oxygen consumption, and myelin degeneration. The geriatric thermoregulatory center is weakened and, therefore, older patients are more susceptible to anesthesia‐induced hypothermia [27]. Bradyarrhythmias, hypotension, decreased minimal alveolar concentration (MAC) of volatile anesthetics, and shivering are associated with hypothermia. Moreover, shivering can increase oxygen consumption up to 400%; therefore, oxygen supplementation is important for any older patient [28]. The incidence of renal disease has been reported to be approximately 0.5–1.5% in dogs and cats [29, 30]. The percentage increases with age; the most common renal disease occurring in elderly dogs and cats is chronic kidney disease [31]. Urinary incontinence, bladder neoplasia, and prostatic disease are also common. Decreased renal mass in senior and geriatric dogs and cats is associated with decreased tubular size and weight, decreased glomerular numbers, and filtration function [29–32]. Protein, water, and sodium reabsorption, aldosterone secretion, anionic and cationic compound secretion and reabsorption, vitamin D and renin formation, and elimination and metabolism of protein‐bound compounds are all compromised in the geriatric patient [29–32]. These changes influence blood pressure regulation, acid–base status, and erythropoietin release, resulting in hyperphosphatemia, azotemia, dehydration, and hypoproteinemia. In general terms, general anesthesia reduces cardiac output leading to an approximately 40% reduction in renal blood flow and glomerular filtration [27]. Older patients with cardiovascular diseases can have an exacerbated response. Additionally, decreases in cardiac output can worsen renal disease after general anesthesia and possibly lead to acute kidney insufficiency. Blood pressure, cardiac and urine output, hydration status, and renal function (urinalysis, blood urea nitrogen [BUN], and creatinine) monitoring is recommended. Preventing and treating any side effects as soon as possible, when necessary, and working to prevent physiologic derangements such as hypoxemia, hypervolemia, hypotension, and hypercarbia are imperative [27]. Patients undergoing anesthesia are at an increased risk of gastroesophageal reflux secondary to factors such as breed predisposition (Poodles, Labrador Retrievers, and German Shepherds) and positioning (Trendelenburg), among others [33]. Increased risk of gastroesophageal reflux during anesthesia, combined with possible laryngeal and pharyngeal function compromise (e.g., laryngeal paralysis or paresis), may be associated with higher risk of aspiration pneumonia in geriatric patients [34]. Proper endotracheal tube inflation and gentle esophageal suctioning before endotracheal extubation may decrease the risk of aspiration pneumonia. However, suctioning the esophagus alone does not change esophageal lumen pH, thus the risk of esophageal damage secondary to acid reflux remains. If regurgitation is observed during anesthesia or if gastric fluid is observed during esophageal suctioning prior to extubation, the following protocol may be followed to decrease the chances of esophageal damage that may lead to esophagitis and/or esophageal stricture: [35]
20
Senior and Geriatric Concerns
Introduction
Physiological Alterations
Respiratory System
Stage
Definition
Adult
Dog has reached physical maturity for breed and is no longer growing, social behavior consistent with adult of breed, lasts until dog transitions to senior
Senior
Follows adult stage and lasts until death, last 25% of dog’s life span for breed
Geriatric
Time period after life expectancy has been reached
Stage
Age (years)
Young adult
3–6
Mature adult
7–10
Senior
11–14
Geriatric
>15
Classification
Definition
ASA status
1
Healthy geriatric patients—minor changes in organ function
II
2
Geriatric patients with subclinical organ dysfunction—decreased function of one or more organs such as the heart, liver, or kidneys
III
3
Geriatric patients with an obvious disease condition—severe clinical organ dysfunction (heart disease, endocrine disease, renal disease, or neoplasia)
IV, V
Cardiovascular System
Hepatic System
Central Nervous System (CNS)
Renal System
Gastrointestinal (GI) System
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