Anesthesia of the geriatric patient

Introduction

A rise in the life expectancy of our domestic pets, and a change in owners’ attitude towards their healthcare, has resulted in geriatric patients being presented to veterinary practices more commonly than in the past, and they are often admitted for surgical or diagnostic procedures requiring general anesthesia. Geriatric animals are typically defined as those which have reached 75-80% of their expected lifespan, although this should perhaps be interpreted with some caution, as the average lifespan is likely due, at least in part, to breed-specific pathology rather than simply variation in biological aging 1. It is also unlikely that a young animal with life-limiting disease is physiologically similar to an aged patient. In general, animals which appear to be old should probably be considered to be so, and if this is incongruous with their physical age further investigation is warranted.

Healthy aging in both humans and animals is associated with changes which result in a reduction in physiological reserve, and older animals are more likely to be affected by clinical and subclinical disease, all of which have implications for anesthesia. A multicenter retrospective analysis concluded that increasing age was an independent risk factor for perioperative mortality; apparently healthy dogs and cats over 12 years of age were approximately 10 and 2.1 times more likely respectively to die from anesthetic-related complications than their younger counterparts 2. This increased risk is likely the result of greater susceptibility to the effects of anesthetic drugs due to decreased functional reserve, hypothermia, prolonged recovery and an increased incidence of subclinical disease.

Age-related physiological changes

Respiratory system 

Within the lungs, the small airways (i.e., less than 2 mm diameter) close at low lung volume, and ventilation to or from alveoli beyond them ceases; in older patients, this closure occurs at higher volumes than in younger animals. Although pulmonary fibrosis and respiratory muscle weakness in older animals results in an increased functional residual capacity (FRC), closing capacity (the maximum volume of air within the lungs at which small airway closure can be detected) also increases with loss of elastic recoil. In time, closing capacity will exceed FRC 3. Atelectasis and resulting ventilation-perfusion (V/Q) mismatch leads to an increased alveolar-arterial (A-a) oxygen gradient and therefore decreased arterial oxygen tension. As a result, geriatric animals are more prone to rapid desaturation and are less able to respond to hypoxia and hypercapnia.

Cardiovascular system

Geriatric patients have less cardiovascular reserve and thus a reduced ability to respond to hypotension. In these patients, blood flow to the organs, including the liver and kidneys, may be reduced, increasing susceptibility to dysfunction in the face of ischemia. In animals without specific cardiovascular pathology, age-related changes include myocardial atrophy and fibrosis, valvular fibrocalcification and ventricular thickening. Involvement of the heart valves may lead to varying degrees of insufficiency, whilst inclusion of the conduction system can predispose patients to the development of arrythmias.

As a result of decreased elasticity of the heart and increased ventricular hypertrophy, cardiac output becomes increasing reliant on the contribution of atrial contraction in increasing ventricular filling. At the same time baroreceptor response in elderly patients is blunted, and whilst decreased vagal tone results in an increased resting heart rate, they are less able to elevate this in response to hypotension. Therefore, maintenance of adequate stroke volume is reliant upon sufficient preload or end diastolic volume. 

The incidence of overt cardiovascular disease also increases in older pets. Canine mitral valve disease is frequently encountered, with an estimated 25% of dogs between the ages of 9-12 years and 33% of dogs over 13 years presenting with some degree of mitral valve insufficiency 4. In cats, hypertrophic cardiomyopathy (HCM) is the most common presentation, affecting approximately 15% of the population, with incidence increasing with age 5. In patients with valvular regurgitation, myocardial work is increased for a given cardiac output, leading to increased oxygen demand, increased myocardial hypoxia and development of arrythmias. HCM patients are at risk of left-ventricular-outflow tract obstruction, myocardial hypoxia and arrythmias. 

Renal system

Loss of kidney function is associated with age. Degenerative changes to the cardiovascular system result in decreased renal blood flow, although glomerular filtration rate (GFR) can remain unchanged in some healthy older individuals 6,7. These hemodynamic changes occur alongside a reduction in renal mass resulting from the loss of glomeruli and nephron units, leading to a reduction in functional reserve. 

Fluid and electrolyte homeostasis is also altered in elderly pets. Renal tubular degeneration and a diminished response to antidiuretic hormone mean that they have a reduced ability to concentrate urine and a lower body water content. However, they may also be less tolerant to volume overload, because their ability to void excess water in urine is impaired by a reduced capacity for renal sodium excretion in response to blood volume and pressure changes. This may be exacerbated by common pathologies including renal insufficiency (which may result in an inability to excrete sodium efficiently) and congestive heart failure (CHF). 

In aged individuals a reduced ability to excrete hydrogen ions is also well documented, resulting in a predisposition to develop acute metabolic acidosis. In individuals with concurrent pulmonary disease this may be exacerbated as a result of limitations in their ability to compensate effectively. 

Hepatic system

Old age is associated with decreased liver mass, thickening of the sinusoidal endothelial cells and a reduction in hepatic blood flow. However, a recent study suggests that median levels of liver enzymes (alanine transaminase (ALT) and alkaline phosphatase (ALP)) are mildly increased in apparently healthy aged patients, perhaps indicating subclinical disease 8. Hepatic metabolism of drugs is dependent upon blood flow, plasma protein binding and distribution within the perisinusoidal space. Consequently, functional limitation may be seen in normally aged patients as the result of reduced delivery of drugs to hepatic enzyme systems. Reduction in liver function may also result in increased clotting times, poor immunological function, hypoalbuminemia, hypocholesterolemia and disorders of glucose metabolism. For example, a decreased ability to incorporate glucose into glycogen predispose elderly patients to hypoglycemia. 

Nervous system 

Changes within the central and peripheral nervous system associated with aging increases susceptibility to anesthetic drugs. Although the underlying pathophysiology is not well characterized, studies in elderly people suggest that the minimum alveolar concentration (MAC) of the volatile agents decrease with age 9 and dose requirements of intravenous anesthetic preparations may also be reduced. 

Post-operative cognitive decline is also well recognized in elderly people and is hypothesized to be the result of neuroapoptosis, neurodegeneration and beta-amyloid accumulation. Studies suggest this may be associated with volatile anesthetics and some injectable agents including midazolam and ketamine 10. Ischemic episodes affecting the central nervous system may similarly result in cell death. Although this phenomenon has not been described in veterinary species, it may also be a risk factor in our small animal patients. 

It should also be remembered that elderly animals may be more prone to stress and difficult to handle as the result of varying degrees of blindness, deafness and age-related neurological disease such as canine cognitive dysfunction and loss of learned behaviors. Unfamiliar hospital environments may exacerbate this, leading to confusion, disorientation and erratic behavior. Olfactory impairment may also lead to reduced appetite or aversion to unfamiliar foods, which may be a consideration when feeding patients postoperatively. 

Gastrointestinal system 

The ability to digest macronutrients probably remains relatively constant with age in dogs, although in cats it may be reduced even in the absence of clinical disease; however, the significance of this is unclear as affected animals tend to increase their dietary intake to compensate 11. Chronic intestinal problems are common in older pets, and can lead to malabsorption and the potential for nutritional deficiencies. Occasionally, these may have implications for anesthesia, for example where a severe cobalamin deficiency results in anemia. 

In older dogs, gastro-esophageal reflux during anesthesia appears to occur more frequently than in younger patients 12. When combined with low gastric pH this leads to an increased risk of post-operative esophagitis. 

Musculoskeletal system 

Elderly patients have a higher incidence of musculoskeletal disorders such as degenerative joint disease and spondylosis deformans leading to chronic pain states. This may be exacerbated by lengthy periods of recumbency or positioning for imaging or surgery, resulting in worsening of the pain and sympathetic stimulation.

Preoperative assessment

Health status is associated with the risk of perioperative morbidity and mortality. Pre-operative assessment is therefore important in all animals undergoing general anesthesia to identify an appropriate anesthetic protocol for the patient, pre-empt potential complications and to advise clients. As a minimum, assessment consists of a thorough clinical history and physical examination which may indicate clinically appreciable, but previously undiagnosed, pathology. The clinician should ascertain if the pet is on any medication, as this may have significance for the anesthetic. Note that owners may not be able to distinguish between healthy aging and clinical signs of pathology, suggesting that client education is important 13 and that history-taking accounts for this. In human patients a reported history of exercise intolerance and poor performance on formal testing is a predictor of perioperative complications and negative outcomes 14, and in veterinary patients assessment of physical activity may also indicate ability to cope with the physiological stresses posed by general anesthesia.

Although current evidence suggests that there may be no benefit to routine preoperative bloodwork as a screening tool, elderly animals are more likely to be affected by subclinical disease, so it may be warranted in this subset of patients 15,16 (Figure 1). Some differences on hematology and biochemistry are expected even in healthy aged animals and, if available, reference ranges formulated for geriatric patients may be used. Otherwise, these changes can be accounted for when interpreting results. For example, seemingly healthy patients may have a decreased hematocrit, mean corpuscular volume and serum iron concentration and an increased platelet count. On biochemistry, total protein, globulins and urea may be increased and albumin is frequently decreased 8. Although hepatic enzyme activity is often mildly increased or unchanged, the potential for reduced liver function (as noted above) remains. Further testing may be warranted, e.g., if hypoglycemia, hypalbuminemia or hypocholesterolemia raise a concern. Such tests might include a bile acid stimulation test and coagulation assays, including prothrombin time and activated partial thromboplastin time.

Both renal and pre-renal azotemia are more prevalent in the aged population, but increased blood urea nitrogen may not be appreciable until there is over 75% impairment. Therefore, where there is evidence of azotemia this should be interpreted in conjunction with urine specific gravity.

The findings on clinical history, physical exam, assessment of exercise tolerance and evaluation of laboratory data can be used to assign an American Society of Anesthesiologists (ASA) classification (Table 1), which grades risk into 5 categories. This is widely used to categorize a human patient’s physiological status and pre-empt anesthetic complications. Strictly speaking, age is not considered to be a criterion when designating ASA classification, but given differences in the physiology of both very young and very old animals and the reduction in reserve, in practice many clinicians will account for this either separate to the classification system or by routinely assigning an ASA II classification to otherwise healthy geriatric patients. Where appropriate, stabilization of comorbidities may be warranted pre-operatively in order to reduce anesthetic risk – for example, a pre-renal azotemia can be corrected prior to surgery to reduce the risk of hypotension leading to acute tubular necrosis.

Premedication and analgesia

As noted above, elderly patients may be more easily distressed in a hospital environment, so careful handling may be necessary to reduce stress. In an ideal situation, intravenous access is obtained on admission to the clinic, but this may not be possible in anxious or aggressive animals without sedation in some cases. It can be appropriate to ask the owner to give a light sedative or anxiolytic prior to admission, and there is some evidence supporting the use of gabapentin (10-20 mg/kg in dogs and 10 mg/kg in cats), trazadone [2-10 mg/kg in dogs and 5-10 mg/kg in cats (i.e., typically one 50 mg tablet)] or alprazolam (0.01-0.1 mg/kg in dogs and 0.125-0.25 mg/kg in cats) for this purpose 17,18. These can be administered alone or in combination, and although they can lead to a mild reduction in blood pressure, may be preferable in many elderly patients to other oral agents such as dexmedetomidine and acepromazine, which cause more marked cardiovascular depression.

In animals with osteoarthritis, flexion and extension of the joints when restraining for intravenous cannula placement can cause significant pain. Here cannulas can be placed in the peripheral vein least likely to require the patient to adopt an uncomfortable position. Alternatively, in dogs with long limbs, placement can be performed in a standing position with the animal weight-bearing on all four legs to avoid the need to flex and extend the joints (Figure 2).

Most patients will be fasted prior to anesthesia, but older animals may be less able to tolerate this as a result of low glycogen reserves in the liver, and six hours is usually considered the maximum appropriate starvation period in these animals. However, those with gastrointestinal disease may have a slower transit time and an increased risk of vomiting, and elderly patients have a higher risk of regurgitation generally. Close observation may therefore be necessary once premedicants are given. The pre-emptive administration of omeprazole (1 mg/kg IV/PO) in patients likely to regurgitate or an antiemetic such as maropitant (1 mg/kg SC/IV/PO) or ondansetron (0.5 mg/kg IV) in those at risk of vomiting may be warranted. Withdrawing water 30 minutes prior to induction or from the time of premedication is usually sufficient, and reduces the likelihood of dehydration in patients unable to tolerate even minimal volume depletion. 

As in all patients, the aim of premedication is to provide anxiolysis and facilitate handling, supply analgesia, minimize requirements for induction and inhalation agents, and allow for smooth recovery from anesthesia. The combination of drugs used, the dose and the route of administration will depend on the individual patient, existing morbidity and the procedure. However, some considerations specific to geriatric patients may also be required. Those with clinical disease may have significantly greater predisposition to hypotension and development of arrythmias where the conduction system is involved or where myocardial hypoxia is a risk. Even in the absence of clinical disease, geriatric patients have less cardiovascular reserve and a poor ability to mount a response to hypotension. Therefore, arrhythmogenic drugs or those with negative inotropic effects may be avoided in some patients and used with caution in others. For this reason, when common premedicants such as alpha-2 agonists (medetomidine, dexmedetomidine) and acepromazine are used in the elderly, it is often at lower doses then in younger patients. 

Renal and hepatic insufficiency in geriatric patients may affect the metabolism and excretion of injectable agents. For example, drugs which are excreted by the kidneys may have a longer elimination half-life in patients with a reduced GFR, prolonging their duration of action. Drugs which cross the blood-brain barrier (and therefore most anesthetic drugs) are lipid soluble and are consequently metabolized by the liver, so in patients with reduced hepatic function this process may be inefficient. Hypoalbuminemia and acidosis, which is seen in some patients, will also affect protein-bound drug metabolism, but this does not necessarily lead to a significant increase in plasma concentrations or free drug or toxicity because the distribution volume is also changed. Therefore, in general, the effects of injectable premedicants can be less predictable, and cautious dosing is warranted with repetition based on requirement rather than at set intervals. 

Analgesia is typically provided as part of premedication for painful procedures, but additional medication may be required in many cases, and a multimodal approach is often appropriate to reduce sympathetic stimulation and post-operative pain. This might include locoregional anesthesia (Figure 3), intraoperative boluses or constant rate infusions of opioids, ketamine or lidocaine, and non-steroidal anti-inflammatory drugs (NSAIDs). However, the intraoperative use of NSAIDs requires particular caution where there is known renal insufficiency or a risk of hypotension. In geriatric patients it may be worth providing analgesia for some procedures normally considered to be non-painful – for example, in animals with chronic pain associated with conditions such as osteoarthritis, positioning for surgery or radiography may worsen this post-operatively.

Preoxygenation

Geriatric patients may be more prone to hypoxemia on induction and less able to cope with its effects. Pre-oxygenation increases the alveolar oxygen reserve by gradually replacing the nitrogen portion of FRC and is therefore protective. Common methods include delivery of 100% oxygen via a face mask and breathing system or by using flow-by. The use of a mask is preferable (Figure 4) if the patient will tolerate it, as this method has been determined to be more effective in increasing the time to desaturation 19. It is also important to ensure an adequate duration is achieved, and this may be especially true of older animals, as human studies suggest that the time taken to adequate denitrogenating in the elderly is significantly longer than in younger patients 20.

Intravenous fluid therapy 

Hypotension is a common sequela to general anesthesia, and in geriatric patients decreased functional reserve means they are particularly susceptible to subsequent organ dysfunction resulting from ischemia. At the same time, the reliance on stroke volume to maintain cardiac output requires adequate preload, and dampened baroreflexes mean they are less able to mount a response to low blood pressure. Intravenous fluid therapy (IVFT) is therefore usually instigated prior to anesthesia in geriatric patients and continued into the recovery period (Figure 5). However, considering that elderly animals are poorly tolerant of volume overload and have reduced body water, caution is required when doing so and comorbidities such as CHF and renal insufficiency need to be considered. Consequences of volume overload include the development of CHF and tissue edema, resulting in reduced perfusion, impaired oxygen exchange in the lungs, reduced gastrointestinal motility, reduced hemostasis (due to the impairment of clotting factors), and delayed wound healing.

In most patients, a balanced crystalloid solution is generally appropriate. Rate of administration can be tailored to the individual’s need but is generally somewhere between 2-5 mL/kg/hour, and fluid boluses of 10-15 mL/kg in dogs and 5-8 mL/kg in cats may be used discriminately to treat hypotension or hypovolemia.

Induction of anesthesia

Most available injectable induction agents are suitable for use in geriatric patients and preference is often based on clinician experience and familiarity. Intravenous propofol, alfaxalone, ketamine and etomidate have been used. All have the potential to cause dose-dependent respiratory and cardiovascular depression, but because requirements for anesthetic drugs are reduced in the elderly, they need careful titrating to effect. A study on propofol in geriatric dogs demonstrated a risk of post-induction apnea in this population and suggested that the elimination half-life was longer and that doses lower than the datasheet recommends may be required 21. Although there are currently no studies specifically considering alfaxalone in geriatric patients, there is evidence to suggest that baroreceptor response may be relatively preserved following administration of this drug 22. 

In patients with unstable cardiovascular disease etomidate is often considered to be the most appropriate induction agent as it results in minimal cardiovascular depression. However, a study in elderly human patients suggests a similar reduction in mean arterial pressure (MAP) and heart rate is seen when compared with propofol 23, and its use is associated with adrenal suppression. Ketamine may be an appropriate choice in some patients as it has the potential to increase cardiac output through sympatheticomimetic activity, but requires cautious dosing in the elderly as inefficient hepatic metabolism and renal clearance may prolong the duration of action. Its use in patients with increased intracranial pressure is, however, controversial and it may not be appropriate in animals with severe HCM or existing tachycardia.

Co-induction techniques involving the administration of a benzodiazepine in combination with propofol or alfaxalone have recently gained interest in small animal patients. These protocols are often discussed in the context of sick or geriatric patients because they have the potential to reduce the dose of induction agent required. Existing evidence, however, suggests that this dose reduction may not be accompanied by the expected improvement in cardiovascular and respiratory stability 24. 

Induction of anesthesia is a risk period for regurgitation and aspiration, and this is increased in the elderly as a result of reduced reflexes 12. Prompt intubation and appropriate inflation of the endotracheal tube cuff, with the patient’s head raised, can help minimize this risk (Figure 6). 

Maintenance of anesthesia

Both isoflurane and sevoflurane are appropriate inhalation agents for the maintenance of anesthesia in elderly patients as they are minimally arrhythmogenic, undergo little metabolism and offer short recovery times. Sevoflurane has a lower blood gas solubility compared with isoflurane, resulting in a more rapid change in anesthetic depth when compared with isoflurane, but there is little difference between the two in terms of their effects on the cardiovascular system, and both offer considerable benefits when compared with halothane. However, given that the MAC of these volatile agents is likely reduced in aged animals, particular attention should be paid to the plane of anesthesia.

Monitoring

Decreased functional reserve means that geriatric patients are less able to compensate for physiological changes induced by anesthesia. Careful monitoring throughout the perioperative period allows for prompt identification of cardiovascular or respiratory depression which could lead to potential complications, and appropriate interventions can therefore be implemented. Physical signs, blood pressure (via invasive or non-invasive means), capnography, peripheral oxygen saturation, echocardiography and body temperature all provide the clinician with useful information. Particular care may be required in animals with specific morbidity which might further reduce their ability to respond to cardiovascular depression and to those on medications which might exacerbate the detrimental effects of hypotension. For example, patients on long term NSAIDs, diuretics or angiotensin-converting-enzyme inhibitors are at a particularly high risk of renal damage. 

Patient warming

Thermoregulation is impaired during general anesthesia and significant reduction in core body temperature can lead to prolonged recovery, sympathetic stimulation, increased post-operative pain and delayed wound healing. Geriatric patients are at an increased risk of hypothermia as a result of altered fat distribution, decreased metabolism and a poor ability to thermoregulate. Maintaining adequate ambient temperature during surgery can help lessen the reduction in body temperature intraoperatively. Other management options include heat and moisture exchange (HME) filter devices, which can be placed between the endotracheal tube and the breathing system (Figure 7) that allow the patient to conserve heat by warming and humidifying inhaled air; warming blankets (Figure 8); minimizing the size of the clipped area; avoiding excessively wetting the coat when prepping for surgery; ensuring that only warmed fluids are administered intra-abdominally; and the use of incubators for small patients during recovery. 

Recovery

Recovery from anesthesia is a time of significant risk 2 and patients therefore require careful monitoring throughout. Concerns specific to geriatric patients include post-operative worsening of chronic pain from positioning under anesthesia, an increased likelihood of regurgitation, hypothermia, hypoglycemia and delayed recovery from injectable anesthetics (as a result of reduced tissue mass leading to increased plasma concentrations). Patients should be closely observed for and treated if regurgitation occurs. Careful positioning and supplying ample soft bedding for patients to lie on during recovery can minimize discomfort in those with a history of osteoarthritis or spinal pain (Figure 9). 

Hypothermia can lead to slower recovery from anesthesia, delayed wound healing and shivering. Severely hypothermic patients may also develop bradycardia which is unresponsive to anticholinergics such as atropine or glycopyrrolate. Shivering results in discomfort, catecholamine release and can lead to hypoxemia as the result of a significant increase in oxygen demand 25. In some cases where hypoxia is of particular concern it might be appropriate to rewarm patients prior to return of the shivering reflex and oxygen can be supplied to animals shivering in recovery.

In order to avoid hypoglycemia, elderly patients should be offered food and water as soon as it is safe to do so. A reduced sense of smell and disorientation in a hospital environment means they are often fussy and adverse to new foods, so asking the owner to provide some of their usual diet and warming the meal can encourage them to eat. 

Conclusion

Although the principles which apply to all anesthetics are the same for elderly patients, an awareness of the physiological changes associated with aging help the clinician to make decisions which optimize patient safety. Careful assessment of the individual can help identify clinical and subclinical pathology and allow the anesthetic plan to be tailored to the patient. Monitoring throughout the perioperative period allows significant cardiovascular and respiratory depression to be identified and corrected promptly in animals with reduced functional organ reserve and a poor ability to cope with physiological stress.