CLINICAL ANESTHESIA 0195-5616/99 $8.00 + .00

MANAGEMENT OF GERIATRIC PATIENTS

A Common Occurrence

Ralph C. Harvey, DVM, MS, and Robert R. Paddleford, DVM

The geriatric patient is defined more by physiological and physical condition than by chronological age. As an approximation, it has been suggested that an animal is "geriatric" when it has reached 75% to 80% of its normal expected life span. Each patient must be evaluated as an individual, however, and many older animals remain remarkably fit, although others appear to age rapidly. The medical management of geriatric patients is a prominent part of veterinary care, and information on the specific needs of older dogs and cats is increasingly available.9

Veterinarians are seeing increasing numbers of geriatric small animal patients as the aging baby boomers in our human population care for the pets obtained when their families were young. It has been estimated that over $1.5 billion is spent annually in the United States on veterinary care of older dogs and cats? As these owners responsibly care for their pets and as we provide more skilled medical care, the life expectancy of dogs and cats can be substantially increased.

PHYSIOLOGICAL CHANGES ASSOCIATED WITH AGING

Although every patient varies with regard to the geriatric changes present, there are some general considerations with regard to altered

From the Department of Small Animal Clinical Sciences, University of Tennessee College of Veterinary Medicine, Knoxville, Tennessee

VETERINARY CLINICS OF NORTH AMERICA: SMALL ANIMAL PRACTICE

VOLUME 29 • NUMBER 3 • MAY 1999 683

684 HARVEY & PADDLEFORD

physiology during aging. The geriatric patient is likely to have a higher incidence of disease and organ dysfunction than a younger patient. Less obvious is the reduction in functional organ reserve capacity relative to that of the younger patient. This decreased organ reserve, termed geriat­ric or elderly normal, may not be apparent until the patient is stressed by disease, hospitalization, anesthesia, or surgery, at which time overt organ failure may occur. Much of the knowledge regarding geriatric changes is based on observations in human patients. It is expected that similar aging changes apply to veterinary patients.

Cardiovascular Changes

There is a reduction in cardiac reserve capacity in geriatric patients compared with that in younger patients. In the geriatric patient, this may limit the ability to compensate for cardiovascular changes that occur during anesthesia. Geriatric patients have decreased baroreceptor activity, blood volume, cardiac output, blood pressure, circulation time, and vagotonia.5 The conduction system of the heart is also affected by aging. In human geriatric patients, left bundle branch block, intraventric­ular conduction delay, ST segment and T wave changes, and atrial fibrillation are commonly noted.6 These may make the older patient more prone to anesthetic-induced cardiac dysrhythmias.

Progressive and degenerative myocardial disease is increasingly likely in geriatric patients. This is usually associated with chronic valvu­lar disease, which can lead to increased myocardial work and oxygen consumption and can demand and make the myocardium extremely sensitive to hypoxia. Geriatric patients often develop thickened elastic fibers and an increase in collagen and calcium in the walls of large arteries, which results in increased peripheral resistance. The ability of the geriatric patient's cardiovascular system to adapt to hypotension is limited, and autoregulation is decreased.20

Pulmonary Changes

Respiratory function progressively diminishes as a patient ages, resulting in a decreased functional reserve. There is a loss of strength in the muscles of respiration, chest wall compliance decreases, and there is a loss of elastic recoil.12 These changes result in an increase in small airway closure, which produces a decrease in vital capacity and an increase in residual volume.17 There is a decrease in lung elasticity, respiratory rate, tidal volume, minute volume, oxygen consumption, carbon dioxide production, maximal diffusion capacity for oxygen capil­lary blood volume, and protective airway reflexes in the aged patient.17, 25

With an age-related increase in closing volume, greater portions of tidal ventilation occur at lung volumes below closing volume, causing air trapping, worsened ventilation/perfusion mismatch, and decreased rest-

MANAGEMENT OF GERIATRIC PATIENTS 685

ing partial pressure of oxygen, arterial.16 A decrease in protective pharyn­geal and laryngeal airway reflexes puts the geriatric patient at increased risk of aspiration in the perioperative period.21 Anatomical dead space and functional residual capacity are increased with ageP Furthermore, human geriatric patients have a marked reduction in their ability to respond to induced hypercarbia and hypoxia.13

Geriatric changes in the respiratory system are significant in that even mild to moderate respiratory depression resulting from anesthesia may produce marked hypoxia and hypercarbia, and any pathological disease of the respiratory system (e.g., pneumonia, edema, pulmonary fibrosis) is greatly exacerbated in the geriatric patient. What is considered mild anesthetic depression in the young patient may be disaster in the aged patient.

Hepatic System

With increasing age, there is a reduction in the functional state of the hepatic microsomal enzyme system. This reduction is present even when the standard biochemical function tests are normal.10 Geriatric patients often have decreased hepatic blood flow, primarily as a result of decreased cardiac output. The plasma half-life of drugs dependent on hepatic excretion, metabolism, or conjugation is often thereby increased in the geriatric patient. Altered hepatic function in the aged patient also may lead to hypoproteinemia, impaired clotting functions, and a greater susceptibility to hypoglycemia.

Renal System

Renal function may be greatly impaired in geriatric animals. This impaired renal function may have marked significance, as the kidney is the major effective organ for fluid and electrolyte balance. Separate from disease processes, normal aging alters renal function in several ways.15

Renal blood flow is decreased, which is most likely a result of reduced cardiac output. There is a decrease in the total number of glomeruli to one half to two thirds of that in young patients, and the nephron mass is reduced. Tubular changes occur, including atrophy, decreased tubular diameter, tubular disruption, and tubular hypertrophy. The glomerular filtration rate may be reduced by 50%, and there is a decreased ability to concentrate urine and excrete hydrogen ion because of decreased function of the distal tubules. The urine volume necessary for excretion of the obligatory solute load increases. Antidiuretic hormone increases with aging due to increased resistance of the distal renal tubules to its effects.5 This change can result in impaired renal concentrating ability.

The result of these changes is a diminished functional renal reserve, which makes the geriatric patient much less tolerant of body water deficits or excessive administration of fluids. Older patients have a

686 HARVEY & PADDLEFORD

decreased capacity to excrete certain drugs and are more prone to acidosis, thus prolonging the plasma half-life of drugs dependent on renal excretion.

The effects of anesthesia and surgery on the kidney can be greatly exacerbated in the geriatric patient. Anesthesia and surgery commonly cause increased activity of the sympathetic nervous system and the renin-angiotensin system, resulting in decreased total renal blood flow and a redistribution of intrarenal blood flow away from the renal cortex. General anesthesia may decrease renal blood flow and glomerular filtra­tion by up to 40%. Anesthesia and surgery also may cause hypovolemia, hypotension, hypoxia, and hypercarbia, all of which compound any preexisting renal dysfunction. These factors contribute to make the geri­atric patient much more susceptible to renal failure following anesthesia and surgery.

Central Nervous System

Aging produces changes in the central nervous system (CNS) in human and animal patients. There are alterations in cognitive, sensory, motor, and autonomic functions. Cerebral perfusion and oxygen con­sumption decline. Aging is also associated with a reduction in brain weight, which is most likely a result of individual neuron degeneration.l4

The loss of brain weight is most evident in the cerebral cortex and the cerebellar cortex.4 Myelin sheaths also degenerate in the aged patient.

There is strong evidence that neurotransmitters change with age, with increased destruction and decreased production of neurotransmit­ters in the geriatric patient. The reduction in neurotransmitter function may be a result of the decreased quantity of neurotransmitters or a change in the receptors themselves.

The overall effects of these CNS alterations on anesthetic manage­ment of the geriatric patient are not fully understood. The effects of local anesthetic agents seem to be enhanced in the aged patient, and the effects of neuromuscular blocking agents are prolonged. In addition, thermoregulatory center function is decreased, making the geriatric pa­tient more susceptible to anesthesia-induced hypothermia.

Autonomic Nervous System

The autonomic nervous system loses some of its ability to respond to stress in the human geriatric patient. This appears to be most marked in the sympathetic nervous system. The aged patient has decreased vasoconstrictor responses and a decreased response to decreased cardiac preload.3 Despite this reduced response, there appears to be an enhanced response to iatrogenic epinephrine and norepinephrine, thus leading to the conclusion that perhaps the autonomic nervous system output is decreased but the receptors are more sensitive.

MANAGEMENT OF GERIATRIC PATIENTS 687

Endocrine System

Although aging does not appear to alter corticotrophin-stimulated plasma cortisol levels, there are reports that human geriatric patients may not have the adrenal gland reserves necessary to protect themselves adequately during stress from anesthesia and surgery.14 Renin and aldo­sterone hormones, which are necessary for water, sodium, and potassium balance as well as blood pressure control, are attenuated when the human geriatric patient is stressed26; thus, the geriatric patient is more susceptible to hypotension.

Various endocrine diseases are more common in the aged patient, and such diseases contribute to cardiac dysfunction, impaired thermo­genesis and ventilation, impaired metabolic elimination of anesthetics, and delayed recovery from anesthesia.

PHARMACOKINETICS

Pharmacokinetics is the study of drug disposition in a patient. The most important age-related changes in the distribution of anesthetic and preanesthetic drugs in the geriatric patient are in the disposition (partitioning within the various distribution volumes) and clearance (excretion, metabolism, or conjugation) of drugs. The slight reduction in plasma albumin concentrations in aged patients is most likely not suffi­cient to produce clinically important changes in the amount of active or unbound drug in plasma after intravenous (IV) drug administration. The aging processes probably do not substantially alter the parenteral uptake of drugs. 19

The physical characteristics of drug molecules make them either lipophilic or hydrophilic and thus determine their ultimate partitioning between lipid tissues (brain, adipose tissue, viscera) and aqueous body compartments (blood, extracellular fluid, skeletal muscle). As a patient ages, there is a progressive decrease in the absolute volume of the aqueous fraction and an increase in the lipid compartments.24 Therefore, for hydrophilic drugs, the decrease in the aqueous fraction of the body of an aged patient could produce higher than expected initial plasma levels after IV administration of recommended doses even when the dose has been adjusted for total body weight. The net pharmacokinetic effect is an enhancement of drug potency.

Because lipophilic drugs undergo some initial distribution to highly perfused, nonadipose, lipid tissues, the effect of the decreased aqueous fraction of the body is offset. Because of the significant increase in total body lipid into which lipophilic drug molecules are ultimately distributed, however, the elimination process is markedly delayed in the geriatric patient-23

Age also alters drug disposition or pharmacokinetics by signifi­cantly reducing both renal and hepatic function and thus decreasing the

688 HARVEY & PADDLEFORD

clearance of drugs eliminated through these pathways, regardless of whether a drug is lipophilic or hydrophilic.

PHARMACODYNAMICS

Pharmacodynamics, or the relationship between drug quantity and drug effect, is frequently altered in the geriatric patient. Geriatric patients have decreased circulating blood volume, which can produce high initial plasma concentrations of anesthetic drugs. The many observations of the need for reduced doses of anesthetic agents in geriatric patients cannot be explained by this pharmacokinetic effect, however.2 Human studies involving tranquilizer and sedative drugs confirm that geriatric patients achieve a given drug effect at plasma concentrations that are significantly lower than those required for young adults.8 In addition, the minimum alveolar concentration of volatile anesthetic agents is less in aged patients than in young adults.22 The mechanism by which age reduces the need for anesthetic agents is unclear, but the decreased anesthetic and analgesic requirements in the aged patient1, 18 are corre­lated with the reduction in brain mass, the decrease in cerebral blood flow, and the decrease in the number and density of neurons and axons in both the CNS and peripheral nervous system. These decreases and reductions occur in geriatric patients even in the absence of disease.

Other factors may play a significant role in reducing anesthetic requirements in the geriatric patient. Alterations in neurotransmitter activity have already been discussed, but other factors may also play a role. It may be that this decreased anesthetic requirement is a result of decreased functional reserve of the nervous system rather than decreased nervous system function.

Regardless of the reasons for the pharmacokinetic and pharmacody­namic alterations of anesthetic agents observed in the geriatric patient, each patient must be examined on an individual basis, because no reliable universal guidelines are available. Some of the clinical conse­quences of these alterations are predictable, but they do not apply consistently to all of the preanesthetic and anesthetic agents used.

ANESTHETIC CONSIDERATIONS

Any geriatric dog or cat presented for anesthesia should be consid­ered on an individual basis. Each geriatric patient has specific physiolog­ical alterations or diseases unique to that individual. Thus, the anesthetic protocol needed for one aged patient may be quite different from that needed for another aged patient.

As is the case with any patient, a thorough and complete preanes­thetic examination should be done. A complete history should be taken, with special emphasis on present and past medical or surgical problems and any current medications the patient is receiving. Any previous

MANAGEMENT OF GERIATRIC PATIENTS 689

anesthetic experience the patient has had should be noted, and close attention should be paid to any anesthetic complications or abnormal responses that occurred. A thorough preanesthetic physical examination should be performed to determine physiological baseline values for future monitoring as well as to help in ascertaining whether the patient has any existing pathological conditions. A complete blood cell count and serum biochemistry profile, with special emphasis on renal and hepatic function and electrolyte balance, should be obtained. Thoracic radiographs and a baseline electrocardiogram should be considered. Any abnormal preanesthetic findings should be thoroughly evaluated, and delaying the anesthesia and surgery should be considered to address the potential problem areas identified.

Preanesthetic Drugs

The preanesthetic medications used in a particular geriatric patient depend on that individual's physical condition, the amount of sedation or analgesia required, and the experience and preference of the veterinar­ian. Commonly used preanesthetic medications include anticholinergic agents, tranquilizers/sedatives, and opioid analgesics (Table 1). Individ­ual drugs as well as drug classes are described in detail elsewhere.

Anticholinergic Agents

Anticholinergic agents are used to decrease respiratory secretions and to counteract sinus bradycardia. The two drugs commonly used are atropine and glycopyrrolate.

The indiscriminant use of anticholinergics should be avoided in the geriatric patient to prevent the occurrence of unwanted and potentially dangerous sinus tachycardia. The heart of the geriatric patient may not be able to tolerate the increased oxygen demand and consumption needed to support a dramatically increased heart rate. Tachycardia in the geriatric patient may thus precipitate acute myocardial hypoxia and failure.

If sinus bradycardia occurs, an anticholinergic may be administered to effect, with careful titration to avoid tachycardia. If potent vagotonic drugs such as some of the opioids like fentanyl or morphine are to be given, an anticholinergic may be warranted. Half of the normal dose of an anticholinergic may be given by intramuscular or subcutaneous injec­tion as a preanesthetic, with additional IV anticholinergic given to effect if needed.

Tranquilizers/Sedatives

There is often less need for substantial sedation or tranquilization as patients age. The excitement of the youngster and the vigilance of the young adult give way to the calmer, and more often tractable, demeanor

C'l 'J:) 0

Table 1. SUGGESTED DOSAGES FOR PREANESTHETIC MEDICATIONS IN THE GERIATRIC DOG AND CAT

Drug

Anticholinergics Atropine sulfate Glycopyrrolate (Robinul-V)

Tranquilizers/sedatives Acetyl promazine

(Acepromazine)

0.22 mg/kg IM, SC 0.01 mg/kg IM, SC

Dog

Dose

0.01-0.05 mg/kg IM, SC, IV; maximum total dose of 1 mg

Diazepam (Valium) 0.2-0.4 mg/kg slowly IV; maximum total dose of 10 mg Midazolarn (Versed) 0.1-0.3 mg/kg IM, SC, IV

Narcotic analgesics (agonists/antagonists) Butorphanol (Torbugesic) 0.1-0.4 mg/kg IM, SC, IV

Buprenorphine (Buprenex) Narcotic analgesics (agonists)

Morphine Meperidine Oxymorphone

0.005-0.01 mg/kg IM, SC, IV

0.05-1 mg/kg SC, IM 0.5-3 mg/kg IM, SC, IV 0.1-0.2 mg/kg IM, SC, IV; maximum total dose of 4 mg

1M = intramuscularly; SC = subcutaneously; IV = intravenously.

Same Same

Same

Same Same

Same

Same

Cat

0.002-0.1 mg/kg SC, IM Same 0.05-0.1 mg/kg IM, SC,

IV

Duration of Action (h)

1-1.5 2-3

3-6

0.5-3 <2

1 (dog) 2-4 (cat) 6-8

3-6 1-2 2-4

MANAGEMENT OF GERIATRIC PATIENTS 691

of the aged patient. Reduction of the stresses associated with hospitaliza­tion, procedures, and anesthesia and surgery may, however, be desirable. The judicious application of tranquilizers or sedatives is often appro­priate in geriatric patients, but careful selection, conservative dosing, and close attention to detail are mandatory. Pharmacodynamic changes associated with aging suggest a general reduction in the dose of agents that depress the CNS.

For the healthy geriatric patient, low-dose acetylpromazine (Acepro­mazine) is a reasonable choice for a preanesthetic. The circulatory actions of acetylpromazine are significant and could be potentially deleterious to the compromised geriatric patient; Yet, at low doses and with appro­priate physiological support, this tranquilizer can be quite useful in aged animals. Remarkably low doses of acetylpromazine can be effective and adequate in the geriatric patient, particularly if the tranquilizer is combined with an opioid analgesic. Volume expansion through IV fluid support minimizes the hypotensive action of acetylpromazine and helps to promote metabolic elimination. Maintenance of acceptable blood pres­sure is verified by monitoring during anesthesia. The use of acetylproma­zine can contribute to undesirably prolonged recovery from anesthesia, particularly in patients with limited hepatic function.

The benzodiazepine "minor tranquilizers" diazepam (Valium) and midazolam (Versed) are more often recommended for geriatric patients than is acetylpromazine. The mild tranquilizing action is reinforced in combination with opioids. The relatively minimal cardiopulmonary depression produced by the benzodiazepines and the lack of severe sedative effects make this drug class appropriate for many geriatric patients. Prolonged mild CNS depression and disorientation can occur infrequently with the benzodiazepines. This is more likely in aged ani­mals, including those with decreased liver function. A specific benzodi­azepine antagonist, flumazenil, may be useful in counteracting residual CNS effects when they do occur or to differentiate between residual benzodiazepine effect and other CNS deficits. The benzodiazepines are also useful for selected geriatric patients in combination with the dissoci­ative anesthetic ketamine.

The alpha2 agonist sedatives xylazine (Rompun, Anased) and mede­tomidine (Domitor) should be used only with great caution in geriatric patients. As useful as this group of drugs can be in younger healthy patients, the substantial circulatory side effects of the alpha2 agonists preclude their use in most geriatric animals. When these drugs are chosen, the availability of specific antagonists to reverse the circulatory, respiratory, and CNS effects does add considerably to the safety of the technique.

Opioid (Narcotic) Analgesics

Opioids are often used alone or in combination with tranquilizers in geriatric patients as preanesthetic medications. Various opioid ago­nists have been used, including morphine, oxymorphone, meperidine

692 HARVEY & PADDLEFORD

(Demerol), and fentanyl (Sublimaze). The opioid agonists/antagonists (e.g., butorphanol, buprenorphine, nalbuphine, pentazocine) also have been used.

The advantages of opioids for many other patients can be particu­larly valuable in geriatric animals. As a group, the opioids produce effective analgesia and sedation with minimal or no direct cardiac de­pression. Bradycardia can be induced by some opioids (notably, mor­phine and fentanyl), but this is a vagal reflex that can be prevented with the use of an anticholinergic if deemed necessary. A reduction in heart rate may be desirable for many aged patients to reduce myocardial oxygen demand and consumption.

The respiratory depressant actions of opioid analgesics are rarely of clinical significance in young healthy patients. Geriatric patients, how­ever, may be at increased risk of any respiratory depression. Additional attention to respiratory function and the potential for depression is required for geriatric patients. The risk of respiratory side effects does not minimize the value of this group of drugs for older animals.

The "ceiling effect" on respiratory depression produced by the partial agonist opioid butorphanol (Torbugesic, Torbutrol, Stadol) has increased the usefulness and popularity of this opioid for geriatric pa­tients. The relatively limited analgesic efficacy of butorphanol is man­aged by the additional use of complementary analgesic strategies. For example, butorphanol may be used to provide a portion of the desired analgesic action along with the concomitant use of a local anesthetic or regional analgesic as part of a "balanced analgesic" approach. Alterna­tively, a pure agonist opioid analgesic such as morphine, oxymorphone, or fentanyl may be used when greater analgesic efficacy is required.

Neuroleptanalgesic Combinations

Combinations of tranquilizers and opioids, neuroleptanalgesic com­binations (Table 2), can be effective in geriatric patients. The pharmaco­logical effects of a neuroleptanalgesic depend on the tranquilizer and

Table 2. NEUROLEPTANALGESIC COMBINATIONS FOR THE GERIATRIC DOG AND CAT

Neuroleptic (Tranquilizer)

Acetylpromazine, 0.01-0.05 mg/kg IM, SC, or IV; not to exceed a maximum total dose of 1 mg

Diazepam, 0.2-0.4 mg/kg slowly IV to a maximum total dose of 10 mg

Acetylpromazine (same as above) Acetylpromazine (same as above)

Diazepam (same as above)

Analgesic (Opioid)

Oxymorphone, 0.1-0.2 mg/kg IM, SC, or IV; total dose of 3 mg

Oxymorphone (same as above)

Meperidine, 1-4.5 mg/kg IM or SC Butorphanol, 0.1-0.4 mg/kg IM, SC,

or IV Butorphanol (same as above)

1M = intramuscularly; SC = subcutaneously; IV = intravenously.

MANAGEMENT OF GERIATRIC PATIENTS 693

opioid used. Neuroleptanalgesics used alone or in combination with local anesthetics may be all that is needed for minor diagnostic or surgical procedures in the geriatric patient. As is the case with any patient, cardiopulmonary function should be closely monitored when neuroleptanalgesics are used, as these drugs may produce respiratory or hemodynamic depression ranging from slight to significant.

Injectable General Anesthetic Agents

Injectable general anesthetic agents can be used in the geriatric patient, but they should be used with care because of the often altered hemodynamics, pharmacokinetics, and pharmacodynamics; decreased plasma protein binding; and decreased ability for hepatic metabolism and renal excretion in the aged patient (Table 3).

Ultrashort-Acting Barbiturates

The ultrashort-acting barbiturates (thiopental and methohexital) can be used to induce anesthesia and for short surgical procedures in some healthy geriatric patients. The lowest possible dose necessary to produce the desired effect should be carefully used. A relative overdose is easily produced, and the margin of safety with this class of anesthetics is slight. The normal aging changes increase multiple risk factors applicable to the use of the barbiturate anesthetics. Although initial arousal from an ultra-short-acting thiobarbiturate is dependent on redistribution of the

Table 3. SUMMARY OF COMPARATIVE PHARMACOLOGICAL PROPERTIES OF INTRAVENOUS INDUCTION AGENTS

Properties Thiopental Etomidate Telazol Propofol

Solubility Water Propylene Water Egg glycol lecithin

Dose (mg/kg) 8-12 (IV) 1-3 (IV) 4-8 (IM) 2-6 (IV) 1-2 (IV)

Onset Rapid Rapid Rapid Rapid Induction Smooth Pain/ Excitatory/ Smooth/

myoclonus smooth pain Cardiovascular Depression Minimal Stimulation Depression Respiratory Depression Minimal Minimal/ Depression

moderate depression

Analgesia None None Superficial (yes) None Deep (visceral)?

Amnesia Minimal Minimal Minimal Minimal Recovery Rapid Quite rapid Intermediate/ Quite

disorientation rapid/ smooth

IV = intravenously; IM = intramuscularly.

694 HARVEY & PADDLEFORD

drug into the various body compartments, ultimately, the drug must be metabolized in the liver to be excreted. If the patient receives an over­dose of a thiobarbiturate or redistribution is hindered by a variety of aging changes, hepatic degradation becomes a prominent pathway for arousal and awakening. Because geriatric dogs and cats do not readily metabolize the thiobarbiturates, prolonged recovery times (6-24 hours) may be observed. If hepatic dysfunction is also evident in the patient, the recovery time from a barbiturate may be even longer if tissue redistribution is altered.

Because of the ability of the barbiturates to depress the cardiovascu­lar system and especially the respiratory system, these anesthetics should be used with extreme care in geriatric patients that may already have cardiopulmonary compromise. In addition, the geriatric patient may have decreased plasma protein-binding capabilities, decreased total body water, hepatic dysfunction, and an increase in the total body lipid into which the thiobarbiturates are ultimately distributed. These factors may cause a marked increase in the physiological effect and duration of action of a given dose of barbiturate.

Dissociative Anesthetic Agents

Ketamine and the dissociate/tranquilizer combination Telazol (tilet­amine/zolazepam) should be used with caution in geriatric dogs and cats with preexisting cardiovascular or pulmonary dysfunction. As is the case with the anticholinergic preanesthetic medications, the sinus tachycardia produced by the dissociative anesthetics may be disadvanta­geous in the geriatric patient because of the marked increase in oxygen demand and consumption. The geriatric patient may not have the car­diac reserve to withstand this increased heart rate. The dissociative agents may also exacerbate any preexisting pulmonary dysfunction and further compromise the patient. Patients with renal or hepatic dysfunc­tion may be expected to have prolonged recovery times. Elimination and recovery from the dissociatives require hepatic metabolism, more so in dogs than in cats. In geriatric patients with impaired liver function, conservative administration of the dissociatives, perhaps supplemented with the use of opioids or other CNS depressants, is recommended. Recovery from dissociative anesthetics can be accompanied by delirium. Administration of a low dose of diazepam (0.1 mg/kg IV) is usually quite effective in smoothing an otherwise rough recovery.

Propofol (PropFio, Diprivan, Rapinovet)

Propofol is well suited for induction of anesthesia in many geriatric patients, as recoveries are consistently rapid and complete. Because its cardiopulmonary depressant effects are similar to those of the thiobarbi­turates, propofol should be used with caution in any geriatric patient with preexisting cardiopulmonary disease or dysfunction. Recovery is rapid following even repeated doses of propofol. It is rapidly redistrib-

MANAGEMENT OF GERIATRIC PATIENTS 695

uted and rapidly metabolized. Residual drug effects are minimal with propofol. There does not appear to be a problem of drug buildup with propofol as compared to barbiturates or dissociative agents when multiple doses or infusions are given. As described elsewhere, multiple doses or infusions of propofol in cats should be limited, as phenols are poorly conjugated and toxicities are noted.

Etomidate (Amidate)

Etomidate is a valuable injectable anesthetic for geriatric patients with clinically significant cardiovascular compromise. The agent pro­duces a rapid induction of anesthesia and a rapid and usually smooth recovery. Occasionally, there are problems with excitement during induc­tion or recovery from etomidate, but this seems less common in geriatric patients and is largely avoided by use of mild preanesthetic sedation. Administration of a small dose of diazepam (0.1 mg/kg IV) is effective in controlling this side effect. The rapid hepatic hydrolysis of etomidate allows for rapid recovery even when multiple doses or infusions of etomidate are used.

At the dose used to produce general anesthesia in dogs and cats (1.0-3.0 mg/kg IV), etomidate produces no change in heart rate, cardiac output, or mean arterial blood pressure. Cardiovascular stability is re­markable with etomidate, in part, because it better maintains barorecep­tor-mediated responses. Etomidate has been demonstrated to temporar­ily inhibit adrenal steroidogenesis in humans and dogs. Whether or not this inhibition is significant, and if so, whether it is detrimental or beneficial to the patient is controversial. Etomidate does produce a mild to moderate dose-dependent respiratory depression.

Inhalant General Anesthetic Agents

Inhalant general anesthetics are probably the anesthetics of choice in the geriatric patient, especially for procedures lasting longer than 10 to 15 minutes and for those in the debilitated patient. The newer inhalants isoflurane (Aerrane), sevoflurane (Ultane), and desflurane (Suprane) of­fer the considerable advantage for geriatric patients of rapid to extremely rapid induction and recovery. Isoflurane has become well established in veterinary anesthesia for use in higher risk patients, including the rela­tively compromised geriatric patient.

Methoxyflurane

Methoxyflurane has limited safety in geriatric patients, particularly those with limited renal function. It may compromise renal function by decreasing renal blood flow, and it has also been shown to produce direct renal toxicity in humans through its metabolic byproducts, primar­ily oxalic acid and inorganic fluoride ion. The renal toxicity produced

696 HARVEY & PADDLEFORD

by methoxyflurane is characterized by proximal renal tubular necrosis with a high-output renal failure, hypematremia, elevated blood urea nitrogen, and dehydration. Although there are no specific contraindica­tions to the use of methoxyflurane, it should be used with caution in the aged patient, particularly if there is renal or hepatic dysfunction.

Halothane

Halothane has been used successfully in many millions of geriatric patients, but there are better inhalants currently available for most of the higher risk patients. Halothane should be used with extreme caution in those patients with cardiac conduction problems or other dysrhyth­mias and in those patients with myocardial insufficiencies. The decrease in cardiac contractility produced by halothane may provide an advan­tage in feline hypertrophic cardiomyopathies.10 Halothane has been im­plicated as a possible cause of postanesthetic hepatitis in human patients. Although the phenomenon is extremely rare even in human patients, chronic liver dysfunction in a geriatric dog or cat is a relative contraindi­cation to the use of halothane.

lsoflurane

Heart rhythm is extremely stable with isoflurane, because unlike halothane, this inhalant does not seem to sensitize the myocardium to catecholamines, and it does not slow the conduction of impulses through the His-Perkinje system. Isoflurane depresses respiration in a dose-re­lated fashion and is a slightly stronger depressant than methoxyflurane or halothane. It does not appear to produce any liver damage even when used for long procedures or during hypoxia. Like the other inhalant anesthetics, it does produce a transitory decrease in renal blood flow, glomerular filtration rate, and urine flow, although no direct renal toxic­ity has been reported. There are no major precautions or contraindica­tions to the use of isoflurane in geriatric patients, and it is probably the volatile inhalant anesthetic of choice for use in the aged animal.

Sevoflurane and Desflurane

These two newer volatile anesthetics provide for an extremely rapid inhalant induction of anesthesia and an extremely rapid recovery. Due to the relative lack of pungency and insolubility of sevoflurane, this inhalant is an excellent choice for inhalant induction in geriatric patients. The more rapid and smooth inhalant induction minimizes the stress associated with the procedure and minimizes the delay in control of the patient's airway. Recovery from these two inhalants is also quite rapid. In order to provide for a smooth and comfortable recovery in geriatric patients, it is necessary to use appropriate clinical finesse and analgesics as well as judicious use of tranquilizers.

MANAGEMENT OF GERIATRIC PATIENTS 697

Nitrous Oxide

Because nitrous oxide has limited anesthetic potency, it must be combined with other CNS depressant drugs or anesthetics for mainte­nance of anesthesia or for inhalant induction of anesthesia. Nitrous oxide has minimal effects on cardiopulmonary function unless hypoxia occurs, and it has no appreciable effects on other organ systems. Nitrous oxide can be used with the more potent inhalant anesthetics during mask induction to induce anesthesia more rapidly in the patient owing to a concentrating or "second gas effect" of the nitrous oxide. The use of nitrous oxide as a component of "balanced anesthesia" allows for reduc­tion of the doses required of other CNS depressants; thereby, the poten­tial toxicity of each agent used may be minimized. In the geriatric patient at risk of specific toxic side effects, the use of nitrous oxide may be warranted. The increased safety afforded by the newer inhaled anesthe­tics, particularly isoflurane, has reduced the relative benefits to be de­rived from the addition of nitrous oxide, however. In geriatric patients with limited oxygenation or those at particular risk of hypoxemia, ni­trous oxide may be relatively contraindicated.

MISCELLANEOUS CONSIDERATIONS

Regardless of the anesthetic techniques used in a particular geriatric patient, certain protocols should be incorporated. Geriatric patient care requires increased vigilance in anesthetic and perioperative monitoring and physiological support. The limited reserve of the aged patient in­creases susceptibility to adverse outcomes, which must be prevented when possible and recognized early when they occur.

Changes in daily routine are less well tolerated in older animals. The lack of restful sleep increases fatigue and may lead to a negative outcome through subtle interference with immune function, nutrition, and hydration, as well as the neuroendocrine response to physiological stressors. Muscle weakness in the older pet complicates recovery, partic­ularly from the trauma of surgical experiences.

For geriatric patients, it is useful to select anesthetics that (1) typi­cally provide for a rapid and complete recovery (propofol), (2) can be fully reversed (opioids), (3) can be totally eliminated by supported ventilation (isoflurane, sevoflurane, desflurane), or (4) have neither sub­stantial intrinsic toxicity nor significant adverse effects should the drug effects persist (glycopyrrolate, diazepam, midazolam, butorphanol). In situations where inadequate metabolism or elimination is recognized, physiological support, including judicious fluid therapy, support of body temperature, and ventilatory support, should be provided as indicated.

Geriatric patients should be preoxygenated for 2 to 5 minutes before anesthetic induction to help prevent hypoxia from developing during induction. The patient should be intubated when a general anesthetic is used to provide a patent airway. The safety that often has been associ-

698 HARVEY & PADDLEFORD

ated with inhalants as opposed to injectable anesthetics is, in part, due to the customary if not obligatory provision of supplemental oxygen as the carrier gas for the volatile anesthetics. Endotracheal intubation and administration of supplemental oxygen can easily be incorporated into injectable general anesthetic techniques and can add substantially to patient safety. If anesthesia is deep enough to allow for placement of an endotracheal tube, the patient is no longer able to protect its airway from either obstruction or aspiration of regurgitated or foreign material. Practically any time that an endotracheal tube can be placed, one is needed. Although not all anesthetized animals require supplemental oxygen administration, the geriatric patient and many other higher risk patients should be considered for this level of support during anesthesia and surgery.

Adequate fluid replacement should be given to prevent a renal crisis and to help maintain a proper hemodynamic state in the geriatric pa­tient. The specific fluid used is dictated by the particular patient's needs; however, in most situations, a balanced electrolyte solution such as lactated Ringer's or Normosol-R solution (Abbott Laboratories, North Chicago, IL) is a reasonable choice. Because hypoglycemia during and after anesthesia can be a problem in the geriatric patient, administering fluids containing glucose may be warranted. The rate of IV fluid admin­istration depends on the particular patient's needs but is generally in the range of 5 to 10 mL/kg/h. Obviously, the rate is decreased in a geriatric patient, where the risk of hypoproteinemia or cardiovascular overload with the subsequent development of pulmonary edema is a concern. Fluid therapy may need to be continued for several hours to several days following anesthesia and surgery.

Close monitoring of cardiovascular and respiratory parameters is essential. If necessary, the geriatric patient's hemodynamics and venti­lation should be supported. Methods should be used to prevent or decrease hypothermia during and after the surgical procedure. The intra­operative monitoring techniques should be continued into the postopera­tive period or until the geriatric patient has returned to a preanesthe­tized state.

References

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Address reprint requests to

Ralph C. Harvey, DVM, MS Department of Small Animal Clinical Sciences

University of Tennessee College of Veterinary Medicine PO Box 1071

Knoxville, TN 37901-1071