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Veterinarian Technician April 2009 (Vol 30, No 4)

Peer Reviewed — Anesthesia: Minimizing Risks in Senior Pets

by Rachael Lucero, RVT

    Anesthesia can be risky in senior dogs and cats, as many age-related changes can impair an animal's ability to compensate for anesthetic effects.

    Because there is greater focus today on preventive health care, dogs and cats are living longer. A decade ago, the average life expectancy for dogs and cats was 6 to 13 years and 7 to 12 years, respectively,1,2 and those life spans are constantly improving. However, large-breed dogs, such as the Great Dane and mastiff, continue to have shorter life expectancies than smaller-breed dogs, such as the Chihuahua and terriers. Therefore, defining geriatric can be difficult, but generally a geriatric animal is one that has reached 75% of its life expectancy.2 In addition, the terms geriatric and senior are often used interchangeably when describing animals that are 7 years of age and older.3

    Advanced age constitutes a discrete risk factor in animals that are undergoing anesthesia. However, thorough preparation and knowledge of organ functions can help minimize the likelihood of an undesirable reaction. When creating an anesthetic protocol for geriatric dogs and cats, therefore, it is important to remember that they have a decreased ability to compensate for changes both intrinsically and extrinsically.

    It is easy to note the common physical characteristics of aging, such as gray fur around the eyes and muzzle, stiff or slow movement, and decreased muscle mass. However, being able to appreciate the age-related impairment of major organs — the heart, lungs, liver, and kidneys — requires diagnostic screening, especially when considering that senior pets are often overweight. Therefore, blood and chemistry panels that test the function of major organs are recommended as part of the preanesthetic workup.

    Minimum laboratory baseline values for healthy-appearing senior pets (see box on laboratory database) are available from AAHA, as are the organization's guidelines for evaluating a sick pet.3 At a minimum for healthy senior pets, AAHA recommends conducting wellness screening tests, along with measuring electrolytes and a platelet count, ideally within 2 weeks preceding the surgical or anesthesia event. If the pet routinely has wellness testing, some preanesthetic tests may be unnecessary, with only a complete blood count and dipstick urinalysis with specific gravity being necessary. If the senior pet is sick, however, additional testing may be appropriate, such as coagulation function testing, free thyroxine testing, and serum protein electrophoresis.3,4

    In some instances, the results of recent blood work may be available, as senior pets are often being treated or monitored for chronic conditions, such as diabetes mellitus, a thyroid disorder, pancreatitis, or arthritis. Any medication that the patient is receiving, such as an NSAID, phenylpropanolamine, or selegiline, should be noted in the medical record to avoid any potentially harmful drug interactions. Obtaining a complete patient history and performing a thorough physical examination also are critical, as the results may directly affect the anesthesia event.

    Numerous considerations need to be addressed before a senior pet is anesthetized.3,4 Of importance are the following concerns regarding the function of aging body systems.

    Cardiovascular and Respiratory

    The geriatric heart has a decreased ability to compensate for stress and hemodynamic changes (e.g., hypovolemia, vasodilation),5 primarily because of a decreased cardiac reserve leading to a lower tolerance of volume depletion (blood loss) and changes in serum and tissue oxygen concentrations.2,4 Cardiac output also is decreased, becoming more dependent on preload as the afterload increases.6 Anticholinergics (Table 1) should be avoided unless absolutely necessary in the anesthesia plan, as an accelerated heart rate in the face of potentially greater afterload increases the myocardial workload, in turn raising myocardial oxygen consumption and demand. Administering an anticholinergic to a geriatric patient could, therefore, induce sinus tachycardia that leads to heart failure.1,4

    Aging also can affect cardiac pacemaking and conduction.5 If heart disease is suspected, thoracic radiography, which is a less-expensive alternative to echocardiography, should be requested to assess the presence and severity of heart disease. Evaluation of the cardiac silhouette and lung fields can be accomplished with left lateral and ventrodorsal views.7 Thoracic radiographs should always be taken at maximum inspiration. Electrocardiography (ECG) also can be requested if an arrhythmia is found during chest auscultation or to obtain a reference base.

    Common cardiac conditions affecting geriatric dogs and cats include congestive heart failure, cardiomegaly, dilated cardiomyopathy, hypertrophic cardiomyopathy, and mitral valve disease. Any cardiac disease should be stabilized as much as possible before anesthesia is administered. Patients with cardiovascular dysfunction are more prone to fluid overload, cardiac dysrhythmias, and heart failure induced by extremes in the heart rate.2

    When evaluating the respiratory system, it is important to recognize that intercostal and diaphragmatic muscles gradually lose elasticity with age, thereby decreasing compliance and conformity.8 Ventilatory volume also is lower, as is the proficiency of gas exchange,7 potentially leading to a ventilation-perfusion mismatch. Geriatric patients also may exhibit diminished response to and recovery from hypoxia and hypercarbia associated with an anesthesia event, predisposing them to apnea.6

    Hepatic

    The geriatric liver often exhibits decreased hepatic blood flow as well as a decrease in liver mass of up to 50%.2,4,5 As clotting factors are produced in the liver, evaluation of coagulability should be added to the preanesthetic workup if liver disease is suspected.2 Coagulopathies are more common in patients with liver disease than in any other scenario.2 Prothrombin time (PT) and activated partial thromboplastin time (PTT) can easily be assessed in-house, as can liver enzymes. Alanine aminotransferase (ALT) levels reflect the approximate number of damaged hepatic cells, and alkaline phosphatase (ALP; alk. phos.) values can be increased with steroid administration.8

    Liver dysfunction can decrease protein synthesis, thereby affecting the binding of drugs and leading to higher amounts of free drug in the circulation.4,8 Evaluation of serum albumin values can help determine the status of protein synthesis. Albumin influences the colloid osmotic pressure (COP), which keeps fluids within the intravascular space.2 Fluid overload may occur because of low albumin levels, necessitating a decrease in the rate of intravenous (IV) fluids or administration of plasma or colloids to help restore the COP.

    The liver is critical to drug metabolism, including many anesthetic agents. Excretion of drugs from the plasma depends on the volume of hepatic blood flow, thereby prolonging the half-life of drugs (e.g., opioids). It may be advisable to choose anesthetic drugs that do not require hepatic metabolism but rather rely on redistribution (e.g., propofol, ketamine).4,6

    Renal

    The geriatric renal mass can be decreased by up to 20%, and function is commonly weakened because of a decrease in renal blood flow and a consequent decrease in the glomerular filtration rate (GFR).4,6,8 A decline in the GFR can impair metabolism of drugs that require renal excretion. The kidneys' ability to overcome ischemic or nephrotoxic events also is affected.4,6 In senior pets, the kidneys are less able to compensate for hypovolemia and hypotension, which may predispose the patient to postanesthesia renal failure.6 Renal impairment should be suspected if the animal has a history of polyuria and polydipsia or if test results indicate increased blood urea nitrogen (BUN) and creatinine values, decreased urine specific gravity, or casts in the urine.

    Central Nervous System and Metabolism

    Enhanced sensitivity to anesthetic agents typically occurs in senior dogs and cats, thereby affecting the cardiovascular control centers in the central nervous system. This can lead to a decrease in the minimum alveolar concentration (MAC) of inhalant anesthetics.5

    Age-related blunting of thermoregulatory function4,5 and the presence of hypothyroidism2,4 also can hinder the geriatric patient's ability to normalize body temperature, especially during the recovery period.2,6 Care should be taken to adequately warm the patient peri- and postoperatively. A circulating warm-water blanket or a forced warm-air mover can be used for this purpose. Rectal temperature should be closely monitored to prevent the patient from overheating and to determine when external warming can be discontinued.

    Closing Remarks

    In general, geriatric patients are more likely than younger animals to have organ dysfunction. They also are less capable of compensating for the physiologic changes associated with anesthesia. The cardiovascular and renal systems are especially sensitive to these changes.

    Astute anesthetists will closely monitor the status of geriatric patients to help prevent complications — even long after anesthesia has concluded. Any existing disease should be stabilized as much as possible before anesthesia is induced.

    1. Hoskins JD, McCurnin DM. Geriatric care in the late 1990s. Vet Clin North Am Small Anim Pract 1997;27(6):1273-1284.

    2. Paddleford RR. Geriatrics. In: Manual of Small Animal Anesthesia and Analgesia, ed 2. Philadelphia: WB Saunders; 1999:274,287-288,291.

    3. Epstein M, Kuehn NF, Lansberg G. AAHA Senior Care Guidelines for Dogs and Cats. JAAHA 2005;41:81-91.

    4. Metzger FL. Senior and geriatric care programs for veterinarians. Vet Clin North Am Small Anim Pract 2005;35(3):743-753.

    5. Stubbs CJ. Providing the best care for senior cats. Vet Med 2006; 101(2):110-116.

    6. Mosley C, Gunkle C. Management of pediatric and geriatric anesthesia. Presented at: The Wild West Veterinary Conference; 2007.

    7. Smith FW Jr. Thoracic radiography of cardiac disease. Veterinary Information Network website. Accessed October 2008 at www.vin.com.

    8. Benson GJ, Thurman JC, Tranquili WJ. Geriatric patients. In: Essentials of Small Animal Anesthesia and Analgesia. Baltimore, Md.: Wiley-Blackwell; 1999:511-514.

    References »

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