Age remains pertinent to anesthetic management

There is no universally accepted definition of old age and as we are all aware, some people and animals age better than others. Because of advances in veterinary medicine, the average lifespan of cats and dogs has increased and according to recent AVMA statistics, about 30 percent of the owned pet population in the United States is considered geriatric (Wise et al, 2002). As senior care becomes a significant component of companion animal practice, we must be aware of the special anesthetic requirements of this population.

First, we should define what is meant by geriatric, since age in itself is not a disease, and physiologic status does not always correlate with chronologic age. Dogs of different breeds and sizes have different life expectancies whereas cats tend to live to a more uniform age. A good working assumption is that if the dog or cat has reached or exceeded 75 percent of its predicted life span, it should be considered a senior patient. Age-related changes are pertinent to anesthetic management. However, the animal's physiological state, the presence of concurrent diseases and use of chronic medications is what ultimately determines the choice of anesthetic protocol. Many age-related changes that impact anesthetic management have been documented in humans and although we lack similar information, we must assume that similar changes occur in our patients. Dental disease is one of the most common disorders seen in veterinary practice. In one large survey (Lund et al, 1999) the overall prevalence of dental calculus and gingivitis in dogs was 20.5 percent and 19.5 percent respectively, and it was 24.2 percent and 13.1 percent in cats. Therefore it follows that many of us will treat these problems in older animals.

As senior populations become more abundant, so should the understanding of anesthetic requirements.

Until recently the anesthetic risks related to age was unknown. A large prospective study of perioperative fatalities, which included 98,000 dogs and 80,000 cats, was completed recently by Dr. David Brodbelt at the Royal Veterinary College in London (personal communication). Regardless of physical status, both dogs and cats more than 12 years of age had a significantly higher risk of anesthetic death; in dogs, this risk was 10 times that of dogs aged between 6 months and 5 years.

As animals age, there is a gradual decrease in the functional reserves of major organs, which alters the response to many anesthetic drugs. In addition, older animals are less tolerant of the stresses of hospitalization and preoperative fasting.

Cardiovascular changes

The most important age-related change in cardiac function is decreased ventricular compliance and cardiac reserve. This renders older animals less tolerant of acute changes in intravascular volume — both dehydration and fluid overload. Many older dogs have audible murmurs, but the impact of these on cardiac function must be assessed. If the owner reports that the dog is exercise intolerant, they should be taken seriously and further work up, including an ECG and echocardiography, might be required. Feline cardiomyopathy is often subclinical due to a cat's lifestyle. Cardiomyopathy might not be uncovered until the cat is stressed or when disease is very advanced.

Degenerative myocardial changes render patients more susceptible to myocardial hypoxia. When a patient is hypothermic in the recovery period, the metabolic demands of shivering dramatically increase oxygen requirements. Therefore, keeping patients normothermic with circulating warm water and forced air blankets is very beneficial. Tachycardia caused by fear, pain, anticholinergic agents and induction agents such as ketamine also increase myocardial oxygen demand.

Respiratory system

Older patients have a decreased respiratory reserve; vital capacity is reduced, the chest wall and lungs become less compliant, and anatomic dead space increases making them more susceptible to hypoxia and hypercapnia. Pre-oxygenation is recommended in geriatric patients. Often a loose-fitting mask will be tolerated by these patients, whereas a tightly fitting one will not. Because animals cannot be asked to take deep breaths of oxygen-enriched air, optimal pre-oxygenation in animals likely takes 4-5 minutes. This technique establishes a safety factor if a difficult intubation is encountered by prolonging the time taken to desaturate.

Hepatic function

With age, the liver loses mass, and the clearance of anesthetic and analgesic drugs may be altered. Unfortunately there are few easy and reliable tests of liver function. Measurement of bile acids is warranted if liver dysfunction is suspected. A low total protein or albumin may indicate decreased production by the liver, and this can affect the action of highly protein-bound drugs such as propofol.

Renal function

As animals age, renal blood flow decreases in addition to glomerular filtration rate and the number of functional glomeruli. Renal tubular changes impair the control of electrolyte, fluid and acid-base balance. This combined with the changes in cardiovascular function render these patients intolerant of fluid losses or excessive fluid administration. Older animals may have underlying renal pathology, which is well compensated for until they are stressed in the perioperative period (fasting, fluid deprivation, hypotension) therefore overt post-anesthetic renal failure is a real concern. Geriatric patients may be on non-steroidal anti-inflammatory drugs for chronic pain or be given these for the acute pain associated with dental procedures. These drugs block prostaglandin production, which is important for maintaining renal blood flow during periods of hypotension. If an NSAID is used in the peri-anesthetic period, great care must be taken to prevent hypotension and to maintain normal fluid balance.

Central nervous system

Brain mass decreases with age as a result of neuronal loss; cerebral blood flow declines, and the quantity of neurotransmitters is reduced. Specific age-related changes indicative of degeneration have been identified in the brainstem of cats (Zhang et al, 2005). Although the exact reasons are unclear, older animals and humans have decreased anesthetic requirements, and this is best documented with inhalant agents.

Guidelines for anesthesia of geriatric patients

As with all patients, a complete history and physical examination are mandatory. Clinical findings will dictate which pre-anesthetic blood work and tests are undertaken, but in general, a complete blood count and chemistry is warranted in this patient population. Fortunately, dental procedures are elective, so there is time to work up these patients to obtain the pertinent information for making an appropriate anesthetic plan.

Premedication

One of the most commonly made mistakes when anesthetizing older patients is to depend primarily on inhalant agents and avoid premedicant agents. Sedation is recommended to decrease anxiety and fear, which lead to increased catecholamine release and can predispose an animal to cardiac arrhythmias, peripheral vasoconstriction, increased cardiac work and decreased tissue perfusion. Acepromazine is not contraindicated in geriatric patients although dose requirements (on an mg/kg basis) may be decreased. Acepromazine is an anti-emetic, anti-arrhythmic and most importantly an anesthetic sparing drug. In dogs, acepromazine decreases the requirements of halothane by 40 percent (Heard et al, 1986) which is beneficial as these are the most cardio-respiratory depressant anesthetic drugs we use. In normovolemic animals, acepromazine does not cause hypotension at clinical doses (0.01-0.05 mg/kg) and in fact may enhance cardiac output by decreasing afterload. Compared with morphine, premedication with acepromazine resulted in greater urine production during anesthesia (Robertson et al, 2001) and in another study (Bostrom et al, 2003) acepromazine appeared to protect renal function in anesthetized dogs despite a decrease in blood pressure.

Benzodiazepines such as midazolam and diazepam produce more reliable sedation in older patients than in younger ones. For premedication, midazolam has the advantage over diazepam because it can be given intramuscularly.

Opioids produce sedation and provide analgesia. In addition, their use as premedicant agents reduces the requirements for induction and maintenance drugs. For dental procedures in dogs and cats, buprenorphine is a good choice because of its relatively long duration of action (six hours) and lack of emetic effect and in cats, there is the added benefit of being able to administer it transmucosally.

Anticholinergics, such as atropine and glycopyrrolate, should not be used routinely because any increase in heart rate results in increased myocardial oxygen demand.

Induction agents

Inhalant agents are not recommended for induction because of the excitement they produce, in addition to the risk of pollution. All intravenous induction agents should be dosed "to effect". Ketamine causes a significant increase in heart rate and blood pressure, which may be detrimental to some patients. Propofol is a good choice because it can be titrated slowly "to effect" without causing excitement. Propofol is rapidly metabolized even in patients with poor liver function and is ideal for "outpatient" and short procedures as recovery is rapid and complete.

Maintenance agents

For all but the shortest procedures, inhalant agents are used for maintenance of anesthesia. All inhalant agents, including the newcomer sevoflurane, are potent cardio-respiratory depressant agents making the use of anesthetic sparing drugs, an important part of pre-anesthetic preparation. If mask induction is performed, then sevoflurane may be the agent of choice because of induction time is shorter than with other agents (Lerche et al, 2002).

Fluid administration

Fluid delivery during anesthesia should be accurately measured in older patients using a syringe pump or a calibrated burretrol. Most dental procedures are associated with minimal blood, therefore the usually recommended intravenous fluid rate during anesthesia of 10mls/kg/hour may not be necessary.

Monitoring

Because of the documented decrease in anesthetic requirements in older patients, the depth of anesthesia must be closely monitored by assessing jaw tone and eye position. Use of pulse oximetry, capnography and blood-pressure monitors is highly recommended.

Pain management

NSAIDS are very effective analgesic agents for acute pain and are an ideal choice in animals with normal renal and hepatic function. Only meloxicam (injectable, one dose peri-operatively) is licensed for use in cats in the United States, all other NSAID drugs are off label as is the oral meloxicam formulation. However, the oral formulation is widely used with success and is easily administered to cats and can be given for several days. In dogs, there are many more NSAIDs to choose from, and the choice is left up to personal preference.

The value of local anesthetic techniques are underestimated greatly for painful dental procedures. They decrease the requirements for maintenance agents and provide excellent analgesia in the intra-and post-operative period. In addition, they are safe, inexpensive and easy to perform. The major techniques employed for dental procedures are the mental, mandibular and maxillary nerve blocks. These blocks are well described in various veterinary texts.

Although geriatric patients may be more delicate and challenging than our younger patients and have some significant changes in physiology, with careful assessment and choice of anesthetic protocols, a good outcome should be the rule, not the exception.

Dr. Sheilah Robertson graduated from Glasgow University Veterinary School in 1980. After a short time in private mixed-animal practice, she spent a year as a surgery resident at Bristol University before pursuing specialization in anesthesia. Dr. Robertson obtained her PhD from Bristol University in 1985. She is board certified by both the European and American Colleges of Veterinary Anesthesia and is a past-president of the ACVA. She has worked at university teaching hospitals in Saskatchewan, Michigan and Florida. Her research interests are primarily focused on the pharmacokinetics and pharmacodynamics of analgesic agents in cats, foals and horses and more recently iguanas. She received a Douglas Houghton Scholarship for 2004-2005 for promoting animal welfare. Currently, she is a professor in the Section of Anesthesia and Pain Management at the University of Florida College of Veterinary Medicine.

Dr. Sheilah Robertson