Reduced cardiac output and contractility, decreased blood volume and blood pressure, and decreased ability to autoregulate blood flow and maintain blood pressure all contribute to a reduction in cardiac vascular reserve (Ko and Galloway 2002). An increased circulation time may result in a delayed response to drug administration.
Older patients may have chronic or degenerative valvular disease and may be susceptible to cardiac arrhythmias due to myocardial fiber atrophy and changes in the conduction system (Muravchick 2000). Drugs that are known to potentiate arrhythmias and/or cause extreme changes in heart rate, such as α2 adrenergic agonists, ketamine, and thiopentone, should be used sparingly or avoided in the geriatric patient (Ko and Galloway 2002).
With increased age there are decreases in respiratory rate as well as tidal and minute volumes. Physical changes include reduced thoracic wall compliance, decreased lung elasticity, and atrophy of the intercostal muscles, and these may contribute to a decreased respiratory reserve in the elderly patient (Paddleford 1995). These patients are highly susceptible to hypoxia and hypercapnia when the respiratory system is depressed by drugs.
Hepatic and renal mass can be significantly decreased in the geriatric patient. A reduction in hepatic and renal blood flow is secondary to a decrease in cardiac output and can result in decreased drug metabolism and clearance. Hypoxia, hypotension, and hypovolemia may exacerbate the reduction of renal blood flow. Due to the reduction of functional organ reserve, these patients have less tolerance for excessive fluid administration, hypovolemia, hypotension, dehydration, and blood loss. Impaired clotting function, hypoproteinemia, and hypoglycemia must also be considered in a patient with reduced hepatic function (Paddleford 1995).
Changes that may be seen in the nervous system of the geriatric patient are altered sensory, motor, cognitive, and autonomic functions. Enhanced effects to anesthetic drugs may occur as a result of decreases in cerebral perfusion, oxygen consumption, thermoregulatory function, and sympathetic responses to stress. Changes in myelination and decreased production of neurotransmitters may also sensitize the elderly patient to local anesthetics (Neiger-Aeschbacher 2007).
All patients must be evaluated on an individual basis. A thorough physical examination, detailed medical history, and laboratory assessment are the bases on which to formulate an anesthesia plan. The animal may present with existing disease — for example, Addison’s, Cushing’s, diabetes — or may be taking medications that will influence the anesthetist’s choice of drugs and supportive therapies. Basic laboratory work should always be performed on the geriatric patient. A complete blood count, biochemistry, and urinalysis may alert the anesthetist to sub-clinical organ dysfunction, which when challenged by general anesthesia may accelerate the disease process. If cardiac murmurs or arrhythmias are noted on examination, a chest radiograph, electrocardiogram, or echocardiograph may be indicated to rule out existing cardiac disease.
Conservative intravenous fluid therapy may be indicated in the preoperative period to ensure that the patient is in the most stable physiological state prior to general anesthesia. A balanced anesthesia and analgesia plan may necessitate the concurrent use of several drugs at reduced doses to achieve the best outcome for the patient with minimal drug-related side effects.
The patient undergoing elective general anesthesia will need to be fasted. It is recommended that fasting be kept to a minimum, generally 8 hours, due to the risk of hypoglycemia. Water should be withheld only for a short period of time to minimize dehydration (Fig. 27.2).
One of the main considerations for drug selection should be minimizing deleterious effects on organ function. All drugs may have exaggerated effects and duration of action may be prolonged in the geriatric patient. Judicious dosing and titration of drugs is advisable until the desired effect is achieved. It is much easier to administer additional drug than to deal with the affects of an overdose.
The classes of drugs that are commonly used for the sedation of geriatric patients are opioids, phenothiazines, benzodiazepines, and α2 adrenergic agonists. These drugs can be used independently or in combination.
Opioids will provide analgesia and sedation in the geriatric patient. They have minimal cardiovascular effects but may cause some respiratory depression. Pure agonists (morphine, pethidine, fentanyl, oxymorphone, hydromorphone and methadone) provide better analgesia than the partial μ agonist (buprenorphine) and the k agonist/μ partial agonists (butorphanol, nalbuphine), but are more likely to cause bradycardia and a centrally mediated respiratory depression (Ko and Galloway 2002). The use of an anticho-linergic may be required to counteract bradycardia (Stoelting 1999). If respiratory depression is evident, this can be exacerbated by a low functional respiratory reserve, and supplemental oxygen should be provided. Some opioids have a relatively short duration of action and this may be beneficial in the geriatric patient. The partial μ agonist and κ agonist/μ partial agonists have a “ceiling effect” on respiratory depression, but do have limited analgesic effectiveness, so are not suitable for severe pain (Ko and Galloway 2002). The perioperative use of opioids may decrease the dose of induction and maintenance agents required. Opioids require hepatic metabolism and should be used judiciously in the patient with severe hepatic disease. A decrease in the dose or frequency of administration may be warranted. If undesirable side effects are seen with the administration of opioids, an opioid antagonist can be administered.
Benzodiazepines (diazepam, midazolam) are popular choices in tranquilization of the elderly patient. They cause minimal cardiovascular and dose-related respiratory depressant effects and have a relatively short duration of action (particularly midazolam). They are commonly used in combination with an opioid for premedication. Benzodiazepines are metabolized by the liver, and a prolonged duration of action may be seen in a patient with hepatic disease. Flumazenil, a benzodiazepine antagonist, can be used as a reversal agent if necessary.
Acepromazine is one of the most widely used tranquilizers in veterinary practice today. It provides excellent antiemetic, anxiolytic, and antiar-rhythmic properties, but should be used with caution on the elderly patient. Hypotension and hypothermia due to peripheral vasodilatation are undesirable side effects seen with the administration of acepromazine. Acepromazine is metabolized by the liver, is not reversible, and has a long duration of action. Blood pressure monitoring and volume support are indicated when acepromazine is administered to a geriatric patient.
Alpha-2 adrenergic agonists
Although alpha-2 adrenergic agonists will provide effective sedation with some analgesia, they will also produce dose-related effects on the cardiovascular, respiratory, and central nervous systems. Alpha-2 adrenergic agonists also require extensive hepatic metabolism. Prolonged and more profound effects of these drugs would be expected in the presence of reduced hepatic function. Their use is best avoided in the geriatric patient.