You have probably heard "Your pet is too old to go under anesthesia" or alternatively "Age is not a disease". The truth is, age in and of itself should not dictate whether a patient is anesthetized or not. Age by itself is not a disease. Frankly, I've anesthetized some of the oldest pets out there and unless they have some underlying medical condition, often do quite well. With that said, with aging comes some significant changes in organ function, which can predispose them to various medical conditions or change how the body responds to stressors and thus anesthetic risk can be higher in some patients. With proper preanesthetic workup, individualized protocol development, close anesthetic and postoperative monitoring, the risk in even those patients with underlying conditions can be dramatically reduced.
The medical community continues to do research in all areas of medicine, allowing more and more insight into the physiology, disease processes, pharmacokinetics and pharmacodynamic of animals. This research has resulted in continual advancements in medical options, allowing patients to live longer lives, increasing the potential need for procedures requiring general anesthesia. As a patient ages, however, organ function changes still occur and there becomes increased potential for onset of new disease process or for the number or severity of diseases already present to be increased. It is therefore important for the anesthetist, to understand what physiologic changes occur with aging, if the patient has any underlying medical conditions, and how those disease process affect things like drug metabolism, drug excretion, ventilation and perfusion, in order to provide the safest anesthesia possible.
Typical physiologic changes that can occur with aging included orthopedic, cardiovascular, pulmonary, renal, and hepatic. Though orthopedic changes generally do not directly affect how a patient will do while anesthetized, they can directly impact how the patient recovers. One should keep this in mind when positioning a patient. Minimizing excessive traction/pressure on joints and maintaining proper padding is important for minimizing joint and muscle pain on recovery. If this cannot be avoided, then assuring proper pain management is provided is vital.
Cardiovascular reserve can decrease with age resulting in decreased ventricular compliance and potential for myocardial fibrosis and valvular changes. Maximal heart rate response is less and response to exogenous drugs such as inotropes & vasopressors, can be decreased. Pulmonary compliance and functional reserve decline with age, making risk of hypoxia more likely, so monitoring of patient EtCO2 and SpO2 perioperatively are important. Liver size decreases with age, as does overall hepatic function resulting in a potential for increased plasma half-life and prolonged metabolism and excretion of drugs. Changes in renal function include decreased renal blood flow, making monitoring for and treatment of hypotension very important.
A thorough preanesthetic physical assessment cannot be overemphasized. Any preexisting medical conditions should be medically managed and, when possible, stabilized prior to anesthesia. Routine bloodwork and urinalysis should be performed prior to anesthesia to assess vital organ function. Several veterinary studies have reported that in geriatric patients specifically, routine pre anesthetic bloodwork diagnosed new or subclinical disease in 30-80% of patients. Thoracic radiographs are generally recommended in older patients, to ensure there is no evidence of metastatic neoplasia, to evaluate heart size, and assess the lungs. This is best medical practice but less important than bloodwork, unless the patient has clinical signs, an underlying disease process, or is a brachycephalic dog, making this diagnostic more important.
When designing an anesthetic protocol, local and regional anesthesia techniques should be considered whenever possible and not contraindicated. These techniques provide analgesia and allow the use of fewer and lower doses of injectable drugs and inhalants, reducing the risk or severity of side effects of systemic drugs. The use of analgesics is incredibly important. There is an enormous amount of literature describing the negative impact that pain has on the sympathetic nervous system, endocrine system, healing and overall morbidity and mortality.
Unless contraindicated, patients should receive sedation/anxiolysis prior to induction of anesthesia. This will minimize the detrimental effects that stress hormones have on organ systems, will enable the use of lower induction dosages and help minimize the need for high concentrations of inhaled anesthetics, as inhalant anesthetics can cause significant hypotension. There is no perfect drug for induction of anesthesia, so consideration of that particular patient’s overall condition is necessary. As these patients may be more prone to decreased cardiac output and more at risk of the side effects of aggressive fluid use, the use of more cardiovascular sparing drugs may be beneficial. Reversible drugs are often a preferred choice, and use of the lowest drug dose possible to achieve the desired effect is best. Regardless of the drugs chosen, titration of drugs to effect should be standard, as these patients are likely to need less than standard doses. Lastly maintenance of normal body temperature is cruicial. Many of these patients have poor body condition and may not be able to maintain body heat well. Hypothermia has many detrimental effects on the body, including adverse effects on the myocardium, decreased healing, and delayed recovery.
Clearly the risks associated with general anesthesia can increase with increasing age, however, not performing some disease controlling procedures (e.g. dentals, mass removals, emergency surgery, etc.) has the potential to actually be more detrimental to the patient's health and quality of life, than the risk of anesthesia itself. One must carefully weigh the risks & benefits of each, while remembering that with appropriate preanesthetic workup, monitoring, interventions, and individualized drug protocols, even older patients can do quite well and even excellent during and after anesthesia!
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