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.
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.
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.
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.