Generally, animals become geriatric when they have reached ~ 75–80% of their life expectancy. Understanding physiological changes in geriatric patients are important factors in choosing appropriate anesthetic methods for the individual patient. Some problems (e.g., hypothermia and anxiety in cats) may be due to a high dose of premedication drugs.
Cardiovascular system: Cardiac output, baroreceptor activity and blood volume may be reduced. However, increased cardiac arrhythmia, circulation time, and higher vagal tome may be encountered. During anesthesia, the animal's ability to maintain blood pressure may decrease and it may take longer for the drug to take effect.
Respiratory system: Respiratory rate, tidal volume, lung elasticity, and oxygen diffusion capacity may be decreased. These changes may affect oxygen saturation. Pre-oxygenation during induction is recommended.
Hepatic system: Hepatic metabolism (both hepatic blood flow and microsomal enzyme activity) may be reduced. During anesthesia, patients may have prolonged drug metabolism and excretion which can lead to prolonged recovery.
Renal system: There are reductions in renal blood flow, glomerular infiltration rate, and ability to concentrate urine. Therefore, ability to clearance drugs may be reduced. In addition, animals will less tolerate to dehydration or overhydration, and blood loss.
Central nervous system: Older animals usually require fewer anesthetics which is thought to be due to reduced neurotransmitters or receptors, reduced cerebral blood flow, and reduced brain weight.
Thorough pre-anesthetic evaluation and physical examination are vital to detect ongoing diseases or medications. Suggested minimum blood panel are PCV, TP, albumin, glucose, BUN, AP, ALT, urine specific gravity. Chest X-rays and plasma T4 levels are also recommended.
Anesthesia: For premedication, keys are reducing stress and drug dose and providing preventive analgesia and smooth induction and recovery. Knowledge of pharmacology is important for anesthetists to choose appropriate anesthetic premedication, induction, and analgesia. Pre-oxygenation when possible without stress is recommended. Premedication should be enough to calm the animal for handling without stress. Examples: premedication is morphine + glycopyrrolate; induction is ketamine/diazepam; maintenance is isoflurane. Throughout anesthesia, balanced anesthesia should be performed (i.e., epidural administration, fentanyl patch, CRI [opioid], local blocks, etc.).
References
1. Hughes J. Anaesthesia for the geriatric dog and cat. Ir Vet J. 2008;61:380–387.
2. Neiger-Aeschbacher G. Geriatric patients. In: Duke-Novakovski T, de Vries M, Seymour C, eds. BSAVA Manual of Canine and Feline Anaesthesia and Analgesia. 2nd ed. Gloucester, UK: British Small Animal Veterinary Association; 2007:303–309.