Anesthesia for the geriatric patient (Proceedings)
What is a geriatric patient? Geriatric patients are pets that have attained 75% of that species or breeds expected life span. Therefore, a 6 year old Yorkshire terrier would not be considered geriatric, but a 6 year old Irish wolfhound would fall under this definition. We can have healthy geriatric patients, but we must take into account how age can affect various mechanical and physiological functions. There are issues that are specific to the geriatric patient.
Geriatric patients have a decrease in cardiac functional reserve so they have a decreased ability to compensate for changes while under anesthesia or sedation. A disease free, older heart will usually have myocardial fibrosis as well as valvular and ventricular wall thickening. This can result in a decreased cardiac output (CO). Geriatrics increase stroke volume (SV), not heart rate (HR), to compensate for decreased CO. The increase in SV is accomplished by increasing preload and atrial contraction (to increase ventricular filling), so geriatric patients can benefit from be preloaded if they are heart healthy. Mitral valve disease (MVD), tricuspid valve disease (TVD) or both are common in geriatric patients. Valvular disease can also cause or may have associated with it left atrial and ventricular enlargement, dilated cardiomyopathy or hypertrophic cardiomyopathy. Geriatrics are more susceptible to arrhythmias (or dysrhythmias) because of cardiac fiber atrophy and age related conduction issues. Avoid anesthetic or sedation drugs that can increase the possibility of dysrhythmias e.g. alpha-2 agonists or ketamine. Anticholinergics should be avoided if being used as part of a standard premed. They can always be given later, if needed. Preloading should be avoided during the preoperative period if moderate to severe cardiac disease is present. Reduction in fluid rates for cardiac patients during the anesthetic period is 5mls/kg/hr.Pulmonary System
Tidal volume (TV) and respiration rate (RR) decrease with age, so geriatric patients will have an overall increase in PaCO2 (ETCO2). Mechanical problems encountered by this group of patients include thoracic rigidity and loss of thoracic wall compliance. Also, intercostal and diaphragmatic muscles will atrophy and alveoli (lungs) lose their elasticity. This will result in a decreased PaO2 and increased PaCO2 when anesthetic & sedation drugs are administered.
Older patients may have a 50% decrease in the number of functional nephrons and have greater difficulty in retaining Na and H20. If possible, preload geriatric renal patients, but not to the point of fluid overloading. Older kidneys have decreased renal blood flow (RBF) and glomerular filtration rates (GFR) so anesthetic or sedation drugs excreted via this route will have longer elimination times (e.g. ketamine in felines). Managing aggressive fluid therapy in the geriatric patient maybe more difficult. Fluid overloading can result in over hydration which leads to edema or heart failure. Geriatric patients are less tolerant of hypovolemia, hypotension, dehydration and blood loss which can result in increased morbidity.
Anesthetic and sedation drugs that are cleared by the liver are slower to be metabolized as hepatic mass decreases. Liver mass may decrease as much as 50% in geriatric patients. As liver mass decreases, hepatic blood flow decreases as well. Less hepatic enzymes are available to metabolize anesthetic drugs - especially lipid soluble drugs. Combined with decreased RBF and GFR, anesthetic drugs dependent on hepatic and renal clearance may have their ½ life increased as well as their duration of effect. Decreases in clotting function, hypoglycemia and hypoabluminemia may also occur in the geriatric patient.