Neurologic System
In general, geriatric patients may respond more profoundly to sedatives and anesthetic agents.7 This is thought to be because of reductions in cerebral blood flow, decreases in neurotransmitters, and receptor affinity as well as changes in drug pharmacodynamics.3,5,9-10 It may be prudent to reduce the dosage and to allow for a longer duration of action.2
Cardiovascular System
Co-existence of primary cardiac disease3,9,11 and cellular structural changes9,12 are prevalent in geriatric patients; however, in some species (eg, Cavalier King Charles spaniels,13,14 bull terriers15), this prevalence is greater than 50%. In general, these physiologic changes render the cardiac reserve intolerant to changes in cardiovascular function. Both heart rate and blood pressure should be kept within normal range. Use of benzodiazepines, opioids, and locoregional techniques can all minimize the deleterious effects induced by anesthetic agents.3,9,10,12,16
Respiratory System
In one study, no differences in arterial blood gases were identified when comparing unanesthetized dogs over 8 years of age to younger dogs17; however, changes to pulmonary mechanics, notably thoracic compliance,16 and ventilatory responses to hypercapnia and hypoxemia9 have been observed. Anesthesia and these mechanical changes to pulmonary mechanics reduce the capacity of geriatric patients to compensate for changes in oxygenation and ventilation. Mechanical ventilation is frequently required to maintain the PaO2 and PaCO2 within normal limits during anesthesia, especially in obese patients.10,12,16
Renal System
Reductions in renal blood flow, glomerular filtration rate,10 and disturbances to sodium and water homeostasis9 render geriatric patients intolerant to hypovolemia and fluid overload.10 Using fluid therapy to maintain a normal urine output is questionable.18,19 General anesthesia causes a reduction in urine output. Trying to maintain a normal urine output during anesthesia may also result in edema, hemodilution, hypoproteinemia, electrolyte disorders, and (potentially) coagulopathies.18 Fluid therapy can cause deleterious effects if used injudiciously.20,21 A prudent approach to hypotension is to give a fluid bolus of 10-20 mL/kg over 15 minutes and optimize the depth of anesthesia, then reassess the blood pressure, hematocrit, and total protein. If the blood pressure is still low, then a perfusion of a positive inotrope such as dobutamine may be instituted.3,9,16 This approach is assuming the original blood volume was normal.
Hepatic System
Liver mass as well as synthetic and metabolic capacities reduce with age.9,10 Body composition also changes with age. Muscle mass decreases and fat content increases. Both of these changes alter the pharmacokinetics and pharmacodynamics of medications.2 Decreases in cardiac output can also prolong duration of action of various medications metabolized by the liver.3,9,10 It may be necessary to reduce initial dose of drugs and to alter dosing intervals.19,20,22