Geriatric Patients: Dosing Differences
Advanced, Clinical Subspecialties
Age-related changes in the elderly include a decrease in muscle mass and an increase in body fat, resulting in a reduction in total body water. This leads to a decreased volume of distribution for water-soluble drugs, causing an increase in plasma concentration. Likewise, due to the increased body fat, there is an increase in lipid-soluble drug deposition causing delayed elimination. There is a progressive reduction in GFR due to a decrease in both renal mass and renal blood flow. However, due to a decrease in muscle mass, serum creatinine (Cr) is often not elevated in older patients. Liver mass and hepatic blood flow also decrease with aging, leading to a reduction in enzyme metabolism and protein synthesis, although a large hepatic reserve make the clinical impact of these changes much less significant than renal changes. A number of the most relevant dosing adjustments for the elderly are reviewed below.
Propofol: Due to the decrease in total body water, propofol will have exaggerated hemodynamic effects in the elderly, even more so if they have reduced intravascular volumes. It is recommended to reduce the initial dose by approximately 50%, as well as increase the time interval between repeated bolus doses.
Etomidate: Similar to propofol, a dose reduction is required with Etomidate due to the lower volume of distribution. It is recommended to decrease the dose by 50% in patients over the age of 80 years.
Opioids: Opioid requirements are reduced in the elderly population. Studies have shown that dosage requirements targeting the same EEG endpoint using Fentanyl and Alfentanyl are 50% lower in elderly patients. The elderly have increased susceptibility to opioid-induced apnea, and due to changes in their cardiac conducting system, they have been shown to have an exaggerated bradycardic response to opioids.
Benzodiazepines: While it is often prudent to avoid benzodiazepines altogether in the elderly population, long acting benzodiazepines such as diazepam and lorazepam should be especially avoided due to their association with delirium secondary to prolonged clearance and active metabolites. Additionally, due to changes in the GABA receptor, the elderly have an increased sensitivity to midazolam causing an increased rate of midazolam-induced apnea. It is recommended to reduce the dose by 50%, and that repeat doses should be in increments no larger than 0.5 mg.
Volatile Anesthetics: MAC requirements for the elderly decrease. This is likely due to age-related cerebral atrophy and alterations in neurotransmitter balance. While exact numbers vary, it is generally accepted that MAC requirements decrease by 4-6% every decade after 40 years old.
Neuromuscular blockers: The response to neuromuscular blockers is unchanged with aging; however, with both a decrease in cardiac output and muscle blood flow, there may be a prolonged onset of action for NMBs. Care should also be taken with certain non-depolarizers (e.g. pancuronium) that can have a prolonged duration of action due to comorbid renal disease.
Spinal/Epidural Anesthesia: Due to age-related anatomic changes, there is often an exaggerated spread of local anesthetic in the epidural space resulting in a higher than expected level. The dose of spinal local anesthetic should be reduced as there is often a longer duration of action for a given dose.
Beta-Agonists: Elderly patients are less responsive to beta receptor stimulation, and as such beta agonists such as isoproterenol will require increased doses to have a similar effect as in a younger patient.
NSAIDs: Due to often prevalent renal disease in the elderly, care should be taken when administering NSAIDs. It is recommended for IV Ketorolac specifically to decrease the doses to 15 mg q6, not to exceed 60 mg in 24 hours.
Anticholinergics: Elderly patient have a decreased cholinergic reserve, and as such, are at risk for side effects from centrally acting anticholinergic medications. Medications to avoid with anticholinergic effects include chlorpheniramine, promethazine and scopolamine.
In general, the geriatric population has a reduced requirement for anesthetic drugs. They are especially at risk for drug interactions as 90% of the geriatric population is on at least one medication, and their co-morbidities can additionally interfere with medications administered. A good general principle is to "start low, and go slow." It's important to titrate medications carefully to avoid unexpected side effects. Short-acting drugs are often the best choice to accomplish this safely.