Which of the following is true regarding the distribution of administered medication in the elderly?
There is a relative reduction of body fat to body water12%A reduced unbound free fraction of administered drugs is typical seen in the elderly11%Older people have higher amounts of albumin when compared to younger adults5%Water soluble drugs tend to have lower levels in the elderly21%As people age, the half-life of a lipid soluble drug increases51%
Prescribing in the elderly
Older people's bodies are different to those of younger adults and this affects how they handle medication. Additionally they are often on a range of other drugs so interactions are an issue.
Both pharmacokinetics and pharmcodynamic differences apply.
Pharmacokinetics
Changes in distribution, metabolism, and excretion are important.
Absorption
In old age there is reduced gastric acid secretion. This is accompanied by decreased gastric motility, and reduced surface area available for absorption. Pharmacokinetic studies on the effect of ageing on drug absorption have provided conflicting results. While some studies have not shown significant age-related differences in absorption rates for different drugs, the absorption of vitamin B12, iron and calcium through active transport mechanisms is reduced, whereas the absorption of levodopa is increased.
Distribution
There is a relative reduction of body water to body fat (the elderly have more fat and less water) as the body ages which results in an altered distribution of lipid soluble drugs. Older people also have less albumin. These changes result in increased blood levels for water soluble drugs, decreased levels for lipid soluble drugs (increased volume of distribution), and a greater unbound free fraction (increased amount of active drug). As people age, the half-life of a lipid soluble drug increases.
Metabolism
Hepatic metabolism of drugs decreases with age as a consequence of reduced hepatic blood flow and reduced enzyme activity.
Excretion
The kidneys become less effective with age (by 65, 35% of renal function is lost, and by 80, 50% is lost). Serum creatinine and urea is often used to estimate renal function but can be misleading in the elderly as they have less muscle and so produce less creatinine.
The e-GFR is a tool that can be used to estimate renal function in the elderly.
Drugs that are primarily excreted by the kidneys (e.g. lithium and sulpiride) will tend to accumulate in the elderly and so small doses may be needed.
Pharmcodynamics
Receptor sensitivity tends to increase during old age and so this group tends to require smaller doses. It is generally recommended that you start with half of the usual adult dose and go on from there. This increase in sensitivity contributes to an increased incidence of side-effects in the elderly.
Older people tend to take longer to respond to treatment (the therapeutic response is delayed).
Above compiled from:
Seminars in clinical psychopharmacology, 2nd Edition
Maudsley Guidelines 11th Edition
Kaplan and Sadock's Synopsis of Psychiatry 11th Edition