Drug therapy in geriatric patients

CHAPTER 11


Drug therapy in geriatric patients


Drug use among the elderly (those 65 years and older) is disproportionately high. Whereas the elderly constitute only 12% of the U.S. population, they consume 31% of the nation’s prescribed drugs. Reasons for this intensive use of drugs include increased severity of illness, multiple pathologies, and excessive prescribing.


Drug therapy in the elderly represents a special therapeutic challenge. As a rule, older patients are more sensitive to drugs than are younger adults, and they show wider individual variation. In addition, the elderly experience more adverse drug reactions and drug-drug interactions. The principal factors underlying these complications are (1) altered pharmacokinetics (secondary to organ system degeneration), (2) multiple and severe illnesses, (3) multidrug therapy, and (4) poor adherence. To help ensure that drug therapy is as safe and effective as possible, individualization of treatment is essential: Each patient must be monitored for desired and adverse responses, and the regimen must be adjusted accordingly. Because the elderly typically suffer from chronic illnesses, the usual objective is to reduce symptoms and improve quality of life, since cure is generally impossible.




Pharmacokinetic changes in the elderly


The aging process can affect all phases of pharmacokinetics. From early adulthood on, there is a gradual, progressive decline in organ function. This decline can alter the absorption, distribution, metabolism, and excretion of drugs. As a rule, these pharmacokinetic changes increase drug sensitivity (largely from reduced hepatic and renal drug elimination). It should be noted, however, that the extent of change varies greatly among patients: Pharmacokinetic changes may be minimal in patients who have remained physically fit, whereas they may be dramatic in patients who have aged less fortunately. Accordingly, you should keep in mind that age-related changes in pharmacokinetics are not only a potential source of increased sensitivity to drugs, they are also a potential source of increased variability. The physiologic changes that underlie alterations in pharmacokinetics are summarized in Table 11–1.






Distribution

Four major factors can alter drug distribution in the elderly: (1) increased percent body fat, (2) decreased percent lean body mass, (3) decreased total body water, and (4) reduced concentration of serum albumin. The increase in body fat seen in the elderly provides a storage depot for lipid-soluble drugs (eg, thiopental). As a result, plasma levels of these drugs are reduced, causing a reduction in responses. Because of the decline in lean body mass and total body water, water-soluble drugs (eg, ethanol) become distributed in a smaller volume than in younger adults. As a result, the concentration of these drugs is increased, causing effects to be more intense. Although albumin levels are only slightly reduced in healthy adults, these levels can be significantly reduced in adults who are malnourished. Because of reduced albumin levels, protein binding of drugs decreases, causing levels of free drug to rise. As a result, drug effects may be more intense.




Excretion

Renal function, and hence renal drug excretion, undergoes progressive decline beginning in early adulthood. Drug accumulation secondary to reduced renal excretion is the most important cause of adverse drug reactions in the elderly. The decline in renal function is the result of reductions in renal blood flow, glomerular filtration rate, active tubular secretion, and number of nephrons. Renal pathology can further compromise kidney function. The degree of decline in renal function varies greatly among individuals. Accordingly, when patients are taking drugs that are eliminated primarily by the kidneys, renal function should be assessed. In the elderly, the proper index of renal function is creatinine clearance, not serum creatinine levels. Creatinine levels do not reflect kidney function in the elderly because the source of serum creatinine—lean muscle mass—declines in parallel with the decline in kidney function. As a result, creatinine levels may be normal even though renal function is greatly reduced.



Pharmacodynamic changes in the elderly


Alterations in receptor properties may underlie altered sensitivity to some drugs. However, information on such pharmacodynamic changes is limited. In support of the possibility of altered pharmacodynamics is the observation that beta-adrenergic blocking agents (drugs used primarily for cardiac disorders) are less effective in the elderly than in younger adults, even when present in the same concentrations. Possible explanations for this observation include (1) a reduction in the number of beta receptors and (2) a reduction in the affinity of beta receptors for beta-receptor blocking agents. Other drugs (warfarin, certain central nervous system depressants) produce effects that are more intense in the elderly, suggesting a possible increase in receptor number, receptor affinity, or both. Unfortunately, our knowledge of pharmacodynamic changes in the elderly is restricted to a few families of drugs.



Adverse drug reactions and drug interactions


Adverse drug reactions (ADRs) are 7 times more common in the elderly than in younger adults, accounting for about 16% of hospital admissions among older individuals and 50% of all medication-related deaths. The vast majority of these reactions are dose related, not idiosyncratic. Symptoms in the elderly are often nonspecific (eg, dizziness, cognitive impairment), making identification of ADRs difficult.


Perhaps surprisingly, the increase in ADRs seen in the elderly is not the direct result of aging per se. Rather, multiple factors predispose older patients to ADRs. The most important are:


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Jul 24, 2016 | Posted by in NURSING | Comments Off on Drug therapy in geriatric patients

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