McKenna's Drug Handbook for Nursing & Midwifery, 7e

3 Drug therapy in the older adult

diminished activity of certain liver enzymes. When an older person takes certain sleep medications, such as nitrazepam, reduced liver metabolisation of the drug may cause a hangover effect the next morning. Decreased hepatic function may lead to the following: • more intense drug effects due to higher blood levels • longer-lasting drug effects due to prolonged blood concentrations • greater incidence of drug toxicity. Renal function Although an older person’s renal function is usually sufficient to eliminate excess body fluid and waste, the ability to elimi- nate some medications may be reduced by 50%or more. Manymedications commonly used by older adults, such as digoxin, are excreted primarily through the kidneys. If renal excretion of the drug decreases, high blood concentrations may result. Digoxin toxicity, therefore, is relatively common in older adults who are not receiving a reduced digoxin dosage to accommodate decreased renal function. Drug dosages can be modified to com- pensate for age-related decreases in renal function. Aided by laboratory tests, such as urea and serum creatinine, prescribers may adjust medication dosages so that the person receives the expected therapeutic benefits without the risk of toxicity. Since creatinine is a by-product of musclemetab- olism, in older individuals serum creatinine may remain in the normal range despite a falling glomerular filtration rate because of low muscle mass. Thus, elderly people should be considered as having impaired renal function and their drug dosage should generally be substantially lower than for younger people. It is common to start with about 50% of the adult dose.

If you are providing drug therapy for older adults, you will want to understand physi- ological and pharmacokinetic changes that may alter appropriate drug dosage or cause common adverse reactions or compliance problems in these people. Physiological changes As a person ages, gradual physiological changes occur. Some of these age-related changesmay alter the therapeutic and toxic effects of medications. Body composition Proportions of fat, lean tissue and water in the body change with age.Total body mass and lean body mass tend to decrease; the proportion of body fat tends to increase. Varying from person to person, these changes in body composition affect the relationship between a drug’s concen- tration and distribution in the body. For example, a water-soluble drug, such as gen- tamicin, is not distributed to fat. Because there is relatively less lean tissue in an older person, more of a drug remains in the blood. GI function In older adults, decreases in gastric acid secretion and GI motility slow the emptying of stomach contents and the movement of intestinal contents through the entire tract. Furthermore, research suggests that older adults may have more difficulty absorbing medications.This is a particularly significant problemwith drugs having a narrow thera- peutic range, such as digoxin, in which any change in absorption can be crucial. Hepatic function The ability of the liver tometabolise certain drugs decreases with age because of dimin- ished blood flow to the liver—a result of the age-related decrease in cardiac output and