Chapter 51: Pharmacokinetics

Pharmacokinetic Factors in Children

• Decreased gastric acid production and slower gastric emptying time affect medication absorption.
• Decreased first-pass medication absorption due to liver immaturity.
• Increased absorption of topical medications due to greater blood flow to the skin and thinner skin.
• Higher body water content dilutes water-soluble medications.
• Decreased protein-binding sites in the blood (until age 1 year) increase blood levels of protein-binding medications.

Pharmacological Factors in Older Adults (65+ Years)

• Decreased kidney and liver function lead to medication accumulation and toxicity.
• Liver disease or failure impairs medication metabolism, causing toxicity.
• Kidney disease or failure prevents or delays medication excretion, causing toxicity.
• Prolonged gastric emptying time delays medication absorption in the intestines.

Physiological Changes with Aging

• Body weight affects medication absorption, with greater body mass requiring larger doses.
• Body surface area (BSA) is a more precise method of regulating medication response.
• Age affects liver and kidney function, leading to heightened sensitivities to medications.
• Sex affects medication response, with varied therapeutic responses in males and females.
• Genetic factors (missing enzymes) alter medication metabolism, enhancing or reducing medication action.
• Biorhythmic cycles affect medication response, with some medications working better at specific times.

Special Considerations for Children

• Pediatric dosages are based on body weight or BSA.
• Newborns and infants (2 days to 1 year old) have immature liver and kidney function, alkaline gastric juices, and an immature blood-brain barrier.
• Be cautious when administering medications to children due to increased risk of medication errors, decreased skeletal bone growth, acute cardiopulmonary failure, and hepatic toxicity.
• Check dosages for accuracy based on weight or BSA.
• Initial pediatric dosages are approximations.
• Most medications do not undergo testing on children.
• Medication forms and concentrations may require dilution, calculation, preparation, and administration of very small doses for children.
• Limited sites exist for IV medication administration.
• Give written and verbal instructions to parents to promote adherence to medication regimens.

NURSING INTERVENTIONS

• Obtain a complete medication history, including prescription medications, OTC medications, recreational drugs, and herbal supplements to reduce the risk of adverse effects.
• Start medication therapy at the lowest effective dosage for the shortest possible time to minimize risk of adverse effects.
• Assess and monitor for therapeutic and adverse effects, and monitor plasma medication levels to provide a rational basis for dosage adjustment.
• Assess and monitor for medication-medication and medication-food interactions.
• Notify the provider of adverse effects and ensure the client understands the effect of prescribed medications on reproductive potential.

REPRODUCTIVE POTENTIAL

• Some medications include special considerations for clients of reproductive potential to reduce the risk of fetal harm and preserve reproductive ability.
• Pregnancy testing before and during medication therapy, and beginning or altering contraceptive use may be necessary.
• Ensure the client understands the effect of prescribed medications on reproductive potential and any specific actions to take during medication use.

PROMOTING ADHERENCE

• Give clear and concise instructions, verbally and in writing, to ensure the client understands the medication regimen.
• Ensure the dosage form is appropriate and administer liquid forms to clients who have difficulty swallowing.
• Provide clearly-marked containers that are easy to open.
• Be particularly alert when administering medications to children due to the increased risk of medication errors.

SPECIAL CONSIDERATIONS

• Providers base some medication dosages on age due to a greater risk of decreased skeletal bone growth, acute cardiopulmonary failure, and hepatic toxicity in children.
• Genetic factors can alter the metabolism of certain medications, thus enhancing or reducing a medication's action.
• Responses to some medications vary with the biologic rhythms of the body, and tolerance to medications can develop over time.
• Limited sites exist for IV medication administration in children, and most medications do not undergo testing on children.

MEDICATION THERAPY IN PREGNANCY AND LACTATION

• Providers manage chronic medical disorders in conjunction with careful maternal-fetal monitoring during pregnancy.
• Pregnancy is a contraindication for live-virus vaccines, and clients who are lactating should avoid medications that have an extended half-life, are sustained-release, or are harmful to infants.
• Administer medications that are safe during lactation immediately after breastfeeding to minimize the medication's concentration in the next feeding.