NURS 232 Normal Aging Physiology and PK (1)

Normal Aging & Pharmacokinetics: What Every Nurse Needs to Know

As we care for older adults, it's crucial to understand how aging affects drug absorption, distribution, metabolism, and elimination. These changes impact medication effectiveness and safety, requiring careful monitoring and dose adjustments.

Pharmacokinetics: How the Body Processes Drugs

Pharmacokinetics refers to the movement of a drug through the body. It consists of four key stages:

  • Absorption – How the drug enters the bloodstream from the site of administration.

  • Distribution – How the drug spreads through the body and reaches its target.

  • Metabolism – How the body breaks down the drug, mainly in the liver.

  • Elimination – How the drug exits the body, primarily through the kidneys.

Absorption in Older Adults

What Changes with Aging?

  • Slower gastric motility → Delays drug movement from stomach to intestines.

  • Reduced gastric blood flow → Affects how quickly drugs are absorbed.

  • Increased gastric pH (less stomach acid) → Alters absorption of certain drugs like calcium carbonate.

🔹 Clinical Example: Older adults may experience delayed pain relief from oral opioids due to slower absorption.

🔹 Key Nursing Consideration: Monitor for delayed onset or incomplete absorption of oral medications.

Distribution: Where the Drug Goes in the Body

After absorption, drugs distribute into different body compartments (fat, water, or proteins like albumin).

How Aging Affects Drug Distribution

  • Lower total body water → Hydrophilic (water-soluble) drugs reach higher concentrations faster.

  • Higher body fat percentage → Lipophilic (fat-soluble) drugs stay in the body longer.

  • Lower albumin levels → More free (active) drug in the bloodstream, increasing drug effects and side effects.

🔹 Clinical Example:

  • Diazepam (a fat-soluble drug) lasts longer in elderly patients due to increased fat stores, leading to prolonged sedation.

  • Warfarin (a protein-bound drug) becomes more potent with low albumin levels, increasing bleeding risk.

🔹 Key Nursing Consideration: Watch for prolonged effects of fat-soluble drugs and monitor albumin levels for protein-bound drugs.

Metabolism: Breaking Down the Drug

Most drug metabolism happens in the liver, where enzymes (like CYP P450) break down medications.

How Aging Affects Metabolism

  • Slower liver metabolism → Drugs stay in the body longer, increasing their effects and side effects.

  • First-pass metabolism declines → Oral drugs that rely on liver processing (like propranolol) may reach higher blood levels than expected.

🔹 Clinical Example: Older adults metabolize benzodiazepines (like lorazepam) more slowly, increasing sedation risk.

🔹 Key Nursing Consideration: Use lower doses and longer intervals between doses for medications processed by the liver.

Elimination: Clearing the Drug from the Body

The kidneys are responsible for clearing many medications. However, kidney function naturally declines with age—even in healthy adults.

What Happens with Aging?

  • Reduced glomerular filtration rate (GFR) → Slower elimination of renally-excreted drugs.

  • Declining renal blood flow → Drugs stay in the system longer.

🔹 High-Risk Drugs in Older Adults (Renally Excreted)

  • Digoxin → Increased risk of toxicity.

  • Aminoglycosides (antibiotics) → Higher risk of kidney damage.

  • NSAIDs → Can further impair kidney function.

🔹 Key Nursing Consideration: Always check renal function (e.g., creatinine, GFR) before giving renally-excreted medications and adjust doses accordingly.

Quick Recap: How Aging Affects Drug Processing

Pharmacokinetics Stage

Changes in Older Adults

Nursing Implications

Absorption

Slower gastric emptying, higher stomach pH

Monitor for delayed drug onset

Distribution

More fat, less water, lower albumin

Watch for prolonged drug effects

Metabolism

Liver enzymes slow down

Reduce doses of hepatically metabolized drugs

Elimination

Kidney function declines

Monitor renal labs, adjust doses

💡 Remember: Start low and go slow when dosing medications in older adults!

Final Thoughts & Key Takeaways

  • Aging changes how drugs are absorbed, distributed, metabolized, and eliminated—affecting drug effectiveness and safety.

  • Monitor for side effects, as older adults are more sensitive to certain medications.

  • Check renal and liver function before giving medications that rely on these organs for clearance.

  • Always reassess medication needs—polypharmacy is a major concern in older patients.