Pharmacology Basics: Pharmacokinetics & Pharmacodynamics

Pharmacology Basics: Pharmacokinetics & Pharmacodynamics

Main Takeaway

  • Safe medication administration requires:

    • A strong understanding of pharmacology, which encompasses:

    • How drugs move through the body (pharmacokinetics)

    • How they affect the body (pharmacodynamics)

    • Factors influencing their actions and patient responses across the lifespan.

Learning Objectives

  • Discuss the processes of pharmacokinetics.

  • Use professional resources to identify drug information.

  • Describe the processes of pharmacodynamics.

  • Consider pharmacodynamic differences across the life span.

  • Differentiate among prescription drugs, over-the-counter drugs, herbals, and dietary supplements.

Pharmacology Basics

  • Safe medication administration is a vital nursing role.

  • Nurses make critical daily decisions regarding:

    • Medication appropriateness.

    • Medication effectiveness.

  • A strong understanding of pharmacology is essential for nurses to address:

    • Drug effects.

    • Patient responses.

Pharmacokinetics

  • Definition: The study of how the body handles a drug.

  • Involves four stages:

    1. Absorption

    2. Distribution

    3. Metabolism

    4. Excretion

1. Absorption

  • Definition: Entry of the drug into the bloodstream.

  • Influencing Factors:

    • Route of administration (oral, IV, topical, etc.).

    • Drug solubility and formulation.

    • Presence of food in the stomach.

  • Considerations:

    • Protein-based medications: Swallowed proteins may be deactivated by enzymes in the GI tract.

    • Oral (PO) / Enteral (NGT, GT, OGT):

    • Includes enteric coating, extended-release formulas, and gastric acidity.

    • Enteric-coated (EK) and extended-release (ER) medications MUST NOT BE CRUSHED:

      • Coating protects the stomach.

      • Extended-release designs ensure absorption over time; crushing leads to a large, immediate dose.

    • Parenteral Injection (Subcutaneous, Intramuscular): Delivers medication faster.

    • Intravenous (IV):

    • Fully available to tissues immediately after administration into the bloodstream.

    • Provides an immediate effect.

    • Pulmonary Inhalation: Rapid absorption directly to the site.

    • Topical and Transdermal Application:

    • Creams, lotions, ointments: Generally for local effect.

    • Transdermal patch formulations: For systemic effect, avoiding the first-pass effect.

2. Distribution

  • Definition: Process by which medication is dispersed throughout the body via the bloodstream.

  • Dependent Factors:

    • Blood flow to tissues:

    • Carries medications to destinations.

    • Decreased flow (due to dehydration), blocked/constricted vessels (hypertension), or weakened heart pumping (heart failure) can impair distribution.

    • Plasma protein binding:

    • Albumin is a key protein; its levels can decrease due to malnutrition or liver disease.

    • High protein binding results in less free drug, leading to slower distribution and action.

    • Low protein levels result in more free drug, increasing risk of toxicity.

    • Lipid solubility

    • Blood-brain barrier

    • Placental barrier

3. Metabolism

  • Definition: Process by which drugs are broken down in the body.

  • Process:

    1. Drug enters the bloodstream.

    2. Carried to the liver (primary site).

  • Altering Factors: Age, weight, other medical conditions.

  • First-Pass Effect:

    • Occurs with orally administered drugs, observed as they are broken down in the liver and intestines before reaching systemic circulation.

    • Reduces bioavailability of the drug.

    • May require a higher oral dose for therapeutic effect.

    • Some drugs cannot be given orally due to extensive first-pass destruction.

4. Excretion

  • Definition: Final stage of medication removal from the body.

  • Routes of Excretion:

    • Kidneys: Most common route; they filter blood and excrete drug byproducts and waste in urine.

    • Sweat, tears, reproductive fluids, breast milk.

  • Rate of Excretion Influencing Factors:

    • Age

    • Weight

    • Kidney function:

    • Measured by lab values such as serum creatinine, glomerular filtration rate (GFR), and creatinine clearance.

    • Decreased kidney function affects excretion, requiring drug dosage alterations for safety.

Pharmacodynamics

  • Definition: Focuses on how drugs affect the body.

  • Key Concepts:

    • Mechanism of Action (MOA): Process by which medications produce the desired effect.

    • Drug-receptor interactions.

    • Dose-response relationships.

    • Therapeutic vs. toxic effects.

Agonistic and Antagonistic Effects

  • Agonistic:

    • Definition: Binds tightly to a receptor to produce a desired effect, mimicking natural substances (e.g., hormone, neurotransmitter).

    • Analogy: A key that fits and turns the lock (receptor) to open a door (cause an effect).

  • Antagonistic:

    • Definition: Binds to a receptor but does not activate it, blocking the receptor and preventing other substances from activating it.

    • Analogy: A key that fits but doesn't turn, preventing other keys from working.

Lifespan Considerations

  • Pediatric: Immature liver/kidneys, which may require dosage adjustments.

  • Geriatric: Decreased renal function, increasing the risk of polypharmacy (taking multiple medications).

  • Pregnancy/Lactation: Teratogenic risks (harm to the fetus) and altered metabolism.

Drug Classifications & Regulation

Generic Medications

  • Definition: Safe alternatives to brand-name counterparts.

  • FDA Law: Must contain the same chemically active ingredients.

  • Note: Fillers or base substances may differ, potentially causing varied patient tolerance.

Over-the-Counter (OTC) Drugs & Supplements

  • OTC Medications:

    • Do not require a prescription.

    • Regulated by the FDA.

    • Some prescription medications can be obtained as OTC in smaller doses.

  • Herbs and Supplements:

    • Includes vitamins, minerals, enzymes, botanicals.

    • NOT regulated by the FDA.

    • Most have not undergone rigorous scientific testing for safety.

    • Potential for adverse effects, especially if interacting with prescription medications or containing similar active compounds.

Key Pharmacological Concepts

Half-Life

  • Definition: The amount of time it takes for 50% of the drug to be eliminated from the body.

  • Serves as a measure of:

    • Metabolism.

    • Excretion.

  • Variability among drugs:

    • Short half-life (a few hours): necessitates administration multiple times a day.

    • Long half-life (>12 hours): allows for a single dose every 24 hours.

Onset, Peak, and Duration

  • Onset: When the medication first begins to take effect.

  • Peak: Maximum concentration of medication in the body, showcasing the greatest therapeutic effect.

  • Duration: Length of time the medication produces its desired therapeutic effect.

Therapeutic Window

  • Definition: The range between a minimally effective dose and a toxic dose, representing the safest and most effective treatment range.

Peak & Trough Levels

  • Peak drug level: Drawn when medication is at its highest level in the bloodstream, often shortly after administration.

  • Trough level: Drawn when the drug is at its lowest level in the bloodstream, typically right before the next dose is given.

  • This measurement is used to assess drug accumulation and guide dosing.

Selectivity

  • Definition: How readily a drug targets specific cells to produce the intended therapeutic effect.

  • Selective drugs: Specifically search out target sites, minimizing side effects.

  • Non-selective drugs: May impact many different types of cells and tissues, potentially causing more side effects.

Side Effects vs. Adverse Effects

  • Side Effect:

    • Occurs when the drug produces effects other than the intended effect.

    • Can be anticipated and often expected.

  • Adverse Effect:

    • Unanticipated and unexpected effects that are dangerous to the patient.

    • Can be severe enough to warrant discontinuation of the medication and are considered unexpected.

Patient Safety Reminders

  • Always check the patient's full medication list.

  • Do not rely on memorizing drug cards; utilize professional resources instead.

  • Use lab values to assess medication safety (e.g., kidney function tests).

  • Be aware of potential drug build-up and toxicity, particularly in cases of impaired liver/kidney function.

  • Consider age-related physiological changes that affect drug metabolism and excretion.