pharmacology

PHARMACOLOGY IN NURSING

  • Definition: Pharmacology is described as the study of the actions of drugs. It encompasses knowledge from other interrelated sciences, especially pharmacokinetics and pharmacodynamics.

  • Importance for Nurses: Understanding different pharmacological classes equips nurses with the knowledge needed to discern how drugs operate within the body to attain therapeutic effects.

PHARMACOKINETICS

  • Definition: The process of drug movement throughout the body that is essential for achieving a drug's therapeutic action.

  • Key Biochemical Processes: Medications go through various processes including absorption, distribution, metabolism, and excretion.

ABSORPTION

  • Definition: The transmission of medications from their site of administration (e.g., gastrointestinal (GI) tract, muscle, skin, or subcutaneous tissue) to the bloodstream.

  • Common Routes of Administration:

    • Enteral (via GI tract)

    • Parenteral (injection)

    • Each route demonstrates a unique absorption pattern.

  • Factors Determining Absorption:

    • Rate of Absorption: Affects how quickly a drug takes effect.

    • Amount of Absorption: Influences the drug's intensity in the body.

    • Route of Administration: Impacts the rate and extent of absorption.

FACTORS AFFECTING ABSORPTION

  • General Factors:

    • Blood circulation

    • Pain and stress levels

    • Food characteristics: texture, fat content, and temperature

    • pH levels

    • Route of administration

    • Epithelial lining of the GI tract

  • Considerations for Geriatric Patients:

    • IM absorption may be erratic

    • Reduced salivary flow complicating swallowing

    • Transdermal absorption is unpredictable

    • Cannot crush timed-release or enteric-coated tablets

    • Increased alkalinity can affect absorption

    • Slower gastric emptying time

    • Decreased GI motility and blood flow

ROUTE: ORAL

  • Mechanism: Drug movement occurs from the GI tract to the liver via the portal vein.

  • Key Concept: The First pass effect is significant, impacting bioavailability.

  • Factors Influencing Absorption Pattern:

    • Stability and solubility of medication

    • GI pH and emptying time

    • Presence of food in the stomach or intestines

    • Concurrent medications and their forms

ROUTE: SUBCUTANEOUS AND INTRAMUSCULAR

  • Barriers to Absorption:

    • The capillary walls have large spaces between cells, presenting no significant barrier.

  • Absorption Pattern Determinants:

    • Solubility of medications; highly soluble ones have rapid absorption (10-30 min)

    • Blood perfusion at injection sites; rapid absorption at sites with high blood perfusion and slow at low blood perfusion.

ROUTE: INTRAVENOUS

  • Barriers to Absorption: There are no barriers as the drug is administered directly into the vein.

  • Absorption Patterns:

    • Immediate entry into the blood

    • Complete absorption, reaching the blood in its entirety.

DISTRIBUTION

  • Definition: The transportation of drugs throughout the body to body fluids and sites of action.

  • Key Factors:

    • Protein binding and fat solubility influence distribution.

    • Organs with the most extensive blood supply receive the drug most rapidly.

    • NOT all drugs can pass the blood-brain barrier or the placental barrier.

FACTORS AFFECTING DISTRIBUTION

  • Impact on Geriatric Patients:

    • Total body water content is reduced.

  • Impact by Gender:

    • Women typically have higher total body fat.

  • Circulatory Factors:

    • Conditions that inhibit blood flow or perfusion can affect distribution.

  • Cell Membrane Permeability: The drug must navigate through tissues and membranes.

  • Plasma Protein Binding: Medications compete for binding sites in the bloodstream, primarily albumin.

  • Pediatric Considerations:

    • Infants have higher total body water content, necessitating higher doses on a mg/kg basis, lower body fat, and reduced protein binding.

METABOLISM – HALF LIFE

  • Definition: Drug metabolism changes medications into less active forms via enzyme action.

  • Half-life (t½): The time required for the concentration of the drug to be reduced by half. Also includes concept of loading dose.

FACTORS AFFECTING METABOLISM & HALF LIFE

  • Age Effects:

    • Infants: Enzymatic activity takes several weeks to develop.

    • Older Adults: Diminished liver cells and reduced blood flow impact metabolism.

  • Other Influencing Factors:

    • Genetics, smoking, diet, other medications, and liver disorders can alter metabolism.

SPECIFIC METABOLIC ENZYMES

  • Some medications increase the activity of metabolism enzymes, leading to quicker drug metabolism and potential need for dosage adjustments to maintain therapeutic levels.

  • CYP2C9 Enzyme: Involved in the metabolism of many common drugs, including:

    • Glipizide (Glucotrol)

    • Losartan (Cozaar)

    • Phenytoin (Dilantin)

    • Warfarin (Coumadin)

FIRST PASS EFFECT

  • Definition: The liver can activate certain medications on their first pass, necessitating non-enteral routes (sublingual, IV) which also exhibit rapid effects.

  • Similar Metabolic Pathways: Problems arise when two drugs share metabolic pathways; one may not be metabolized effectively.

  • Nutritional Status: Malnutrition can lead to deficits in enzyme production needed for metabolism.

EXCRETION

  • Definition: The elimination of medications from the body, primarily through the kidneys, though they can also exit through the liver, lungs, intestines, and exocrine glands (e.g., breast milk).

  • Methods of Excretion: Both active drug and its metabolites are excreted from the body.

  • Renal Function in Infants:

    • Preterm infants have about 15% renal capacity relative to adults.

    • Neonates have 35%.

    • Adult function is achieved at 9 to 12 months.

PHARMACODYNAMICS

  • Definition: The study of effects drugs have on the body and how they influence cellular physiology.

  • Interaction Dynamics: How drugs affect target cells, body systems, and organs to produce effects.

  • Primary Effect: The desired response from a drug.

  • Secondary Effect: Borrows effects that may be desirable or undesirable and are often termed side effects.

MEDICATION RESPONSES

  • Dosing: Aims to regulate medication responses to maintain plasma levels within therapeutic and toxic concentrations.

  • Therapeutic Range: When plasma medication levels are effective and non-toxic. Nurses monitor client responses using these therapeutic levels.

PHARMACODYNAMICS

  • Onset: The duration required for a drug to reach its minimum effective concentration.

  • Peak: The highest concentration of drug in the bloodstream.

  • Duration: The length of time a drug exerts a therapeutic effect.

THERAPEUTIC INDEX

  • Definition: A measure of a drug's safety margin. High Therapeutic Index (TI) indicates that monitoring blood levels is unnecessary.

  • Route Consideration: It's critical to consider the route of administration when monitoring for peak levels, which occur when absorption equals elimination.

EXAMPLES OF THERAPEUTIC INDEX

  • Oral Medications: Typically peak plasma levels occur 1 to 3 hours after ingestion.

  • IV Medications: Can peak within 10 minutes.

  • Trough Levels: Blood samples collected just before the next dose, irrespective of the administration route.

  • Plateau: Indicates the drug concentration remains stable in plasma during a series of doses.

HALF-LIFE

  • Definition: The period required for the concentration of a drug in the body to decline to half its initial value. Factors such as liver and kidney function significantly influence half-life.

  • Steady State: Typically, it requires four half-lives to achieve a stable blood concentration wherein medication intake equals metabolism and excretion.

  • Example Calculation:

    • If a drug at 25 mg has a half-life of 6 hours post-administration, it would be as follows:

    • 6 hours: 12.5 mg

    • 12 hours: 6.25 mg

    • 18 hours: 3.13 mg

    • 24 hours: 1.56 mg

AUDIENCE RESPONSE QUESTION

  • Given a 50 mg dose with a half-life of 6 hours, how many milligrams will remain at 24 hours? Answers include: a) 25 mg, b) 12.5 mg, c) 6.25 mg, d) 3.13 mg, e) 1.56 mg.

THERAPEUTIC DRUG MONITORING

  • Significance: Involves measuring drug concentration in blood samples. Crucial for neonates, infants, and children.

  • Purpose: Adjust dosing and frequency to sustain therapeutic levels for potentially toxic drugs.

DRUG TO DRUG INTERACTIONS

  • Effects:

    • Additive Effect: The combined impact of two drugs equals the sum of their effects.

    • Synergistic Effect: The outcome exceeds the effects anticipated from either drug alone.

    • Antagonistic Effect: One drug diminishes the effectiveness of another.

    • Displacement: One drug displaces another from its protein-binding site.

    • Interference: The first drug inhibits the metabolism/excretion of the second drug, enhancing the latter's activity.

    • Incompatibility: Results when two drugs are chemically incompatible.

PEDIATRIC PATIENTS

  • Dosage adjustments are typical throughout various growth stages.

  • Measure liquid medications in mL and ensure correct dilution.

  • Verify dosages before administration.

  • Caution: Aspirin use in children is associated with Reye’s syndrome, and allergic reactions can occur quickly.

TERATOGENS

  • Categories:

    • A, B, C, D, X—different categories denote varying levels of risk in pregnancy and lactation, for both males and females of reproductive potential.

USE IN PREGNANCY CATEGORIES

  • Category A: No demonstrable risk of fetal abnormalities based on well-controlled studies in pregnant women.

  • Category B: Animal studies reveal no harm, but no control studies in pregnant women OR adverse effects in animal studies without risks in human trials observed.

  • Category C: Adverse effects in animal studies, no human studies, or lack of adequate studies both in humans and animals.

  • Category D: Demonstrated risk but benefits may outweigh potential risks in pregnant women.

  • Category X: Proven fetal abnormalities based on studies, contraindicated in pregnant women or those who may become pregnant.

DRUGS KNOWN TO BE TERATOGENIC

  • Notable classifications include:

    • Androgens and estrogenic hormones

    • ACE inhibitors, ethanol, tetracycline

    • Thalidomide, vitamin A, warfarin

    • Angiotensin II receptor antagonists

    • Anticonvulsants, antimanic agents, antithyroid agents

    • Chemotherapy, statins, cocaine

QUESTION

  • Which patient has the greatest percentage of body water? Options: a) Older adult, b) Middle-aged person, c) Infant, d) Toddler.

GERIATRIC PATIENTS

  • Conduct a thorough drug history that includes prescription, OTC, herbal medications, and assessment of nutritional status.

  • Evaluate vision and motor skills.

  • For new symptoms, ascertain whether they result from previously prescribed medications.

  • It is always recommended to start with smaller doses and gradually increase them.

GERIATRIC CONSIDERATIONS

  • Simplify multi-drug regimens.

  • Regularly review the necessity of each medication, evaluating for discontinuation where appropriate.

  • Assess patient’s ability to finance their medications.

  • Awareness of polypharmacy (the concurrent use of multiple medications).

NURSING IMPLICATIONS: OLDER ADULT DRUG DOSING AND MONITORING

  • Black Box Warnings: Indicate notable risks or life-threatening complications.

  • Beers Criteria: Identifies potentially inappropriate medication use in older adults.

  • Renal Dosing: Ensures medications are safe based on renal function.

  • Drug-Drug Interactions: Remain vigilant of potential interactions.

  • Safety Information: Always provide necessary safety information to patients regarding their medications.

QUESTIONS ON GERIATRIC PATIENTS

  • Older adults are at risk for polypharmacy; what is that? Options include: tachyphylaxis, drug interaction, polypharmacy, tolerance.

  • When assessing renal function in older adults, which lab value is monitored? Options include: liver enzymes, serum electrolytes, complete blood count, blood urea nitrogen, and creatinine.