Drug Action Pharmacokinetics

Understanding Pharmacokinetics

Overview of Pharmacokinetics

• Pharmacokinetics refers to the processes that describe what the body does to a drug, focusing on its movement and action within the body.

• The acronym ADME represents the four key processes: Absorption, Distribution, Metabolism (or Biotransformation), and Excretion (or Elimination).

Absorption Process

• Absorption is defined as the movement of a drug from its site of administration into the bloodstream.

• There are three major mechanisms for absorption: passive diffusion, active transport requiring energy or carriers, and pinocytosis involving vesicle formation.

Factors Affecting Drug Absorption

• Key factors influencing absorption include:

• Rate of dissolution: Liquid forms absorb faster than tablets or capsules.

• Surface area: Larger areas enhance absorption rates.

• Blood flow: Increased blood flow correlates with improved absorption.

• Lipid solubility: Lipid-soluble drugs pass through membranes more easily than water-soluble ones.

pH Influence on Absorption

• The pH level affects drug ionization; for example, weak acids like aspirin are less ionized in acidic environments (like the stomach), facilitating easier passage through gastric lining.

Bioavailability Concept

• Bioavailability measures how much of an administered drug dose reaches systemic circulation. Oral drugs often have less than 100% bioavailability due to first-pass metabolism in the liver before reaching systemic circulation.

Distribution of Drugs

Mechanisms of Distribution

• Distribution involves moving drugs from systemic circulation to target sites. It is influenced by factors such as blood flow and tissue affinity.

Protein Binding Dynamics

• Drugs can form reversible bonds with proteins in the body; albumin is a significant protein involved due to its abundance and size which keeps it in circulation.

Free vs Bound Drug Molecules

• Only unbound (free) drug molecules can exit blood vessels to exert pharmacological effects. Bound drugs remain inactive as they cannot penetrate capillary walls.

Balance Between Free and Bound Drugs

• As free drug levels decrease in circulation, more bound drugs are released from proteins to maintain equilibrium. This balance is crucial for effective therapeutic responses.

Clinical Implications

Considerations for Nursing Practice

Drug Binding and Bioavailability

Understanding Drug Protein Binding

• The degree to which a drug binds to proteins affects its bioavailability, influencing how much of the drug is available for therapeutic effect.

• Highly protein-bound drugs can lead to drug-to-drug interactions and potential toxicity, especially in patients with liver or kidney disease or malnourishment due to lower serum albumin levels.

Implications of Serum Albumin Levels

• Low serum albumin levels result in fewer protein binding sites, leading to increased free drug circulation and potential toxicity.

• The blood-brain barrier (BBB) selectively allows lipid-soluble drugs or those with transport systems to penetrate, while both lipid-soluble and insoluble drugs can cross the placental barrier during pregnancy.

Nursing Responsibilities During Pregnancy and Lactation

Educating Patients on Drug Safety

• Nurses must educate pregnant mothers about the implications of taking medications due to the selective nature of the placental barrier.

• It is crucial for nurses to check which drugs may be secreted into breast milk before administering them to lactating patients.

Metabolism and Drug Elimination

Liver's Role in Drug Metabolism

• Most drug metabolism occurs in the liver, primarily through cytochrome P450 enzymes; liver conditions like hepatitis can alter this process.

• Monitoring liver function through laboratory tests such as SGPT (ALT) is essential for assessing potential drug toxicity.

Understanding Half-Life

• Half-life refers to the time required for a drug's concentration in the body to decrease by 50%, affecting dosing intervals.

• For example, aspirin has a half-life of 3 hours; after this period, its concentration reduces significantly, impacting how often it should be administered.

Excretion Mechanisms

Pathways for Drug Removal

• Drugs are eliminated from the body via urine, bile, sweat, saliva, breast milk, and expired air; understanding these pathways is vital for patient education.