PHARMACOLOGY FOR NURSES A Pathophysiological Approach FOURTH EDITION

PHARMACOLOGY FOR NURSES A Pathophysiological Approach FOURTH EDITION CHAPTER 4 Pharmacokinetics Copyright © 2014, © 2011, © 2008 by Pearson Education, Inc. All Rights Reserved

Application of Pharmacokinetics to Clinical Practice • Pharmacokinetics: the study of drug movement throughout the body • Know how the body handles medication • Understand actions and side effects of drugs • Understand obstacles that a drug faces to reach target cells Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Drugs in the Body • Greatest barrier for many drugs is crossing many membranes. • Enteral route drugs are broken down by stomach acids and enzymes. • Organs attempt to excrete medicines. • Phagocytes may attempt to remove medicines seen as foreign. Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Four Components of Pharmacokinetics • • Absorption Distribution Metabolism Excretion Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Figure 4. 1 The four processes of pharmacokinetics: absorption, distribution, metabolism, and excretion Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Drugs Cross Plasma Membranes to Produce Effects • Diffusion or passive transport • Active transport Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Diffusion or Passive Transport • Molecules move from higher to lower concentration. • Usually small, nonionized, or lipidsoluble molecules Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Active Transport • Chemicals move against concentration or electrochemical gradient. • Usually large, ionized, or water-soluble molecules • Cotransport involves the movement of two or more chemicals across the membrane. Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Absorption • Movement from site of administration, across body membranes, to circulating fluids • Primary factor determining length of time for effect of drug to occur Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Factors Affecting Drug Absorption • • • Route of administration Drug formulation Drug dosage Digestive motility Digestive tract enzymes Blood flow at administration site Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Factors Affecting Drug Absorption • Degree of ionization of drug – In acid of stomach, aspirin is nonionized and easily absorbed by bloodstream. – In alkaline of small intestine, aspirin is ionized and less likely to be absorbed. • p. H of surrounding environment • Drug-drug/drug-food interactions • Dietary supplement/herbal product– drug interactions Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Metabolism of Medications • • • Also known as biotransformation Changes drug so it can be excreted Involves biochemical reactions Liver—primary site Addition of side chains, known as conjugates, makes drugs more water soluble and more easily excreted by the kidneys. Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Metabolism in the Liver • Hepatic microsomal enzyme system (P 450 system) – Inactivates drug – Accelerates drug excretion – Some agents, known as prodrugs, have no pharmacologic activity unless first metabolized to active form by body. Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Oral Drugs Enter Hepatic-Portal Circulation (First-Pass Effect) • Drug is absorbed. • Drug enters hepatic circulation, goes to liver. • Drug is metabolized to inactive form. • Drug conjugates and leaves liver. • Drug is distributed to general circulation • Many drugs are rendered inactive by first-pass effect. Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Figure 4. 4 First-pass effect: (a) drugs are absorbed; (b) drugs enter hepatic portal circulation and go directly to liver; (c) hepatic microsomal enzymes metabolize drugs to inactive forms; (d) drug conjugates, leaving liver; (e) drug is distributed to general circulation Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Metabolism and Pharmacotherapy • Can be decreased metabolic activity in some patients: – Infants and elderly – Patients with severe liver disease – Patients with certain genetic disorders – Dosages need to be adjusted in these patients. Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Distribution of Medications • Distribution involves the transport of pharmacologic agents throughout the body. – Simplest factor determining distribution is the amount of blood flow to body tissues. Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Distribution of Medications • Physical properties of drug have big influence. • Certain tissues, such as bone marrow, have a high affinity, or attraction, for certain medications. Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Drugs Bind with Plasma Proteins • Many drug molecules form drug–protein complexes—binding reversibly to plasma proteins—and thus never reach target cells. • Cannot cross capillary membranes • Drug not distributed to body tissues Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Figure 4. 3 Plasma protein binding and drug availability: (a) drug exists in a free state or bound to plasma protein; (b) drug–protein complexes are too large to cross membranes Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Distribution of Medications • Drugs and other chemicals compete for plasma protein–binding sites. – Drug–drug and drug–food interactions may occur when one drug displaces another from plasma proteins. • Some have greater affinity. • Displaced drug can reach high levels. – Can produce adverse effects Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Distribution of Medications • Blood-brain barrier and fetal-placenta barrier: special anatomic barriers that prevent many chemicals and medications from entering – Makes brain tumors difficult to treat – Fetal-placenta barrier protects fetus; no pregnant woman should be given medication without strong consideration of condition. Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Primary Site of Excretion of Drugs Is Kidneys • Free drugs, water-soluble agents, electrolytes, and small molecules are filtered. • Drug-protein complexes are secreted into distal tubule. • Secretion mechanism is less active in infants and older adults. • p. H of filtrate can increase excretion. Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Renal Failure Diminishes Excretion of Medications • Drugs are retained for extended times. • Dosages must be reduced. Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Other Organs Can Be Sites of Excretion • Respiratory system • Glands • Biliary system Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Enterohepatic Recirculation of Drugs • Drugs are excreted in bile. • Bile recirculates to liver. • Percentage of drug recirculated numerous times • Prolongs activity of drug – Activity of drug may last after discontinuation. Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Figure 4. 5 Enterohepatic recirculation Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Drug Plasma Concentration and Therapeutic Response • Concentration of medication in target tissue is often impossible to measure, so it must be measured in plasma. – Minimum effective concentration— amount of drug required to produce a therapeutic effect – Toxic concentration—level of drug that will result in serious adverse effects Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Drug Plasma Concentration and Therapeutic Response • Concentration of medication in target tissue is often impossible to measure, so it must be measured in plasma. – Therapeutic range—plasma drug concentration between the minimum effective concentration and the toxic concentration Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Plasma Half-Life (t½) of Drugs • Length of time needed to decrease drug plasma concentration by one half • The greater the half-life, the longer it takes to excrete. • Determines frequency and dosages Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

How Drug Reaches and Maintains Therapeutic Range • • Repeated doses of drug are given Drug accumulates in bloodstream. Plateau is reached. Amount administered equals amount eliminated. Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Figure 4. 6 Single-dose drug administration: pharmacokinetic values for this drug are as follows: onset of action = 2 hours; duration of action = 6 hours; termination of action = 8 hours after administration; peak plasma concentration = 10 mcg/m. L; time to peak drug effect =5 hours; t½= 4 hours Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Figure 4. 7 Multiple-dose drug administration: drug A and drug B are administered every 12 hours; drug B reaches therapeutic range faster, because the first dose is a loading dose Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Loading Dose • Higher amount of drug given • Plateau reached faster • Quickly produces therapeutic response Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban

Maintenance Dose • Keeps plasma-drug concentration in therapeutic range Pharmacology for Nursing: A Pathophysiology Approach , Fourth Edition Michael Patrick Adams | Leland N. Holland | Carol Urban