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Chapter 4. Pharmacokinetics
Terms in this set (36)
absorption (page 37)
is a process involving the movement of a substance from its site of administration, across body membranes, to circulating fluids. Drugs may be absorbed across the skin and associated mucous membranes, or they may move across membranes that line the gastrointestinal (GI) or respiratory tract. Most drugs, with the exception of a few topical medications, intestinal anti-infectives, and some radiologic contrast agents, must be absorbed to produce an effect. Absorption is the primary pharmacokinetic factor determining the length of time it takes a drug to produce its effect.
affinity (page 39)
AKA attraction. The bone marrow, teeth, eyes, and adipose tissue have an especially high affinity, or attraction, for certain medications. Examples of agents that are attracted to adipose tissue are thiopental (Pentothal), diazepam (Valium), and lipid-soluble vitamins. Tetracycline binds to calcium salts and accumulates in the bones and teeth. Once stored in tissues, drugs may remain in the body for many months and are released very slowly back to the circulation.
blood-brain barrier (page 40)
anatomical structure that prevents certain substances from gaining access to the brain
conjugates (page 40)
side chains that, during metabolism, make drugs more water soluble and more easily excreted by the kidney
dissolution (page 37)
determines how quickly the drug disintegrates and disperses into simpler forms; therefore, drug formulation is an important factor of bioavailability. In general, the more rapid the dissolution, the faster the drug absorption and the faster the onset of drug action
distribution (page 39)
involves the transport of drugs throughout the body. The simplest factor determining distribution is the amount of blood flow to body tissues. The heart, liver, kidneys, and brain receive the most blood supply. Skin, bone, and adipose tissue receive a lower blood supply; therefore, it is more difficult to deliver high concentrations of drugs to these areas.
drug-protein complex (page 39)
rugs bind reversibly to plasma proteins, particularly albumin. Drug-protein complexes are too large to cross capillary membranes; thus, the drug is not available for distribution to body tissues. Drugs bound to proteins circulate in the plasma until they are released or displaced from the drug-protein complex. Only unbound (free) drugs can reach their target cells or be excreted by the kidneys.
duration of drug action (page 43)
is the amount of time it takes for a drug to maintain its desired effect until termination of action.
enterohepatic recirculation (page 42)
recycling of drugs and other substances by the circulation of bile through the intestine and liver. Biliary reabsorption is extremely influential in prolonging the activity of cardiac glycosides, some antibiotics, and phenothiazines. Recirculated drugs are ultimately metabolized by the liver and excreted by the kidneys. Recirculation and elimination of drugs through biliary excretion may continue for several weeks after therapy has been discontinued.
enzyme induction (page 40)
process in which a drug changes the function of the hepatic microsomal enzymes and increases metabolic activity in the liver. For example, phenobarbital causes the liver to synthesize more microsomal enzymes. By doing so, phenobarbital increases the rate of its own metabolism as well as that of other drugs metabolized in the liver. In these patients, higher doses of medication may be required to achieve the optimum therapeutic effect.
excretion (page 41)
Drugs are removed from the body. The rate at which medications are excreted determines the concentration of the drugs in the bloodstream and tissues. This is important because the concentration of drugs in the bloodstream determines their duration of action. Pathologic states, such as liver disease or renal failure, often increase the duration of drug action in the body because they interfere with natural excretion mechanisms. Dosing regimens must be carefully adjusted in these patients.
fetal-placental barrier (page 40)
special anatomical structure that inhibits entry of many chemicals and drugs to the fetus. It prevents potentially harmful substances from passing from the mother's bloodstream to the fetus. Substances such as alcohol, cocaine, caffeine, and certain prescription medications, however, easily cross the placental barrier and can potentially harm the fetus. Consequently, a patient who is pregnant should not take any prescription medication, over-the-counter (OTC) drug, or herbal therapy without first consulting with a health care provider
first-pass effect (page 41)
mechanism whereby drugs are absorbed across the intestinal wall and enter into the hepatic portal circulation.
(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]
hepatic microsomal enzyme system (page 40)
as it relates to pharmacotherapy, liver enzymes that inactivate drugs and accelerate their excretion; sometimes called the P-450 system. The primary actions of the hepatic microsomal enzymes are to inactivate drugs and accelerate their excretion. In some cases, however, metabolism can produce a chemical alteration that makes the resulting molecule more active than the original.
loading dose (page 44)
is a higher amount of drug, often given only once or twice to "prime" the bloodstream with a sufficient level of drug. Loading doses are particularly important for drugs with prolonged half-lives and for situations in which it is critical to raise drug plasma levels quickly, as might be the case when administering an antibiotic for a severe infection.
maintenance dose (page 44)
Before plasma levels can drop back toward zero, intermittent doses are given to keep the plasma drug concentration in the therapeutic range.
metabolism (page 40)
also called biotransformation, is the process of chemically converting a drug to a form that is usually more easily removed from the body. Metabolism involves complex biochemical pathways and reactions that alter drugs, nutrients, vitamins, and minerals. The liver is the primary site of drug metabolism, although the kidneys and cells of the intestinal tract also have high metabolic rates.
minimum effective concentration (page 43)
the amount of drug required to produce a therapeutic effect
onset of drug action (page 43)
represents the amount of time it takes to produce a therapeutic effect after drug administration. Factors that affect drug onset may be many, depending on numerous pharmacokinetic variables. As the drug is absorbed and then begins to circulate throughout the body, the level of medication reaches its peak
peak plasma level (page 43)
occurs when the medication has reached its highest concentration in the bloodstream. It should be mentioned that depending on accessibility of medications to their targets, peak drug levels are not necessarily associated with optimal therapeutic effect. In addition, multiple doses of medication may be necessary to reach therapeutic drug levels.
pharmacokinetics (page 37)
is derived from the root words pharmaco, which means "medicine," and kinetics, which means "movement or motion." Pharmacokinetics is thus the study of drug movement throughout the body. In practical terms, it describes how the body deals with medications
plasma half-life (t 1/2) (page 43)
the length of time required for the plasma concentration of a medication to decrease by one-half after administration. Some drugs have a half-life of only a few minutes, whereas others have a half-life of several hours or days. The longer it takes a medication to be excreted, the greater the half-life. For example, a drug with a t 1/2 of 10 hours would take longer to be excreted and thus produce a longer effect in the body than a drug with a t 1/2 of 5 hours. If a patient has extensive renal or hepatic disease, the plasma half-life of a drug will increase, and the drug concentration may reach toxic levels. In these patients, medications must be given less frequently, or the dosages must be reduced.
prodrugs (page 40)
have no pharmacologic activity unless they are first metabolized to their active form by the body. Examples of prodrugs include benazepril (Lotensin) and losartan (Cozaar).
therapeutic range (page 43)
The plasma drug concentration between the minimum effective concentration and the toxic concentration
toxic concentration (page 43)
the level of drug that will result in serious adverse effects
what are the four processes of pharmacokinetics?
absorption, distribution, metabolism, and excretion
What are the two processes that drugs primarily use to cross body membranes? How do they work?
1. Active transport. This is movement of a chemical against a concentration or electrochemical gradient; cotransport involves the movement of two or more chemicals across the membrane.
2. Diffusion or passive transport. This is movement of a chemical from an area of higher concentration to an area of lower concentration.
Explain the different types of ways drug molecules can enter into a cell based on their size and attraction.
drug molecules that are small, nonionized, and lipid soluble will usually pass through plasma membranes by simple diffusion and more easily reach their target cells. Small water-soluble agents such as urea, alcohol, and water can enter through pores in the plasma membrane. Large molecules, ionized drugs, and water-soluble agents, however, will have more difficulty crossing plasma membranes. These agents may use other means to gain entry, such as protein carriers or active transport. Drugs may not need to enter the cell to produce their effects. Once bound to receptors, located on the plasma membrane, some drugs activate a second messenger within the cell, which produces the physiological change
what are the four types of types of drug-drug interactions? what does each of them do?
These include the following:
• Addition. The action of drugs taken together as a total.
• Synergism. The action of drugs resulting in a potentiated (more than total) effect.
• Antagonism. Drugs taken together with blocked or opposite effects.
• Displacement. When drugs are taken together, one drug may shift another drug at a nonspecific protein-binding site (e.g., plasma albumin), thereby altering the desired effect.
what factors can affect drug excretion of a drug?
• Liver or kidney impairment.
• Blood flow.
• Degree of ionization.
• Lipid solubility.
• Drug-protein complexes.
• Metabolic activity.
• Acidity or alkalinity (pH).
• Respiratory, glandular or biliary activity.
what factors can affect absorption of a drug?
• Drug formulation and dose.
• Size of the drug molecule.
• Surface area of the absorptive site.
• Digestive motility or blood flow.
• Lipid solubility.
• Degree of ionization.
• Acidity or alkalinity (pH).
• Interactions with food and other medications.
what factors affect the duration of a drug?
Drug concentration (amount of drug given).
• Dosage (how often a drug is given or scheduled).
• Route of drug administration (oral, parenteral, or topical).
• Drug-food interactions.
• Drug-supplement interactions.
• Drug-herbal interactions.
• Drug-drug interactions.
Describe the types of barriers drugs encounter from the time they are administered until they reach their target cells.
For most medications, the greatest barrier is crossing the many membranes that separate the drug from its target cells. A drug taken by mouth must cross the plasma membranes of the mucosal cells of the gastrointestinal tract and the capillary endothelial cells to enter the bloodstream. To leave the bloodstream, it must again cross capillary cells, travel through interstitial fluid, and enter target cells by passing through their plasma membranes. Depending on the mechanism of action, the drug may also need to enter cellular organelles, such as the nucleus, which are surrounded by additional membranes. While seeking their target cells and attempting to pass through the various membranes, drugs are subjected to numerous physiological substances such as stomach acids and digestive enzymes.
Why is the drug's plasma half-life important to nurses?
The plasma half-life is the time required for the concentration of the medication in the plasma to decrease to half its initial value after administration. This value is important to the nurse because the longer the half-life, the longer it takes the medication to be excreted. The medication will then produce a longer effect in the body. The half-life determines how often a medication will be administered. Renal and hepatic diseases will prolong the half-life of drugs, increasing the potential for toxicity.
Describe how the excretion process of pharmacokinetics may place patients at risk for adverse drug effects.
The process of eliminating drugs from the body most often occurs by excretion through the kidneys. Renal impairment will alter this excretion, placing the patient at risk for adverse drug effects, often drug toxicity. Gaseous forms of drugs are eliminated through respiration; patients with impaired respiratory effort or those with respiratory disease may also experience adverse drug effects. Because water-soluble forms of drugs may be eliminated through breast milk, infants of breast-feeding mothers may be at risk for adverse drug effects if the drug crosses through the milk in large enough quantities.
Explain why drugs metabolized through the first-pass effect might need to be administered by the parenteral route.
Many oral drugs are rendered inactive by hepatic metabolism as the drug first passes through that system. Alternative routes of delivery that bypass the first-pass effect (sublingual, rectal, or parenteral routes) may need to be considered for these drugs.
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