Foundations and Principles of Pharmacology
Objectives
1. Define the key words used in pharmacology and about giving drugs.
2. Explain the differences between the chemical, generic, official, and brand names of medicines.
3. List the basic types of drug actions.
4. Describe the four basic physiologic processes that affect medications in the body.
5. Discuss the differences between side effects and adverse effects.
Key Terms
absorption (ăb-SŎRP-shŭn, p. 32)
additive effect (ĂD-ĭ-tĭv, p. 36)
adverse reactions (ăd-VŬRS, p. 35)
agonists (ĂG-ō-nĭsts, p. 32)
allergy (ĂL-ĕr-jē, p. 35)
anaphylactic reaction (ăn-ă-fĭ-LĂK-tĭk, p.35)
antagonistic effect (ăn-tăg-ŏ-NĬS-tĭk, p. 36)
antagonists (ăn-TĂG-ŏ-nĭsts, p. 32)
bioequivalent (BĪ-ō-ĭ-KWĬ V-ĭ-lent, p. 36)
biotransformation (BĪ-ō-trăns-fŏr-MĀ-shŭn, p. 34)
chemical name (KĔM-ĭ-kăl, p. 32)
desired action (ĂK-shŭn, p. 35)
displacement (dĭs-PLĀS-mĕnt, p. 36)
distribution (dĭs-trĭ-BŪ-shŭn, p. 33)
drug interaction (ĭn-tĕr-ĂK-shŭn, p. 36)
enteral (route) (ĔN-tĕr-ăl, p. 33)
excretion (ĕks-KRĒ-shŭn, p. 34)
first-pass (effect) (p. 34)
generic name (jĕn-ĔR-ĭk, p. 31)
half-life (p. 34)
hepatotoxic (hĕp-ă-tō-TŎK-sĭk, p. 35)
hypersensitivity (hĭ-pĕr-sĕn-sĭ-TĬV-ĭ-tē, p. 35)
idiosyncratic response (ĭd-ē-ō-sĭn-KRĂ-tĭk, p. 35)
incompatibility (ĭn-kŏm-păt-ĭ-BĬL-ĭ-tē, p. 36)
interference (ĭn-tŭr-FĔR-ĕns, p. 36)
nephrotoxic (nĕf-rō-TŎK-sĭk, p. 35)
official name (ō-FĬSH-ŭl, p. 32)
parenteral (route) (pĕ-RĔN-tĕr-ăl, p. 33)
partial agonists (PĂR-shăl ĂG-ō-nĭsts, p. 32)
percutaneous (route) (pĕr-kū-TĀ-nē-ŭs, p. 33)
pharmacodynamics (FĂRM-ă-kō-dĭ-NĂM-ĭks, p. 31)
pharmacokinetics (FĂRM-ă-kō-kĭNĔT-ĭks, p. 31)
pharmacotherapeutics (FĂRM-ă-kō-thĕr-ă-PŪ-tĭks, p. 31)
receptor site (rē-SĔP-tŏr, p. 32)
side effects (SĪD ĕf-FĔCTS, p. 35)
solubility (sŏl-ū-BĬL-ĭ-tē, p. 33)
synergistic effect (sĭn-ĕr-JĬS-tĭk, p. 37)
trade name (TRĀD, p. 32)
Overview
http://evolve.elsevier.com/Edmunds/LPN/
This chapter provides an overview of very basic information from chemistry, physics, anatomy, and physiology that explains the action of drugs in the body (pharmacokinetics, or what the body does to the drug). It also covers basic information on the effects of drugs on the functions of the body (pharmacodynamics, or what the drug does to the body). This information is vital in understanding pharmacotherapeutics, or the use of drugs in the treatment of disease (Box 4-1).
Drug Names
Medications have several different names that may be confusing when you first learn to work with drugs. It is very important to know the different names of a medication so that the wrong drug is not given to a patient. Sometimes a medication is ordered by one name for the drug and the pharmacist labels it with another name for the same drug. For example, Valium (trade name) is the same as diazepam (generic name). It is important to know whether the medication is the same or a different drug.
The most common drug name used is the generic name. This is the name the drug manufacturer uses for a drug, and it is the same in all countries. It is the name given to a drug before there is an official name, or when the drug has been available for many years and more than one company makes the drug. Examples would be penicillin and tetracycline. The American Pharmaceutical Association, the American Medical Association, and the U.S. Adopted Names Council assign generic names. Generic names are not capitalized when written. An example is warfarin.
Another common drug name is the trade name, or brand name. This name is often followed by the symbol ®, which indicates that the name is registered to a specific drug maker or owner and no one else can use it. This is the drug name used in written or TV advertisements and other marketing, and it is often descriptive, easy to spell, or catchy sounding so that prescribers will remember it easily and be more likely to use it. The first letter of the trade name, and sometimes other letters, are capitalized. Examples of trade names are Dimetapp, Lanoxin, and Augmentin.
Chemical names are often the most difficult to remember, because they include the chemicals that make up the drug. These names are usually long and hyphenated, and they describe the atomic or molecular structure. An example is ethyl 1-methyl-4-phenylisonipecotate hydrochloride, the chemical name for meperidine (Demerol).
The final type of name is the official name, which is given by the Food and Drug Administration. Sometimes this name is similar to the brand or chemical name. The first letter of the official name is also capitalized. An example is Ethacrynic acid.
Types of Drug Actions
Drug Attachment (Drug-Receptor Binding)
Drugs take part in chemical reactions that change the way the body acts. They do this most commonly when the medication forms a chemical bond at a specific site in the body called a receptor site (Figure 4-1). The chemical reactions between a drug and a receptor site are possible only when the receptor site and the drug can fit together like pieces of a jigsaw puzzle or a key fitting into a lock. The drug attaches to the receptor site and activates the receptor; the drug has an action similar to the body’s own chemicals.
Some drugs attach to the receptor site but produce only a small chemical response. These drugs are called partial agonists. When a drug attaches at a drug receptor site but there is no chemical drug response these are called antagonists. Some partial agonists and antagonists are able to compete with other chemicals or drugs already bonded to a receptor site and replace them. The Memory Jogger box summarizes the various types of receptor site activity.
Memory Jogger
Drug Receptor Sites
Basic Drug Processes
Drugs must be changed chemically in the body to become usable. There are four basic processes involved in drug utilization in the body. These processes are absorption, distribution, metabolism, and excretion. Drugs have different characteristics, or pharmacokinetics, that determine to what extent these processes will be used. To understand how a drug works, the nurse must understand each of these processes for the specific drug being given.
The Process of Absorption
Absorption involves the way a drug enters the body and passes into the circulation. Absorption takes place through processes of diffusion, filtration, and osmosis. These mechanisms of absorption are more fully described in Box 4-2. How fast the drug is absorbed into the body through these processes depends on the solubility of the drug, the route of administration, and the degree of blood flow through the tissue where the medication is found.
Box 4-2
Mechanisms Involved in Absorption
Diffusion
Diffusion is the tendency of the molecules of a substance (gas, liquid, or solid) to move from a region of high concentration to one of lower concentration.
Osmosis
Osmosis is the diffusion of fluid through a semipermeable membrane; the flow is primarily from the less thick or more concentrated solution to the thinner or less concentrated solution.
Filtration
Filtration is the passage of a substance through a filter or through a material that prevents passage of certain molecules.
All medication must be dissolved in body fluid before it can enter body tissues. The ability of the medication to dissolve is called solubility. To achieve the best possible drug action, sometimes the medication must be dissolved quickly; at other times it should be dissolved slowly. Solubility of the drug is often controlled by the form of the medication; for example, solutions are more soluble than capsules because a liquid is absorbed faster than a tablet, which must dissolve. An injection with an oil base must be chemically changed before absorption can take place, and this holds the drug in the tissues longer, which may be the desired action. When the patient takes water with a tablet, it not only helps in swallowing but also helps dissolve the medication.
The route of administration also influences absorption. The most common medication routes are enteral (directly into the gastrointestinal [GI] tract through oral, nasogastric tube, or rectal administration); parenteral (directly into dermal, subcutaneous, or intramuscular tissue, epidurally into the cerebrospinal fluid, or into the bloodstream through intravenous [IV] injections); and percutaneous (through topical [skin], sublingual [under the tongue], buccal [against the cheek], or inhalation [breathing] administration).
In areas where the blood flow through tissues is very high, medication is rapidly absorbed. Examples of this include placing a nitroglycerin tablet under the tongue right next to blood vessels or spraying steroids into the nose and lungs through a nebulizer. IV medications injected into the bloodstream have the fastest action. Oral or rectal medications usually take much longer because they must dissolve and diffuse across the barrier tissue in the GI tract (the gastric mucosa) before being carried to the body tissues where they will have their action.
The Process of Distribution
Once the medication is absorbed, it must travel throughout the body. The term distribution refers to the ways that drugs move by means of circulating body fluids to their sites of action in the body. The bloodstream and lymphatic system usually carry the drug throughout the whole body. The organs that have the biggest blood supply receive the medication faster, and areas of skin and fat receive the medication more slowly. Some drugs do not pass well through cell membranes with very small passages, such as those covering the placenta and the brain. These are referred to as placental and blood-brain barriers, although the barrier is not a complete barrier, because some drugs and some conditions make it possible for drugs to easily pass through these areas. The various types of tissues, including bone, fat, and muscle, do not absorb equal amounts of the drug. Thus the distribution is different for different drugs.
The chemical properties of a drug also affect how the drug is distributed. Some chemicals bind together with proteins such as albumin (found in the blood plasma), which serve as carriers giving a ride to drugs that are not easily dissolved. The ratio of bound chemical compared with free chemical remains the same in the blood. As more of the free chemical diffuses into the tissues, more of the bound chemical becomes unlocked and thus also available to diffuse.
Some medications are attracted to tissues other than the target receptor sites. For example, medications that dissolve easily in lipids (fats) prefer adipose, or fat tissue, and stores of the medication may build up in these areas. As the medication moving throughout the body binds at the receptor sites, more of the medication stored in adipose tissue will gradually be given up by those cells. Thus a drug that can be stored in the fat cells may remain in the body for a long time while it is slowly released.
The Process of Metabolism
Once the medication is absorbed and distributed in the body, the body’s enzymes use it in chemical reactions through the process of metabolism. Some drugs that are breathed into the lungs or injected into the tissue may go directly into the bloodstream and be carried quickly to the site of action. But many medications have to be stimulated or have activation of pro-drugs before they become usable, while drugs are broken down into smaller usable parts, primarily in the liver, through a series of complex chemical reactions until they become chemically inactive. This process is called biotransformation. When most of a medicine goes very quickly to the liver, a lot of the medication is inactivated on its “first-pass” through the liver before it can be distributed to other parts of the body. That is why some medicines are given sublingually or intravenously; otherwise, patients do not get the amount of medication they require. (For example, only 1 mg of propranolol is required intravenously, but 40 mg are required when the drug is given by mouth.)
Genetic differences in the enzyme pathways in the liver also explain why people respond differently to a drug—whether they grow tolerant of the drug and seem to need larger doses, or whether they are sensitive to the drug and only need a small dose. These enzyme pathways, known as the cytochrome P-450 system, play an important role in the adverse drug reactions patients may have when taking several drugs at the same time or when there are drug-food interactions.
The Process of Excretion or Elimination
All inactive chemicals, chemical by-products, and waste (often referred to as metabolites) finally break down through metabolism and are removed from the body through the process of excretion
