McKenna's Pharmacology, 2e

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P A R T 1  Introduction to nursing pharmacology

Routes of administration Drug absorption is influenced by the route of admin­ istration. Generally, drugs given by the oral route are absorbed more slowly than those given parenterally. Of the parenteral route, intravenously administered drugs are ‘absorbed’ the fastest. 1 The oral route is the most frequently used drug administration route in clinical practice. Oral adminis­ tration is not invasive and, as a rule, is less expensive than drug administration by other routes. It is also the safest way to deliver drugs. People can easily continue their drug regimen at home when they are taking oral medications. Oral administration subjects the drug to a number of barriers aimed at destroying ingested foreign com­ pounds including drugs. The acidic environment of the stomach is one of the first barriers to foreign chemicals. The acid breaks down many compounds and inactivates others. This fact is taken into account by pharmaceuti­ cal companies when preparing drugs in capsule or tablet form. The binders that are used are often designed to break down in a certain acidity and release the active drug to be absorbed. When food is present, stomach acidity is higher and the stomach empties more slowly, thus exposing the drug to the acidic environment for a longer period. Certain foods that increase stomach acidity, such as milk products, alcohol and protein, also speed the breakdown of many drugs. Other foods may chemically bind drugs or block their absorption. To decrease the effects of this acid barrier and the direct effects of certain foods, oral drugs ideally are to be given 1 hour before or 2 hours after a meal. Some drugs that cannot survive in sufficient quantity when given orally are administered by injection directly into the body. Drugs that are injected intravenously (IV) reach their maximum plasma concentration almost at the time of injection, as there is no initial breakdown of the drug nor delay due to absorption. Basically, these drugs have an immediate onset and are fully “absorbed” at administration because they directly enter the blood stream. These drugs are more likely to cause toxic effects because much higher peak concentration of the drug is reached and the margin for error in dose is much smaller. Drugs that are injected intramuscularly (IM) are absorbed directly into the capillaries in the muscle. This takes time because the drug must be picked up by the capillaries and transferred into the veins and the general circulation. Men have more vascular muscles than women do. As a result, drugs administered to men via the IM route reach a peak level faster than they do in women. Subcutaneous injections deposit the drug just

The therapeutic index usually measures the differ­ ence between: • An effective dose for 50% of the people treated • The minimal dose at which adverse reactions occur A drug with a low or narrow therapeutic index has a narrow range of safety between an effective dose and a lethal one. Examples of drugs with narrow margins of safety are warfarin, digoxin and gentamycin. On the other hand, a drug with a high therapeutic index has a wide range of safety and less risk of toxic effects. Loading dose Some drugs may take a prolonged period to reach an effective concentration. If their effects are needed quickly, a loading dose is recommended. Digoxin ( Lanoxin )—a drug used to increase the strength of heart contrac­ tions—and many of the xanthine bronchodilators (e.g. aminophylline, theophylline) used to treat asthma attacks are often started with a loading dose (a higher dose than that usually used for treatment) to reach the effective con­ centration. The effective concentration is then maintained by using the recommended dosing schedule. Dynamic equilibrium The actual concentration (generally this concentration refers to blood or plasma concentration) that a drug reaches in the body results from a dynamic equilibrium involving several processes: • Absorption from the site of entry • Distribution to the active site • Biotransformation (metabolism) in the liver • Excretion from the body These processes are key elements in determining the amount of drug (dose) and the frequency of dose repetition (scheduling) required to achieve the effec­ tive concentration for the desired length of time. When administering a drug, the nurse or midwife needs to consider the phases of pharmacokinetics so that the drug regimen can be made as effective as possible. Absorption To reach responsive tissues, a drug must first make its way into the circulating fluids of the body. Absorption refers to what happens to a drug from the time it is introduced to the body until it reaches the circulating fluids and tissues. Drugs can be absorbed from many different areas in the body: through the GI tract either orally or rectally, through mucous membranes, through the skin, through the lung, or through muscle or subcutaneous tissues (Figure 2.2). Pharmacology: Absorption

1 In case of intravenous administration the drug does not need to be absorbed through any membranes, thus using the term absorption in relation to intravenous administration of a drug is somewhat of a misnomer.