McKenna's Pharmacology, 2e

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C H A P T E R 2  Drugs and the body

by other routes often reach responsive tissues before passing through the liver for biotransformation. ■■ Drug excretion is removal of the drug from the body. This occurs mainly through the kidneys. ■■ The half-life of a drug is the period of time it takes for an amount of drug in the body to decrease to half of the peak level it previously achieved. The half- life is affected by all aspects of pharmacokinetics. Knowing the half-life of a drug helps in predicting dosing schedules and duration of effects. ■■ The actual effects of a drug are determined by its pharmacokinetics, its pharmacodynamics and many human factors that can change the drug’s effectiveness. ■■ To provide the safest and most effective drug therapy, the nurse or midwife must consider all of the possible factors that influence drug concentration and effectiveness. Knowing your strengths and weaknesses helps you to study more effectively. Take a PrepU Practice Quiz to find out how you measure up! ONLINE RESOURCES An extensive range of additional resources to enhance teaching and learning and to facilitate understanding of this chapter may be found online at the text’s accompanying website, located on thePoint at http://thepoint.lww.com. These include Watch and Learn videos, Concepts in Action animations, journal articles, review questions, case studies, discussion topics and quizzes. Barrett, K. E. & Ganong, W. F. (2010). Ganong’s Review of Medical Physiology (23rd edn). New York: McGraw-Hill. Dale, M. M. & Rang, H.P. (2012). Pharmacology (7th edn). Edinburgh: Elsevier, Churchill Livingstone. Dempsey, J., Hillege, S. & Hill, R. (2014). Fundamentals of Nursing and Midwifery: A Person-centred Approach to Care (2nd Australian and New Zealand edn). Sydney: Lippincott Williams & Wilkins. Goodman, L. S., Brunton, L. L., Chabner, B. & Knollmann, B. C. (2011). Goodman and Gilman’s Pharmacological Basis of Therapeutics (12th edn). New York: McGraw-Hill. Gudin, J. & Gudin, J. (2012). “Opioid therapies and cytochrome P450 interactions.” Journal of Pain & Symptom Management, 44(6 Suppl) , S4–14. Guyton, A. & Hall, J. (2011). Textbook of Medical Physiology (12th edn) . Philadelphia: Saunders Elsevier. Martin, J. H. (2009). Pharmacogenetics of warfarin—Is testing clinically indicated? Australian Prescriber, 32, 76–80. Miksys, S., Tyndale, R. F., Miksys, S. & Tyndale, R. F. (2013). Cytochrome P450-mediated drug metabolism in the brain: 2011 CCNP Heinz Lehmann Award paper. Journal of Psychiatry & Neuroscience , 38(3) , 152–163. Porth, C. M. (2011). Essentials of Pathophysiology: Concepts of Altered Health States (3rd edn). Philadelphia: Lippincott Williams & Wilkins. Porth, C. M. (2009). Pathophysiology: Concepts of Altered Health States (8th edn). Philadelphia: Lippincott Williams & Wilkins. BIBLIOGRAPHY

into any plan of care so that obvious problems can be spotted and handled promptly. If a drug just does not do what it is expected to do, further examine the factors that are known to influence drug effects (Box 2.2). Fre­ quently, the drug regimen can be modified to deal with that influence. Rarely is it necessary to completely stop a needed drug regimen because of adverse or intolerable effects. In many cases, the nurse is the caregiver in the best position to assess problems early. ■■ Pharmacodynamics is the study of the way that drugs affect the body. ■■ Most drugs work by replacing natural (endogenous) chemicals, by stimulating normal cell activity or by depressing normal cell activity. ■■ Chemotherapeutic agents work by interfering with normal cell functioning, causing cell death. The most desirable chemotherapeutic agents are those with selective toxicity to foreign cells and foreign cell activities. ■■ Drugs frequently act at specific receptor sites on cell membranes to stimulate enzyme systems within the cell and to alter the cell’s activities. ■■ Pharmacokinetics—the study of the way the body deals with drugs—includes absorption, distribution, biotransformation and excretion of drugs. ■■ The goal of established dosing schedules is to achieve an effective concentration of the drug in the body. This effective concentration is the amount of the drug necessary to achieve the drug’s therapeutic effects. ■■ Arriving at an effective concentration involves a dynamic equilibrium among the processes of drug absorption, distribution, metabolism or biotransformation and excretion. ■■ Absorption involves moving a drug into the body for circulation. Oral drugs are absorbed from the small intestine, undergo many changes and are affected by many things in the process. IV drugs are injected directly into the circulation and do not need additional absorption. ■■ Drugs are distributed to various tissues throughout the body depending on their solubility and ionisation. Most drugs are bound to plasma proteins for transport to responsive tissues. ■■ Drugs are metabolised or biotransformed into less toxic chemicals by various enzyme systems in the body. The liver is the primary site of drug metabolism or biotransformation. The liver uses the cytochrome P450 enzyme system to alter the drug and start its biotransformation. ■■ The first-pass effect is the breakdown of oral drugs in the liver immediately after absorption. Drugs given CHAPTER SUMMARY