McKenna's Pharmacology, 2e

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P A R T 1  Introduction to nursing pharmacology

Half-life The half-life of a drug is the time it takes for the amount of drug in the body to decrease to half of the peak level it previously achieved. For instance, if a person takes 20 mg of a drug with a half-life of 2 hours, 10 mg of the drug will remain 2 hours after administration. Two hours later, 5 mg will be left (half of the previous level); in 2 more hours, only 2.5 mg will remain. This information is important in determining the appropri­ ate timing for a drug dose or determining the duration of a drug’s effect on the body. (See Box 2.1 Focus on calculations.) The absorption rate, the distribution to the tissues, the speed of biotransformation and how fast a drug is excreted are all taken into consideration when deter­ mining the half-life of a drug. The half-life that is indicated in any drug monograph is the half-life for a healthy person. Using this information, one can estimate the half-life of a drug for a person with kidney or liver dysfunction (which could prolong the biotrans­ formation and the time required for excretion of a drug), allowing the prescriber to make changes in the dosing schedule. The timing of drug administration is important to achieve the most effective drug therapy. Nurses and midwives can use their knowledge of drug half-life to explain the importance of following a schedule of drug administration in the hospital or at home. Figure 2.4 shows the effects of drug administration on the effective concentration of a drug. A person is taking a drug that has a half-life of 12 hours. You are trying to determine when a 50-mg dose of the drug will be gone from the body: • In 12 hours, half of the 50 mg (25 mg) would be in the body. • In another 12 hours (24 hours), half of 25 mg (12.5 mg) would remain in the body. • After 36 hours, half of 12.5 mg (6.25 mg) would remain. • After 48 hours, half of 6.25 mg (3.125 mg) would remain. • After 60 hours, half of 3.125 (1.56 mg) would remain. • After 72 hours, half of 1.56 (0.78 mg) would remain. • After 84 hours, half of 0.78 (0.39 mg) would remain. • Twelve more hours (for a total of 96 hours) would reduce the drug amount to 0.195 mg. • Finally, 12 more hours (108 hours) would reduce the amount of the drug in the body to 0.097 mg, which would be quite negligible. • Therefore, it would take 4½ to 5 days to clear the drug from the body. Calculations BOX 2.1 Determining the impact of half-life on drug levels

dysfunction can lead to toxic levels of a drug in the body because the drug cannot be excreted. Figure 2.3 outlines the pharmacokinetic processes that occur when a drug is administered orally. Pharmacology: Excretion

Safe medication administration

The liver is very important in metabolising drugs in the body and the kidneys are responsible for a large part of the excretion of drugs from the body. One should get into the habit of always checking a person’s liver and renal function before they start a drug regimen. If the liver is not functioning properly, the drug may not be metabolised correctly and may reach toxic levels in the body. If the kidneys are not functioning properly, the drug may not be excreted properly and could accumulate in the body. Dose adjustment needs to be considered if a person has problems with either the liver or the kidneys.

Drug swallowed

Not dissolved, lost in faeces

Drug dissolved in gastrointestinal fluids

Lost in acid

Dissolved drug reaches intestine

Lost in food, acid, digestion

Drug absorbed by portal system

Drug in liver

Biotransformed to non-effective state Bound to plasma proteins

Drug in circulation

Broken down in tissues Bound to plasma proteins

Drug distributed throughout body

Reaches reactive tissue Excreted by kidneys, lungs, skin, etc. Bound to fat tissue

Drug “does its thing”

FIGURE 2.3  Pharmacokinetics affects the amount of a drug that reaches reactive tissues. Very little of an oral dose of a drug actually reaches reactive sites.