Plasma Halflife And Steadystate Concentration

The manner in which plasma drug concentration rises or falls when dosing is begun, altered or ceased follows certain simple rules, which provide a means for rational control of drug effect. Central to understanding these is the concept of half-life (t¥.) or half-time. Consider the time course of a drug in the blood after an i.v. bolus injection, i.e. a single dose injected in a period of seconds as distinct from a continuous infusion. Plasma concentration will rise quickly as drug enters the blood to reach a peak; there will then be a sharp drop as the drug distributes round the body (distribution phase), which will be followed by a steady decline as drug is removed from the blood by the liver or kidneys (elimination phase). If the elimination processes are first-order, the time taken for any concentration point in the elimination phase to fall to half its value is always the same; in other words, the t]/2, which is the time taken for the plasma concentration to fall by half, is a constant, as is illustrated in Figure 7.2. Note that from the peak concentration the drug is virtually eliminated from the plasma in 5 t^ periods.

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