Not Eliminated by First-Order Kinetics

For drugs eliminated by first-order kinetics, the relationship between dosing rate and steady-state plasma concentration is given by rearranging Equation 2.3 as follows  

FIGURE 2.8 Metabolism of phenytoin to form p-HPPH and p-HPPH glucuronide. The first step in this enzymatic reaction sequence is rate limiting and follows Michaelis-Menten kinetics, showing progressive saturation as plasma concentrations rise within the range that is required for anticonvulsant therapy to be effective. 

For most drugs, the amount of drug eliminated from the body during any time interval is proportional to the total amount of drug present in the body. In pharmacokinetic terms, this is called/i rsf-order elimination and is described by the equation 

Although these equations deal with total amounts of drug in the body, the equation C — XjV provides a general relationship between X and drug concentration (C) at any time after the drug dose is administered. Therefore, C can be substituted for X in Equations 2.7 and 2.8 as follows  

In the following sections we will review the mathematical basis of some of the important relationships that are used when pharmacokinetic principles are applied to the care of patients. The reader also is 

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