PharmSim

Formulas for zero order infusion

During zero order infusion, the plasma concentration of a drug at a certain moment in time depends on:

• the infusion rate

• the volume of distribution

• the clearance

The infusion rate and the clearance determine the steady state plasma level (Css). The elimination rate is a measure for the speed with which the drug leaves the plasma. Again, the decerase in plasma concentration is describes by an a power. Css minus the portion that is eliminated from the plasma yields the plasma concentration:

where and

The drug action over time depends on the plasma concentration, and, in addition on the maximal effect of the drug (Emax), which is 100% in this applet, and the EC50. If the plasma concentration equals the EC50, the drug action should be 50% by definition. Furthermore, when the plasma concentration is approaches 0, the action should also approach zero, and when the plasma concentration equals a number of times the EC50, the action equals 1. In a formula, this looks as follows:

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