To illustrate first order kinetics we might consider what would happen if we were to give a drug by iv bolus injection, collect blood samples at various times and measure the plasma concentrations of the drug. We might see a steady decrease in concentration as the drug is eliminated, as shown in Figure IV-1.

Figure IV-1 Linear Plot of Concentration versus Time

If we measure the slope of this curve at a number of times we are actually measuring the rate of change of concentration at each time point represented by the straight line tangents in Figure IV-2.

Figure IV-2, Same as Figure IV-1 with Tangents shown.

Now if we plot this rate of change versus the plasma concentration, for each data point, we will get a straight line when first order kinetics are obeyed. This is shown in Figure IV-3.

Figure IV-3, Plot of Cp/t versus Cp for first order plot

This behavior can be expressed mathematically as:-

Equation IV-1

Copyright 2001 David W.A. Bourne