The equation for Cp versus time is shown above. At the end of the infusion period when t = T the plasma concentration can be calculated as:
Equation VI5. Concentration at the End of an IV Infusion
Once the infusion is stopped all we have is a first order elimination.
Then
Equation VI6. Concentration after an IV Infusion has Stopped
where t is time counted from the start of the infusion. Then t  T is the time since the end of the infusion. Then
Equation VI7. Concentration during and after an IV Infusion
for t > T
If t is less than or equal to T then set T = t. Then the term e^{kel * (tT)} becomes = 1 and is not needed.
Figure VI5. Linear Plot of Cp versus Time for Interrupted Infusion. Showing Monoexponential Rise and Fall
That is if we stop the loading infusion and don't start the maintenance infusion.
Figure VI6. Semilog Plot of Cp versus Time. NOTE: Intercept is not Cp^{0}
= 14.1 x 0.5 = 7.05 mg/L
Thus 4 hours after the infusion was stopped the Cp has fallen to half the value at the end of the infusion, remember we started with the drug halflife equal to 4 hours.
Time (hr)  3  5  9  12  18  24 
Cp (mg/L)  12  9  8  5  3.9  1.7 
Figure VI7. Plot of Cp Versus Time after a TwoHour Infusion
The red line drawn through the data points and back to the Yaxis represents the bestfit line.
Calculate kel and V given post infusion Cp versus time data

Copyright 2002 David W.A. Bourne