Sampling blood following cessation of infusion

In this example, the infusion of procainamide HCl is initiated at a rate of 6.367 mgkg h and the dmg is infused for 3h (one half life of the dmg). At 3h, the infusion is switched to the second infusion rate of 3.183 mgkg h.  

The first infusion rate will enable the desired procainamide plasma concentration of 6pgmL to be reached in this patient at 3h, and the second infusion rate will maintain the procainamide plasma concentration at that level (6 pg mL ) until the cessation of the second infusion rate. 

In this example, please note, if it is desired to attain a higher procainamide plasma concentration at a given time (3h in this example), the first and second infusion rates will be higher however, the ratio of two infusion rates will remain unaffected. If, however, it desired to attain the same procainamide plasma concentration (i.e. 6pgmL ) sooner than 3h (i.e. sooner than one half life of the drug), the ratio of first to second infusion rates will be greater. 

Equation 10.24 is essentially the same as Eq. 1.17 (p. 15). In essence, once the infusion is stopped, there is no drug entering the blood and the dmg present in the blood follows a mono-exponential decline that is identical to that seen for an intravenous bolus injection. 

Equation 10.25 is virtually identical to Eq. 1.18 (p. 15) and, therefore, the profile will look similar to that for intravenous bolus administration (Fig. 10.14). 

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