Infusion Kinetics

Infusion, as a means of drug administration, involves an effectively continuous flow of a drug solution into the blood stream over a relatively long period of time. (Intravascular injections are separate administrations of drug solutions, each over a short period of time.) A major purpose of an infusion is to maintain a steady blood or plasma concentration of drug over a long period of time, i.e. to achieve and maintain Css.
The Css achieved during infusion of a drug is directly proportional to the rate of drug administration (D/T, or k0), and inversely proportional to both the rate of elimination (kel), and to the volume of body throughout which the drug is distributed: Css =D/TkelVd. Since, kelVd equals total clearance: Css = D/TClT, or Css =k0/ClT. The concentration finally achieved varies directly with the infusion rate and indirectly with the total clearance of the drug (always assuming first-order elimination and a single compartment system).

For a drug given by infusion, and eliminated by first-order kinetics from a one-compartment system, the rate at which Css is achieved depends only on the half-life of the drug. In the absence of other
doses (such as a loading dose [q.v.]) the plasma concentration at any time after beginning the infusion (CT), expressed as a fraction of the Css to be achieved, is given by (1 - f):

CT/Css = 1 - 0.5T/t1/2

After duration of infusion of one half-life, 50% of the final concentration will have been achieved; after a duration of infusion of 4 half-lives, about 95% of the final concentration will have been achieved.

Cf. Css(Css), F, Multiple Dose Regimens, First-Order Kinetics, Compartment