The following pharmacology definition has been taken from the Pharmacology and Experimental Therapeutics Department Glossary at Boston University School of Medicine.

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/T)/kelVd. Since, kelVd equals total clearance: Css = (D/T)/ClT, 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/t½

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.

The copyright of the quoted is hold by Trustees of Boston University. Permission has been granted for its use in this blog.

Recommended pharmacokinetics reading

  • Pocket Guide: Pharmacokinetics Made Easy (2009)
  • Basic Clinical Pharmacokinetics (2009)
  • Concepts in Clinical Pharmacokinetics (2010)
  • Clinical Pharmacokinetics, 4th Edition (2008)
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