“Drug administration through the intravenous route over a negligible period of time”
Description
Administering a drug intravenously ensures that the entire dose enters the general circulation. Intravenous administration bypasses the absorption phase and the hepatic first-pass effect. Bioavailability is therefore complete. The drug is then distributed throughout the body and then eliminated by the liver and/or kidney.
Three parameters determine the drug concentration-time profile after administration of an iv bolus:
- Dose: with higher doses, the initial drug concentration is also higher, but its relative rate of decline remains identical.
- Volume of distribution: a larger Vd implies a lower initial drug plasma concentration, but also a longer half-life (t1/2).
- Clearance: greater clearance of the drug leads to a faster rate of decline in the drug plasma concentration, and a shorter half-life (t1/2).
Clinical implications
An iv bolus injection ensures the rapid achievement of very high peak concentrations, as may be required for some drugs, but contra-indicated for others.
With an iv bolus administration the amount of drug delivery is precisely controlled.
Related terms
Distribution phase (early phase): After entering the systemic circulation, the drug is distributed throughout the body. Distribution can determine an early rapid decline in plasma concentration.
Elimination phase (late phase): Once the drug in the plasma and tissues has reached equilibrium, the decline of plasma concentration is driven by elimination of the drug from the body.
Chart Pharmacokinetics
Assessment
Analysing the plasma concentration-time profile after an intravenous bolus injection is very useful to calculate the different parameters such as clearance (CL), half-life (t1/2) and volume of distribution (Vd) of a given drug.
The equation of the plasma concentration-time curve for a drug with a negligible distribution phase is:
$$C=C(O)*e^{-\lambda*t}$$
C(0) = plasma drug concentration at time 0
extrapolated value = D/Vd
D = dose
λ = elimination constant rate = CL/Vd