PHARM/PCS V5 · Procedure 26
Pharmacokinetics I: Constant Infusion, First-Order Elimination
Work out the steady-state concentration of a drug given by constant IV infusion, or the infusion rate needed to reach a target level.
What it computes
Describes plasma drug concentration during a constant-rate IV infusion when the drug is cleared by first-order elimination. Two modes: solve the steady-state (saturation) equation Csat = R/(V·ke) for any one unknown given the other three; or work the approach-to-steady-state time course. If t½ is supplied instead of ke, it is converted via ke = 0.693/t½.
Inputs and outputs
You provide
- For the steady-state equation: any three of Csat, the infusion rate R, the volume of distribution V, and ke (or the half-life in place of ke)
- For the approach to steady state: time and C/Cmax, one pair per line
PHARM/PCS reports
- Whichever term of Csat = R/(V·ke) you left blank
- ke and the half-life
- A plot of concentration rising toward Csat over time; for time-course data, C/Cmax against time with the fitted approach to saturation
When to use it
When a drug is given by continuous infusion and you need the steady-state concentration (or the infusion rate to hit a target), or want to know how quickly that plateau is reached. It's the first of the seven-part pharmacokinetics series (Procedures 26-32).
Reading the results
Steady state depends only on rate-in vs. clearance (R, V, ke), not on how you got there, so the saturation equation is exact at plateau, but the approach takes roughly 4-5 half-lives, and a reading taken too early reads low. Confirm ke and V come from the same subject and model as the infusion.
Keep your units consistent
The book's first example mixes micrograms per milliliter, milligrams per hour, and milliliters. The procedure does not convert units, so reconcile them before entering values: with R in mg/hr and V in mL, a Csat of 8 µg/mL goes in as 0.008. The 1986 program could only solve for ke; V5 solves for whichever of the four terms you leave blank.
This page describes the procedure. Calculations run inside the licensed PHARM/PCS V5 application. PHARM/PCS is a trademark of MCS Scientific LLC.