M06.01.020 Rate of Infusion

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Learning Objective

By the end of this lesson, the learner will be able to explain how the rate of infusion (k₀) affects the time to steady state and the steady-state plasma concentration, and relate these principles to linear (first-order) pharmacokinetics.

Rate of Infusion

When a drug is administered by constant IV infusion, the plasma concentration gradually increases until it reaches steady state (Css).

No matter the infusion rate:

Time to reach steady state depends ONLY on the elimination half-life (t½)
It does NOT depend on the infusion rate.

Thus, slow infusion, moderate infusion, or rapid infusion → all reach steady state in the same time (4–5 half-lives).

Effect of Infusion Rate on Css

The rate of infusion (k₀) determines the steady-state plasma concentration:

$C_{ss} \propto k_0$

Double k₀ → Double Css
Triple k₀ → Triple Css

Because clearance (Cl) is constant for first-order kinetics: $C_{ss} = \frac{k_0}{Cl}$

This is why:

Doubling an oral maintenance dose also doubles average plasma concentration,
as long as the drug follows linear kinetics.
Dose–concentration plots are linear for most drugs using first-order kinetics.

Important Note

The loading dose equation can be used to estimate the amount of drug in the body at any time: $LD = V_d \times C_p$ Given a plasma concentration, one can compute the total amount in the body using Vd.