A drug following first order kinetics is being administered by constant i.v. infusion at a rate of 10 mg/min. Its steady state plasma concentration is 2 mg/min. If the dose rate is increased to 20 mg/dl, what will be the new steady state plasma concentration ?
**Core Concept**
The question tests the understanding of first-order kinetics and the concept of steady-state plasma concentration. First-order kinetics describes the relationship between the rate of drug elimination and its plasma concentration, where the rate of elimination is directly proportional to the plasma concentration. Steady-state plasma concentration is achieved when the rate of drug administration equals the rate of drug elimination.
**Why the Correct Answer is Right**
When a drug is administered by constant IV infusion, the steady-state plasma concentration is directly proportional to the dose rate. In this case, the initial dose rate is 10 mg/min, and the steady-state plasma concentration is 2 mg/L. If the dose rate is doubled to 20 mg/min, the new steady-state plasma concentration will also double, assuming the volume of distribution and clearance remain constant. This is because the rate of drug administration is directly proportional to the dose rate, and the rate of elimination remains constant.
**Why Each Wrong Option is Incorrect**
**Option A:** This option suggests that the new steady-state plasma concentration will remain the same as the initial concentration, which is incorrect because the dose rate has been doubled.
**Option B:** This option suggests that the new steady-state plasma concentration will be halved, which is incorrect because the dose rate has been doubled, not halved.
**Option C:** This option suggests that the new steady-state plasma concentration will be increased by a factor of 4, which is incorrect because the dose rate has been doubled, not quadrupled.
**Clinical Pearl/High-Yield Fact**
When a drug is administered by constant IV infusion, the steady-state plasma concentration can be adjusted by changing the dose rate. However, the volume of distribution and clearance must remain constant for this relationship to hold true.
**Correct Answer:** B. 4 mg/L