Tue28September1204PM 3

An intravenous infusion of a drug is commenced at a rate of 20 ml/hour. The concentration of the drug in the syringe is 5 mg/mL. The plasma clearance of the drug is 20 L/hour.

Which of the following values best approximates to the plasma concentration of the drug at steady state assuming that the infusion rate remains unchanged?

(Please select 1 option)

5 mg/mL

50 mcg/mL

5 mcg/mL Correct

50 mg/mL

500 mcg/mL

Explanation

When a drug is administered by intravenous infusion the plasma concentration rises in an exponential fashion as a wash-in curve until it attains steady state concentration (the point at which the infusion rate is balanced by the elimination rate or clearance). It will take 4-5 half-lives of the drug to reach this steady state point.

The factors that determine the infusion rate or dose (ID) in mg/hour of a drug are Cpss (target plasma concentration at steady state) and the clearance (CL) in ml/minute or litre/hour.

ID = Cpss × CL

In this example we know the infusion rate of 20 ml/hour. The concentration of the drug is 5 mg/mL. The patient is receiving 100 mg of the drug/hour and the clearance of the drug is 20 L/hour.

ID = Cpss × 20

Therefore,

Cpss = 100 mg/20000 ml

Cpss = 0.005 mg/mL or 5 mcg/mL

The factors which determine a loading dose (LD) include:

Cp - target plasma concentration (mcg/L)

Vd - volume of distribution

F - bioavailability (if not intravenously administered)

LD = Cp × Vd/F

Answer Statistics

1

20%

2

28%

3

39%

4

9%

5

7%

Times answered: 231