Question

A drug (CL = 6 L/h, Va= 150 L) is to be administered as an IV infusion. A steady-state plasma concentration of 1.2 mg L is desired. Calculate the following: (a) a suitable infusion rate, Rint (b) assuming it would take 4 half-lives to achieve the steady-state, how long would it take this drug to achieve this steady-state, tss (c) the value of a loading dose that would immediately achieve the desired steady-state plasma concentration of 1.2 mg/L, DL (d) when the infusion is stopped once it has reached the steady-state, what would be the plasma concentration, Cp, one hour after termination of the infusion?

Answer 1

Solution:

(a). Elimination rate constant should be calculated as

Ke = CL /Vd

Ke = 6 /150

Ke = 0.04

Infusion rate K0 = Cl. Cpss / S.F

Cl = 6L/hr

CPss = 1.2mg/L

S = 1

F =1

Ko = 1.2 X 6 / 1X 1

K0 = 7.2mg/h

Infusion rate is 7.2mg/h

(b) Half life t_1/2 = 0.693 Vd / Cl

= 0.693 X 150 / 6

= 17.325

tss = 4 x 17.325

= 69.3

Time to steady state is 69h.

(c) Loading Dose D_L = CPss . Vd / S.F

CPss = 1.2mg/L

Vd = 150

S = 1

F = 1

By substituting on the above formula,

D_{L =} 1.2 X 150 / 1 X 1

Loading Dose D_L = 180mg.

(d). Plasma concentration after termination of infusion Cp = CPss .e^-KT

CPss = Plasma steady state concentration =1.2mg/L

e^-KT = time elapsed since termination = 1h

CP = 1.2 X 1

CP = 1.2