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The halflife of a drug is 4 hours and its volume of distribution in a patient is 45
L.
(A) What infusion rate is required to maintain a concentration of 20 mg/L in
this patient?
Q = C
ss
* k * V and k = 0.693/t
1/2
= 0.693/4 = 0.173 hr
1
.
Therefore Q = 20 mg/L * 0.173 hr
1
* 45 L =
155.9 mg/hr
(B) What is the clearance of this drug?
Cl = kV = 0.173 hr
1
* 45 L =
7.8 L/hr
(C) What loading dose would you have to give this patient to bring him to 20
mg/L?
Loading dose = 20 mg/L * 45 L = 900 mg
(D) If you stopped infusing drug into this patient and he had a drug
concentration of 20 mg/L at that time, how much time would elapse until
his body concentration was 3.5 mg/L?
t
0
kt
t
0
1
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View Full DocumentIn accordance with the plateau principle, if the halflife of a drug is 4 hours,
how many hours will it take to reach 75% of the steady state concentration.
f = 1 – e
kt
, f = 0.75, k = 0.693/4 = 0.173, and you solve for t
So 0.75 = 1 – e
0.173t
, so 0.25 = e
0.173t
.
Now take ln of both sides, 1.39 = 0.173t, so t = 8 hrs
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 Spring '08
 SISSON

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