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#1
01-25-2012
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Calculating the steady state concentration of Drug X

A pharmacy fellow is trying to determine what the plasma concentration of an experimental antiarrhythmic agent (Drug X) will be at steady-state. A continuous intravenous infusion of the agent began 6 hours earlier at a rate of 3 mg/min. Drug X has a half-life of 3 hours, a volume of distribution of 120 L, and a clearance of 0.6 L/min. If the rate of infusion remains constant, what will the plasma concentration be at steady-state?

A. 0.005 mg/L
B. 0.4 mg/L
C. 2.0 mg/L
D. 5.0 mg/L
E. 40.0 mg/L
F. 200.0 mg/L
 The above post was thanked by: docusmle9 (01-26-2012), step_enhancer (01-25-2012), zainsyed (05-01-2014)

#2
01-25-2012
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Ans D........

infusion rate = Css * Cl ( clearance )

thus Css = infusion rate / clearance

3/0.6 = 5 mg/ml .......

god i am bad at these ........
 The above post was thanked by: Moloch (01-27-2012), step_enhancer (01-25-2012)
#3
01-26-2012
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The given infusion rate is a maintanence dose so I agree with @ Hitman using the formula answer is D. 5
#4
01-26-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,406 Threads: 118 Thanked 642 Times in 407 Posts Reputation: 652

D. 5

MD = (Cl x Css x infusion rate) / bioavailability
3 = (0.6 x Css x 1) / 1
3 = 0.6 x Css
Css = 3 / 0.6
Css = 5
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#5
01-26-2012
 USMLE Forums Addict Steps History: Not yet Posts: 124 Threads: 13 Thanked 59 Times in 43 Posts Reputation: 69

yup!!!! u guys r rite.
the ans is....... DDDDD.... 5.0mg/L
 The above post was thanked by: docusmle9 (01-26-2012), Moloch (01-27-2012)
#6
04-15-2013
 USMLE Forums Scout Steps History: Not yet Posts: 13 Threads: 4 Thanked 3 Times in 3 Posts Reputation: 13

Quote:
 Originally Posted by Dr.NickRiviera D. 5 MD = (Cl x Css x infusion rate) / bioavailability 3 = (0.6 x Css x 1) / 1 3 = 0.6 x Css Css = 3 / 0.6 Css = 5

MD=Cl x Css x DOSING INTERVAL ( not infusion rate) / f
#7
11-08-2016
 USMLE Forums Newbie Steps History: --- Posts: 1 Threads: 0 Thanked 0 Times in 0 Posts Reputation: 10

I have a more or less equal problem.

I want to calculate the Css with only know the urine concentration (0,5µg/l) and the half life time (60days).

It is a toxin that everybody takes up during the day by inhalation. So the Problem is that everybody takes up a different dose.

Has somebody any idea?

 Tags Pharmacology-, Step-1-Questions

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