new_n_lost
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A new antifungal medication is being tested in Phase I clinical trials. Examination of the pharmacokinetic properties of the drug reveals that the half-life of the drug is 6 hours. If a continuous intravenous infusion of this drug were started on a research subject, how long would it take to reach 75% of steady state?
A. 3 hours
B. 6 hours
C. 9 hours
D. 12 hours
E. 18 hours
F. 24 hours
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| hope4dabest
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D
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| icandoittoo
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| DRFP
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| imnalini
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sumbody xplain.................
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| milee99
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pls explain
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| milee99
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for continuous infusion,
concentration of drug in plasma at time t is given by
Cp=[K0/(V.kel)] [1-e^(-kel.t)]
where k0 is infusion rate , kel is elimination constant =0.693/(t1/2) , e is the logarithmic e, V is volume of distribution
also steady state conc at continuous infusion Cpss=k0/(kel.V)=k0/Clearance
substitute 75%cpss =3/4*k0/(kel.V) in first equation
e^(-kel.t)=0.25=1/4
so, e^(kel.t)=4
taking log,
log e 4= kel.t
so, log e (2^2)=2(log e 2)=kel*t
that was log 4 to base e
kel=0.693/(t1/2)=0.693/6=(log e 2 )/6
so, substitute ,2 (log e 2 )=[(log e 2)/6] * t
t/6=2
t=12
so , 12 hrs
i donno if we have to kn ow all these formulas
its not in kaplan
its not in katzung
plus we need log knowledge refreshed
people i hope u all answered it by sum clear logic
please explain if yes
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| new_n_lost
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hope4dabest wrote:
D
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| new_n_lost
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Its quite simple actually
The correct answer is D.
The rule of thumb is that the plasma concentration will reach 50% in one half-life, 75% in two half-lives, 87.5% in three half-lives, etc., so that the difference between the current drug level and 100% halves with each half-life. In this instance, it takes two half-lives to reach 75%. The half-live of this drug is 6 hours, so two half-lives is 12 hours.
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| new_n_lost
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Katzung Board Review Pg 23 Figure 3-3 7th ed. Explains it well.
Kaplan 2004ed. pg 14 explains the Steady state effects.
The trick in this question was the continuous infusion whereas if we look at the graph given in Kaplan Fig 1-1-18 explains it all.
If a single bolus was given the plasma conc wud have decreased by 50% every 6 hrs now if the infusion was going at a steady state the after 6 hrs the drug falls to 50% of initial conc. and after another 6hrs the state becomes half of infused drug in 6hrs plus the leftover in the plasma.
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FORUM RULES-- Those who believe in telekinesis, raise my hand. I get enough exercise just by pushing my luck --P4U World.." The pure and simple truth is rarely pure and never simple."
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| new_n_lost
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The tricky word was Continous infusion now if it were asked wht wud be the time taken to achieve the 75% of the required steady state we all wud have reached the answer.
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FORUM RULES-- Those who believe in telekinesis, raise my hand. I get enough exercise just by pushing my luck --P4U World.." The pure and simple truth is rarely pure and never simple."
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| milee99
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right
thanks
i got confused d/t continuous infusion
and got psyched out wid this equation
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| milee99
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| Ig F
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thnx nnl..u made it sound really easy.......D is the correct answer
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