Jackofknives
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A 50-year-old woman with metastatic breast cancer has elected to participate in the trial of a new chemotherapeutic agent. It is given by constant intravenous infusion of 8 mg/h. Plasma conc (Cp) are measued with the results shown in the table.
Time after start of infusion (h); plasma conc (mg/L)
1, 0.8
2, 1.3
4, 2.0
8, 3.0
10, 3.6
16, 3.7
20, 3.84
25, 3.95
30, 4.0
40, 4.0
(A) Vd is 30L
(B) CL is 2L/h
(C) Elimination follows zero order kinetics
(D) T½ is 8h
(E) Doubling the rate of infusion would result in a plasma conc of 16 mg/L at 40h
Edited by Jackofknives on 08/21/07 - 07:22 AM
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| angel^Doctor
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B
plz provide explanation
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| new_n_lost
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Where is the Question line, its seems to be missing ??
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| keepgoing
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at 30,40h plasma con is constant->saturation point,but cant expand more ,plzz tell JOk interesting now
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| Jackofknives
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yea steady state is 4mg/L
50% of the conc is reached after 4h of infusion, so according to constant infusion principle, half life of the drug is 4h.
dosing rate = CL * Cp => CL = 8 / 4 = 2L/h
t½ = 0.7 * Vd / CL => Vd = t½ * CL * / 0.7 = 11.5L
this drug follows 1st order kinetics
so answer is B
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| angel^Doctor
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[quote=Jackofknives]yea steady state is 4mg/L
50% of the conc is reached after 4h of infusion, so according to constant infusion principle, half life of the drug is 4h. ????? WHY
dosing rate = CL * Cp => CL = 8 / 4 = 2L/h
t½ = 0.7 * Vd / CL => Vd = t½ * CL * / 0.7 = 11.5L HOW ?????
this drug follows 1st order kinetics WHY ?????
PLZ MORE EXPLAINATION
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| Jackofknives
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when drug is infused into the body, drug accumulation and elimination takes place simultaneously.
So if you infuse 100% by the first t½, you will accumulate 100% - t½ amount = 50%
By the next half life: (50% + 100%) – [t½] = 75%
Next one : (75% + 100%) – [t½] = 87.5%
And so on.
t½ = 0.7 * Vd / CL is just a t½ equation dependant on CL and Vd, I dunno how its derived since ivnt had pharm yet but ill let u know when I get hold of a teacher.
Its first order bc accumulation is progressive and clearly elimination process is not saturated.
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| angel^Doctor
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thank u now i get it
except 1 part which is elimination not saturated but it not a big deal 
thank u again
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