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Old 07-17-2013
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Listening Pharmacology question, someone please explain

I am confused here pls explain this question one for me

Q:if drug is known to be distributed into TBW, what dose (mg) is needed to obtain an initial plasma level of 5mg/L in patient weighing 70kg?

a)210
b)150
c)110
d)50
e)35

The answer is 210, but i cant find a way to get there, please someone explain
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Old 07-17-2013
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Arrow Stuck

Im stuck on the TOTAL body weight part, i know is a loading dose question but how can you get the total body weight in liters with a given weight?
or only cause i know 70kg equals 42liters is the way that i can do this?
what i mean is what if the test gives me another weight..
thats why i want to know how do you get with the 42 liters part
please!!
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Quote:
Originally Posted by drpisho View Post
Im stuck on the TOTAL body weight part, i know is a loading dose question but how can you get the total body weight in liters with a given weight?
or only cause i know 70kg equals 42liters is the way that i can do this?
what i mean is what if the test gives me another weight..
thats why i want to know how do you get with the 42 liters part
please!!
That's because Total body weight(kg) consists 40% nowater mass and 60% total body water(L). It's in renal physiology, just read your book.
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Old 07-17-2013
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TBW = 70 percent of body weight.
in this case its 42 kgs. Tbw multiplied by the reqd plasma level will give u the amt of dose.
ie 42 times 5 =210 in this case.
when a drug diffuses in the body,it enters all the compartments.hence u have to consider TBW.
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Old 07-17-2013
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Quote:
Originally Posted by drpisho View Post
I am confused here pls explain this question one for me

Q:if drug is known to be distributed into TBW, what dose (mg) is needed to obtain an initial plasma level of 5mg/L in patient weighing 70kg?

a)210
b)150
c)110
d)50
e)35

The answer is 210, but i cant find a way to get there, please someone explain
Here I believe they're asking you about loading dose. How is loading dose calculated? by multiplying plasma conc. with volume of distribution(Vd). so LD= Cp x Vd/F. F is bioavailability and is usually 100% when drug is administered intravenously so it should be 1. When they don't mention bioavailability and they say it's given By IV route than you should assume that F=1.
TBW is total body water. Approximately 60% of the body is made of water(varies between male and female and with age). 60% of 70 kg is around 42L. multiply 42 by 5mg/L and you get 210mg.
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Old 07-17-2013
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Wink

so basically everyone is answering this just cause we KNOW that a 70kg person has 42lt?
what if they give me a 56kg person and i have to figure out what the total body water of them is... thats my worry.. whats the formula to get total body water?

thanks everyone for answering
these pharm questions are driving me crazy, i was never good at math no wonder i became a doctor
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Old 07-17-2013
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Quote:
Originally Posted by drpisho View Post
so basically everyone is answering this just cause we KNOW that a 70kg person has 42lt?
what if they give me a 56kg person and i have to figure out what the total body water of them is... thats my worry.. whats the formula to get total body water?

thanks everyone for answering
these pharm questions are driving me crazy, i was never good at math no wonder i became a doctor
you're complicating things buddy. 60% of the body is made of water and 60% of 70 is 42. 60% of 56 is about 34. Have you watched the Conrad Fisher lectures physiology lectures? I remember him saying that he weighed 100kg and has 60 L of water, likening himself to an aquarium, lol.
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Drpisho, it's not just that we know it's 42L, but it's that our current understanding of total body water is that it roughly amounts to 60% of body weight. So the magic number here isn't 42 but 60% instead. 60% of 70kg is 42L and for your earlier comment,total body water of a patient weighing 56kg should come to [60% of 56kg] around 33.6 L and not 42L. Hope it helps clear the confusion.

Quote:
Originally Posted by drpisho View Post
so basically everyone is answering this just cause we KNOW that a 70kg person has 42lt?
what if they give me a 56kg person and i have to figure out what the total body water of them is... thats my worry.. whats the formula to get total body water?

thanks everyone for answering
these pharm questions are driving me crazy, i was never good at math no wonder i became a doctor
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dr mumble (07-17-2013), drpisho (07-17-2013), Leeby (07-18-2013), Novobiocin (07-17-2013)



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