Mailing List Archive

Mailing List: techdiver

Message Display

John Dunk wrote:

>>You keep making this claim but have not supplied any proof.
>>
>>So in the case of *dissolved* N2, just how is it that it's
>>distribution and elimination depend on Vd ?
>
>The statement about drug elimination is correct.I'm still not sure if
>it is correct to consider N2 as a drug (waiting for some clarification
>from Pete myself on this) but his premise on volume of distribution is
>a known pharmacological relationship.
>
>Volume of distribution is based on a known quantity of a drug
>divided by the concentration achieved when that drug is dissolved.
>Example: say you had 10 grams of a drug,and a swimming pool of unknown
>volume.you dissolve the drug in the pool & then take a sample and
>assay it for concentration.Let's say it shows a concentration of
>01grams/liter.You can then determine that the volume of the pool is
>1,000 liters by a simple math relationship.In the body,the fluid
>sampled is blood.Now here's where Vd comes into play.Not all of a
>dissolved drug is contained in the bloodstream.Sometimes the drug is
>taken up in the tissues,resulting in a  higher than expected Vd.For
>instance,disolve that same 10 grams of a particular drug in the
>body,and you might find a concentration of .001 grams/liter,giving you
>an APPARENT volume of distribution of 10,000 liters.Now we all know
>that the body does not contain 10,000 liters of fluid;what has
>happened is some of the drug has been taken up by the tissues,and does
>not show up as concentration in the bloodstream.In order for this
>concentration to be eliminated,it must first move out of the tissues &
>into the bloodstream.

You use the total quantity and the volume to derive a concentration.
I assume you agree to that the amount of drug and the volume of
the pool (Vd) is unimportant and that the significant parameter
is the *concentration*.

If you agree to that the blood is always saturated with N2 you have
your concentration directly.

You state that sometimes N2 is taken up in tissue.
I submit that the *concentration* of N2 in tissue groups
are mathematically related to the concentration of N2 in
the blood.  I.e., the Haldanean model has for about 100 years
done directly what Peter wants to do through measurements
of N2 uptake and Vd measurements.



>Clearance is defined as the rate the drug is
>eliminated ( in liters/hour) and is equal to Vd times k (k being an
>elimination constant for that particular drug).

And if the lung's k(N2) is very large, ppN2 in the blood is fixed
to ppN2 ambient.  Then use ppN2(tissue) = function of ppN2(blood)
and Vd never has to enter.

>There can be many other factors that influence the rate at which a
>drug is absorbed/eliminated in the body,but Vd and k are two of the
>major ones FOR A DRUG.

But maybe there is a better way to specify elimination of drugs
diven given based on partial pressures as opposed to doses measured
in grams ?

John
cc015012@br*.br*.ed*

Navigate by Author: [Previous] [Next] [Author Search Index]
Navigate by Subject: [Previous] [Next] [Subject Search Index]

[Send Reply] [Send Message with New Topic]

[Search Selection] [Mailing List Home] [Home]