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efrex
13th February 2003, 10:09 AM
I'm going through some medical literature that I have to summarize, and found dosages listed in "mg/m^2." It's not that critical to my work, but I figure the more I know the better. What exactly is that unit? Milligrams per meter squared? Kinda weird (and wrong, too, I'd think: wouldn't it be in terms of mass per volume?).

All help, as always, greatly appreciated.

rwald
13th February 2003, 12:59 PM
I'm not sure, but could it be surface area of the person? If this dosage was recommend for a cream or something, it would make sense.

efrex
14th February 2003, 04:27 AM
Nope, it's for an intravenous drug... I see it in other places in the literature as well, so it must be a standard of some sort... :confused:

Agammamon
14th February 2003, 08:27 AM
http://www.remission.org/abvd.shtml
Acording to info on this site it is milligrams per square meter.
The m2 is body area calculated somehow by using height and weight.

Kate
14th February 2003, 09:32 AM
Dosage pertains to the scaling of metabolic rate. Large mammals use less food per gram of body weight than smaller ones. Scaling is about how much less.

Consider two spherical animals. The ratio of their weights (as a stand-in for their volumes, since most flesh is of similar density) is equal to the ratio of their diameters cubed. The ratio of their surface areas is equal to the ratio of their diameters squared.

It turns out, however, that metabolism scales approximately with surface area, not weight. The best way to approximate surface area is to raise the weight (which is easy to measure) to the two-thirds power.

The dosage is often expressed in mg/m^2, but you estimate the "area" via the volume to the 2/3 trick.

(Actually, it seems that the "real" scale factor power is 3/4, not 2/3, and has something to do with the branching patterns of capillaries and other small tubules, rather than heat loss across exposed surfaces. See, for example:

http://www.sciam.com/article.cfm?articleID=000381E9-208F-1C75-9B81809EC588EF21)

efrex
14th February 2003, 09:43 AM
Agammamon & Kate: thanks for the info! and Kate: thanks for bringing up spherical animals... it's been a long time since I've used those approximations ("spherical cow" was a long running joke back in my undergrad days, lo these many years ago).

Soapy Sam
15th February 2003, 09:46 AM
Kate- that's fascinating but worrying. I know adult humans 6'6"@ 350 lbs and 5'0" @ 80lbs. Do their metabolic rates actually vary much?
I presume there must be another correction factor for children and infants?

Mr. Turquoise
17th February 2003, 05:21 AM
Originally posted by Soapy Sam
I presume there must be another correction factor for children and infants?

I believe that pediatricians have their own reference values for dealing with children and infants. I would guess that this is because infants grow more rapidly than children, who grow more rapidly than adults, and thus have different metabolic rates. Vets also have different reference tables for dealing with the various species they encounter. Unfortunately, I do not know the degree to which these values vary.

Mr. Turquoise