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PROGRESS IN ATOMIC MEDICINE
Fluid Compartments
The measurement of the body fluid compartments has been reviewed
by Edelman and Leibman™ and by Levitt and Gaudino.”* This section
will review the measurement of total body water. extracellular fluid, and
blood volume.
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Measurement of Total Body Water: The requirements for measurement
by an isotope dilution technique can be met for total body water. Total
hody water may be measured bythe dilution of deuterated water,*7**?°
triliated water, Hea labelled urea."*7* TS antipyrine. and N-acetyl-+ amino
antipyrine. $8.88 Aniodinated antipyrine derivative has also been prepared
with 1°7.°" Unfortunately. both antipyrine and iodoantipyrine undergo
relatively rapid catabolism or excretion of the test substances, which
precludes their use in patients with loculated fluid collections, such
as in ascites and hydrothorax or in edematous states in which the
diffusion of these molecules is slow. In contrast. tritiated and deuterated
water are clistributed throughout all fluid spaces as rapidly as natural
water and share the same relatively long excretion half-time of approximately 10 days. The tritium method requires a measurement of the
isotope concentration of tritium which emits a very weak § particle.
The liquid scintillation spectrometer. however, has made the measure-
ment of tritium reliable and technically easy.*’ For the measurement of
deuterium. a mass spectrometer or a falling drop apparatus is required.”°*’ The cost of instrumentation and the exacting requirements
of technique of both the mass spectrometer and the liquid falling drop
measurement are such as to make the use of tritium an attractive alternative. However. it should be recognized that tritium is radioactive and
where it is desired to avoid the use of radioisotopes. deuterium does
afford a satisfactory method for measurement of total body water. A
small amount of labelled water may be given either intravenously, intra-
muscularly. or orally and adequate time. usually several hours. allowed
for mixing. Fither urine or plastna maybe analyzed for isotope content.
If plasma is used for radio assay. it is suggested that samples be taken
at about 3. 6. and 9 hours and the specific activity extrapolated to the
time of administration. In the case of urine. a similar procedure can be
used but care must be taken to insure emptying of the bladder 1 hour
hefore the first sample is taken.
The values obtained for total hody water by deuterium and tritium
agree well with that obtained by antipyrine®*** but appear to beslightly.