Measurement of total body water in intensive care patients with fluid overload

The measurement of total body water (TBW) in critically ill intensive care patients with greatly expanded TBW allows body composition studies to be undertaken in such patients with potentially important clinical consequences. Previous workers in this field have stressed the importance of the distortion of compartmental specific activity resulting from continued intravenous (IV) fluid administration during the period of equilibration and have made attempts to predict the equilibrium value of specific activity from the early arterial kinetics. In this paper a method for the measurement of TBW in critically ill intensive care patients is presented together with results of 16 studies on 11 such patients (mean TBW 54.61). It is shown that the effect of continued IV fluid administration in association with prolonged equilibration is small and that the prediction of TBW from analysis of the early (first hour) arterial kinetics is inappropriate. It is concluded that in such patients the volume of distribution of the isotope is constant after four hours from IV injection and that TBW can be measured with a mean precision of 0.7% (SD) from the fourth, fifth, and sixth hour measurements.