Comparison of extra-cellular fluid volume measurement in children by ⁹⁹Tc^m-DPTA clearance and multi-frequency impedance techniques

Comparisons of extra-cellular fluid (ECF) volume estimates made by isotope dilution and electrical impedance techniques have been made in a group of 16 children. For each patient an estimate of ECF volume (V_t) was obtained from measurements of the blood clearance of ⁹⁹Tc^m-diethylene triamine penta-acetate (DPTA) which was compared with two estimates (V_i1 and V_i2) of ECF volume obtained from measurements of the whole-body electrical impedance at 50 frequencies in the range 1 kHz to 1.348 MHz and a third estimate V_h based on patient height, L, alone. The observed frequency response of the impedance measurements was fitted to a three-element equivalent-circuit model of whole-body impedance and gave a value of the ECF resistance R. V_i1 was obtained from V_i1=a(L²/R)+b. V_i2 was given by c(W¹2/L²/R)^2/3 where W is the patient weight, and V_h was given by dL²+e. The constants a, b, c, d, e were determined by comparison with V_t and were equal to 0.335 1 Omega m^-2 (standard error = 0.01 1 Omega m^-2, 0.42 1 (0.251), 0.33 1 ( Omega ²kg^-1m^-4)^1/2 0.007 1 ( Omega ²kg^-1m^-4)^1/3, 4.92 1 m^-2 (2.8*10^-5 1m^-2), 0.13 1 (0.41 1), respectively. V_i1, V_i2, V_h Were linearly correlated with V_t (r²=0.98, 0.99, 0.95, respectively, p<0.001), and upper and lower levels of agreement were given by +or-0.95 1 (V_t and V_i1), 1.44 1 and -1.12 1 (V_t and V_i2), +or-1.5 1 (V_t and V_h), respectively. Thus inclusion of the impedance data accounted for greater volume variation, but differences between the techniques were not significant (paired t-test and Mann-Whitney analysis) suggesting that more accurate and detailed measurements are required.