COMPARISON OF EXTRACELLULAR FLUID VOLUME MEASUREMENT IN CHILDREN BY TC-99(M)-DPTA CLEARANCE AND MULTIFREQUENCY IMPEDANCE TECHNIQUES

Comparisons of extra-cellular fluid (ECF) volume estimates made by iso tope 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 Tc-99(m)-die thylene triamine penta-acetate (DPTA) which was compared with two esti mates (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 t o 1.348 MHz and a third estimate V-h based on patient height, L, alone . The observed frequency response of the impedance measurements was fi tted to a three-element equivalent-circuit model of whole-body impedan ce and gave a value of the ECF resistance R. V-i1 was obtained from V- i1 = a(L(2)/R) + b. V-i2 was given by c(W(1/2)L(2)/R)(2/3) where W is the patient weight, and V-h was given by dL(2) + e. The constants a, b , c, d, e were determined by comparison with V-t and were equal to 0.3 35 1 Omega m(-2) (standard error = 0.01 1 Omega m(-2), 0.421 (0.251), 0.331 (Omega(2)kg(-1)m(-4))(1/2) 0.0071 (Omega(2)kg(-1)m(-4))(1/3), 4. 991 m(-2) (2.8 x 10(-5) 1m(-2)), 0.131 (0.411), respectively. V-i1. V- i2, V-h were linearly correlated with V-t (r(2) = 0.98, 0.99, 0.95, re spectively, p < 0.001), and upper and lower levels of agreement were g iven by +/- 0.951 (V-t and V-i1), 1.441 and -1.121 (V-t and V-i2), 1.1 51 (V-t and V-h), respectively. Thus inclusion of the impedance data a ccounted for greater volume variation, but differences between the tec hniques were not significant (paired t-test and Mann-Whitney analysis) suggesting that more accurate and detailed measurements are required.