Extra-cellular volume estimation by electrical impedance--phase measurement or curve fitting: a comparative study.
In order to determine body fluid shifts between the intra- and extra-cellular spaces, multifrequency impedance measurement is performed. According to the Cole-Cole extrapolation, lumped values of intra- and extra-cellular conduction can be estimated which are commonly expressed in resistances Ri and Re respectively. For this purpose the magnitude and phase of the impedance under study are determined at a number of frequencies in the range between 5 kHz and 1 MHz. An approach to determine intra- and extra-cellular conduction on the basis of Bode analysis is presented in this article. On this basis, estimation of the ratio between intra- and extra-cellular conduction could be performed by phase measurement only, midrange in the bandwidth of interest. An important feature is that the relation between intra- and extra-cellular conduction can be continuously monitored by phase measurement and no curve fitting whatsoever is required. Based on a two frequency measurement determining Re at 4 kHz and phi(max) at 64 kHz it proved possible to estimate extra-cellular volume (ECV) more accurately compared with the estimation based on extrapolation according to the Cole-Cole model in 26 patients. Reference values of ECV were determined by sodium bromide. The results show a correlation of 0.90 with the reference method. The average error of ECV estimation was -3.6% (SD 8.4), whereas the Cole-Cole extrapolation showed an error of 13.2% (SD 9.5). An important feature of the proposed approach is that the relation between intra- and extra-cellular conduction can be continuously monitored by phase measurement and no curve fitting whatsoever is required.
Goovaerts
H G
HG
Department of Clinical Physics and Informatics, University Hospital Vrije Universiteit, Amsterdam, The Netherlands. hg.goovaerts@azvu.nl
Faes
T J
TJ
de Valk-de Roo
G W
GW
ten Bolscher
M
M
Netelenbosch
J C
JC
van der Vijgh
W J
WJ
Heethaar
R M
RM
D017097:Electric Impedance; D005110:Extracellular Space / Q000502:physiology*; D006801:Humans; D007424:Intracellular Fluid / Q000502:physiology*; D008954:Models, Biological; D010992:Plethysmography, Impedance; D012016:Reference Values