Monitoring of segmental intra- and extracellular volume changes using electrical impedance spectroscopy.

Osmotically induced cellular volume changes in the perfused rat hindlimb were used to validate the use of bioelectrical impedance spectroscopy as a method for observing fluid shifts between the intracellular and extracellular spaces. Electrical impedance spectra were measured as cell volumes were manipulated by perfusion with Krebs-Henseleit solutions having different concentrations of NaCl. A simple equivalent circuit model of current conduction through the monitored tissue was fit to each measured spectrum to obtain segmental values of the equivalent intracellular resistance, membrane capacitance, and extracellular resistance. These parameters are theoretically governed by variations in the average cell volume fraction and ionic concentrations in the intra- and extracellular fluid spaces. In accord with this theoretical dependence, the parameters changed systematically and reversibly in conformance with both the magnitudes and directions of the perfusate concentration changes and the resultant cell volume changes. Results indicate that bioelectrical impedance spectroscopy, coupled with computer-aided equivalent circuit analysis, can be used to monitor segmental intercompartmental fluid shifts at minute-by-minute resolution.