BIOIMPEDANCE MEASUREMENT OF SEGMENTAL FLUID VOLUMES AND HEMODYNAMICS
Leslie D. Montgomery, Ph.D.*, Yi-Chang Wu, M.D. and Wayne A. Gerth, Ph.D.
Background: Bioimpedance has become a useful tool to measure changes in body fluid compartment volumes. An Electrical Impedance Spectroscopic (EIS) system is described that extends the capabilities of conventional fixed frequency impedance plethysmographic (IPG) methods to allow examination of the redistribution of fluids between the intracellular and extracellular compartments of body segments. Methods: The combination of EIS and IPG techniques was evaluated in the human calf, thigh and torso segments of eight healthy men during 90 min of 6° head-down tilt (HDT). Results: After 90 min HDT the calf and thigh segments significantly (P<0.05) lost conductive volume (8 and 4%, respectively) while the torso significantly (P<0.05) gained volume (approximately 3%). Hemodynamic responses calculated from pulsatile IPG data also showed a segmental pattern consistent with vascular fluid loss from the lower extremities and vascular engorgement in the torso. Lumped-parameter equivalent circuit analyses of EIS data for the calf and thigh indicated that the overall volume decreases in these segments arose from reduced extracellular volume that was not completely balanced by increased intracellular volume. Conclusion: The combined use of IPG and EIS techniques enables noninvasive tracking of multi-segment volumetric and hemodynamic responses to environmental and physiological stresses.
Keywords: Bio-Impedance, Impedance Spectroscopy, Fluid redistribution, Hemodynamics, Head-down tilt, Intracellular volume, Extracellular volume
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