A LABORATORY ACTIVITY IN BIOELECTRICAL IMPEDANCE ANALYSIS

Elliot Mylott (emylott@pdx.edu)Sabrina HoffmanRalf Widenhorn (ralfw@pdx.edu)

Physics Department

Portland State University, Portland, OR

European Journal of Clinical Nutrition 63, 619-626 (May 2009)

OMRON Body Fat Analyzer (healthgoods.com)

Bioelectrical Impedance Analysis

OMRON Body Fat Analyzer (healthgoods.com)

RJL Bioelectric Impedance Analyzer (rjlsystems.com\products)

The Body as an RC Circuit

(Liedtke, Principles of Bioelectrical Impedance Analysis 1997)

•Reactive components due to cellular walls•Resistive components due to both intra- and extracellular fluid•Current flows primarily through Fat Free Mass

Series Equivalent Circuit

•Body impedance frequency dependant

•Single Frequency BIA uses 50kHz

•More complex body models approximated with series circuit

Single Frequency Series Equivalent

Bio-Impedance Analysis BIAEvaluation of Nutrition and Hydration Status Using Vectorgram and BiaGram,(rjlsystems.com)

Circuit Analysis

22cXRZ

RcXtan tanRcX

2tan1

2

ZR

• Measured Impedance and Phase Leads to Reactance and Resistance

• Comparison of voltage across a known resistor gives gain and phase

•Current

•Impedance

•Phase

Measurement I – NI ELVIS and LabView

Tto 1360 DAQ must have a high sampling rate.

Measurement II – Apparatus Competition

Main Components:• Function Generator• Differential Amplifier• Gain / Phase Detector

• Outputs DC Voltage proportional to Z and

Mass Calculation

Estimate mass of a resistor

Wikipedia, Electrical resistivity and conductivity

In human testing • Cylinder model• Measure of Fat Free Mass• Length Height• Density approximately equal

to that of water• Coefficients and fitting factors

empirically derived

http://nutrition.uvm.edu/bodycomp/bia/lesson4.html

• Example equation to determine total body water (TBW)

Lukaski and Bolonchuk, Aviation Space and Environmental Medicine 59 1163-1169, 1988)

Multi-frequency BIA

American Journal of Clinical Nutrition 1996; 64: 388S-96S

Students can explore

• Series and Parallel impedance combinations

• More accurate models for human electrical characteristics

• Relationship between frequency and impedance

Summary

Introductory Physics Level Laboratory Exercise Exploring:

• Resistance, reactance

• Parallel and Series Impedance

• Phasor diagrams and phase shifts

• Waveform measurement

• Resistivity, density and mass

• RC Circuits

• Electrical Characteristics of the body