Transcript Bioelectrical Impedance Analysis

A LABORATORY ACTIVITY IN BIOELECTRICAL
IMPEDANCE ANALYSIS
OMRON Body Fat Analyzer (healthgoods.com)
Elliot Mylott ([email protected])
Sabrina Hoffman
Ralf Widenhorn ([email protected])
European Journal of Clinical Nutrition 63, 619-626 (May 2009)
Physics Department
Portland State University, Portland, OR
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
Bio-Impedance Analysis BIA
Evaluation of Nutrition and Hydration Status Using Vectorgram and BiaGram,
(rjlsystems.com)
•Body impedance frequency dependant
•Single Frequency BIA uses 50kHz
•More complex body models approximated
with series circuit
Single Frequency
Series Equivalent
Circuit Analysis
• Comparison of voltage across a known
resistor gives gain and phase
𝑉1
𝑉2
=
𝑍∗𝐼
𝑅𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒 ∗𝐼
=
𝑍
𝑅𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒
• Measured Impedance and Phase Leads to
Reactance and Resistance
Z  R2  X c 2
tan  
Xc
R
R
Z2
1  tan2 
X c  R tan
Measurement I – NI ELVIS and LabView
•Current
•Impedance
•Phase
  360o  t  1T
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 𝜑
Mass Calculation
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
•
Wikipedia, Electrical resistivity and conductivity
Estimate mass of a resistor
2
𝑙
𝑙
𝑅=𝜌
↔ 𝑉=𝜌
𝐴
𝑅
𝑚 = 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 ∙ 𝑉
Example equation to determine total
body water (TBW)
𝑇𝐵𝑊 =
𝐻𝑡 2
0.372
𝑅
+ 3.05 ∙ 𝐺𝑒𝑛𝑑𝑒𝑟 +
0.142 ∙ 𝑊𝑒𝑖𝑔ℎ𝑡 − 0.069 ∙ 𝐴𝑔𝑒
Lukaski and Bolonchuk, Aviation Space and Environmental Medicine 59 1163-1169, 1988)
http://nutrition.uvm.edu/bodycomp/bia/lesson4.html
Multi-frequency BIA
Students can explore
• Series and Parallel
impedance combinations
• More accurate models for
human electrical
characteristics
• Relationship between
frequency and impedance
American Journal of Clinical Nutrition 1996; 64: 388S-96S
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