Jovan Popovich & Michael Moeller
Projects 1
Professor Nasir
October 27, 2013
Project Report: Peripheral Edema Sensor
Abstract
Edema is excess fluid buildup in the interstitial tissue due to a number of several major
disorders such as heart failure, liver disease, kidney disease, and blood clots. Currently there is
no diagnostic tool to measure peripheral edema, only a test for pitting edema which does not
have a set standard for the different levels of severity. A possible method for measuring edema
uses bioelectrical impedance values measured in the extremities. Impedance analysis is a
technique that can display the differences between the resistive and reactive components which
change as the swelling increases. The proposed solution will measure the bioelectric impedance
of the lower limbs which can be compared to a baseline value of the patient when there is no
edema present. The impedance values will change as the swelling in the limbs increases or
decreases, providing doctors with concrete statistical data on the patient’s edema level instead of
relying on a visual test. This is useful because it can show how effective a particular treatment
for the underlying cause of the edema is.
Bioelectrical impedance testing will be conducted first on pig legs and then on human test
subjects using a handheld LCR meter. Electrode placement, around the ankles, must be
determined and secured in order to maintain testing fidelity. Results will be compiled and
checked against one another using mean value comparisons. Eventually, hospital patients with
leg edema should be tested.
Background and Project Goal
Edema is swelling caused by excess fluid being trapped in interstitial tissues. Edema is
actually a symptom of several disorders, some more dire than others. The main culprit is
congestive heart failure, specifically, right ventricular failure which causes a decreased blood
volume to be pumped to lungs. This decrease in efficiency causes blood that is not returned to
the heart, which can eventually pool in the legs. Right sided ventricular failure can be linked to
chronic pulmonary disease and is often seen in conjunction with right ventricular failure. This
eventually leads to fluid leakage from the capillaries to gather in the surrounding tissue causing
swelling. Often these effects are seen in conjunction with venous insufficiency in the legs.
Kidney failure, specifically nephrotic syndrome is another prime cause of edema. This
causes a high loss of albumin from the urine. The loss of albumin means that there is less
regulation of homeostatic water balance in the blood vessels, allowing for leakage into the
interstitium. Other causes of edema include liver disease, blood clots, and tumors. When edema
is present, it can cause painful swelling and difficulty walking, as well as increasing heart rate
and blood pressure. Figure 1 below shows a significant case of pitting edema.
Figure 1 - An example of a healthy foot versus one that has severe swelling due to edema
The goal of this project is to develop a testing method for assessing the severity of
peripheral edema in lower extremities. This would be accomplished by measuring the impedance
between different areas on the legs and comparing the result to a known, healthy value.
Instrumentation
We currently are using an Agilent U1733C LCR meter which uses a two probe
measurement system to send a constant voltage signal (either 0.70Vrms or 0.74Vrms) and
measure the change in current between the two, yielding an impedance value comprising the
resistance-reactance magnitude and phase angle. The meter can send out a signal at five
different frequency levels: 100Hz, 120Hz, 1kHz, 10kHz, and 100kHz. These five frequencies will
be used for each testing trial to give a representation of the reactance characteristics of the
tissues due to boundary capacitances. The measurement setup is outlined below in figure 2.
Figure 2. LCR meter and test set-up
In conducting these tests, it is crucial to lower the skin contact impedance, as that is not the
target measurement. This is why use of silver/silver-chloride test electrodes along with
electrode gel is a viable option for testing electrical impedance. These electrodes have been
used as measurement contacts for EKG tests for decades as they are consistent and
reliable. The figure below shows the conduction path of the signal for measurement.
Figure 3. Measurement Signal
As in figure 2, this diagram shows a two electrode system signal. This is essentially how
common bioimpedance analysis weight scales operate. Because of this, the scale will be one of
our testing considerations.
Setup and Tests
In order to obtain meaningful data which will translate to a better understanding of edema
levels and their implications, it is crucial to design a system which yields consistent results
according to a patient’s symptom severity. The first step in the design approach is to determine
the number and placement of electrodes in the ankle region, for best analysis. Trial on the
experimenters, themselves, will lead to a more refined plan for electrode placement. This
process will involve the testing of several electrode leads: Ag/AgCl gel coated (ECG-type)
compared with dry electrode types. A special, compression sock will be used to ensure stability
of the electrodes and peripherals during measurement. The signal producing component, at this
stage, will involve a function generator which will send a signal through a known impedance as
part of a current source. Measuring the voltages through the sensor system will yield the
components needed to calculate an impedance and plot the impedance values to generate the
components of a complex impedance curve. Preliminary single measurements will need to be
taken first, in order to assess how data acquisition will be handled.
Ideally for the initial testing, it would be very beneficial to have some sort of human
tissue analogue for comparison. We plan to conduct impedance testing on pig legs, injected with
a saline solution to emulate the effects of swelling. We will then proceed to test the pig legs
using our LCR meter and silver-silver chloride electrodes under dry skin contact conditions. We
plan to reach an appropriate level of consistency with our data and use it to establish some sort of
reference for our human tests. Eventually, trial testing on hospital patients is the goal for the end
of the project.
Research Plan
October
Further research on Bioimpedance Spectroscopy methods
November
Contact/lead Interface preliminary design
December Mechanical design and reinforcement of sensors, IRB approval
January
Determination of sensor placement for best results
February
Data Acquisition and Interpretation of BIA
March
Modifications to test and beginning of patient trials
April
Compilation of results and conclusions about design
May
Finish research report and present results
There is a need for initial pilot studies to determine the level of effectiveness of our test
setup. The need to establish baseline impedance values is crucial in this investigatory research
project. Obtaining data corresponding to average edema levels is crucial to show the ranges of
impedance values for a healthy individual, as a preliminary measure used to help determine how
much edema will vary in these impedance levels. The first lab measurements can easily be
conducted on healthy individuals (researchers themselves), to determine the efficacy of the
signal transduction system. Once it is established that the testing is conclusive and yields
definitive, repeatable results, testing can proceed for a longer period of time. This would be
carried out for a few hours at the least, to determine levels of impedance variation. IRB forms
were submitted in order for permission to test on other volunteering individuals. The goal is to
try to achieve a continuous monitoring system for edema sufferers, which would track symptom
severity over a span of a couple days. If possible, clinical trials could be carried out during the
later stages of our research, to assess patients with actual chronic edema symptoms.
Expected Outcomes
For this project, we expect to be able to correctly correlate the impedance measurement
to identify edema. Another thing that we should be able to tell is over how long it takes for
edema to set in and its severity. The device setup may not be portable yet, but the testing
methods should be well established and provide repeatable results.
Anticipated Challenges
One potential problem with this project is being able to accurately calculate the skin
impedance. Being able to overcome this problem is very important because the viability of this
project is determined by how well our results correlate to the real values. A second problem to
overcome is mechanical stability of the electrodes. This would include making the electrode
contacts have a reliable connection with the skin to get as accurate measurements as possible.
Another challenge would be determining what impedance values are for edema compared to a
baseline reading and then to correlate our results to symptoms. For our results to have any
credibility, our method needs to be able to have reproducible data so that when the results
indicate a person has edema, there actually is a noticeable swelling present in the patient’s limb.
A final problem would be getting participants for obtaining trial data. This challenge may be
resolved soon as we have a meeting to discuss patient trials at St. John Providence hospital.
Implications
The results of this research would yield very important data about patients suffering from
circulatory disorders (i.e. thrombosis, varicose veins), congestive heart failure, and peripheral
diabetic disorders like kidney and liver malfunctions. In these diseases and others, often,
patients experience varying levels of pedal edema in correspondence to the severity of their
disorder and their activity levels. Currently, medical professionals can only visually test to see if
a patient has edema by compression of the swollen area. In some disciplines, a four level system
is used to assess the severity of the swelling. With a continuous monitoring system for edema
measurement, physicians and their medical team can get a better idea of the chronology and
severity of the disorder causing the swelling and what activities these edema levels may
correspond to. This design project would lead to new ways of investigating patient symptoms in
severe cases of circulatory and cardiac pathology and may very well be a key indicator to patient
health or morbidity.
Future Directions
We would like to continue doing research on edema symptoms and causation as well as
bioelectrical impedance applications. This would lead to better accuracy in the system as well as
a better understanding of the results. System portability would be another focus. Ideally, there
would be minimal impact on how a person walks with the sensor on. This would mean limiting
any wires and making sure that there is solid contact between the electrodes and the skin, even
during movement. A battery operated, wireless system could be developed to be able to
distribute the sensor device to outpatients for symptom measurement. This would provide many
advantages to doctors as they would have access to a patient’s data as long as the patient was
wearing the sensor. This would provide the physician with a more complete understanding as to
the cause of the patient’s edema and how any prescribed medications are affecting the
condition’s severity.
Costs
Price
Material
Handheld LCR Meter
$575
ECG Monitor Electrodes
$40
Wire
$50
Connectors/clips
$10
Gallium 99.99% Pure (40[g]) $100
Weight Scale (BMI capable)
$50
Total Cost
$825
The greatest cost would come from the LCR meter which is crucial for recording impedance
characteristic data. Another necessity is the use of ECG electrodes which will be changed out
frequently and from test to test. Testing gallium electrodes in comparison to Ag/Ag-cl electrodes
would provide an interesting comparison and would, perhaps, provide for a means to integrate
the testing apparatus into a compact wearable form. The BMI scale would be helpful as a
comparison test for a product already on the market. We could test ourselves on a daily basis to
monitor how accurate the measurements of commercial bioimpedance analysis tests are.
References
Kim, S., Koeny, M., Leonhardt, S., Roethlingshoefer, L., Schlebusch, T., On the road to a textile
integrated bioimpedance early warning system for lung edema, Body Sensor Networks
(BSN), 2010 International Conference on , vol., no., pp.302-307, 7-9 June 2010, doi:
10.1109/BSN.2010.21
Ursula G. Kyle et al., Bioelectrical impedance analysis—part I: review of principles
and methods. Clinical Nutrition Volume 23, Issue 5, Pages 1226–1243, October 2004.
Nordqvist, Kevin. “What Is Edema? What Causes Edema?” Medical News Today. 29 July 2009.
http://www.medicalnewstoday.com/articles/159111.php