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UW-Madison BME 300 - Finger Plethysmograph to Measure Blood Resistivity

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Finger Plethysmograph to Measure Blood Resistivity Tim Balgemann, Team Leader Lucas Vitzthum, Communications Nick Harrison, BSAC Tyler Lark, BWIG BME 400 Department of Biomedical Engineering University of Wisconsin-Madison December 10, 2008 Clients John G. Webster, Ph.D Department of Biomedical Engineering Ravi Shankar, Ph.D Advisor Thomas Yen, Ph.D. Department of Biomedical Engineering2Abstract Impedance plethysmography can be used to measure arterial volume change that occurs with propagation of the blood pressure pulse in a limb segment. For this measurement, we assume a constant value of blood resistivity. However, blood resistivity may change under both physiological and pathological conditions. Use of an impedance plethysmograph on a finger immersed in a saline filled beaker may yield a method for determining this change in blood resistivity. This may develop into a method that diabetics can use to measure glucose levels non-invasively. The goal of our project is to design a finger plethysmograph to measure blood resistivity.3Table of Contents Page Abstract 2 Table of Contents 3 Problem Statement 4 Background Information 4-8 Current Devices 8 Design Constraints 8-9 Design Alternatives 9-15 Final Design 15-18 Testing and Results 18-22 Future Work 22-23 Conclusion 24 References 25 Appendix A: Circuit 26 Appendix B: PDS 27-29 Appendix C: Project Expenditures 30 4Problem Statement Our goal is to design a finger plethysmograph to measure blood resistivity. In order to accomplish this, we will need to design and build a data acquisition device to acquire the signal from the finger. The device should mechanically immobilize the test subjects’ finger such that motion artifacts are kept to a minimum. This device should be able to detect the electrical potential (voltage) change across the finger so that the change in resistance may be determined. It should be able to detect the velocity-dependent change in blood resistivity due to arterial blood pulsations. In addition, we will need to build an electrical circuit to perform signal processing and analysis. This circuit should be capable of rectifying the alternating current (AC) signal from the finger data acquisition device and modulate it into a direct current (DC) signal to be analyzed. The circuit should be capable of discerning or visually displaying the voltage changes caused by correlated changes in blood resistivity. As an added feature, this circuit may contain an automatic reset function capable of adjusting one of the differential amplifier inputs to that of the output from the data acquisition (finger holder) device. This will allow the device to easily accommodate fingers having different electrical resistances and will prevent having to manually adjust voltages using a potentiometer to match independences with each new test subject or finger position. Background Information Diabetes Diabetes is a disease characterized by the body’s inability to manage glucose levels. It is a chronic condition caused by the pancreas’s lack of ability to produce enough insulin or the5failure of the body to effectively use the insulin produced. Individuals with type 1 diabetes produce little to no insulin and as a result need to self-administer doses of insulin on a daily basis. People with type 2 diabetes cannot use insulin effectively. The condition is usually treated through lifestyle changes and if necessary, oral drugs [1] (WHO 2008). Regardless of type, however, it is important for all diabetics to closely monitor their blood glucose concentration. The prevelence of diabetes is astounding. In the United States alone there are 17.9 million diagnosed cases of diabetes, and it is estimated that there are an additional 5.7 million undiagnosed individuals living with the disease [2]. The disease is also on the rise. Over the last three years, the number of diagnosed individuals has risen 13.5 percent. In addition, the American Diabetes Association projects 57 million people as pre-diabetic. Globally, occurance rates of diabetes are even higher; at least 171 million people worldwide have diabetes [1]. As a result, the economic burdens of diabetes are substantial. In the US, the total estimated costs of diabetes in 2007 include $116 billion in excess medical expenditures and an additional $58 billion in reduced national productivity [3]. The indirect costs of diabetes that affect productivity include increased work absenteeism, reduced productivity while at work, unemployment from disease-related disability, and lost productive capacity due to early mortality. As highlighted by the large number of individuals living with the disease as well as the large associated economic burdends, there are exceptionally strong motivations (both moral and financial) to perform research in the area of diabetes treatment and monitoring. Thus, an improved method for diabetics to monitor their blood sugar is highly desired. Electrical Theory6Four electrode impedance plethysmography uses two electrodes to pass current through the finger and two electrodes to measure the voltage output across the finger. For this project, the finger will be inserted downward into a tube similar to that shown in Figure 1. The electrode at the top, near the base of the finger, is the current input. The electrode at the bottom acts as the ground where the current exits the system. The two center electrodes measure the voltage across the middle section of the finger. By passing current through the finger (which provides resistance), the resulting voltage drop can be measured across these electrodes [4]. Figure 1: Four electrode impedance plethysmography [4] The voltage measurements obtained will vary depending upon physiological changes in the blood and due to the blood pulse itself. It is expected that these measurements will be small. In order to observe and analyze the signal, the wires from the middle electrodes are connected to a circuit where the signal will be amplified and processed. Finally, the voltage output can be used to calculate the impedance and resistivity of the blood in the finger. It is thought that the resistivity can be correlated with different blood compositions.7Biological Theory During high blood flow, such as when the blood pulses through the


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UW-Madison BME 300 - Finger Plethysmograph to Measure Blood Resistivity

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