1 Impedance Cardiography Client: Professor John Webster Advisor : Professor Willis Thompkins Leader: Kim Safarik Communicator: Jacob Meyer BWIG: David Schreier BSAC: Terra Gahlman2 Table of Contents I. Problem Statement………………………………………………………3 II. Background……………………………………………………………...3 III. Client Specifications……………………………………………………..6 IV. Designs…………………………………………………………………...7 i. Design: Electrode Track………………………………………7 ii. Design: Mesh Liner System………………………………….8 iii. Design: Gel Electrode Matrix Design………………………..9 iv. Design: Gel Liner System Design…………………………...10 V. Final Design……………………………………………………………...11 VI. Final Design Construction……………………………………………….12 VII. Materials and Methods…………………………………………………..13 VIII. Testing and Results………………………………………………………15 i. Prototype Testing…………………………………………….15 ii. Comparison…………………………………………………..17 IX. Ethical Considerations……………………………………………………18 X. Conclusion and Future Work……………………………………………..19 XI. Appendices……………………………………………………………….21 i. References…………………………………………………….21 ii. Production Design Specifications……………………………..213 I. Problem Statement Impedance cardiography is a medical procedure utilized in order to noninvasively analyze and depict the flow of blood through the body2. Traditionally, four electrodes are attached to the body, two on the neck and two on the chest, which take beat by beat measurements of blood volume and velocity changes in the aorta. However, this system suffers from degrees of inaccuracy, possibly due to the fact that the electrodes are placed too far from the heart1. As a result, it is our collective goal to design an accurate, reusable, and spatially specific impedance cardiograph system that ensures accurate and reliable readings. . II. Background: It is frequently necessary in the hospital setting to assess the state of a patient’s circulation. Here the determination of simple measurements, such as heart rate and blood pressure, may be adequate for most patients, but if there is a cardiovascular abnormality then more detailed approach is needed3. In order to non-invasively gather specific measurements on the volume of blood pumped by the heart (cardiac output) through the aorta, the technique of cardiography can be used. Cardiographic measurements are useful both in establishing a patient’s initial cardiovascular state and in measuring one’s response to various therapeutic interventions such as transfusion, infusion of inotropic drugs, and infusion of vasoactive drugs or altering heart rate2. Existing methods of measuring cardiac output are unsatisfactory for various reasons. If carefully carried out, the Fick method is accurate but requires a pulmonary artery catheter that is not practical in routine clinical practice. Several variants of this method have been4 Figure 1: Current Impedance Cardiography electrode placement. devised, but their accuracy leaves something to be desired. Transoesophageal echocardiography (TOE) provides diagnosis and monitoring of a variety of structural and functional abnormalities of the heart. It can also be used to derive cardiac output from measurement of blood flow velocity by recording the Doppler shift of ultrasound reflected from the red blood cells. However, reviews of this method have been mixed. The main disadvantages of this method are that a skilled operator is needed to utilize it, the probe is large and therefore heavy sedation or anesthesia is needed, the equipment is very expensive, and the probe cannot be fixed so as to give continuous cardiac output readings without an expert user being present2. A final existing option that holds promise is impedance cardiography, first described by Nyoer in 19403. The conventional impedance cardiogram is a record of variations of chest impedance (resistance to current flow), obtained by using an electric current passed in the head to foot axis from the neck to the upper abdomen as seen in Figure 12. This high frequency, non-stimulating current is not only noninvasive, but painless to the patient as well. The frequency of the current (about 100 kHz) passing through the chest and heart is high enough to prevent sensation or muscle stimulation but low enough so that the pattern of current flow is similar to that of direct current4.5 Traditionally, chest impedance is recorded between the thoracic inlet at the base of the neck and the thoracic outlet at the level of the diaphragm. The impedance signal is related to changes in the size and composition of blood-containing structures within the chest, and it is by this reasoning the impedance cardiogram promises to reveal meaningful information concerning cardiac output and the effectiveness of the heart as a pump on a beat-by-beat basis. This would be extremely useful for monitoring critically ill patients and patients undergoing anesthesia, especially in cases where blood volume or cardiac output may change significantly1. Although indices derived from the chest impedance signal track cardiac stroke volume very well, the absolute values of stroke volume in units of milliliters of blood per heartbeat have been considered unreliable. This is especially the case in a situation involving congestive heart failure when the ventricular ejection fraction is greatly diminished, or in patients with either reduced or increased peripheral vascular resistance3. In addition, comparisons of impedance based stroke volume and cardiac output with results from the green dye dilution or the Fick methods show that the impedance cardiogram tends to overestimate stroke volume by 5 to 10 percent, with rather wide standard deviations, leading to the conclusion that the impedance method is not accurate. Recent studies have emphasized that changes in the impedance of lungs, great
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