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Combating Heart Disease with FEA PUBLIC ACCESS

Biomedical Researchers are Combining Electrostatic Finite-Element-Analysis Software with Laboratory Testing to Improve Treatments for Arrhythmia.

[+] Author Notes

Tamara C. Baynham is a. Ph.D . student in the Department of Biomedical Engineering at the University of Alabama in Birmingham.Dr. Stephen B. Knisley is an Established Investigator Awardee of the American Heart Association.

Mechanical Engineering 120(10), 70-72 (Oct 01, 1998) (3 pages) doi:10.1115/1.1998-OCT-2

Abstract

Biomedical researchers are combining electrostatic finite element analysis (FEA) software with laboratory testing to improve treatments for arrhythmia. The researchers used Algor's Superdraw Ill, a precision finite-element model-building tool, to model a 100-by-100-centimeter sheet, which represented a conductive area of the heart. They applied a resistivity value based on a thickness of 1cm to simulate a uniform resistance over heart fibers. The finite element model showed that current through the element faces at the ends of the electrodes was 151 percent larger than current near the electrode center. The researchers also used FEA to determine that the length of the line electrode does not affect the current distribution. In order to confirm the results of the analysis and further test the positioning of the electrode with respect to heart fibers, researchers applied line electrodes in varying positions andorientations on 13 hearts from New Zealand rabbits. Finally, researchers could determine the distribution of the change in transmembrane voltage from a line electrode, made up of a summation of points, using electrostatic analysis.

Article

Anyone who has listened through a stethoscope to a healthy human heart has probably heard its soft, rhythmic beating. However, for about 4.3 million Americans who experience arrhythmias, their heartbeats may be anything but rhythmic; they may even be life threatening.

Arrhythmias are abnormal rhytlm1s of the heart that cause it to pump less effectively. Arrhythmias factor into half of all sudden cardiac deaths in the United States each yea r. Increased demand for improved technology for arrhythmia treatment and prevention, especially electronic implantable devices that apply an electric shock to the heart, is fueling research at the Cardiac Rhythm. Management Laboratory in the Department of Biomedical Engineering at the university of Alabama in Birmingham.

With the aid of laboratory testing on rabbit hearts and finite-element analysis, researchers have learned how the structure of heart fibers may affect the application and distribution of an electric current. The findings of this research will be applied to improve existing implantable devices to better regulate heart rhythms.

A normal, healthy heart is about the size of an orange and pumps thousands of gallons of blood each day. T he four chambers of the heart, the upper left and right atria and the lower left and right ventricles, must work in precise order. A specialized group of cells, called the sinus node, is located in the right atrium and sends electric impulses at a regular interval through heart fibers of the atria and ventricles. As the electric impulse propagates and moves uniformly through the fibrous membrane of the heart, the heart contracts to pump blood and then expands as the signal passes. Throughout this contraction/expansion cycle, the heart exhibits changes in the amount of current or voltage across its fibers, called transmembrane voltage.

For the heart to function normally, the distribution of the transmembrane voltage must be approximately uniform throughout the heart. If the values differ greatly, arrhythmia can occur because the electric impulses of the heart becol11.e disorganized. Heart disease or blocked arteries are often to blame for this disorganization. For example, if a main artery becomes blocked, blood flow is stopped to part of the heart. The heart loses its synchrony because all parts of it no longer function properly and arrhythmia occurs. Arrhythmias can cause the heart to beat either slower or faster than the normal rate of 60 to 100 beats per minute. This situation can lead to a potentially fatal condition called ventricular fibrillation, in which the heart quivers rapidly instead of pumping blood. If left untreated, the patient could die within minutes.

Treatments vary for each type of arrhythmia. Excessive slowing of the heart often requires the implantation of an electronic pacemaker under the skin. Arrhythmias that result in an accelerated heart rate can be classified as either atrial or ventricular fibrillation. According to the American Heart Association in Dallas, atrial fibrillation causes about 15 percent of strokes per year when blood clots develop in the atria and move to an artery in the brain. This type of fibrillation is often treated with medication to prevent clotting.

Ventricular fibrillation, the most serious type of fibrillation, is involved in a large number of the 250,000 sudden cardiac deaths in the United States each year. Ventricular fibrillation must be corrected immediately with electric shock therapy. The restored cardiac rhythm is maintained with medication or an electronic device called an automatic implantable cardioverter/ defibrillator (AICD).

In recent years, more physicians have chosen AICDs to prevent death from fibrillation, as studies have proved their effectiveness to be equal to or better than medications. Furthermore, not all patients who survive fibrillation can be effectively treated by drug therapy. Because fibrillation occurs across all age groups, these devices provide an alternative to lifelong medication and can improve the overall quality of life.

Common AICDs consist of batteries and a pulse generator insulated in a flat case, which is inserted under the skin. Insulated metal wires run from the case through veins to the heart cavity, where electrodes are positioned directly within the heart. Sensor electrodes automatically detect when the heart is out of rhythm and signal the pulse generator to send an electric current through an exposed electrode. Therefore, fibrillation is halted quickly after irregular rhythms are detected.

With support from the National Institutes of Health in Atlanta and the American Heart Association, researchers at the Cardiac Rhythm Management Laboratory are working to improve current AICD technology. They are doing so by exploring better options for the most effective shapes and positions of electrodes on heart fibers as well as learning more about the fiber structure of the heart. Using electrostatic finite-element-analysis (FEA) software from Algor Inc. in Pittsburgh to study the distribution of an electric current as it flows through the heart, the researchers hoped to develop techniques that would create a more uniform change in membrane voltage to halt ventricular fibrillation more efficiently.

Current AICDs contain electrodes that are essentially small cylinders or the rounded tips of wire leads. Experiments to study how an electric current affects the heart have been performed with these small electrodes, called point electrodes. Such electrodes emit electric current, which spreads radially from the source much like ripples of water that result when a pebble is dropped into a pond. However, unlike the smooth surface of a pond, the fibers of the heart can create resistance to the current, causing an uneven spread of current and a very nonuniform distribution of transmembrane voltage.

Because of a point electrode's shape, it cannot be oriented with respect to the fibers' direction to reduce resistance. The researchers hypothesized that, by using a longer line electrode instead of just a point, an electrode could be positioned either parallel or perpendicular to heart fibers to best transfer current and create a more uniform distribution of transmembrane voltage. A line electrode can be a series of points adjacent to each other to form a line or an entire segment of exposed wire applied directly to the heart.

Before the researchers could test this hypothesis, they had to determine the density or distribution of the electric current emitted from a line electrode. Up to this point, classical electrodynamic theory had been applied only to conventional shapes, such as the disc-shaped electrode with radial symmetry. Based on classical theory, researchers knew that more current is emitted from the edge of a disc than from the center. The researchers used Algor electrostatic- analysis software to determine how much current would be emitted from each point on the line. It was possible that more current could have been distributed from the ends of the electrode than from the middle segment.

The researchers used Algor's Superdraw Ill, a precision finite- element model-building tool, to model a 100-by-100-centimeter sheet, which represented a conductive area of the .heart. They applied a resistivity value based on a thickness of 1• cm to simulate a uniform resistance over heart fibers. A 3.6-by-3.6-cm central region contained a 1- cm-long electrode in the center. Voltage boundary elements were applied to points on the sheet that were in contact with the electrode.

Smaller two-dimensional planar elements were used around the electrode to achieve more detailed results in this area of concern and larger elements were used for outlying areas. Researchers specified that a voltage of 100 volts be applied at the electrode and that the voltage be zero at the perimeter of the sheet. They found that the values chosen for the voltage and resistivity values affected the total current values, but did not affect how the current was distributed along the electrode.

The finite-element model showed that current through the element faces at the ends of the electrodes was 151 percent larger than current near the electrode center. The researchers ran another electrostatic analysis to determine if the same effect would occur with a coarser mesh. These results indicated only a 113-percent increase near the electrode ends compared with the electrode center. This agrees with the theory that the increase at the ends is greater with a finer mesh and smaller with a coarser mesh. Researchers were able to show that the ends of a line electrode emit a higher concentration of current---5imilar to that previously known for the edge of a disc electrode.

The researchers also used FEA to determine that the length of the line electrode does not affect the current distribution. Another finite-element model was constructed with a 3-millimeter-Iong electrode located at the center. The analysis results indicated that the percentage of current coming from regions near the electrode ends made up about 50 percent of the total current, regardless of electrode length.

To confirm the results of the analysis and further test the To confirm the results of the analysis and further test the searchers applied line electrodes in varying positions and orientations on 13 hearts from New Zealand rabbits. The researchers used line electrodes made up of a series of points applied adjacent to each other, since the sum of the current from all points would correspond to current from a line. Rabbit hearts were chosen because their fibrous structure is similar to human hearts. Furthermore, they are smaller in size, comparable to that of a small peach. Smaller hearts are more feasible to study because of the small amount of artificial blood needed to keep them "alive."

The researchers began by applying line electrodes parallel to heart fibers to determine if the changes in transmembrane voltage of the fibers would become more uniform, as was hypothesized. As described previously, producing an efficient change of voltage from very irregular to uniform is vital to restoring regular heart rhythms.

The magnitude of changes in transmembrane voltage on either side of the electrode remained constant or increased in the central region for the first few rnillin1eters away from the electrode and then began to decrease. The most significant changes in magnitude of trans membrane voltage occurred at the electrode ends. This finding correlates with the results of the electrostatic analysis, which showed a high concentration of current distribution in this area. A region near the center of the electrode experienced a negligible change in transmembrane voltage. The changes in transmembrane voltage had the same sign (that is, they were more uniform) when the line electrodes were parallel to heart fibers. The results indicated that the change in transmembrane voltage was less uniform across the fibers. Notably, the change in transmembrane voltage had different signs. Regions of small changes in voltage were detected 4 mm away from and on either side of the electrode's approximate center.

The research showed that the parallel orientation of a line electrode to heart fibers enhances the uniformity of the change in transmembrane voltage. Therefore, the application of a line electrode, instead of a point electrode, parallel to fibers enables better control over the distribution of the responses by the fibers to the application of electric current. This means that the change in transmembrane voltage would be more uniform and homogeneous, closer to that of a normally functioning heart. This knowledge could be applied to future types of AICDs to regionally block the areas of the heart from be coming out of synch and prone to fibrillation.

Furthermore, line electrodes may play an important role in developing new, less invasive therapies for arrhythmias. This includes inserting line electrodes in the heart through the cardiac veins for therapeutic treatment.

This method may increase the efficiency with which electric current is applied to the heart by distributing current from several electrodes. Furthermore, it is less traumatic than some previous methods used that required surgical procedures to open the chest.

Finally, researchers were able to determine the distribution of the change in transmembrane voltage from a line electrode, made up of a summation of points, using electrostatic analysis. In the future, electrodes of other shapes can @e studied using FEA to determine potential advantages of new types of electrode configuration.

With the prevalence of heart disease, arrhythmias, and sudden death due to heart problems, it is important to learn more about the structure of the heart and its responsiveness to both electric current therapies and medications.

This knowledge will help arrhythmia victims, both young and old, to live longer and lead more productive lives.

This view of a rabbit heart shows the directions of fibers on the surface over which electric current flows.

Grahic Jump LocationThis view of a rabbit heart shows the directions of fibers on the surface over which electric current flows.

In this computer model, electric current dissipates over a conductive sheet corresponding to the surface of the heart; voltage across a 3.6-by-3.6-cm central region containing a line electrode is shown in the insert.

Grahic Jump LocationIn this computer model, electric current dissipates over a conductive sheet corresponding to the surface of the heart; voltage across a 3.6-by-3.6-cm central region containing a line electrode is shown in the insert.

A line electrode aligned perpendicular to heart fibers resulted in a less uniform change in transmembrane voltage.

Grahic Jump LocationA line electrode aligned perpendicular to heart fibers resulted in a less uniform change in transmembrane voltage.

Copyright © 1998 by ASME
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