Mar. 9, 1998
Ventricular fibrillation kills thousands of Americans each week by inducing abnormal electrical signals that turn their hearts into quivering "bags of worms" no longer able to pump blood. Victims die within minutes unless the erratic heart rhythms can be halted with massive jolts of electricity from a defibrillator.
Medical researchers have now moved one step closer to understanding the causes of ventricular fibrillation through a remarkable series of high-resolution movies that clearly show how the condition disrupts the electrical signals that normally govern the heart. The unique high-speed imaging system produced for the research also revealed for the first time that ventricular fibrillation may develop in two distinct phases.
Details of the work, done by an international team of medical researchers and physicists from the United States and Canada, appear in the March 5 issue of the journal Nature.
"We have now seen the smoking gun of fibrillation," said Dr. William L. Ditto, professor of physics at the Georgia Institute of Technology and one of the study's co-authors. "We now have evidence of what is going on. This dramatically increases the possibility that we could develop a new defibrillator or improve existing defibrillators."
The movies reveal a series of unusual spiral waves that originate with "rotors" near the surface of the heart. The waves rapidly expand, flow across the heart muscle, merge and even interfere with each other, causing heart cells to contract in an uncoordinated way.
Knowing how these unique waves form and behave could provide the information needed to design and test control techniques that may provide an alternative to existing defibrillators -- which deliver the electrical equivalent of "a bowling ball dropped onto your chest from a two-story building."
Because the spiral waves seem chaotic in their behavior, researchers hope they can apply newly discovered chaos control techniques to restore normal heartbeat. Instead of the massive jolt of electricity, the chaos control technique might bring the heart back into normal rhythm using carefully-applied electrical signals of much less energy.
"The idea behind chaos control is that very small changes to a truly chaotic system dramatically change its behavior," Ditto explained. Reducing the amount of energy could also allow defibrillators, both portable devices used by emergency medical teams and the implantable devices put into chests of people vulnerable to fibrillation, to be smaller and operate longer on their batteries.
Besides visualizing spiral waves in the heart, this study of canine hearts also showed that ventricular fibrillation goes through two distinct stages. That previously-unknown information should also help improve control techniques.
"The electro-physiology of the heart evolves in time as ventricular fibrillation develops, and that has implications for how we attempt to control it," said Dr. Francis X. Witkowski, the study's lead author and a Professor of Medicine at the University of Alberta in Edmonton and a Medical Scientist of the Alberta Heritage Foundation for Medical Research. "The initial form, which occurs in the first several seconds of ventricular fibrillation, is a different entity from what develops over time."
Medical researchers have long known that the longer the fibrillation episode lasts, the more difficult it is to stop. They had blamed that on declining blood flow in a fibrillating heart, but Witkowski says the newly-discovered two-phase pattern suggests the explanation may be more complicated.
The unique imaging system used by the research team produces detailed information from as many as 16,000 points on a portion of the exterior surface of the heart. Operating at 838 frames per second, the system allows researchers to capture the rapid and disorganized waveforms for analysis.
The system relies on flourescent dyes that respond to electrical changes in the cells of the canine heart muscle. Researchers expose the beating heart to high intensity lights, then image and intensify specific wavelengths of light returned by the dyes. Pioneered by Witkowksi, who is also trained as an electrical engineer, the system produces images with improved resolution compared to earlier techniques.
The next step in the work is to try out chaos control techniques, using the imaging system to observe the effects. Once they find promising techniques, the researchers would hope to try them on surgery patients whose hearts go into fibrillation on the operating table. The technique could also have application to atrial fibrillation, a less-serious disruption of the heart's atrium.
The stakes are high, notes Witkowski, who as a practicing cardiologist regularly sees the consequences of sudden cardiac death caused by ventricular fibrillation.
"Sudden cardiac death kills more Americans than anything else," he said. "The median age is 59, so these are not people who are very old. This often happens with people who are suffering a first heart attack from which they could have recovered."
Sponsors for the work include the U.S. Office of Naval Research, the U.S. National Science Foundation, the Medical Research Council of Canada, and the Alberta Heritage Foundation for Medical Research.
Besides the researchers mentioned, the team also included L. Joshua Leon of the Ecole Polytechnique in Montreal, Patricia A. Penkoske from the University of Alberta, Wayne R. Giles from the University of Calgary, Mark L. Spano from the Naval Surface Warfare Center, and Arthur T. Winfree of the University of Arizona.
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