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Physics can save lives: a new type of defibrillation aims to reduce the voltage needed to shock out-of-control hearts back to a normal beating pattern. Ordinarily the beating heart is an orderly process (called systole) in which the heart muscle cells contract cooperatively to insure that blood is pumped about once every second. If, however, some portions of cardiac tissue are electrically triggered in a non-coordinated way, the overall activity of the heart can become chaotic. An irregular systole (fibrillation) in the atrial chambers of the heart can be tolerated for some time, but fibrillation of the ventricles can kill a person within a few minutes. The most extreme remedy for ventricular fibrillation (VF) is the application of a huge electrical shock (administered by paddles applied to the chest). Conventional defibrillators applied to the outside of the body can deliver a voltage difference of up to 5000 volts and a current of 20 amps. The shock delivered by implanted defibrillators is much less, but can still result in trauma. The goal of the shock is to overwhelm the electrical environment of the entire heart---disrupting electrical waves even in the parts of the heart beating normally---hoping a global coordinated rhythm will resume. (One could compare this to brute-force method of chemotherapy, in which toxic chemical meant to kill cancer cells will also kill many healthy cells, resulting in unpleasant side effects.) To see how the general assault on fibrillation can be modified, consider that the threatening arrhythmias take the form of rotating waves (spirals) of electrical excitation passing across the volume of the heart. These spirals are enhanced (and dangerously pinned in position) by the presence of scars (dead tissue) on the heart caused at the scene of previous attacks and even by other heterogeneities present in healthy hearts such as blood vessels, connective tissues, and oriented bundles of cardiac muscle fibers. Alain Pumir and Valentin Krinsky and their colleagues at the University of Nice, France and at the Centre National de la Recherche Scientifique (CNRS) try to undo threatening vortices not by jolting the whole heart but by aiming their countermeasures at the vortices exclusively. This permits a much smaller voltage to be used, and hence less trauma to the patient and less damage to the heart itself. One of their earlier efforts in this direction (Physical Review Letters , 30 July 2004) allowed a rotating vortex in the heart to be removed using an input electrical energy lower by a factor of 20. Later the approach was confirmed to be effective using rabbit hearts. Now Pumir and Krinsky (
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,
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) have designed an even better scheme, one that would counteract a chaotic cardiac crisis consisting of many vortices. In addition, this approach permits the energy to be reduced by a factor of a hundred or a thousand from present levels. A sophisticated implant device, programmed to mitigate potential fibrillation with the new shock method, would be almost unnoticeable
to the patient. Teams led by R. Gilmour (Cornell University, USA) and E.Bodenschatz (Max Planck Institute for Dynamics and Self-Organization, Göttingen, Germany ) are currently testing the method. An estimated 250,000 people have implanted defibrillators, so the scope for medical benefits are enormous. (Pumir et al., Physical Review Letters , upcoming article) Source: American Institute of Physics |
