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Physics News Update
Number 840, September 27 , 2007 by Phil Schewe

Controlling Cardiac Chaos

Controlling cardiac chaos-a gentler approach. 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) Nonlinear Institute 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 (33-6-6844-1415, alain.pumir@unice.fr , valentin.krinski@inln.cnrs.fr) 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) and E.Bodenschatz (Max Planck Institute, Goettingen eberhard.bodenschatz@ds.mpg.de ) 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)

Thermal Logic Gates

Information processing in the world's computers is mostly carried out in compact electronic devices, which use the flow of electrons both to carry and control information. There are, however, other potential information carriers, such as photons, which are parcels of light. Indeed a major industry, photonics, has developed around the sending of messages encoded in pulsed light.

Heat pulses, or phonons, rippling through a crystal might also become a major carrier, says Baowen Li of the National University of Singapore (phylibw@nus.edu.sg). Li, with his colleague Lei Wang, have now shown how circuitry could use heat---energy already present in abundance in electronic devices---to carry and process information.

They suggest that thermal transistors (also proposed by Li's group in Applied Physics Letters, 3 April 2006) could be combined into all the type of logic gates---such as OR, AND, NOT, etc.-used in conventional processors and that therefore a thermal computer, one that manipulates heat on the microscopic level, should be possible.

Given the fact that a solid state thermal rectifier has been demonstrated experimentally in nanotubes by a group at UC Berkeley (Chang et al., Science, 17 November 2006) only a few years after the theoretical proposal of "thermal diode," the heat analog of an electrical diode which would oblige heat to flow preferentially in one direction (Li et al, Physical Review Letters, 29 October 2004). Li is confident that thermal devices can be successfully realized in the foreseeable future. (Wang and Li, Physical Review Letters, upcoming article

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