Identifying chaos in heart quakes.

Every year, more than 350,000 people in the United States die suddenly of heart attacks, most caused by an abrupt shift from rhythmic pumping to spas- modic convulsions of the heart.

Now, researchers report evidence suggesting that an ailing heart's erratic shivering, known as cardiac fibrillation, is a form of chaos. In other words, the irregular beating is not completely random but results from a specific type of deterioration in the organization of the heart's electric activity.

Alan Garfinkel, a cardiology professor at the University of California, Los Angeles, and his colleagues describe their findings in the Jan. 15 Journal of Clinical Investigation.

The observations represent the "best evidence to date implicating chaotic behavior in fibrillation in human and canine hearts," says Richard L. Verrier of the Beth Israel Deaconess Medical Center in Boston.

The discovery of a specific pathway to chaos in cardiac fibrillation also opens up the possibility of new therapeutic strategies that may help prevent fibrillation, Garfinkel suggests.

Normally, with each successive heartbeat, an electric wave front propagates across the interconnected muscle fibers of the heart, causing them to con- tract. However, such a wave can occasionally get stuck-perhaps because of some abnormality in the tissue-and begin rotating as a spiral wave (SN: 9/5/92, p. 156). Scientists have predicted that this rotating disturbance can break free and begin to circulate as a unit in the heart tissue. It may also break up into a small number of additional spiral waves, all rotating and meandering at different rates.

Garfinkel and his group looked for evidence of this pattern of behavior in cardiac recordings of five patients who suffer from chronic atrial fibrilla- tion, in multi-electrode recordings of electric activity during ventricular fibrillation in the excised, intact heart of a dog, and in maps of electric activity across thin slices of human and canine cardiac tissue in the lab.

Their measurements showed waves of electric activity consistent with a scenario in which a meandering spiral wave breaks up into additional spiral waves, causing the system to destabilize and become chaotic.

"The data suggest that fibrillation is a form of . . . chaos," Garfinkel says. "Characteristic oscillations precede and precipitate fibrillation."

The chaotic behavior appears to arise because of interactions among distinct spiral waves meandering at different rates. A computer simulation of the pro- cess in a sheet of cardiac tissue supports the findings. The effect resembles the way in which a smoothly flowing liquid breaks up into vortices, becoming turbulent.

"They've made a very strong case that, at least in some situations, the transition to fibrillation involves spiral waves," says physicist William L.

Ditto of the Georgia Institute of Technology in Atlanta, who studies cardiac excitations.

"There's a lot of interest in understanding the stability of spiral waves in the heart," notes physiologist Leon M. Glass of McGill University in Montreal. Identifying the instabilities involved in fibrillation is difficult both theoretically and experimentally. "It's important to try to document them," he says.

Garfinkel and his colleagues attribute spiral wave instabilities to a charac- teristic known as restitution, which governs how quickly a heart cell recovers from one beat to the next. Changes in the recovery rate of a subset of cells can trigger spiral wave meandering, the researchers say.

The UCLA study offers clues as to what kinds of electrocardiogram signals might serve as predictors of cardiac fibrillation, Verrier remarks. "The chances are quite reasonable that this kind of an approach will work." "With these kinds of studies," Ditto adds, "we have a real chance to make a major breakthrough."