Chaos in the Heart

Chaotic behavior in chemical reactions and heart fibrillation can be selectively influenced and suppressed

January 30, 2003

A healthy heart beats at a regular pace determined by electrical pulses generated inside the heart which spread as waves through the cardiac muscle. Such excitation waves produce normal heart contractions. Sometimes this well-organized behavior is however replaced by irregular excitation patterns which lead to cardiac fibrillation and can be viewed as a form of chaotic behavior. Similar chaotic wave patterns may also develop in systems with chemical reactions. In a collaboration with Spanish scientists, A.S. Mikhailov (Fritz Haber Institute) has now discovered that this kind of chaos can be suppressed by weak periodic modulation of the medium excitability. If verified for the heart, this discovery could lead to new methods for terminating cardiac fibrillation.

An electric excitation wave runs through the heart and causes it to contract once every second. Sometimes, however, the ordered wave propagation breaks down and, as a result, normal physiological contractions disappear and fibrillation sets in. If this condition is not immediately terminated, the outcome is fatal.

From a mathematical viewpoint, fibrillation can be considered as a special form of chaos. Similar forms of chaos are also possible in chemically excitable media, such as for example, the famous periodic Belousov-Zhabotinsky reaction. In experimental investigations of this reaction, American scientist Arthur Winfree discovered the so-called rotating scroll waves in 1973. In its transversal cross-section, a scroll wave looks like a spiral. Such spirals are stacked one upon another to form a scroll-shaped pattern, resembling a loosely rolled sheet of paper. The scroll rotates around a central filament. This filament can be straight or curved; it may also build loops and rings. Subsequently, Winfree suggested that chaos in three-dimensional excitable media can emerge through disordered dynamics of such filaments. In his opinion, this can often explain the development of fibrillation and the phenomenon of sudden cardiac death.

Investigations by A.S. Mikhailov (Fritz Haber Institute) together with S. Alonso and F. Sagues (University of Barcelona) have now shown that the Winfree chaos of scroll waves is typical and can be observed in general models of excitable media. The figure gives an example of such a chaotic wave pattern.

Two years ago, Mikhailov and his colleagues at the Fritz Haber Institute succeeded in observing and controlling chaotic patterns in a chemical reaction [1]. In his new publication in "Science", the scientists show that chaos of scroll waves can generally be controlled - suppressed or induced - by weak periodic modulation of parameters determining excitation threshold of a medium. This discovery, which was discovered for abstract mathematical models, could in the future lead to new methods for suppressing cardiac fibrillation.

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