Chaos in the heart: signals and models

How does the heart's rhythm become chaotic and turn fatal? This question is addressed in a combined experimental, modeling and signal analysis study. Under normal electrophysiological conditions, electrical conduction in the heart is maintained by a network of electrically coupled cells. Action potential generation and conduction in this network of cells can be mathematically modeled and simulated on a computer. A well timed, ectopic stimulation can introduce abnormal conduction leading to a self-sustaining rhythm described as reentry. Computer simulations as well as experiments show that the reentrant rhythm is generated by spiral waves that continue to perpetuate the rhythm at a fast rate, causing tachycardia or fibrillation. Signal analysis of ventricular fibrillation rhythm reveals that this rhythm is indeed chaotic but a has high correlation dimension. Surprisingly, the rhythm of a single cell in a fibrillating heart exhibits only a low dimensional chaos. Use of chaos as a signal analysis tool is useful for discriminating ventricular fibrillation from non-life threatening rhythms such as sinus and ventricular tachycardia. In summary, the fatal rhythm of the heart can be modeled and analyzed with the help of sophisticated mathematical tools. These technique should find applications in research and development associated with external or implantable defibrillators and cardiac rhythm management devices.