Electrical signals, conducted through five billion cells in the human heart, synchronize the contraction and relaxation of the cardiac muscle, culminating in a perfect heartbeat.
When the electrical impulses, however, become disorganized, the rhythm disturbances lead to critical conditions such as atrial and ventricular fibrillation, which are characterized by weak, uncoordinated and chaotic contractions. Scientists have had trouble understanding the causes behind these diseases, but a new way of looking at how the heart works may help.
With an electrocardiogram, the type of arrhythmia can be diagnosed, though currently existing methods are limited in the exact identification of where these disturbances come from.
To develop a new model of the heart, Dr. Hiroshi Ashikaga, a cardiologist and biomedical engineer at the Hopkins School of Medicine, led his team in using information theory, the mathematical theory of communication, to suggest that rhythm disturbances can be alternatively explained as communication breakdowns. The study, which was published online on March 4 in the Journal of the Royal Society Interface, describes the different types of cardiac fibrillation as disruptions in the transmission of information throughout the heart.
Interpreting the heart as a communication system, the researchers measured the amount of information that was transmitted during normal and abnormal heart rhythms, quantifying the electrical signals and mapping the electrical communication flow with computer representations. Within this framework, they can think of an electrical propagating through the heart as information being conveyed among the cardiac muscle cells.
The normal heartbeat as well as three mechanisms of arrhythmia were simulated across a two-dimensional lattice of cells using the two most commonly used models of cardiac action potential propagation: the mono-domain reaction-diffusion model and cellular automata model. Having derived a profile for each heart rhythm from data based on the information shared between the cells, the scientists were surprised to discover that the information sharing was different for each arrhythmia and each model.
“If one accepts the traditional view of the heart as a functional syncytium [a single cell with many nuclei], one can imagine that the information sharing among the cells within the heart should be freely available at different locations,” Ashikaga said. “Contrary to the classical teaching that the heart can be considered to be one big cell, we found that the heart has a very heterogeneous structure.”
The researchers also discovered that while the four heart rhythms all demonstrated heterogeneous spatial profiling, they significantly differed in the entropy of individual cardiac cells. Entropy, a term utilized in information theory, quantifies the uncertainty about a certain variable, the extent to which information is lacking.
Using entropy, the scientists intended to track the electrical signals that are too chaotic to trace with the current clinical method. Of particular interest was the application of the information theory framework to spiral reentry arrhythmia, a condition distinguished by spiral waves that circulate in the heart tissue.
The center of the arrhythmia, an area that does not get excited or otherwise affected by this electrical wave, does not display much uncertainty and is easy to pinpoint by considering the entropy of the heart.
“Spiral waves, which are commonly observed in any kind of excitable system, have been successfully measured in the hearts of mammals, such as sheep, goats and rabbits,” Ashikaga said. The means to directly measure the spiral wave in the human heart, however, had been lacking. “One of the motivations behind the project was to measure the spiral wave in the human heart by modeling the heart based on the information theory.”
With the quantification of entropy in the human heart, its core, a therapeutically important location, can be more precisely targeted by treatments such as catheter ablation, which removes the heart tissue that is the cause of the abnormal rhythm.
“The framework will introduce a more personalized, efficient way to treat individual patients,” Ashikaga said.
The consequent line of investigation from the findings of this study will be to discern if the paradigm proves to be valid in human patients. Before the framework is applied to human patients, however, Ashikaga and his team cite two limitations of their investigation, one being the cardiac simulation across a two-dimensional, homogeneous lattice and the other being the omission of the conduction delay in their computation.
The real cardiac tissue of a human heart is three-dimensional and heterogeneous. Moreover, there is a chance that due to the finite distance between the cells in the cardiac muscle, the delay before the electrical signal had reached some cells may have been underestimated, and the amount of shared information may actually be more than what was measured by the researchers.
Nevertheless, the paradigm of the heart that was constructed from the information theory has revealed the heterogeneous nature of the heart and shown the clinical importance of entropy in refining therapeutic methods.
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