In a paper published recently in the journal Chaos, researchers from Sergio Arboleda University (Bogotá, Colombia) and the Georgia Institute of Technology (Atlanta, USA) used an electrophysiological computer model of the heart’s electrical circuits to examine the effect of the applied voltage field in multiple fibrillation-defibrillation scenarios—and discovered that far less energy is needed compared to what is currently used in state-of-the-art defibrillation techniques.
“The results were not at all what we expected,” said study author Roman Grigoriev (Georgia Institute of Technology, Atlanta, USA). “We learned the mechanism for ultra-low-energy defibrillation is not related to synchronisation of the excitation waves like we thought, but is instead related to whether the waves manage to propagate across regions of the tissue which have not had the time to fully recover from a previous excitation.
“Our focus was on finding the optimal variation in time of the applied electric field over an extended time interval. Since the length of the time interval is not known a priori, it was incremented until a defibrillating protocol was found.”
The authors applied an adjoint optimisation method, which aims to achieve a desired result—defibrillation, in this case—by solving the electrophysiologic model for a given voltage input and looping backward through time to determine the correction to the voltage profile that will successfully defibrillate irregular heart activity while reducing the energy by the largest amount.
They ultimately discovered that adjusting the duration and the ‘smooth variation’ in time of the voltage supplied by defibrillation devices appears to be a more efficient mechanism, potentially reducing the energy needed to stop fibrillation by three orders of magnitude, or by 1,000 times.
According to the researchers, energy reduction in defibrillation devices is an active area of research and, while defibrillators are often successful in putting a stop to dangerous arrhythmias in patients, they are painful and cause damage to the cardiac tissue.
“Existing low-energy defibrillation protocols yield only a moderate reduction in tissue damage and pain,” Grigoriev stated. “Our study shows these can be completely eliminated. Conventional protocols require substantial power for implantable cardioverter-defibrillators (ICDs), and replacement surgeries carry substantial health risks.”
In a normal rhythm, electrochemical waves triggered by pacemaker cells at the top of the atria propagate through the heart, causing synchronised contractions. During arrhythmias, such as fibrillation, the excitation waves start to quickly rotate instead of propagating through and leaving the tissue, as is the case with a normal rhythm.
“Under some conditions, an excitation wave may or may not be able to propagate through the tissue. This is called the ‘vulnerable window’,” Grigoriev added. “The outcome depends on very small changes in the timing of the excitation wave or very small external perturbations.
“The mechanism of ultra-low-energy defibrillation we uncovered exploits this sensitivity. Varying the electrical field profile over a relatively long time interval allows blocking the propagation of the rotating excitation waves through the ‘sensitive’ regions of tissue, successfully terminating the irregular electric activity in the heart.”
