Arrhythmogenesis as the failure of repolarization

Contemporary theories of cardiac fibrillation typically rely on the emergence of rotors to explain the transition from regular sinus rhythm to disordered electrophysiological activity. How those rotors spontaneously arise in the absence of re-entrant anatomical circuits is not fully understood. Here we propose a novel mechanism where arrhythmias are initiated by cardiac cells that fail to repolarize following a normal heartbeat. Those cells subsequently act as a focal ectopic source that drive the ensuing fibrillation. We used a simple computational model to investigate the impact of such cells in both homogeneous and heterogeneous excitable media. We found that heterogeneous media can tolerate a surprisingly large number of abnormal cells and still support normal rhythmic activity. At a critical limit the medium becomes chronically arrhythmogenic. Numerical analysis revealed that the critical threshold for arrhythmogenesis depends on both the strength of the coupling between cells and the extent to which the abnormal cells resist repolarization. Arrhythmogenesis was also found to emerge first at tissue boundaries where cells naturally have fewer neighbors to influence their behavior. These findings may explain why atrial fibrillation typically originates from the cuff of the pulmonary vein.Author summaryCardiac fibrillation is a medical condition where normal heart function is compromised as electrical activity becomes disordered. How fibrillation arises spontaneously is not fully understood. It is generally thought to be triggered by premature depolarization of the cardiac action potential in one or more cells. Those premature beats, known as early-afterdepolarizations, subsequently initiate a self-sustaining rotor in the otherwise normal heart tissue. In this study, we propose an alternative mechanism whereby arrhythmias are initiated by cardiac cells that fail to repolarize of their own accord but still operate normally when embedded in functional heart tissue. We find that such cells can act as focal ectopic sources under appropriate conditions of inter-cellular coupling. Moreover, cells on natural tissue boundaries are more susceptible to arrhythmia because they are coupled to fewer cells. This may explain why the pulmonary vein is often implicated as a source of atrial fibrillation.

DC SHOCK SIMULATOR

Defibrillators are electronic devices that carry shock electrical signals (pulses) to the heart muscle to maintain myocardial depolarization that is undergoing cardiac fibrillation (ventricular fibrillation or atrial fibrillation) (Bronzino, 2000). There are several conditions that must be met for the occurrence of shock processes including shock time, energy to be provided, patient and operator safety. In this defibrillator the use of selectors / energy selection is linear in the range 1-30 Joules with the use of tools at 10, 15, 20, 25, 30 Joules. The energy will then be discarded or given to the patient via a paddle when pressed the Discharge / shock button. The result of the signal given to the patient is monophasic. This study used a pre-experimental type with a One Group post test design research design. Measurements were made 5 times the volt meter at the test points determined by the compiler.

Renewal theory provides a universal quantitative framework to characterise the continuous regeneration of rotational events in cardiac fibrillation

ABSTRACTBackgroundCardiac fibrillation is thought to be maintained by rotational activity, with pivoting regions called phase singularities (PSs). Despite a century of research, no clear quantitative framework exists to model the fundamental processes responsible for the continuous formation and destruction of rotors in fibrillation.ObjectiveWe conducted a multi-modality, multi-species study of AF/VF under the hypothesis that PS formation/destruction in fibrillation can be modelled as self-regenerating renewal processes, producing exponential distributions of inter-event times governed by constant rate-parameters defined by the prevailing properties of the system.MethodsPS formation/destruction was studied and cross-validated in 5 models, using basket recordings and optical mapping from: i) human persistent AF (n = 20), ii) tachypaced sheep AF (n = 5), iii) rat AF (n = 4), iv) rat VF (n = 11) and v) computer simulated AF (SIM). Hilbert phase maps were constructed. PS lifetime data were fitted by exponential probability distribution functions (PDFs) computed using maximum entropy theory, and the rate parameter (λ) determined. A systematic review was conducted to cross-validate with source data from literature.ResultsPS destruction/formation distributions showed good fits to an exponential in all systems (R2≥ 0.90). In humans,λ= 4.6%/ms (95%CI,4.3,4.9)), sheep 4.4%/ms (95%CI,4.1,4.7)), rat AF 38%/ms (95%CI,22,55), rat VF 46%/ms (95%CI,31.2,60.2) and SIM 5.4%/ms (95%CI,4.1,6.7). All PS distributions identified through systematic review were exponential with λ comparable to experimental data.ConclusionThese results provide a universal quantitative framework to explain rotor formation and destruction in AF/VF, and a platform for therapeutic advances in cardiac fibrillation.

Ricardo Miledi - an outstanding neurophysiologist of 20th-21st centuries (1927-2017)

Ricardo Miledi (16.09.1927-18.12.2017) is an outstanding neurophysiologist and biophysicist who made a great contribution to the study of synaptic transmission functions. He proved the key role of сalcium ions in the release of neuromediators, developed methods of receptor expression and membrane fragments integration into large oocytes that provided huge possibilities for thousands of researchers to study subtle mechanisms of transmembrane proteins function in norm and pathology. Ricardo Miledi received his MD degree in the National Autonomous University of Mexico and in 1954 he defensed his dissertation on the study of electrical nature of cardiac fibrillation in the National Institute of Cardiology (Mexico). In 1956-1958 he underwent training in Canberra Health Research Institute (Australia) in the laboratory headed by John Eccles (Nobel Prize 1963). In 1958 R. Miledi was invited to the Department of biophysics of University College London where in cooperation with Bernard Katz (Nobel Prize 1970) made a number of important discoveries in the analysis of acetylcholine receptor expression in denervated mucle; determination of the role of calcium in neuromediators release; analysis of membrane noise on neuromediator application to neuromuscular synapses; study of the effect of antibodies from patients with myasthenia gravis on neuromuscular transmission. In the early 1980s Ricardo Miledi implemented the method of functional expression in Xenopus frog oocytes of receptors and ion channels from messenger ribonucleic acid (mRNA). His heritage running the gamut is presented in more than 500 articles.