Excito-oscillatory dynamics as a mechanism of ventricular fibrillation

Life and death saddles in the heart

Saddle points are responsible for threshold phenomena of many biological systems. In the heart, saddle points determine the normal excitability and conduction, but are also responsible for certain abnormal action potential behaviors associated with lethal arrhythmias. We investigate the dynamical mechanisms for the genesis of lethal extra heartbeats in heterogeneous cardiac tissue under two diseased conditions. For both conditions, the lethal events occur when the system is close to the saddle point, implying the pivotal role of the saddle point in cardiac arrhythmogenesis.

Optimization of an In silico Cardiac Cell Model for Proarrhythmia Risk Assessment

Drug-induced Torsade-de-Pointes (TdP) has been responsible for the withdrawal of many drugs from the market and is therefore of major concern to global regulatory agencies and the pharmaceutical industry. The Comprehensive in vitro Proarrhythmia Assay (CiPA) was proposed to improve prediction of TdP risk, using in silico models and in vitro multi-channel pharmacology data as integral parts of this initiative. Previously, we reported that combining dynamic interactions between drugs and the rapid delayed rectifier potassium current (IKr) with multi-channel pharmacology is important for TdP risk classification, and we modified the original O'Hara Rudy ventricular cell mathematical model to include a Markov model of IKr to represent dynamic drug-IKr interactions (IKr-dynamic ORd model). We also developed a novel metric that could separate drugs with different TdP liabilities at high concentrations based on total electronic charge carried by the major inward ionic currents during the action potential. In this study, we further optimized the IKr-dynamic ORd model by refining model parameters using published human cardiomyocyte experimental data under control and drug block conditions. Using this optimized model and manual patch clamp data, we developed an updated version of the metric that quantifies the net electronic charge carried by major inward and outward ionic currents during the steady state action potential, which could classify the level of drug-induced TdP risk across a wide range of concentrations and pacing rates. We also established a framework to quantitatively evaluate a system's robustness against the induction of early afterdepolarizations (EADs), and demonstrated that the new metric is correlated with the cell's robustness to the pro-EAD perturbation of IKr conductance reduction. In summary, in this work we present an optimized model that is more consistent with experimental data, an improved metric that can classify drugs at concentrations both near and higher than clinical exposure, and a physiological framework to check the relationship between a metric and EAD. These findings provide a solid foundation for using in silico models for the regulatory assessment of TdP risk under the CiPA paradigm.

Filament Dynamics during Simulated Ventricular Fibrillation in a High-Resolution Rabbit Heart

The mechanisms underlying ventricular fibrillation (VF) are not well understood. The electrical activity on the heart surface during VF has been recorded extensively in the experimental setting and in some cases clinically; however, corresponding transmural activation patterns are prohibitively difficult to measure. In this paper, we use a high-resolution biventricular heart model to study three-dimensional electrical activity during fibrillation, focusing on the driving sources of VF: “filaments,” the organising centres of unstable reentrant scroll waves. We show, for the first time, specific 3D filament dynamics during simulated VF in a whole heart geometry that includes fine-scale anatomical structures. Our results suggest that transmural activity is much more complex than what would be expected from surface observations alone. We present examples of complex intramural activity, including filament breakup and reattachment, anchoring to the thin right ventricular apex; rapid transitions among various filament shapes; and filament lengths much greater than wall thickness. We also present evidence for anatomy playing a major role in VF development and coronary vessels and trabeculae influencing filament dynamics. Overall, our results indicate that intramural activity during simulated VF is extraordinarily complex and suggest that further investigation of 3D filaments is necessary to fully comprehend recorded surface patterns.

Quantification of transmembrane currents during action potential propagation in the heart

The measurement, quantitative analysis, theory, and mathematical modeling of transmembrane potential and currents have been an integral part of the field of electrophysiology since its inception. Biophysical modeling of action potential propagation begins with detailed ionic current models for a patch of membrane within a distributed cable model. Voltage-clamp techniques have revolutionized clinical electrophysiology via the characterization of the transmembrane current gating variables; however, this kinetic information alone is insufficient to accurately represent propagation. Other factors, including channel density, membrane area, surface/volume ratio, axial conductivities, etc., are also crucial determinants of transmembrane currents in multicellular tissue but are extremely difficult to measure. Here, we provide, to our knowledge, a novel analytical approach to compute transmembrane currents directly from experimental data, which involves high-temporal (200 kHz) recordings of intra- and extracellular potential with glass microelectrodes from the epicardial surface of isolated rabbit hearts during propagation. We show for the first time, to our knowledge, that during stable planar propagation the biphasic total transmembrane current (I(m)) dipole density during depolarization was ∼0.25 ms in duration and asymmetric in amplitude (peak outward current was ∼95 μA/cm(2) and peak inward current was ∼140 μA/cm(2)), and the peak inward ionic current (I(ion)) during depolarization was ∼260 μA/cm(2) with duration of ∼1.0 ms. Simulations of stable propagation using the ionic current versus transmembrane potential relationship fit from the experimental data reproduced these values better than traditional ionic models. During ventricular fibrillation, peak I(m) was decreased by 50% and peak I(ion) was decreased by 70%. Our results provide, to our knowledge, novel quantitative information that complements voltage- and patch-clamp data.

Depolarization-induced automaticity in rat ventricular cardiomyocytes is based on the gating properties of L-type calcium and slow Kv channels

Depolarization-induced automaticity (DIA) of cardiomyocytes is the property of those cells to generate pacemaker cell-like spontaneous electrical activity when subjected to a depolarizing current. This property provides a candidate mechanism for generation of pathogenic ectopy in cardiac tissue. The purpose of this study was to determine the biophysical mechanism of DIA in terms of the ion conductance properties of the cardiomyocyte membrane. First, we determined, by use of the conventional whole-cell patch-clamp technique, the membrane conductance and DIA properties of ventricular cardiomyocytes isolated from adult rat heart. Second, we reproduced and analysed DIA properties by using an adapted version of the experimentally based mathematical cardiomyocyte model of Pandit et al. (Biophys J 81:3029-3051 2001, Biophys J 84:832-841 2003) and Padmala and Demir (J Cardiovasc Electrophysiol 14:990-995 2003). DIA in 23 rat cardiomyocytes was a damped membrane potential oscillation with a variable number of action potentials and/or waves, depending on the strength of the depolarizing current and the particular cell. The adapted model was used to reconstruct the DIA properties of a particular cardiomyocyte from its whole-cell voltage-clamp currents. The main currents involved in DIA were an L-type calcium current (I CaL) and a slowly activating and inactivating Kv current (I ss), with linear (I B) and inward rectifier (I K1) currents acting as background currents and I Na and I t as modulators. Essential for DIA is a sufficiently large window current of a slowly inactivating I CaL combined with a critically sized repolarizing current I ss. Slow inactivation of I ss makes DIA transient. In conclusion, we established a membrane mechanism of DIA primarily based on I CaL, I ss and inward rectifier properties; this may be helpful in understanding cardiac ectopy and its treatment.