Spatiotemporally controlled cardiac conduction block using high-frequency electrical stimulation

Effects of high-frequency biphasic shocks on ventricular vulnerability and defibrillation outcomes through synchronized virtual electrode responses

Electrical defibrillation is a well-established treatment for cardiac dysrhythmias. Studies have suggested that shock-induced spatial sawtooth patterns and virtual electrodes are responsible for defibrillation efficacy. We hypothesize that high-frequency shocks enhance defibrillation efficacy by generating temporal sawtooth patterns and using rapid virtual electrodes synchronized with shock frequency. High-speed optical mapping was performed on isolated rat hearts at 2000 frames/s. Two defibrillation electrodes were placed on opposite sides of the ventricles. An S1-S2 pacing protocol was used to induce ventricular tachyarrhythmia (VTA). High-frequency shocks of equal energy but varying frequencies of 125–1000 Hz were used to evaluate VTA vulnerability and defibrillation success rate. The 1000-Hz shock had the highest VTA induction rate in the shorter S1-S2 intervals (50 and 100 ms) and the highest VTA defibrillation rate (70%) among all frequencies. Temporal sawtooth patterns and synchronous shock-induced virtual electrode responses could be observed with frequencies of up to 1000 Hz. The improved defibrillation outcome with high-frequency shocks suggests a lower energy requirement than that of low-frequency shocks for successful ventricular defibrillation.

Nanostructured cavity devices for extracellular stimulation of HL-1 cells

Microelectrode arrays (MEAs) are state-of-the-art devices for extracellular recording and stimulation on biological tissue. Furthermore, they are a relevant tool for the development of biomedical applications like retina, cochlear and motor prostheses, cardiac pacemakers and drug screening. Hence, research on functional cell-sensor interfaces, as well as the development of new surface structures and modifications for improved electrode characteristics, is a vivid and well established field. However, combining single-cell resolution with sufficient signal coupling remains challenging due to poor cell-electrode sealing. Furthermore, electrodes with diameters below 20 µm often suffer from a high electrical impedance affecting the noise during voltage recordings. In this study, we report on a nanocavity sensor array for voltage-controlled stimulation and extracellular action potential recordings on cellular networks. Nanocavity devices combine the advantages of low-impedance electrodes with small cell-chip interfaces, preserving a high spatial resolution for recording and stimulation. A reservoir between opening aperture and electrode is provided, allowing the cell to access the structure for a tight cell-sensor sealing. We present the well-controlled fabrication process and the effect of cavity formation and electrode patterning on the sensor's impedance. Further, we demonstrate reliable voltage-controlled stimulation using nanostructured cavity devices by capturing the pacemaker of an HL-1 cell network.

High frequency stimulation of cardiac myocytes: a theoretical and computational study

High-frequency stimulation (HFS) has recently been identified as a novel approach for terminating life-threatening cardiac arrhythmias. HFS elevates myocyte membrane potential and blocks electrical conduction for the duration of the stimulus. However, low amplitude HFS can induce rapidly firing action potentials, which may reinitiate an arrhythmia. The cellular level mechanisms underlying HFS-induced electrical activity are not well understood. Using a multiscale method, we show that a minimal myocyte model qualitatively reproduces the influence of HFS on cardiac electrical activity. Theoretical analysis and simulations suggest that persistent activation and de-inactivation of ionic currents, in particular a fast inward window current, underlie HFS-induced action potentials and membrane potential elevation, providing hypotheses for future experiments. We derive analytical expressions to describe how HFS modifies ionic current amplitude and gating dynamics. We show how fast inward current parameters influence the parameter regimes for HFS-induced electrical activity, demonstrating how the efficacy of HFS as a therapy for terminating arrhythmias may depend on the presence of pathological conditions or pharmacological treatments. Finally, we demonstrate that HFS terminates cardiac arrhythmias in a one-dimensional ring of cardiac tissue. In this study, we demonstrate a novel approach to characterize the influence of HFS on ionic current gating dynamics, provide new insight into HFS of the myocardium, and suggest mechanisms underlying HFS-induced electrical activity.

Defibrillation success with high frequency electric fields is related to degree and location of conduction block

BACKGROUND

We recently demonstrated that high frequency alternating current (HFAC) electric fields can reversibly block propagation in the heart by inducing an oscillating, elevated transmembrane potential (Vm) that maintains myocytes in a refractory state for the field duration and can terminate arrhythmias, including ventricular fibrillation (VF).

OBJECTIVES

To quantify and characterize conduction block (CB) induced by HFAC fields and to determine whether the degree of CB can be used to predict defibrillation success.

METHODS

Optical mapping was performed in adult guinea pig hearts (n = 14), and simulations were performed in an anatomically accurate rabbit ventricular model. HFAC fields (50-500 Hz) were applied to the ventricles. A novel power spectrum metric of CB-the loss of spectral power in the 1-30 Hz range, termed loss of conduction power (LCP)-was assessed during the HFAC field and compared with defibrillation success and VF vulnerability.

RESULTS

LCP increased with field strength and decreased with frequency. Optical mapping experiments conducted on the epicardial surface showed that LCP and the size of CB regions were significantly correlated with VF initiation and termination. In simulations, subsurface myocardial LCP and CB sizes were more closely correlated with VF termination than surface values. Multilinear regression analysis of simulation results revealed that while CB on both the surface and the subsurface myocardium was predictive, subsurface myocardial CB was the better predictor of defibrillation success.

CONCLUSIONS

HFAC fields induce a field-dependent state of CB, and defibrillation success is related to the degree and location of the CB.