Correlation among fibrillation, defibrillation, and cardiac pacing

Optimal entrainment for removal of pinned spiral waves

Cardiac fibrillation is caused by self-sustaining spiral waves that occur in the myocardium, some of which can be pinned to anatomical obstacles, making them more difficult to eliminate. A small electrical stimulation is often sufficient to unpin these spirals but only if it is applied during the vulnerable unpinning window. Even if these unpinning windows can be inferred from data, when multiple pinned spirals exist, their unpinning windows will not generally overlap. Using phase-based reduction techniques, we formulate and solve an optimal control problem to yield a time-varying external voltage gradient that can synchronize a collection of spiral waves that are pinned to a collection of heterogeneous obstacles. Upon synchronization, the unpinning windows overlap so that they can be simultaneously unpinned by applying an external voltage gradient pulse at an appropriate moment. Numerical validation is presented in bidomain model simulations. Results represent a proof-of-concept illustration of the proposed unpinning strategy which explicitly incorporates heterogeneity in the problem formulation and requires no real-time feedback about the system state.

Heart defibrillation: relationship between pacing threshold and defibrillation probability

BACKGROUND

Considering the clinical importance of the ventricular fibrillation and that the most used therapy to reverse it has a critical side effect on the cardiac tissue, it is desirable to optimize defibrillation parameters to increase its efficiency. In this study, we investigated the influence of stimuli duration on the relationship between pacing threshold and defibrillation probability.

RESULTS

We found out that 0.5-ms-long pulses had a lower ratio of defibrillation probability to the pacing threshold, although the higher the pulse duration the lower is the electric field intensity required to defibrillate the hearts.

CONCLUSION

The appropriate choice of defibrillatory shock parameters is able to increase the efficiency of the defibrillation improving the survival chances after the occurrence of a severe arrhythmia. The relationship between pulse duration and the probability of reversal of fibrillation shows that this parameter cannot be underestimated in defibrillator design since different pulse durations have different levels of safety.

Toward a More Efficient Implementation of Antifibrillation Pacing

We devise a methodology to determine an optimal pattern of inputs to synchronize firing patterns of cardiac cells which only requires the ability to measure action potential durations in individual cells. In numerical bidomain simulations, the resulting synchronizing inputs are shown to terminate spiral waves with a higher probability than comparable inputs that do not synchronize the cells as strongly. These results suggest that designing stimuli which promote synchronization in cardiac tissue could improve the success rate of defibrillation, and point towards novel strategies for optimizing antifibrillation pacing.

Exact coherent structures and chaotic dynamics in a model of cardiac tissue

Unstable nonchaotic solutions embedded in the chaotic attractor can provide significant new insight into chaotic dynamics of both low- and high-dimensional systems. In particular, in turbulent fluid flows, such unstable solutions are referred to as exact coherent structures (ECS) and play an important role in both initiating and sustaining turbulence. The nature of ECS and their role in organizing spatiotemporally chaotic dynamics, however, is reasonably well understood only for systems on relatively small spatial domains lacking continuous Euclidean symmetries. Construction of ECS on large domains and in the presence of continuous translational and/or rotational symmetries remains a challenge. This is especially true for models of excitable media which display spiral turbulence and for which the standard approach to computing ECS completely breaks down. This paper uses the Karma model of cardiac tissue to illustrate a potential approach that could allow computing a new class of ECS on large domains of arbitrary shape by decomposing them into a patchwork of solutions on smaller domains, or tiles, which retain Euclidean symmetries locally.

High-energy defibrillation impairs myocyte contractility and intracellular calcium dynamics

OBJECTIVES

We examined the effects of energy delivered with electrical defibrillation on myocyte contractility and intracellular Ca2+ dynamics. We hypothesized that increasing the defibrillation energy would produce correspondent reduction in myocyte contractility and intracellular Ca2+ dynamics.

DESIGN

Randomized prospective study.

SETTING

University-affiliated research laboratory.

SUBJECTS

Ventricular myocytes from male Sprague-Dawley rat hearts.

MATERIALS AND METHODS

Ventricular cardiomyocytes loaded with Fura-2/AM were placed in a chamber mounted on an inverted microscope and superfused with a buffer solution at 37 degrees C. The cells were field stimulated to contract and mechanical properties were assessed using a video-based edge-detection system. Intracellular Ca2+ dynamics were evaluated with a dual-excitation fluorescence photomultiplier system. Myocytes were then randomized to receive 1) a single 0.5-J biphasic shock; 2) a single 1-J biphasic shock; 3) a single 2-J biphasic shock; and 4) a control group without shock. After the shock, myocytes were paced for an additional 4 mins.

RESULTS

A single 0.5-J shock did not have effects on contractility and intracellular Ca2+ dynamics. Higher energy shocks, i.e., 1- or 2-J shocks, significantly impaired contractility and intracellular Ca2+ dynamics. The adverse effects were greater after a 2-J shock compared with a 1-J shock.

CONCLUSIONS

Higher defibrillation energy significantly impairs ventricular contractility at the myocyte level. Reductions in cardiomyocyte shortening and intracellular Ca2+ dynamics abnormalities were greater when higher energy shock was used.

Lethal effect of electric fields on isolated ventricular myocytes

Defibrillator-type shocks may cause electric and contractile dysfunction. In this study, we determined the relationship between probability of lethal injury and electric field intensity (E in isolated rat ventricular myocytes, with emphasis on field orientation and stimulus waveform. This relationship was sigmoidal with irreversible injury for E > 50 V/cm . During both threshold and lethal stimulation, cells were twofold more sensitive to the field when it was applied longitudinally (versus transversally) to the cell major axis. For a given E, the estimated maximum variation of transmembrane potential (Delta V(max)) was greater for longitudinal stimuli, which might account for the greater sensitivity to the field. Cell death, however, occurred at lower maximum Delta V(max) values for transversal shocks. This might be explained by a less steep spatial decay of transmembrane potential predicted for transversal stimulation, which would possibly result in occurrence of electroporation in a larger membrane area. For the same stimulus duration, cells were less sensitive to field-induced injury when shocks were biphasic (versus monophasic). Ours results indicate that, although significant myocyte death may occur in the E range expected during clinical defibrillation, biphasic shocks are less likely to produce irreversible cell injury.

Termination of atrial fibrillation using pulsed low-energy far-field stimulation

BACKGROUND

Electrically based therapies for terminating atrial fibrillation (AF) currently fall into 2 categories: antitachycardia pacing and cardioversion. Antitachycardia pacing uses low-intensity pacing stimuli delivered via a single electrode and is effective for terminating slower tachycardias but is less effective for treating AF. In contrast, cardioversion uses a single high-voltage shock to terminate AF reliably, but the voltages required produce undesirable side effects, including tissue damage and pain. We propose a new method to terminate AF called far-field antifibrillation pacing, which delivers a short train of low-intensity electric pulses at the frequency of antitachycardia pacing but from field electrodes. Prior theoretical work has suggested that this approach can create a large number of activation sites ("virtual" electrodes) that emit propagating waves within the tissue without implanting physical electrodes and thereby may be more effective than point-source stimulation.

METHODS AND RESULTS

Using optical mapping in isolated perfused canine atrial preparations, we show that a series of pulses at low field strength (0.9 to 1.4 V/cm) is sufficient to entrain and subsequently extinguish AF with a success rate of 93% (69 of 74 trials in 8 preparations). We further demonstrate that the mechanism behind far-field antifibrillation pacing success is the generation of wave emission sites within the tissue by the applied electric field, which entrains the tissue as the field is pulsed.

CONCLUSIONS

AF in our model can be terminated by far-field antifibrillation pacing with only 13% of the energy required for cardioversion. Further studies are needed to determine whether this marked reduction in energy can increase the effectiveness and safety of terminating atrial tachyarrhythmias clinically.

Effect of electrode surface area on thresholds for AC stimulation and ventricular fibrillation

Unintended, weak AC stimulation (leakage currents) from medical devices can cause blood pressure collapse and ventricular fibrillation (VF), potentially even death. Yet, little is understood about AC cardiac stimulation. The objective of this paper is to establish the relationship between the stimulation and VF thresholds for electrode size and stimulation frequency. Twenty-four retired male breeder guinea pigs were anesthetized with isoflurane, a tracheotomy and thoracotomy were performed, and vitals were monitored using the lead II ECG and an optical plethysmograph. The circular flat ends of eleven stainless steel rods were used as electrodes with areas ranging from 0.1 to 26.79 mm2. In the first study, 60-Hz AC stimuli of 5 s duration were delivered with strengths from 25-3000 microA or until VF was induced. In the second group, the current thresholds at 20, 40, 80, and 160 Hz were determined at electrode areas of 0.2, 2.01, and 16.4 mm2. Reactions were categorized as having no effect, having some effect (EFFECT, typically blood pressure collapse), and inducing VF. On a log-log scale, electrode radii had a piecewise-linear relationship with the current thresholds for EFFECT (p < 0.005) and VF (p < 0.01). The liminal area determined by the piecewise-linear fit was 2.0 and 2.84 mm2 for EFFECT and VF, respectively. Above the liminal area, the threshold increased proportional to r(1.25) and r(0.95) (r = radius of electrode), for EFFECT and VF, respectively. Based on these experimental results, we present a theoretical framework to explain the electrode size-stimulation threshold variation for both low strength AC stimulation and VF initiation.

Systematic evaluation of the determinants of defibrillation efficacy

OBJECTIVES

We studied the effect of varying shock capacitance, shock impedance, and pulse duration on defibrillation efficacy in a randomized, crossover manner for biphasic shocks.

BACKGROUND

The relationship between the electrical determinants of defibrillation efficacy is incompletely understood.

METHODS

Biphasic shocks were delivered to 12 dogs through epicardial patches (to vary impedance) after 15 seconds of ventricular fibrillation using one of 100- or 155-muF capacitors at each of four pulse durations (2.5, 5, 10, 20 ms), in a balanced random order. There were two impedance groups: six with higher impedance (mean 97 +/- 15 Omega, range 80-120) and six with lower impedance (mean 39 +/- 3 Omega, range 34-44). Voltage requirements were estimated as the average of three defibrillation threshold (DFT) tests.

RESULTS

Shock capacitance, resistance, and pulse duration all had significant effects upon the minimum voltage DFT (P = .0065, P = .0066, and P = .0001, respectively). The tilt associated with the lowest voltage and current requirement for each of the four capacitance/resistance combinations varied widely, between 34 +/- 5% and 63 +/- 3%, depending on capacitance and impedance. The optimal pulse duration associated with minimum DFT lies between 5.11 and 5.34 ms.

CONCLUSIONS

Defibrillation voltage requirements for biphasic shocks are affected by pulse duration, capacitance and impedance, but not "tilt."

Effects of Electrical Shocks on Ca i 2+ and V m in Myocyte Cultures

Changes in intracellular calcium concentration (ΔCa i 2+ ) induced by electrical shocks may play an important role in defibrillation, but high-resolution ΔCa i 2+ measurements in a multicellular cardiac tissue and their relationship to corresponding V m changes (ΔV m ) are lacking. Here, we measured shock-induced ΔCa i 2+ and ΔV m in geometrically defined myocyte cultures. Cell strands (width=0.8 mm) were double-stained with V m -sensitive dye RH-237 and a low-affinity Ca i 2+ -sensitive dye Fluo-4FF. Shocks (E≈5 to 40 V/cm) were applied during the action potential plateau. Shocks caused transient Ca i 2+ decrease at sites of both negative and positive ΔV m . Similar Ca i 2+ changes were observed in an ionic model of adult rat myocytes. Simulations showed that the Ca i 2+ decrease at sites of ΔV + m was caused by the outward flow of I CaL and troponin binding; at sites of ΔV − m it was caused by inactivation of I CaL combined with extrusion by Na–Ca exchanger and troponin binding. The important role of I CaL was supported by experiments in which application of nifedipine eliminated Ca i 2+ decrease at ΔV + m sites. Largest ΔCa i 2+ were observed during shocks of ≈10 V/cm causing simple monophasic ΔV m . Shocks stronger than ≈20 V/cm caused smaller ΔCa i 2+ and postshock elevation of diastolic Ca i 2+ . This was paralleled with occurrence of biphasic negative ΔV m that indicated membrane electroporation. Thus, these data indicate that shocks transiently decrease Ca i 2+ at sites of both ΔV − m and ΔV + m . Outward flow of I CaL plays an important role in Ca i 2+ decrease in the ΔV + m areas. Very strong shocks caused smaller negative ΔCa i 2+ and postshock elevation of diastolic Ca i 2+ , likely caused by membrane electroporation.

Nonlinear changes of transmembrane potential during electrical shocks: role of membrane electroporation

Defibrillation shocks induce nonlinear changes of transmembrane potential (DeltaVm) that determine the outcome of defibrillation. As shown earlier, strong shocks applied during action potential plateau cause nonmonotonic negative DeltaVm, where an initial hyperpolarization is followed by Vm shift to a more positive level. The biphasic negative DeltaVm can be attributable to (1) an inward ionic current or (2) membrane electroporation. These hypotheses were tested in cell cultures by measuring the effects of ionic channel blockers on DeltaVm and measuring uptake of membrane-impermeable dye. Experiments were performed in cell strands (width approximately 0.8 mm) produced using a technique of patterned cell growth. Uniform-field shocks were applied during the action potential plateau, and DeltaVm was measured by optical mapping. Shock-induced negative DeltaVm exhibited a biphasic shape starting at a shock strength of approximately 15 V/cm when estimated peak DeltaV-m was approximately -180 mV; positive DeltaVm remained monophasic. Application of a series of shocks with a strength of 23+/-1 V/cm resulted in uptake of membrane-impermeable dye propidium iodide. Dye uptake was restricted to the anodal side of strands with the largest negative DeltaVm, indicating the occurrence of membrane electroporation at these locations. The occurrence of biphasic negative DeltaVm was also paralleled with after-shock elevation of diastolic Vm. Inhibition of I(f) and I(K1) currents that are active at large negative potentials by CsCl and BaCl2, respectively, did not affect DeltaVm, indicating that these currents were not responsible for biphasic DeltaVm. These results provide evidence that the biphasic shape of DeltaVm at sites of shock-induced hyperpolarization is caused by membrane electroporation.

The effects of electrode position on the excitability of rat atria during postnatal development

Cardiac excitability is determined by the direction of the electric field, which is defined by the positioning of electrodes. However, important morphological and physiological modifications that happen during the postnatal development of the heart may affect the cardiac threshold. In this work we have evaluated the effect of electrode positioning on the excitability threshold of isolated Wistar rat atria (left auricles) during postnatal development. This was performed by determining the parameters of strength-duration curves for stimuli (rheobase, chronaxie and normalized minimum pulse energy) of atria from rats at ages (days) 5, 15, 30, 60, 90 and 120. These parameters were determined using electric field stimulation in four different orientations (apex-base, base-apex, left-right and right-left). Atrial rheobase decreased by 1.5- to 4-fold with animal age and was altered by electric field orientation in a diversified way, whereas atrial chronaxie increased only with animal age. The minimum pulse energy decreased two- to nine-fold with ageing. This was mainly due to rheobase dependence with electric field direction. We showed that the appropriate cardiac stimulation depends on the effects of three combined factors (pulse parameters, electrode position and animal age) on the atrial tissue excitability.

Defibrillation and the geometry of the heart: a novel measurement with implications for defibrillation mechanisms

We present a novel measurement for studying defibrillation mechanisms: the time course of changes in the size of the left ventricular (LV) cavity within 500 ms following defibrillation. Mechanical changes can be linked to electrical mechanisms via an understanding of excitation-contraction coupling. Eight mongrel dogs were internally defibrillated 5-50 seconds (including backup shocks) after the onset of 20 ventricular fibrillation (VF) episodes per animal. Two dimensional, short axis, LV cavity, ultrasound images were recorded at 30 frames per second just prior to inducing VF, during defibrillation and following the shock. Each frame was individually analysed to yield the LV cavity area as a function of time. Defibrillation shocks were followed by a highly reproducible phenomenon: (1) a dramatic and rapid increase in LV area, (2) a more or less prominent LV area plateau and (3) a decrease in the LV area. The peak fractional area increase ranged from 1.65 to 4.64 times larger than the baseline (LV area just prior to defibrillation), averaging 2.18 +/- 0.686. Successful shocks took significantly longer (p < 0.01) to return to 1.3 times the baseline (407 +/- 209 ms) than unsuccessful shocks (296 +/- 130 ms). Extrapolating to electrical mechanisms, our novel measurement demonstrates that defibrillation causes immediate relaxation and therefore suggests a significant role for deexcitation in defibrillation.

Hemodynamic collapse, geometry, and the rapidly paced upper limit of ventricular vulnerability to fibrillation by T-wave stimulation

There is an upper limit to the vulnerability (ULV) of the ventricles to fibrillation (VF) induced by T-wave stimuli. Across species, disease states, and pharmacological treatments, the ULV is correlated to the defibrillation threshold (DF50). However, one factor known to increase the ULV far above the DF50 is rapid pacing. In this article we test the hypothesis that this increase is owing to an accompanying hemodynamic collapse or geometric change. In 18 dogs, T-wave stimuli were delivered from transvenous defibrillating electrodes. The T-wave shock strength that induced VF 50% of the time (the ULV50) was measured using a 10-step Bayesian up-down protocol. T-wave stimuli were delivered after 15 paced beats at one of several rates: normal (80% of the R-R interval), rapid (the interval just fast enough to cause hemodynamic collapse), or 10 milliseconds greater than rapid (which did not cause hypotension). We measured the geometry of the left ventricle at the moment of T-wave stimulation using linear ultrasound. Rapid pacing significantly increased the ULV50 above the normal rate ULV (507 +/- 62.9 vs 379 +/- 70.6 V, P < .005, n = 18), even in the subset without hemodynamic collapse (505 +/- 84.4 vs 394 +/- 66.5 V, P < .005, n = 6). No significant geometric changes were noted between rapid (19.8 mm) and normal (20.6 mm, n = 6, P < NS) pacing, but QT interval reduction appears to correlate with the ULV50 (QT vs ULV50, r > 0, P < .01). Rapid pacing can dramatically increase the measured ULV50. The most likely cause is a concurrent change in the electrophysiology, eg, QT or APD, of the myocardium. As the only known factor to consistently alter the relationship between ULV and the DF50, rapid pacing offers a unique opportunity for the study of the link between defibrillation and ULV testing.

Progressive depolarization: a unified hypothesis for defibrillation and fibrillation induction by shocks

Experimental studies of defibrillation have burgeoned since the introduction of the upper limit of vulnerability (ULV) hypothesis for defibrillation. Much of this progress is due to the valuable work carried out in pursuit of this hypothesis. The ULV hypothesis presented a unified electrophysiologic scheme for linking the processes of defibrillation and shock-induced fibrillation. In addition to its scientific ramifications, this work also raised the possibility of simpler and safer means for clinical defibrillation threshold testing. Recent results from an optical mapping study of defibrillation suggest, however, that the experimental data supporting the ULV hypothesis could instead be interpreted in a manner consistent with traditional views of defibrillation such as the critical mass hypothesis. This review will describe the evidence calling for such a reinterpretation. In one regard the ULV hypothesis superseded the critical mass hypothesis by linking the defibrillation and shock-induced fibrillation processes. Therefore, this review also will discuss the rationale for developing a new defibrillation hypothesis. This new hypothesis, progressive depolarization, uses traditional defibrillation concepts to cover the same ground as the ULV hypothesis in mechanistically unifying defibrillation and shock-induced fibrillation. It does so in a manner consistent with experimental data supporting the ULV hypothesis but which also takes advantage of what has been learned from optical studies of defibrillation. This review will briefly describe how this new hypothesis relates to other contemporary viewpoints and related experimental results.

Effects of transvenous electrode polarity and waveform duration on the relationship between defibrillation threshold and upper limit of vulnerability

BACKGROUND

The upper limit of vulnerability (ULV) hypothesis for defibrillation predicts that maneuvers that alter the ULV will cause a similar alteration in the defibrillation threshold (DFT). The purpose of this study was to test this prediction by evaluating the effects of electrode polarity and waveform duration on the relationship between the DFT and the ULV.

METHODS AND RESULTS

Platinum spring electrodes were placed in the right ventricular (RV) apex and the superior vena cava in 12 pigs. Strength-duration curves were constructed for the DFT and ULV for each electrode polarity with monophasic waveforms (6 pigs) of different durations (2 to 14 ms) and biphasic truncated exponential waveforms (6 pigs) having phase 1 equal to 4 ms and phase 2 of different durations (0 to 10 ms). ULV data were gathered by scanning of the T wave. The ventricular pacing threshold (VPT) and ventricular fibrillation threshold (VFT) were also determined with these same waveforms. For the RV electrode as a cathode for monophasic and the first phase of biphasic stimuli, VPTs for the same waveform duration were significantly lower than for the configuration with the RV electrode as an anode. VFTs were not significantly different for the two electrode polarities with either monophasic or biphasic waveforms. The DFT changed in a fashion similar to the ULV with changes in electrode polarity and phase duration for both monophasic and biphasic waveforms. The ULV and DFT for each waveform duration for each polarity were strongly correlated (r=.83 to .99).

CONCLUSIONS

The almost identical changes in ULV and DFT with changes in electrode polarity and waveform duration provide new evidence to support the ULV hypothesis of defibrillation.