Effects of monophasic and biphasic shocks on action potentials during ventricular fibrillation in dogs

A percutaneous catheter-based system for the measurement of potential gradients applicable to the study of transthoracic defibrillation

BACKGROUND

The local electric (E) field or potential gradient produced by a shock reliably predicts VF termination. In this study we evaluated a multiple electrode, catheter-based device for closed-chest 3D measurements of E field from transthoracic defibrillation shocks.

METHODS

Catheters with multiple electrodes on the tip were placed in intracardiac locations in anesthetized swine. An empirically derived calibration matrix and custom microprocessor was used to transform simultaneously measured voltages into orthogonal E field vector components. E fields produced in six intracardiac locations by 30 and 300 J shocks were compared in eight animals. Correlations were determined for measured current and E field at various shock strengths at two different transthoracic impedances in five additional animals. VF was induced in 12 animals and E field measured during defibrillation attempts.

RESULTS

The E field measurements resulting for 30 J transthoracic shocks were not significantly different among different intracardiac sites. At 300 J, however, significant differences were observed between sites with the greatest intensities recorded in the coronary sinus and right ventricle. Within animals, the variability of the measurement at each site was small, ranging from 2.8 +/- 1.6% to 5.7 +/- 4.5%. Significant correlations (P < 0.001) between measured E field and peak current were observed at native impedance (34 +/- 4 Omega, r = 0.81) and at adjusted impedance (76 +/- 4 Omega, r = 0.78) with transthoracic shocks of 200, 300, and 360 J. In VF studies, the probability of defibrillation was closely fit by a sigmoidal dose response curve in the coronary sinus E field with an approximate threshold of 4.7 V/cm with 50% defibrillation success at 9.3 V/cm.

CONCLUSIONS

The measured intracardiac E field variability within animals and at a specific site was small, exhibiting a median value of 5.1%, contrasted to median variabilities across animals of 5-11% suggesting the capacity of this measurement system to provide subject specific information on the distribution of E fields. The measured E field magnitudes across animals in the coronary sinus were linearly correlated with applied shock current with a very strong linear relation to effective shock voltage observed in vitro in a saline tank. When evaluated as a predictor of shock success, the observed values were consistent with previously reported critical fields. This technique may be of value in evaluating waveforms for transthoracic defibrillation as well as electrode size, placement, and composition.

Transmural recording of monophasic action potentials

To investigate the possibility of transmural recording of repolarization through the ventricular wall, KCl monophasic action potential (MAP) electrodes positioned along plunge needles were developed and tested. The MAP electrode consists of a silver wire surrounded by agarose gel containing KCl, which slowly eluted into the adjacent tissue to depolarize it. In six dogs, a plunge needle containing three KCl MAP electrodes was inserted into the left ventricle to simultaneously record from the subepicardium, midwall, and subendocardium. In six pigs, eight plunge needles containing three KCl MAP electrodes and two plunge needles containing similar electrodes except for the absence of KCl were inserted into the ventricles. In three guinea pig papillary muscles, a KCl electrode was used to record MAPs along with two microelectrodes for recording transmembrane potentials. Transmural MAP recordings could be made for >1 h in dogs and >2 h in pigs with a significant decrease in MAP amplitude over time but without a significant change in MAP duration. With the electrodes without KCl in pigs, the injury potentials subsided in <30 min. When the pacing rate was changed to alter the action potential duration and refractory period in dogs, the MAP duration correlated with the local effective refractory period (r = 0.94). The time course of the MAP duration recorded with a KCl MAP electrode in guinea pig papillary muscles corresponded well with that of the transmembrane potential recorded with an adjacent microelectrode. It is possible to record transmural repolarization of the ventricles with KCl MAP electrodes on plunge needles. The MAP is caused by the KCl rather than being a nonspecific injury potential.

Regional differences in arrhythmogenic aftereffects of high intensity DC stimulation in the ventricles

Regional differences of the aftereffects of high intensity DC stimulation were investigated in isolated rabbit hearts stained with a voltage-sensitive dye (di-4-ANEPPS). Optical action potential signals were recorded from the epicardial surface of the right and left ventricular free wall (RVep, LVep) and from the right endocardial surface of the interventricular septum (IVS). Ten-millisecond monophasic DC stimulation (S2, 20-120 V) was applied to the signal recording spots during the early plateau phase of the action potential induced by basic stimuli (S1, 2.5 Hz). There was a linear relationship between S2 voltage and the S2 field intensity (FI). S2 caused postshock additional depolarization, giving rise to a prolongation of the shocked action potential. With S2 > or = 40 V (FI > or = approximately 20 V/cm), terminal repolarization of action potential was inhibited, and subsequent postshock S1 action potentials for 1-5 minutes were characterized by a decrease in the maximum diastolic potential and a decrease in the amplitude and a slowing of their upstroke phase. The higher the S2 voltage, the larger the aftereffects. The changes in postshock action potential configuration in RVep were significantly greater than those observed in LVep and IVS when compared at the same levels of S2 intensity. In RVep, 12 of 20 shocks of 120 V resulted in a prolonged refractoriness to S1 (> 1 s), and the arrest was often followed by oscillation of membrane potential. Ventricular tachycardia or fibrillation ensued from the oscillation in five cases. No such long arrest or serious arrhythmias were elicited in LVep and IVS. These results suggest that RVep is more susceptible than LVep and IVS for arrhythmogenic aftereffects of high intensity DC stimulation.

Arrhythmogenic changes in action potential configuration in the ventricle induced by DC shocks

Failure of defibrillation by direct current (DC) shocks is the result in part of new ventricular tachyarrhythmias induced by the shocks. We investigated the arrhythmogenic substrate produced by the shocks. Fluorescent action potential (AP) signals were recorded from rabbit hearts perfused in vitro with the use of our original optical recording system. Localized application of 10-ms shocks (S2) during the plateau phase of APs by basic stimuli (S1) caused field intensity (FI)-dependent changes in APs: (a) S2 > 7 V/cm caused additional depolarization, giving rise to a prolongation of AP duration (APD); (b) With S2 > 20 V/cm, terminal repolarization was inhibited, and subsequent postshock S1 APs for 1 to 5 min were characterized by decreases in the maximum diastolic potential and amplitude of APs; and (c) S2 > 30 V/cm often resulted in a prolonged refractoriness, oscillation of membrane potential leading to ventricular tachycardia or fibrillation (VT/VF). The right ventricle was more susceptible than other regions for the aftereffects of high-intensity shocks. Using an 8-channel recording system, we compared the effect of 10-ms monophasic (M) and 5/5-ms biphasic (B) shocks applied to the whole ventricles with FI of 1 to 20 V/cm at the signal recording sites. B shocks were less potent than M shocks in the FI-dependent action potential duration (APD) prolongation, and in the shock-induced enhancement of APD dispersion. Incidence and duration of VT/VF induced by M shocks were significantly greater than those by B shocks. These findings suggest that DC shocks will cause two types of arrhythmogenic substrate: one induced at sites of high FI, and the other at sites with moderate FI. The former would produce local block or focal repetitive excitation due to prolonged depolarization and oscillation of membrane potential, and the latter circuitous movement of wavefronts through an enhancement of spatial inhomogeneity of repolarization.

Using an artificial neural network to detect activations during ventricular fibrillation

Ventricular fibrillation is a cardiac arrhythmia that can result in sudden death. Understanding and treatment of this disorder would be improved if patterns of electrical activation could be accurately identified and studied during fibrillation. A feedforward artificial neural network using backpropagation was trained with the Rule-Based Method and the Current Source Density Method to identify cardiac tissue activation during fibrillation. Another feedforward artificial neural network that used backpropagation was trained with data preprocessed by those methods and the Transmembrane Current Method. Staged training, a new method that uses different sets of training examples in different stages, was used to improve the ability of the artificial neural networks to detect activation. Both artificial neural networks were able to correctly classify more than 92% of new test examples. The performance of both artificial neural networks improved when staged training was used. Thus, artificial neural networks may beuseful for identifying activation during ventricular fibrillation.

The biphasic mystery: why a biphasic shock is more effective than a monophasic shock for defibrillation

We demonstrate that a biphasic shock is more effective than a monophasic shock at eliminating reentrant electrical activity in an ionic model of cardiac ventricular electrical activity. This effectiveness results from early hyperpolarization that enhances the recovery of sodium inactivation, thereby enabling earlier activation of recovering cells. The effect can be seen easily in a model of a single cell and also in a cable model with a ring of excitable cells. Finally, we demonstrate the phenomenon in a two-dimensional model of cardiac tissue.

Electrophysiologic properties and ventricular fibrillation in normal and myopathic hearts

This study tests the hypothesis that moderate myocardial dysfunction is associated with altered myocardial anisotropic properties and structurally altered ventricular fibrillation (VF). Mongrel dogs were randomized to either a control group or a group that was rapidly paced at 250 beats/min until the left ventricular ejection fraction was [Formula: see text] 40%. Changes in anisotropic properties and the electrical characteristics of VF associated with the development of moderate myocardial dysfunction were assessed by microminiature epicardial mapping studies. In vivo conduction, refractory periods, and repolarization times were prolonged in both longitudinal and transverse directions in myopathic animals versus controls. VF was different in myopathic versus control animals. There were significantly more conducted deflections during VF in normal hearts compared with myopathic hearts. Propagated deflection-to-deflection intervals during VF were significantly longer in myopathic hearts compared with controls (125.5 ± 49.06 versus 103.4 ± 32.9 ms, p = 0.009). There were no abnormalities in cell size, cell shape, or the number of intercellular gap junctions and there was no detectable change in the expression of the gap junction proteins Cx43 and Cx45. Moderate myocardial dysfunction is associated with significant electrophysiological abnormalities in the absence of changes in myocardial cell morphology or intercellular connections, suggesting a functional abnormality in cell-to-cell communication.Key words: cardiomyopathy, anisotropy, fibrillation, defibrillation.

Energy levels for defibrillation: what is of real clinical importance?

Today, transthoracic and intracardiac defibrillation offer a well-accepted and widely used form of therapy for patients with life-threatening ventricular arrhythmias. Despite the wide clinical use of defibrillators, the mechanisms by which an electrical shock halts fibrillation are still not completely understood. During a shock, different amounts of current flow through the different parts of the heart and the current distribution is highly uneven. This current distribution is affected by changes in the shock potential gradient through the heart, changes in fiber orientation, and changes in myocardial conductivity caused by connective tissue barriers. It would be ideal if the potential gradient distribution throughout the ventricles could be measured directly for each individual patient during defibrillator implantation and follow-up and the shock strength could be programmed based on this measurement, but so far this is not possible. A more feasible approach is to determine, by trial and error, the magnitude of the shock strength delivered through the defibrillation electrodes for successful defibrillation. There is no distinct threshold value above which all shocks succeed and below which all shocks fail to defibrillate. Rather, increasing shock strength increases the likelihood the shock will succeed. Therefore, instead of a distinct defibrillation threshold, a probability of success curve exists. However, increasing the shock strength above an optimal range can actually decrease the success rate for defibrillation. One possible explanation is that the high voltage gradients caused by such large shocks damage cells and result in postshock arrhythmias that may reinitiate fibrillation. Another problem that can affect the probability of defibrillation success for a particular programmed energy setting is that the shock strength required for defibrillation may increase over time due to (1) the growth of fibrotic tissue around the defibrillation electrode; (2) migration of the lead; (3) acute ischemia; or (4) other changes in the underlying cardiac disease (e.g., worsening of heart failure). Such possible increases in the defibrillation shock strength requirement should be compensated for before they occur by adding a margin of safety to the shock strength needed for effective defibrillation. When programming an implantable defibrillator, it is important to keep in mind that the defibrillation shock should be (1) strong enough to defibrillate at least 98% of the time with the first shock; (2) weak enough not to cause severe post-shock arrhythmias or reinitiation of fibrillation; but (3) strong enough to compensate for changes of defibrillation energy requirements over time. This usually can be accomplished by setting the defibrillator 7-10 J higher than the defibrillation threshold determined by a standard step-down protocol.

Transmembrane potential changes caused by monophasic and biphasic shocks

Transmembrane potential change (DeltaVm) during shocks was recorded by a double-barrel microelectrode in 12 isolated guinea pig papillary muscles. After 10 S1 stimuli, square-wave S2 shocks of both polarities were given consisting of 10-ms monophasic and 10/10-ms and 5/5-ms biphasic waveforms that created potential gradients from 1.1 +/- 0.3 to 11.9 +/- 0.4 V/cm. S2 shocks were applied with 30, 60- to 70-, and 90- to 130-ms S1-S2 coupling intervals so that they occurred during the plateau, late portion of the plateau, and phase 3 of the action potential, respectively. Some shocks were given across as well as along the fiber orientation. The shocks caused hyperpolarization with one polarity and depolarization with the opposite polarity. The ratio of the magnitude of hyperpolarization to that of depolarization at the three S1-S2 coupling intervals was 1.5 +/- 0.3, 1.1 +/- 0.2, and 0.5 +/- 0.2, respectively. DeltaVm during the shock was significantly greater for the monophasic than for the two biphasic shocks. The prolongation of total repolarizing time (TRT) was significantly greater for monophasic (119.8 +/- 19.1%) and 10/10-ms biphasic (120.5 +/- 18.2%) than for 5/5-ms biphasic (113.0 +/- 12.9%) waveforms. The dispersion of the normalized TRT between instances of hyperpolarization and depolarization caused by the two shock polarities was 7.4 +/- 7.1% for monophasic, 3.0 +/- 4.1% for 10/10-ms biphasic, and 2.8 +/- 3.1% for 5/5-ms biphasic shocks (P < 0.05 for monophasic vs. biphasic). Shock fields along fibers produced a larger DeltaVm and prolongation of TRT than those across fibers. We conclude that 1) a change in shock polarity causes an asymmetrical change in membrane polarization depending on shock timing; 2) the 5/5-ms biphasic waveform causes the smallest DeltaVm, prolongs repolarization the least, and causes the smallest polarity-dependent dispersion; and 3) the changes in transmembrane potential and repolarization are influenced by fiber orientation.

Postshock potential gradients and dispersion of repolarization in cells stimulated with monophasic and biphasic waveforms

INTRODUCTION

Even though the clinical advantage of biphasic defibrillation waveforms is well documented, the mechanisms that underlie this greater efficacy remain incompletely understood. It is established, though, that the response of relatively refractory cells to the shock is important in determining defibrillation success or failure. We used two computer models of an isolated ventricular cell to test the hypothesis that biphasic stimuli cause a more uniform response than the equivalent monophasic shocks, decreasing the likelihood that fibrillation will be reinduced.

METHODS AND RESULTS

Models of reciprocally polarized and uniformly polarized cells were used. Rapid pacing and elevated [K]o were simulated, and either 10-msec rectangular monophasic or 5-msec/5-msec symmetric biphasic stimuli were delivered in the relative refractory period. The effects of stimulus intensity and coupling interval on response duration and postshock transmembrane potential (Vm) were quantified for each waveform. With reciprocal polarization, biphasic stimuli caused a more uniform response than monophasic stimuli, resulting in fewer large gradients of Vm (only for shock strengths < or = 1.25x threshold vs < or = 2.125x threshold) and a smaller dispersion of repolarization (1611 msec2 vs 1835 msec2). The reverse was observed with uniform polarization: monophasic pulses caused a more uniform response than did biphasic stimuli.

CONCLUSION

These results show that the response of relatively refractory cardiac cells to biphasic stimuli is less dependent on the coupling interval and stimulus strength than the response to monophasic stimuli under conditions of reciprocal polarization. Because this may lead to fewer and smaller spatial gradients in Vm, these data support the hypothesis that biphasic defibrillation waveforms will be less likely to reinduce fibrillation. Further, published experimental results correlate to a greater degree with conditions of reciprocal polarization than of uniform polarization, providing indirect evidence that interactions between depolarized and hyperpolarized regions play a role in determining the effects of defibrillation shocks on cardiac tissue.

Disparate effects of biphasic and monophasic shocks on postshock refractory period dispersion

The magnitude by which a defibrillation shock extends the refractory period immediately postshock (refractory period extension, RPE) does not explain why biphasic shocks defibrillate with greater efficacy than monophasic shocks. It may be that spatial heterogeneity of RPE is a more important regulator of defibrillation efficacy. We measured RPE in 15 pentobarbital-anesthetized swine using 400-V biphasic and monophasic shocks of equal pulse duration at three discrete myocardial sites. Spatial heterogeneity of RPE was calculated as the difference between the maximum and minimum values of the three recording sites. Monophasic shocks produced greater magnitude of RPE than biphasic shocks at all sites tested (82 +/- 6 to 99 +/- 13 and 64 +/- 6 to 68 +/- 5 ms, respectively; P < 0.05). However, RPE dispersion was significantly less with biphasic shocks versus monophasic shocks (29 +/- 4 and 48 +/- 7 ms, respectively; P < 0.05). This suggests that one potential mechanism by which biphasic shocks defibrillate with greater efficacy is limiting postshock spatial heterogeneity of refractoriness. Thus these data support our hypothesis that RPE heterogeneity is a more likely predictor of defibrillation efficacy than magnitude of RPE.

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.

[Mechanisms of electrical defibrillation]

Ventricular fibrillation has been described as a "chaotic, random, asynchronous electrical activity of the ventricles due to repetitive reentrant excitation and/or rapid focal discharge". Reentrant and non-reentrant mechanisms are responsible for the initiation of ventricular fibrillation. After fibrillation has been induced, it is thought that multiple, disorganized, wandering wavelets follow constantly changing reentrant pathways. Electrical defibrillation is the only valid therapeutic approach for ventricular fibrillation. A successful defibrillation shock must be of sufficient strength to stop fibrillation but must not be so strong that damage to the myocardium occurs. The clinical use of the implantable cardioverter/defibrillator device has significantly stimulated research in the field of cardiac defibrillation. In order to develop more efficient shock waveforms and electrode configurations for smaller, and also longer lasting devices, we need a better understanding of the basic mechanisms of defibrillation. The development of computerized electrical mapping systems, capable of recording before, during and after a defibrillation shock, optical recording systems and microelectrodes, for action potential recording before and after the shock application and mathematical models have contributed much to the understanding of defibrillation mechanisms.An electrical shock hits the cardiac cells in different phases of their action potential. This results in 1) direct activation, 2) a "graded response", or 3) no effect. "Graded response" produces prolongation of the action potential and prolongs refractoriness without giving rise to a propagated activation front. Refractory period prolongation in an area that is still refractory at the time of the shock is critical for successful defibrillation. Mapping studies have shown that for successful defibrillation with monophasic shocks a minimal potential gradient of 5-7 V/cm is necessary (the exact value depends on the waveform and the orientation of the cells with respect to the electric field).Several hypotheses have been developed in order to explain the mechanisms that underlie successful defibrillation shocks. This paper will discuss the various theories. The "upper limit of vulnerability" hypothesis for defibrillation states that a successful defibrillation shock must stop existing activation fronts by directly exciting or by prolonging refractoriness just in front of the upcoming activation fronts and must not give rise to new activation fronts at the border of the directly excited area. Shocks slightly weaker then necessary to defibrillate stop fibrillation activation fronts, but give rise to new activation fronts that reinitiate fibrillation. These new activation fronts arise at a "critical point," where a critical shock potential gradient interferes with a critical degree of tissue refractoriness. Mappping studies support the "upper limit of vulnerability" hypothesis of defibrillation but not all defibrillation failures, however, can be explained by this hypothesis.Clinical data and experimental results have shown that biphasic shocks may have lower defibrillation thresholds than monophasic shocks. The advantage of defibrillation with a biphasic waveform is not yet clearly understood. We discuss some possible reasons why some biphasic waveforms have lower defibrillation thresholds than monophasic waveforms.

Refractory period extension during ventricular pacing at fibrillatory pacing rates

Refractory period extension (RPE) has been proposed as a basic mechanism for defibrillation but it remains unclear if RPE exists at the fast rates associated with ventricular fibrillation. In 7 pentobarbital anesthetized dogs, we measured refractory periods with and without 8 ms rectangular transcardiac shocks at left ventricular pacing rates of 200-600 beats/min. To achieve these high rates, an incremental rate pacing method was used to produce pacing train timing sequences requiring 4.5-27 seconds. A variably timed premature stimulus followed the last stimulus in each pacing train. To determine refractoriness, a 128 electrode array (4 x 4 cm) was used to detect the presence, or absence of an activation sequence sweeping away from the pacing site. At each rate, a control refractory period (RPc) was measured and refractory periods were also measured for 8 and 12 V/cm shocks with coupling intervals of 60% to 90% of RPc. RPc decreased as the rate increased with a minimum RPc of 94 ms at a rate of 600 beats/min (100 ms cycle length). RPE/RPc versus shock coupling interval was similar at all pacing rates. RPE/RPc increased with increased coupling interval or higher shock intensity. We conclude that during ventricular pacing at fibrillatory rates tissue is nearly always in a refractory state; that RPE exists at fibrillatory activation rates; and that RPE/RPc versus shock coupling interval does not vary strongly with pacing rate. These findings support the hypothesis that RPE contributes to defibrillation.

Induction of ventricular fibrillation by T-wave field-shocks in the isolated perfused rabbit heart: role of nonuniform shock responses

OBJECTIVES

Single electrical field shocks are able to induce ventricular fibrillation (VF) if applied during the vulnerable period. During this period, a shock can either prolong the action potential duration or induce a new action potential. Whether the occurrence of different shock responses contributes to the induction of VF has not been investigated directly in the intact heart.

METHODS

In 12 isolated Langendorff-perfused rabbit hearts seven monophasic action potentials (MAPs) were recorded simultaneously during the application of 838 T-wave shocks. Post-shock repolarization was assessed by classifying the shock-induced response in each MAP recording either as a full action potential or an action potential prolongation. Heterogeneity of post-shock repolarization was defined if both response patterns were present in different MAP recordings at the same time. The heterogeneity of post-shock activation was measured as the dispersion of the post-shock activation time (PS-AT). The arrhythmogeneity of a shock was quantified as the number of rapid shock-induced repetitive responses.

RESULTS

Shocks inducing nonuniform repolarization were associated with greater arrhythmogeneity than shocks inducing uniform repolarization (17.6 +/- 30.0 versus 1.6 +/- 1.1 shock-induced repetitive responses, p < 0.001). The severity of the induced arrhythmia increased gradually with increasing nonuniformity of repolarization (p < 0.01 for a 10% increase), being maximal when the shock initiated near equal numbers of both full action potentials and action potential prolongations. The induction of severe arrhythmias by T-wave shocks was also associated with a higher dispersion of PS-AT (29 +/- 14 ms for the induction of VF, 19 +/- 12 ms for non-sustained arrhythmia, and 12 +/- 8 ms for no arrhythmic response, all p < 0.001). For VF inducing shocks, an increase in shock strength towards the upper limit of vulnerability decreased the dispersion of PS-AT from 34 +/- 15 ms to 23 +/- 11 ms (p < 0.001).

CONCLUSIONS

Nonuniform post-shock repolarization and dispersed post-shock activation contribute to the induction of VF by T-wave shocks. A decreasing dispersion of PS-AT towards higher shock strengths may contribute to the decreased or abolished inducibility by shocks above the upper limit of vulnerability.

Modeling the interaction between propagating cardiac waves and monophasic and biphasic field stimuli: the importance of the induced spatial excitatory response

INTRODUCTION

Biphasic (BP) defibrillation waveforms have been shown to be significantly more efficacious than equivalent monophasic (MP) waveforms. However, when defibrillation fails, it tends to do so first in distal regions of the heart where induced field gradient magnitudes are lowest. We tested the hypothesis that the improved efficacy of BP waveforms results from their enhanced ability to prevent the initiation of new postshock activation fronts behind preexisting wavetails, rather than from any significantly improved ability to terminate preexisting wavefronts.

METHODS AND RESULTS

An idealized computer model of a one-dimensional cardiac strand was used to investigate the spatial and temporal interactions between an underlying propagation front (or tail) and uniform MP or BP field stimuli of various intensities. Axial discontinuities from intercellular junctions induced sawtooth patterns of polarization during such field stimuli, enabling the shocks to interact directly with all cells. MP and BP diastolic thresholds were essentially equal. All suprathreshold MP and BP field stimuli successfully terminated preexisting wavefronts by directly depolarizing tissue ahead of those fronts, thus blocking their continued progression. However, the postshock response at the wavetail was significantly dependent on the shape and strength of the administered field. Low-strength MP stimuli induced an all-or-none excitation response across the wavetail, producing a sharp spatial transmembrane voltage gradient from which a new sustained anterogradely propagating wavefront was initiated. In contrast, low-strength BP field stimuli induced a spatially graded excitatory response whose voltage gradient was insufficient to initiate such a wavefront. Higher-strength MP and BP stimuli both produced graded excitatory responses with no subsequent propagation.

CONCLUSIONS

Shock-induced spatial "all-or-none" excitatory responses facilitate, and graded excitatory responses prevent, the postshock initiation of new propagating wavefronts. Moreover, BP field stimuli can induce such graded excitatory responses at significantly lower stimulus strengths than otherwise equivalent MP stimuli. Therefore, these results support an alternative "graded excitatory response" mechanism for the improved efficacy of BP over MP field stimuli in low gradient regions.

A biophysical model for defibrillation of cardiac tissue

We propose a new model for electrical activity of cardiac tissue that incorporates the effects of cellular microstructure. As such, this model provides insight into the mechanism of direct stimulation and defibrillation of cardiac tissue after injection of large currents. To illustrate the usefulness of the model, numerical stimulations are used to show the difference between successful and unsuccessful defibrillation of large pieces of tissue.

Class III antiarrhythmic effects of LY-190147 on defibrillation threshold

Defibrillation strength shocks delivered within an action potential (AP) delay repolarization. Shock-induced AP duration extension (APDE) may prolong refractoriness and terminate or prevent reinitiation of reentry, favoring defibrillation. This study examined LY-190147 (LY) effects on defibrillation threshold (DFT) in 11 dogs. Ventricular effective refractory period (VERP) and epicardial monophasic AP duration at 75% repolarization (APD75) were recorded at 300-, 400-, 500-, and 600-ms pacing cycle length (CL). APDE was measured as the time to 50% repolarization after a DFT strength shock delivered at 50, 25, and 0 ms before or 25 ms after VERP during pacing at 300 ms CL in 4 of the dogs. We made all recordings before drug administration and after infusions of 0.03, 0.3, and 3.0 mg/kg LY, using 1.5-h dosing intervals. LY lowered DFT in a saturating dose-response manner whether expressed as shock peak voltage (V) or energy. LY decreased DFT-V from 357 +/- 77 V before drug to 331 +/- 60 V (-6 +/- 12%), 290 +/- 43 V (-17 +/- 13%, p < 0.001), and 312 +/- 45 V (-11 +/- 12%, p < 0.05) at 0.03, 0.3, and 3.0 mg/kg, respectively. Similarly, LY treatment decreased defibrillation energy requirements from 6.9 +/- 2.7 J before drug by 7 +/- 25%, 26 +/- 24%, and 12 +/- 25% at the same doses. At 300-600 ms CL, LY prolonged APD75 by an average of 10 +/- 8% at 0.03 mg/kg, 17 +/- 6% at 0.3 mg/kg, and 24 +/- 9% at 3 mg/kg. At these CL, LY prolonged VERP by an average of 4 +/- 6% at 0.03 mg/kg, 15 +/- 10% at 0.3 mg/kg, and 11 +/- 9% at 3 mg/kg. APDE was increased from 62 +/- 9 ms before to 68 +/- 14, 80 +/- 16 (p < 0.001) and 72 +/- 13 ms (p < 0.05) at 0.03, 0.3, and 3.0 mg/kg LY, respectively. Therefore, LY prolonged VERP and APDE and affected DFT in the same saturating dose-response manner. LY may facilitate defibrillation by increasing the duration of postshock refractoriness.

Effects of optical enantiomers CK-4000(S) and CK-4001(R) on defibrillation and enhancement of shock-induced extension of action potential duration

INTRODUCTION

Class III antiarrhythmics have been reported to lower defibrillation threshold (DFT); however, the mechanism(s) of this effect is unknown. Recent evidence suggests that DFT strength DC shocks may terminate reentrant arrhythmias through prolongation of action potential duration and refractory periods. Since Class III antiarrhythmic drugs prolong repolarization, we examined the hypothesis that these drugs enhance shock-induced action potential duration extension (APDE), which might contribute to lowering of DFT.

METHODS AND RESULTS

In order to investigate the specificity of drug effects on action potential repolarization following a shock, an optical enantiomer with mixed beta-blocking and Class III effects (CK-4000) and its enantiomer with "pure" Class III antiarrhythmic effects (CK-4001) were compared against placebo in a canine defibrillation model (n = 8 per group). At the 3 mg/kg dose, the enantiomers nonstereoselectively lowered DFT voltage by 19 +/- 15% (CK-4000, P < 0.05 compared to placebo) and 25 +/- 12% (CK-4001, P < 0.05 compared to placebo), indicating that Class III antiarrhythmic actions alone were sufficient for this effect. Similarly, CK-4000 and CK-4001 at 3 mg/kg enhanced APDE (P < 0.01 compared to placebo) by 20 +/- 11% and 24 +/- 17%, respectively. APDE prolongation significantly correlated with reduction in DFT voltage for both CK-4000 (r = -0.55, P < 0.03) and CK4001 (r = -0.63, P < 0.01). At 3 mg/kg, the enantiomers stereoselectively prolonged action potential duration (APD75) by an average of 37 +/- 14% (CK-4000, P < 0.001) and 23 +/- 14% (CK-4001, P < 0.001), and ventricular effective refractory period (VERP) by 38 +/- 15% (CK-4000, P < 0.01) and 27 +/- 13% (CK-4001, P < 0.05). Prolongations of APD75 and VERP did not correlate with reductions of DFT in individual dogs.

CONCLUSIONS

These results show that Class III antiarrhythmics and DFT strength shocks additively delay repolarization, which suggests that drug enhancement of APDE may contribute to their effects on DFT.

Alteration of ventricular fibrillation by propranolol and isoproterenol detected by epicardial mapping with 506 electrodes

INTRODUCTION

We hypothesized that drugs which alter ventricular refractoriness or excitability produce quantifiable changes in ventricular fibrillation.

METHODS AND RESULTS

We used a 528-channel mapping system to quantify the effects of the beta-antagonist, propranolol, and the beta-agonist, isoproterenol, on activation patterns in ventricular fibrillation. A plaque of 506 (22 x 23) electrodes spaced 1.12 mm apart and covering about 5% of the ventricular epicardium was sewn to the anterior right ventricle in 18 pigs (30 kg). Propranolol (0.25 to 0.4 mg/kg) increased the refractory period at a right ventricular epicardial site while isoproterenol (3 to 5 micrograms/min) shortened it. Ventricular fibrillation was induced by programmed stimulation, and unipolar electrograms were recorded from the 506 plaque electrodes for 2 seconds beginning 1, 15, and 30 seconds after the onset of fibrillation. Active epicardial recording sites were identified from the first derivative of the unipolar potentials (dV/dt) detected at each electrode. Then, neighboring active sites were grouped into activation fronts by computer analysis. In six pigs the effect of repeated inductions of ventricular fibrillation was assessed by comparing ventricular fibrillation after saline with a preceding control episode of fibrillation. Each activation front excited 40% +/- 46% of the mapped region before blocking. No changes were observed with saline and multiple inductions of fibrillation. In another six pigs, ventricular fibrillation after propranolol was compared with a preceding control episode of fibrillation. Ventricular fibrillation after propranolol exhibited a decreased activation rate per epicardial recording site and fewer activation fronts per second. There was no change in the amount of tissue excited by each activation front or the number of reentry cycles per activation front compared with control. In addition, there was no change in the maximum negative dV/dt detected per activation at an epicardial site. In six pigs ventricular fibrillation during isoproterenol was compared with control episodes of ventricular fibrillation before and 45 minutes after washout of the drug. The control episodes of fibrillation were not different from each other. Compared with control, ventricular fibrillation during isoproterenol exhibited an increased activation rate per epicardial site, an increased amount of tissue excited by each activation front, and an increased maximum negative dV/dt for each activation. There was no change in the number of activation fronts per second or the number of reentry cycles per activation front compared with control.

CONCLUSION

Quantitative analysis revealed that propranolol and isoproterenol do not have symmetrically opposite effects on ventricular fibrillation. Propranolol decreased the number of activation fronts while isoproterenol increased the amount of tissue excited by each activation front. Thus, drugs that alter ventricular refractoriness or excitability alter ventricular fibrillation.

Responses of the transmembrane potential of myocardial cells during a shock

INTRODUCTION

The purpose of this investigation was to study the transmembrane potential changes (delta Vm) during extracellular electrical field stimulation.

METHODS AND RESULTS

Vm was recorded in seven guinea pig papillary muscles in a tissue bath by a double-barrel microelectrode with one barrel in and the other just outside a cell while shocks were given across the bath. The short distance (15 to 30 microns) between the two microelectrode tips and alignment of the tips parallel to the shock electrodes eliminated the shock artifact. Following ten S1 stimuli, an S2 shock field created by a 10-msec square wave was delivered during the action potential plateau or during diastole through shock electrodes 1 cm on either side of the tissue. Four shock strengths creating field strengths of 1.7 +/- 0.1, 2.9 +/- 0.2, 6.1 +/- 0.6, and 8.8 +/- 0.9 V/cm were given for the same impalement. Both shock polarities were given at each shock strength. For shocks delivered during the action potential plateau, the magnitudes of the peak delta Vm caused by the above four potential gradients were 21.1 +/- 8.2, 33.6 +/- 13.6, 49.9 +/- 24.2, and 52.3 +/- 28.0 mV (P < 0.05 among the four groups) for the shocks causing depolarization and 37.9 +/- 14.2, 56.6 +/- 16.4, 83.1 +/- 19.4, and 92.9 +/- 29.1 mV (P < 0.05 among the four groups) for the shocks causing hyperpolarization. Though delta Vm increased as potential gradients increased, the relationship was not linear. The magnitude of hyperpolarization was 1.9 +/- 0.5 times that of depolarization when the shock polarity was reversed (P < 0.05). As potential gradients increased from 1.7 +/- 0.1 to 8.8 +/- 0.9 V/cm, the time constant of the membrane response decreased significantly from 3.5 +/- 1.8 to 1.6 +/- 0.7 msec for depolarizing shocks and from 6.0 +/- 3.1 to 3.4 +/- 1.9 msec for hyperpolarizing shocks (P < 0.01 vs depolarizing shocks). For shocks delivered during diastole, hyperpolarizing shocks induced triphasic changes in Vm during the shock, i.e., initial hyperpolarization, than depolarization, followed again by hyperpolarization.

CONCLUSION

During the action potential plateau, the membrane response cannot be represented by a classic passive RC membrane model. During diastole, activation upstrokes occur even during hyperpolarization caused by shocks creating potential gradients between approximately 2 and 9 V/cm.

Correlation among fibrillation, defibrillation, and cardiac pacing

An electrical stimulus must create an electric field of approximately 1 V/cm in the extracellular space to stimulate myocardium during diastole. To initiate fibrillation by premature stimulation during the vulnerable period or to defibrillate, an extracellular electric field of approximately 6 V/cm is required, a value approximately six times greater than that necessary for diastolic pacing. Yet, the current strength of the pulse given to the stimulating electrode to initiate fibrillation or to defibrillate is much greater than six times the diastolic pacing threshold. The ventricular fibrillation threshold is typically 40 times greater than the diastolic pacing threshold expressed in terms of current. The defibrillation threshold in terms of current is typically thousands of times greater than the diastolic pacing threshold. The reason that these thresholds vary so much more in terms of stimulus current than in terms of extracellular potential gradient is that each of the three thresholds requires creation of the required potential gradient at different distances from the stimulating electrode. Pacing requires a potential gradient of approximately 1 V/cm only in a small liminal volume of tissue immediately adjacent to the electrode. Initiation of ventricular fibrillation by premature stimulation during the vulnerable period requires a potential gradient of approximately 6 V/cm about 1 cm away from the stimulating electrode to allow sufficient space for the central common pathway of a figure-eight reentrant circuit to form. Since the potential gradient falls off rapidly with distance from the stimulating electrode, a stimulating current about 40 times greater than the diastolic pacing threshold is required to generate an electric field of 6 V/cm approximately 1 cm away from the stimulating electrode. Defibrillation requires an electric field of approximately 6 V/cm throughout all or almost all of the ventricular myocardium. Since some portions of the ventricles can be more than 10 cm away from the defibrillation electrodes, a shock of several amps is required to create this field, a current thousands of times greater than the pacing threshold.

Mechanisms of defibrillation. Critical points and the upper limit of vulnerability

The upper limit of vulnerability hypothesis for defibrillation states that a successful defibrillation shock must both stop the fibrillation wave fronts on the heart at the time that the shock is delivered and not start new wave fronts that will lead to reentrant circuits being formed, causing the heart to refibrillate. Mapping studies have demonstrated that defibrillation shocks can halt all wave fronts on the heart but fibrillation will begin again with an initial activation pattern that is different from the activation pattern on the heart just before the shock is delivered. In a fashion similar to the reinitiation of fibrillation following a failed defibrillation shock, properly sized and timed shocks can be delivered to the heart during paced rhythm to induce functional reentry that will initiate fibrillation. If the shocks are made incrementally larger, a shock level will be reached that is high enough not to start fibrillation in regular rhythm regardless of when it is delivered during the cardiac cycle. This shock level is called the upper limit of vulnerability. In this study, the formation of reentrant circuits with defibrillation-sized shocks and how this formation of reentrant circuits may be related to mechanism of defibrillation, via the upper limit of vulnerability hypothesis are discussed.

Animated images of cardiac membrane voltage during defibrillation

Optical recording using voltage-sensitive dyes has been used to investigate the mechanisms of defibrillation because it (1) is immune to the artifacts produced by high-voltage shocks, (2) provides the time course of the membrane action potential, and (3) can be used to make simultaneous recordings at many sites. The authors used the laser scanning technique to optically record action potentials from 100 sites with 1-ms resolution on the surface of the isolated, perfused rabbit heart during defibrillation. The data were typically analyzed by constructing maps of impulse propagation and examining individual recordings from sites of interest. Described here is a new analysis method that creates millisecond-by-millisecond images of the spatial distribution of membrane potentials. The experimental protocol applied a test shock to the fibrillating heart, followed by a rescue shock and a paced beat. Optical recordings were calibrated to yield membrane voltage as a percentage of the resting and overshoot levels of the postrescue stimulated action potential. The positions of the recording sites and the membrane voltage levels for all 100 sites during a single 1-ms interval were used to interpolate membrane voltage levels at points within a 128 x 128 pixel frame using the biharmonic interpolation method. The level of membrane potential was encoded by pixel color and surface elevation. Sequential frames were viewed as a face-on two dimensional or as a three-dimensional perspective of the colored surface. Animation of membrane voltage distributions enabled the visualization of the interaction between the shock-induced electrophysiologic response and the propagation of electrical activity preceding and following a defibrillation shock. Successful defibrillation shocks synchronized repolarization across the surface of the heart following the shock.

Cardiac responses to premature monophasic and biphasic field stimuli. Results from cell and tissue modeling studies

Experimental and clinical observations confirm that certain biphasic (BP) defibrillation shocks are significantly more efficacious than equivalent monophasic (MP) shocks, yet the mechanisms underlying these improvements are still not well understood. The authors used two separate, but related, computer models to investigate in detail the excitation responses of active cardiac cells and tissue to idealized premature extracellular MP and BP field stimuli. The results revealed a large disparity in MP and BP excitation responses to low-strength, but not high-strength, fields. In particular, at these low-strength levels, the polarity reversal within BP shocks effectively extends excitability to earlier cellular refractory states than can be achieved with simple MP shocks. Moreover, whereas low-strength MP shocks induce a distinct all-or-none excitatory response to varying shock prematurities, the excitatory response to equivalent BP shocks remains highly graded. In tissue simulations where such field stimuli intersected propagating wave fronts, the all-or-none excitatory response elicited by low-strength MP shocks created a postshock discontinuity in the spatial transmembrane voltage profile, which initiated a new propagation wave front. In contrast, the graded excitatory response elicited by BP waveforms effectively prevented the formation of postshock wave fronts. High-strength MP and BP stimuli prevented renewed propagation equally well. In conclusion, these results suggest a new mechanisms for BP defibrillation superiority over MP waveforms: that the graded excitatory response to BP stimuli at low-field strengths effectively prevents the formation of large spatial transmembrane voltage gradients, which can lead to renewal of propagated wave fronts.

Threshold reduction with biphasic defibrillator waveforms. Role of charge balance

Mechanism underlying improved defibrillation efficacy of biphasic waveforms at low shock intensities remain poorly understood. Recent studies suggest that biphasic waveforms produce a longer mean postshock response throughout the ventricle. This prolongs the cellular refractory period, blocks fibrillation wave fronts, and causes fibrillation to cease. Previous studies showed that hyperpolarizing monophasic waveforms, delivered during the refractory period, can shorten action potential duration (APD90), which would be deleterious for defibrillation. This study tested the hypothesis that a balanced-charge biphasic waveform produces a longer mean total mean APD than a comparable monophasic waveform by preventing this shortening in hyperpolarized regions as well as by prolonging APD in depolarized regions. To test this hypothesis, the authors examined transmembrane potential changes produced by hyperpolarizing and depolarizing monophasic and balanced-charge symmetrical biphasic waveforms using a computer model of the ventricular action potential. Shock intensities within the low-intensity "window," where biphasic waveforms defibrillate with higher efficacy than monophasic waveforms (1.5-3 times diastolic threshold), were used. Results show that biphasic S2 produced a significantly longer response both under hyperpolarizing and depolarizing conditions. The hyperpolarizing/depolarizing biphasic S2 produced a prolonged response with a well-defined plateau. Following the depolarizing/hyperpolarizing S2, APD90 did not shorten as with the hyperpolarizing monophasic S2. Rather, repolarization continued near the original S1 times course, but with slight prolongation of S1 APD90. These results suggest that biphasic waveforms enhance the prolonged refractory periods required for defibrillation throughout the heart, including regions exposed to both anodal and cathodal stimulation.

Mechanisms of electrical defibrillation: impact of new experimental defibrillator waveforms

Six possible explanations for why some biphasic waveforms have lower defibrillation thresholds than monophasic waveforms of the same duration are as follows: (1) the impedance for the second phase of the biphasic shock is very low because electrode polarization develops during the first phase; (2) the large change in voltage between the first and second phases of a biphasic waveform is responsible for the increased defibrillation efficacy; (3) biphasic waveforms cause less severe detrimental effects in regions of high potential gradient; (4) the first phase of the biphasic waveform restores activity of the sodium channels, which makes defibrillation easier for the second phase; (5) the potential gradient required for defibrillation is less for biphasic waveforms than for monophasic waveforms; and (6) biphasic waveforms are better able to stimulate the myocardium to induce new action potentials or to cause refractory period prolongation. Evidence shows that, while a few of these proposed mechanisms are incorrect, several of the others may together contribute to the general superiority of biphasic waveforms.