The Electrical Thresholds of Ventricular Myocardium

Basic mechanisms of cardiac impulse propagation and associated arrhythmias

Propagation of excitation in the heart involves action potential (AP) generation by cardiac cells and its propagation in the multicellular tissue. AP conduction is the outcome of complex interactions between cellular electrical activity, electrical cell-to-cell communication, and the cardiac tissue structure. As shown in this review, strong interactions occur among these determinants of electrical impulse propagation. A special form of conduction that underlies many cardiac arrhythmias involves circulating excitation. In this situation, the curvature of the propagating excitation wavefront and the interaction of the wavefront with the repolarization tail of the preceding wave are additional important determinants of impulse propagation. This review attempts to synthesize results from computer simulations and experimental preparations to define mechanisms and biophysical principles that govern normal and abnormal conduction in the heart.

Field Stimulation of 2-D Sheets of Excitable Tissue

This paper develops equations for the transmembrane potentials (Vm) that occur in two-dimensional (2-D) sheets of tissue in response to field stimulation from an electrode near but not on the surface of the tissue. Comparison of results with those for one dimension shows that an additional term is present in the 2-D equations that influences the evolution of Vm in the interval between the end of the stimulus and the active propagation that may follow. The results provide an analytical framework for understanding Vm in response to field stimulation in two dimensions, both during the tissue's critical linear phase and thereafter.

Calculation of electric fields induced by body and head motion in high-field MRI

In modern magnetic resonance imaging (MRI), patients are exposed to strong, nonuniform static magnetic fields outside the central imaging region, in which the movement of the body may be able to induce electric currents in tissues which could be possibly harmful. This paper presents theoretical investigations into the spatial distribution of induced electric fields and currents in the patient when moving into the MRI scanner and also for head motion at various positions in the magnet. The numerical calculations are based on an efficient, quasi-static, finite-difference scheme and an anatomically realistic, full-body, male model. 3D field profiles from an actively shielded 4T magnet system are used and the body model projected through the field profile with a range of velocities. The simulation shows that it possible to induce electric fields/currents near the level of physiological significance under some circumstances and provides insight into the spatial characteristics of the induced fields. The results are extrapolated to very high field strengths and tabulated data shows the expected induced currents and fields with both movement velocity and field strength.

Effect of regional differences in cardiac cellular electrophysiology on the stability of ventricular arrhythmias: a computational study

Re-entry is an important mechanism of cardiac arrhythmias. During re-entry a wave of electrical activation repeatedly propagates into recovered tissue, rotating around a rod-like filament. Breakdown of a single re-entrant wave into multiple waves is believed to underlie the transition from ventricular tachycardia to ventricular fibrillation. Several mechanisms of breakup have been identified including the effect of anisotropic conduction in the ventricular wall. Cells in the inner and outer layers of the ventricular wall have different action potential durations (APD), and support re-entrant waves with different periods. The aim of this study was to use a computational approach to study twisting and breakdown in a transmural re-entrant wave spanning these regions, and examine the relative role of this effect and anisotropic conduction. We used a simplified model of action potential conduction in the ventricular wall that we modified so that it supported stable re-entry in an anisotropic model with uniform APD. We first examined the effect of regional differences on breakdown in an isotropic model with transmural differences in APD, and found that twisting of the re-entrant filament resulted in buckling and breakdown during the second cycle of re-entry. We found that breakdown was amplified in the anisotropic model, resulting in complex activation in the region of longest APD. This study shows that regional differences in cardiac electrophysiology are a potentially important mechanism for destabilizing re-entry and may act synergistically with other mechanisms to mediate the transition from ventricular tachycardia to ventricular fibrillation.

Wave front fragmentation due to ventricular geometry in a model of the rabbit heart

The role of the heart's complex shape in causing the fragmentation of activation wave fronts characteristic of ventricular fibrillation (VF) has not been well studied. We used a finite element model of cardiac propagation capable of simulating functional reentry on curved two-dimensional surfaces to test the hypothesis that uneven surface curvature can cause local propagation block leading to proliferation of reentrant wave fronts. We found that when reentry was induced on a flat sheet, it rotated in a repeatable meander pattern without breaking up. However, when a model of the rabbit ventricles was formed from the same medium, reentrant wave fronts followed complex, nonrepeating trajectories. Local propagation block often occurred when wave fronts propagated across regions where the Gaussian curvature of the surface changed rapidly. This type of block did not occur every time wave fronts crossed such a region; rather, it only occurred when the wave front was very close behind the previous wave in the cycle and was therefore propagating into relatively inexcitable tissue. Close wave front spacing resulted from nonstationary reentrant propagation. Thus, uneven surface curvature and nonstationary reentrant propagation worked in concert to produce wave front fragmentation and complex activation patterns. None of the factors previously thought to be necessary for local propagation block (e.g., heterogeneous refractory period, steep action potential duration restitution) were present. We conclude that the complex geometry of the heart may be an important determinant of VF activation patterns. (c) 2002 American Institute of Physics.

Ventricular fibrillation: mechanisms of initiation and maintenance

Ventricular fibrillation (VF) is the major immediate cause of sudden cardiac death. Traditionally, VF has been defined as turbulent cardiac electrical activity, which implies a large amount of irregularity in the electrical waves that underlie ventricular excitation. During VF, the heart rate is too high (> 550 excitations/minute) to allow adequate pumping of blood. In the electrocardiogram (ECG), ventricular complexes that are ever-changing in frequency, contour, and amplitude characterize VF. This article reviews prevailing theories for the initiation and maintenance of VF, as well as its spatio-temporal organization. Particular attention is given to recent experiments and computer simulations suggesting that VF may be explained in terms of highly periodic three-dimensional rotors that activate the ventricles at exceedingly high frequency. Such rotors may show at least two different behaviors: (a) At one extreme, they may drift throughout the heart at high speeds producing beat-to-beat changes in the activation sequence. (b) At the other extreme, rotors may be relatively stationary, activating the ventricles at such high frequencies that the wave fronts emanating from them breakup at varying distances, resulting in complex spatio-temporal patterns of fibrillatory conduction. In either case, the recorded ECG patterns are indistinguishable from VF. The data discussed have paved the way for a better understanding of the mechanisms of VF in the normal, as well as the diseased, human heart.

How electrode size affects the electric potential distribution in cardiac tissue

We investigate the effect of electrode size on the transmembrane potential distribution in the heart during electrical stimulation. The bidomain model is used to calculate the transmembrane potential in a three-dimensional slab of cardiac tissue. Depolarization is strongest under the electrode edge. Regions of depolarization are adjacent to regions of hyperpolarization. The average ratio of peak depolarization to peak hyperpolarization is a function of electrode radius, but over a broad range is close to three.

Safety of strong, static magnetic fields

Issues associated with the exposure of patients to strong, static magnetic fields during magnetic resonance imaging (MRI) are reviewed and discussed. The history of human exposure to magnetic fields is reviewed, and the contradictory nature of the literature regarding effects on human health is described. In the absence of ferromagnetic foreign bodies, there is no replicated scientific study showing a health hazard associated with magnetic field exposure and no evidence for hazards associated with cumulative exposure to these fields. The very high degree of patient safety in strong magnetic fields is attributed to the small value of the magnetic susceptibility of human tissues and to the lack of ferromagnetic components in these tissues. The wide range of susceptibility values between magnetic materials and human tissues is shown to lead to qualitatively differing behaviors of these materials when they are exposed to magnetic fields. Mathematical expressions are provided for the calculation of forces and torques.

Effect of pacing site on ventricular fibrillation initiation by shocks during the vulnerable period

The critical point hypothesis for the upper limit of vulnerability (ULV) states that the site of S1 pacing should not affect the ULV S2 shock strength for a single S2 shock electrode configuration but may affect the S1-S2 interval at which sub-ULV shocks induce ventricular fibrillation (VF). Furthermore, early post-S2 activations leading to VF should arise in areas with low potential gradients of similar magnitude, regardless of the S1 site. This hypothesis was tested in 10 pigs by determining ULVs for three S1 sites [left ventricular apex (LVA), LV base (LVB), and right ventricular outflow tract (RVOT)] with one S2 configuration (LVA patch to superior vena cava catheter). T-wave scanning was performed with biphasic S2 shocks incremented from 60 V in 40-V steps and stepped up or down in 20- and 10-V steps. Activations and S2 potential gradients were recorded at 528 epicardial sites. Although shocks just below the ULV induced VF significantly earlier in the T wave when the S1 site was the RVOT than when it was the LVA or LVB, ULVs were not significantly different for the three S1 pacing sites. Early post-S2 activations arose closer to the S2 electrode for weak S2s but moved to distant low potential gradient areas as the S2 strengthened. Just below the ULV, early post-S2 activations arose in the RVOT when the S1 site was the LVA or LVB but arose along the RV base when the S1 site was the RVOT. Early site potential gradients were not significantly different just below the ULV (LVA: 8.2 +/- 4.1 V/cm; LVB: 8.6 +/- 4. 9 V/cm; RVOT: 8.7 +/- 4.4 V/cm). At the ULV, early post-S2 activations arose from the same areas but did not induce VF. The results support the critical point hypothesis for the ULV. For this S2 configuration, no single point in the T wave could be used to determine the ULV for all S1 sites.

A spatial scale factor for electrophysiological models of myocardium

In the United States among males 20-64 years old about 1/3 of deaths are classified as 'sudden cardiac death', and 1/4 had no forewarning, nor did autopsy turn up any visible cause. The heart just switches from its normally periodic pumping to an alternative mode called 'fibrillation' more resembling electrical turbulence. In normal tissue its mechanism is a geometrically re-entrant mode of normal propagation. Everything about this spatial pattern depends upon the magnitude of 'D', the one term with dimension involving space in the pertinent biophysical equations. Explicit or implicit estimates in current literature span orders of magnitude. In this article I argue from a diversity of recent experiments for a narrower range of realistic values. It has an important role in the spatio-temporal evolution of fibrillation and in defibrillation.

Attachment of meandering reentrant wave fronts to anatomic obstacles in the atrium. Role of the obstacle size

Acetylcholine chloride (ACh) induces nonstationary meandering reentrant wave fronts in the atrium. We hypothesized that an anatomic obstacle of a suitable size prevents meandering by causing attachment of the reentrant wave front tip to the obstacle. Eight isolated canine right atrial tissues (area, 3.8 x 3.2 cm) were mounted in a tissue bath and superfused with Tyrode's solution containing 10 to 15 mumol/L ACh. Holes with 2- to 10-mm diameters were sequentially created in the center of the tissue with biopsy punches. Reentry was induced by a premature stimulus after eight regular stimuli at 400-ms cycle length. The endocardial activation maps and the motion of the induced reentry were visualized dynamically before and after each test lesion using 509 bipolar electrodes. In the absence of a lesion (n = 8), the induced single reentrant wave front, in the form of a spiral wave, meandered irregularly from one site to another before terminating at the tissue border. Holes with 2- to 4-mm diameters (n = 6) had no effect on meandering. However, when the hole diameters were increased to 6 mm (n = 8), 8 mm (n = 8), and 10 mm (n = 6), the tip of the spiral wave attached to the holes, and reentry became stationary. Transition from meandering to an attached state converted the irregular and polymorphic electrogram to a periodic and monomorphic activity with longer cycle lengths (101 +/- 11 versus 131 +/- 9 ms for no hole versus 10-mm hole, respectively; P < .001). Regression analysis showed a significant positive linear correlation between the cycle length of the reentry and the hole diameter (r = .89, P < .01) and between the cycle length of the reentry and the excitable gap (r = .89, P < .05). We conclude that a critically sized anatomic obstacle converts a nonstationary meandering reentrant wave front to a stationary one. This transition converts an irregular "fibrillation-like" activity into regular monomorphic activity.

On measuring curvature and electrical diffusion coefficients in anisotropic myocardium: comments on "effects of bipolar point and line simulation in anisotropic rabbit epicardium: assessment of the critical radius of curvature for longitudinal block"

Notion of "curvature" and "propagation perpendicular to the activation front" inherited from electrophysiological theory of isotropic reaction-diffusion media do not apply directly to experimental data taken in uniformly anisotropic myocardium. They apply directly only after the concept of "curvature" is normalized and propagation is made to look perpendicular to activation fronts by rescaling distances to achieve isotropy. Without rescaling, the curvature-dependence of longitudinal propagation speed turns out counter-intuitively to measure transverse intercellular electrical coupling.

Shock-induced depolarization of refractory myocardium prevents wave-front propagation in defibrillation

The elimination of most, if not all, propagating wave fronts of electrical activation by a shock constitutes a minimum prerequisite for successful defibrillation. However, the factors responsible for the prevention of postshock propagating activity are unknown. We investigated the determinants of this effect of defibrillation shocks in 23 Langendorff-perfused rabbit hearts by optically mapping cardiac cellular electrical activity by means of laser scanning. The optical action potentials obtained by this method were continuously recorded from 100 ventricular epicardial sites before, during, and after shock delivery during fibrillation. Analysis of activation maps showed that postshock propagating activity arose from areas depolarized by the shock. In 273 shock episodes, 898 sites at the border of shock-depolarized areas (BSDAs) from which wave-front propagation could have arisen were identified. The incidence of postshock propagation from BSDA sites was inversely related to refractoriness, as indexed by coupling interval (CI) or the optical takeoff potential (Vm). Specifically, there was a near-zero probability of postshock propagation if the shock caused depolarization at CIs < 50% of the fibrillation cycle length or from myocardium still depolarized to > or = 60% of the amplitude of a paced action potential (APA). Furthermore, incidences of wave-front propagation following shocks were consistently lower than the propagation incidences of naturally occurring unshocked fibrillation wave fronts, at comparable CIs and Vms. We conclude that the incidence of postshock wave-front propagation decreases with increasing refractoriness at the BSDA and that shock-induced depolarization of effectively refractory myocardium (ie, depolarized to > or = 60% APA) is required to guarantee the cessation of continued wave-front propagation in defibrillation.

Spiral waves, organizers of temporal and spatial order in excitable media

The notion "excitable medium' appears to be a unifying concept for the description of a class of wave phenomena observed in many fields of research, from material science to medicine. The analysis of the spatio temporal organization in excitable media reveals that significantly different time series of local activity at various sites may have a common origin in cooperative dynamic excitation structures; rotating spirals. A spiral-shaped pattern emanates from a small organizing center, the core of the spiral. The motion of the spiral tip within the core determines the frequency and the wavelength of a spiral and the peculiar filter properties of an excitable medium allow for the formation of regular spirals even in the case of an highly irregular motion of the tip. Computer-assisted video techniques are recommended to measure these intimate neighbourhood of regular and irregular time-space structures in living matter.

A mathematical model of make and break electrical stimulation of cardiac tissue by a unipolar anode or cathode

Numerical simulations of electrical stimulation of cardiac tissue using a unipolar extracellular electrode were performed. The bidomain model with unequal anisotropy ratios represented the tissue, and the Beeler-Reuter model represented the active membrane properties. Four types of excitation were considered: cathode make (CM), anode make (AM), cathode break (CB), and anode break (AB). The mechanisms of excitation were: for CM, tissue under the cathode was depolarized to threshold; for AM, tissue at a virtual cathode was depolarized to threshold; for CB, a long cathodal pulse produced a steady-state depolarization under the cathode and hyperpolarization at a virtual anode. At the end (break) of the pulse, the depolarization diffused into the hyperpolarized tissue, resulting in excitation. For AB, a long anodal pulse produced a steady-state hyperpolarization under the anode and depolarization at a virtual cathode. At the end (break) of the pulse, the depolarization diffused into the hyperpolarized tissue, resulting in excitation. For AB stimulation, decay of the hyperpolarization faster than that of the depolarization was necessary. The thresholds for rheobase and diastolic CM, AM, CB, and AB stimulation were 0.038, 0.41, 0.49, and 5.3 mA, respectively, for an electrode length of 1 mm and a surface area of 1.5 mm2. Threshold increased as the size of the electrode increased. The strength-duration curves for CM and AM were similar except when the duration was shorter than 0.2 ms, in which case the AM threshold rose more quickly with decreasing duration than did the CM threshold. CM and AM resulted in similar strength-frequency curves. The model agrees qualitatively, but (in some cases) not quantitatively, with experiments.

Effects of bipolar point and line stimulation in anisotropic rabbit epicardium: assessment of the critical radius of curvature for longitudinal block

Excitation front shape and velocity were studied in anisotropic perfused rabbit epicardium stained with potentiometric fluorescent dye. In the combined results from all experiments, convex excitation fronts produced by stimulation with a single electrode propagated longitudinally 13.3% slower than flat excitation fronts produced by stimulation with a line of electrodes. For transverse propagation, the two stimulation methods produced similar flat excitation fronts and velocities. The critical excitation front radius of curvature for longitudinal block (Rcr), calculated from excitable media theory, was 92 microns in control hearts. In hearts exposed to diacetyl monoxime (20 mmol/L), which decreases inward sodium current, Rcr was 175 microns. The slower longitudinal propagation velocity of convex fronts versus flat fronts and the theoretically predicted critical radius of curvature may be important for propagation and block of ectopic depolarizations in the heart.

Regional capture of fibrillating ventricular myocardium. Evidence of an excitable gap

Previous investigations have suggested that during ventricular fibrillation (VF) pacing stimuli are incapable of evoking propagated ventricular activations. To determine whether regional myocardial capture could be achieved during rapid pacing in VF, extracellular unipolar potentials were sampled (2 kHz) and recorded from 506 Ag-AgCl electrodes arranged in a rectangular grid (22 x 23, 1.12-mm spacing) embedded in a plaque overlying two pacing electrodes in the epicardium of the anterobasal right ventricle in pentobarbital-anesthetized pigs (25 to 30 kg, n = 6). During separate episodes of electrically induced VF, two bursts of 40 monophasic stimuli (10 mA, 2-millisecond duration) were asynchronously applied to the stimulating electrodes in either a bipolar, unipolar anodal, or unipolar cathodal mode. Evidence of regional capture was provided by (1) animating the first temporal derivative of the extracellular potentials, (2) analyzing inter-beat interval patterns, and (3) employing the Karhunen-Loeve decomposition method to quantify the repetitiveness of spatio-temporal patterns of activation. Regional capture of ventricular myocardium during VF was observed when pacing stimuli fell late in the local myocardial activation interval and when the pacing cycle length was 80% to 115% of the mean subplaque activation cycle length. When myocardial activations became phase locked to the pacing stimuli, repeatable spatiotemporal patterns of activation followed each stimulus. Poincaré sections at the plaque border revealed that during VF prior to pacing, interbeat intervals were irregular but were driven by pacing to stable fixed values at times corresponding to our qualitative declaration of regional capture. A similar correspondence was demonstrated between the time of capture, defined by direct observation of the activation patterns, and a rise in the power contained in the first two spatial modes of a Karhunen-Loeve decomposition. These data demonstrate that appropriately timed stimuli produce regional capture of fibrillating right ventricular myocardium in the pig and support the existence of an excitable gap during VF in this model.

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.

Comparison of simultaneous versus sequential defibrillation pulsing techniques using a nonthoracotomy system

The defibrillation threshold (DFT) using simultaneous (SIML) versus sequential (SEQ) pathways for shock delivery was compared in 16 patients with an implanted cardioverter defibrillator. All patients had three-lead nonthoracotomy systems (NTL) using a left chest subcutaneous patch, a right ventricular endocardial lead, and a lead in the coronary sinus (n = 5) or superior vena cava (n = 11). The DFT were determined 2-44 days (17 +/- 17 days) after implantation. The DFT was defined as the lowest energy shock that resulted in successful defibrillation. The first pathway tested was SIML in 12 and SEQ in 4 patients with output beginning at or above the intraoperative DFT, routinely 18 J. The second pathway was tested beginning 2-4 J above the DFT of the first tested pathway. All shocks were delivered in 2-4 J decrement or increment steps. The SEQ pathway shocks resulted in a significantly lower DFT than SIML pathway shocks (14 +/- 6 vs 18 +/- 6 J; P < 0.01). There was no difference in the time delay after ventricular fibrillation initiation before shock delivery for the successful defibrillation between SIML versus SEQ pathways (7 +/- 2 secs for both pathways). In 7 of 16 patients, defibrillation using SEQ pathway resulted in a > 5 J lowering of DFT, while only one patient had > 5 J lowering of DFT using SIML shocks (P < 0.05). These results have important implications for selecting the optimal pathway for implantable cardioverter defibrillator therapy with a multilead NTL system.

Strength-interval curves in canine myocardium at very short cycle lengths

While ventricular electrophysiological properties have been intensively studied at normal heart rates, little is known about these properties at the very short cycle lengths (approximately 100 msec), which are present in ventricular fibrillation. We examined refractoriness in the right ventricles of six dogs at stimulation intervals of 80 to 300 msec. Starting at 300 msec, the basic (S1) cycle length was decremented by 10 msec each beat to 200, 150, or 125 msec. A 1-msec premature (S2) stimulus of 1, 5, 10, or 20 mA was then introduced. The S1-S2 interval was decremented until capture was lost. The refractory period was considered to be the shortest interval that captured the heart for each S2 strength. Only pacing episodes that did not induce fibrillation were included. Strength-interval curves maintained the same hyperbolic shape but shifted to very short refractory periods as the S1-S1 interval was decreased. At the shortest S1-S1 intervals, premature stimuli were capable of capturing the heart without inducing ventricular fibrillation for S1-S2 intervals as short as 83 +/- 3 msec. Thus, decremental rapid pacing can produce refractory periods shorter than the cycle length during ventricular fibrillation. This finding suggests that there is no need to postulate a discontinuous jump to new electrophysiological properties or relationships at the onset of fibrillation, but that the capability for fibrillation is an integral part of normal electrophysiological parameters when they are pushed to values that do not occur normally. The results of this study should be useful in the further development of active membrane models and cellular automata models of cellular electrical behavior.

Effect of sotalol on ventricular fibrillation and defibrillation in humans

Antiarrhythmic drugs are frequently administered to patients receiving implanted cardioverter defibrillators. Some of these drugs may decrease the efficacy of defibrillation shocks from the defibrillator. Sotalol, a drug with beta-blocking and class III antiarrhythmic properties, lowers defibrillation energy requirements in experimental animals and may do so in humans. Oral sotalol 171 +/- 58 mg was administered before and after device implantation in 25 patients receiving implanted defibrillators. During sotalol therapy, the lowest energy required for successful defibrillation was 5.9 +/- 3.4 J (range 2-15J). In a concurrent nonrandomized comparison group of 23 patients, including 18 treated with amiodarone, the lowest successful energy was 16 +/- 10 J (p < 0.01). In 5 sotalol patients, ventricular fibrillation (VF) could not be induced at all (1 patient) or more than 2 or 3 times (4 patients) despite repeated 60 Hz stimulation. The induced VF had a pronounced tendency to terminate spontaneously, with the termination occurring at up to 23 seconds after the offset of 60 Hz stimulation. The cycle length of the VF was 236 +/- 34 msec, significantly greater than in patients not given drug therapy (191 +/- 21 msec, p < 0.01). In 10 patients, but none of the controls, intracardiac electrograms during surface electrocardiographic VF were regular, monoform, and without low-amplitude diastolic activity. In addition, monophasic action potentials during apparent VF showed maintenance of distinct and normal morphology. The ventricular effective refractory period increased after sotalol (249.4 +/- 19 to 278.4 +/- 24 msec; p < 0.03) and the maximum heart rate response to exercise was limited to 120 +/- 28 beats/min.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac electrophysiological experiments in numero, Part III: Simulation of arrhythmias and pacing

This paper is the third and final part of a series of articles reviewing mathematical and computer models of the electrophysiological processes. This section reviews the arrhythmia simulation and discusses models of arrhythmogenic processes, fibrillation and defibrillation, and of heart-pacemaker interaction. The models of arrhythmogenesis are classified into three main sections: models of reentry and vortex reentry, models of myocardial electrotonic interactions, and models of macroreentrant supraventricular tachycardias. This final part of the review discusses the future potential of mathematical and computer models of different cardiac processes.