Dynamics of circus movement re-entry across canine Purkinje fibre-muscle junctions

State of the art in Purkinje bioengineering

This review article will cover the recent developments in the new evolving field of Purkinje bioengineering and the development of human Purkinje networks. Recent work has progressed to the point of a methodological and systematic process to bioengineer Purkinje networks. This involves the development of 3D models based on human anatomy, followed by the development of tunable biomaterials, and strategies to reprogram stem cells to Purkinje cells. Subsequently, the reprogrammed cells and the biomaterials are coupled to bioengineer Purkinje networks, which are then tested using a small animal injury model. In this article, we discuss this process as a whole and then each step separately. We then describe potential applications of bioengineered Purkinje networks and challenges in the field that need to be overcome to move this field forward. Although the field of Purkinje bioengineering is new and in a state of infancy, it holds tremendous potential, both for therapeutic applications and to develop tools that can be used for disease modeling.

The Initial Part of Polymorphic Ventricular Tachycardia as a Clue for the Sustainability of Tachycardia and Ablation Success: A Varying Degree of Purkinje-Myocardial Complicity?

The cardiac Purkinje system is capable of very rapid burst activity suggestive of its potential role in being a driver of polymorphic ventricular tachycardia (VT) (PMVT) or ventricular fibrillation (VF). It plays a pivotal role, however, not only in the triggering of but also the perpetuation of ventricular arrhythmias. A varying degree of Purkinje-myocardial complicity has been blamed in determining not only the sustained or non-sustained nature of PMVT but also the pleomorphism of the non-sustained runs. The initial part of PMVT before cascading to the whole ventricle to establish disorganized VF can give important clues for ablation of PMVT and VF. We present a case of an electrical storm after acute myocardial infarction that was successfully ablated after identifying Purkinje potentials that triggered polymorphic, monomorphic, and pleiomorphic VTs and VF.

Pro-Arrhythmic Effects of Discontinuous Conduction at the Purkinje Fiber-Ventricle Junction Arising From Heart Failure-Induced Ionic Remodeling - Insights From Computational Modelling

Heart failure is associated with electrical remodeling of the electrical properties and kinetics of the ion channels and transporters that are responsible for cardiac action potentials. However, it is still unclear whether heart failure-induced ionic remodeling can affect the conduction of excitation waves at the Purkinje fiber-ventricle junction contributing to pro-arrhythmic effects of heart failure, as the complexity of the heart impedes a detailed experimental analysis. The aim of this study was to employ computational models to investigate the pro-arrhythmic effects of heart failure-induced ionic remodeling on the cardiac action potentials and excitation wave conduction at the Purkinje fiber-ventricle junction. Single cell models of canine Purkinje fiber and ventricular myocytes were developed for control and heart failure. These single cell models were then incorporated into one-dimensional strand and three-dimensional wedge models to investigate the effects of heart failure-induced remodeling on propagation of action potentials in Purkinje fiber and ventricular tissue and at the Purkinje fiber-ventricle junction. This revealed that heart failure-induced ionic remodeling of Purkinje fiber and ventricular tissue reduced conduction safety and increased tissue vulnerability to the genesis of the unidirectional conduction block. This was marked at the Purkinje fiber-ventricle junction, forming a potential substrate for the genesis of conduction failure that led to re-entry. This study provides new insights into proarrhythmic consequences of heart failure-induced ionic remodeling.

Short coupled torsade de pointes: Critical timing of the ventricular premature beats

In this case report, we describe a 73 year old female with structuraly normal heart that developed shortcoupled torsades de pointes (TdP) resulting in an electrical storm unresponsible to several antiarrhythmic drugs, but fully controlled with verapamil. The critical timing of the ventricular premature beats that initiated TdP corresponded to those that occurred at the peak of the previous T wave. This behavior differentiates this entity from other forms of malignant ventricular arrhythmias in patients with structurally normal heart. It is imperative that the clinical set-up and unique electrocardiographic fingerprint of this unusual malignant entity be assiduously recognized since verapamil can be life-saving in this condition.

Structure, function and clinical relevance of the cardiac conduction system, including the atrioventricular ring and outflow tract tissues

It is now over 100years since the discovery of the cardiac conduction system, consisting of three main parts, the sinus node, the atrioventricular node and the His-Purkinje system. The system is vital for the initiation and coordination of the heartbeat. Over the last decade, immense strides have been made in our understanding of the cardiac conduction system and these recent developments are reviewed here. It has been shown that the system has a unique embryological origin, distinct from that of the working myocardium, and is more extensive than originally thought with additional structures: atrioventricular rings, a third node (so called retroaortic node) and pulmonary and aortic sleeves. It has been shown that the expression of ion channels, intracellular Ca(2+)-handling proteins and gap junction channels in the system is specialised (different from that in the ordinary working myocardium), but appropriate to explain the functioning of the system, although there is continued debate concerning the ionic basis of pacemaking. We are beginning to understand the mechanisms (fibrosis and remodelling of ion channels and related proteins) responsible for dysfunction of the system (bradycardia, heart block and bundle branch block) associated with atrial fibrillation and heart failure and even athletic training. Equally, we are beginning to appreciate how naturally occurring mutations in ion channels cause congenital cardiac conduction system dysfunction. Finally, current therapies, the status of a new therapeutic strategy (use of a specific heart rate lowering drug) and a potential new therapeutic strategy (biopacemaking) are reviewed.

MEMBRANE AND TISSUE LEVEL CONTRIBUTIONS TO PURKINJE-VENTRICULAR INTERACTIONS: A MODEL STUDY

Purkinje-to-ventricular (P-to-V) propagation and electrotonic modulation of repolarization at discrete Purkinje-ventricular junctions (PVJs) depend on differences in the ionic currents and tissue structure of the P network and the V myocardium. We used computer simulations to assess these membrane and tissue level contributions to P-V interactions. At the membrane level, we used the DiFrancesco-Noble membrane equations to model P ionic kinetics and the Luo-Rudy dynamic membrane equations to model V ionic kinetics. At the tissue level, we modeled the P network as a layer of branching cables, and we modeled a single myocardial sheet with an anisotropic layer of excitable cells. P-to-V propagation was enhanced at the tissue level when multiple wavefronts in the branching P network collided at the PVJ. At the membrane level, P-to-V propagation was enhanced by a reduced transient outward current (Ito) in the P layer. Repolarization at the PVJ was also modulated by both membrane and tissue level contributions. Under nominal conditions, action potential duration (APD) shortened in the P layer and prolonged in the V layer. However, when the V mass was reduced, both P and V cell APDs shortened during coupling with nominal Ito. Subsequent Ito inhibition restored coupling-induced prolongation of the V action potential in the reduced V mass. These results suggest that under physiologic conditions, both membrane and tissue level contributions to P-V interactions are important, while membrane level contributions become even more important under pathologies that reduce the difference in P and V tissue size, particularly in the setting of healed myocardial infarction.

New drugs vs. old concepts: a fresh look at antiarrhythmics

Common arrhythmias, particularly atrial fibrillation (AF) and ventricular tachycardia/fibrillation (VT/VF) are a major public health concern. Classic antiarrhythmic (AA) drugs for AF are of limited effectiveness, and pose the risk of life-threatening VT/VF. For VT/VF, implantable cardiac defibrillators appear to be the unique, yet unsatisfactory, solution. Very few AA drugs have been successful in the last few decades, due to safety concerns or limited benefits in comparison to existing therapy. The Vaughan-Williams classification (one drug for one molecular target) appears too restrictive in light of current knowledge of molecular and cellular mechanisms. New AA drugs such as atrial-specific and/or multichannel blockers, upstream therapy and anti-remodeling drugs, are emerging. We focus on the cellular mechanisms related to abnormal Na⁺ and Ca²⁺ handling in AF, heart failure, and inherited arrhythmias, and on novel strategies aimed at normalizing ionic homeostasis. Drugs that prevent excessive Na⁺ entry (ranolazine) and aberrant diastolic Ca²⁺ release via the ryanodine receptor RyR2 (rycals, dantrolene, and flecainide) exhibit very interesting antiarrhythmic properties. These drugs act by normalizing, rather than blocking, channel activity. Ranolazine preferentially blocks abnormal persistent (vs. normal peak) Na⁺ currents, with minimal effects on normal channel function (cell excitability, and conduction). A similar "normalization" concept also applies to RyR2 stabilizers, which only prevent aberrant opening and diastolic Ca²⁺ leakage in diseased tissues, with no effect on normal function during systole. The different mechanisms of action of AA drugs may increase the therapeutic options available for the safe treatment of arrhythmias in a wide variety of pathophysiological situations.

Ionic mechanisms for electrical heterogeneity between rabbit Purkinje fiber and ventricular cells

The intrinsic heterogeneity of electrical action potential (AP) properties between Purkinje fibers (PFs) and the ventricular wall, as well as within the wall, plays an important role in ensuring successful excitation of the ventricles. It can also be proarrhythmic due to nonuniform repolarization across the Purkinje-ventricular junction. However, the ionic mechanisms that underlie the marked AP differences between PFs and ventricular cells are not fully characterized. We studied such mechanisms by developing a new family of biophysically detailed AP models for rabbit PF cells and three transmural ventricular cell types. The models were based on and validated against experimental data recorded from rabbit at ionic channel, single cell, and tissue levels. They were then used to determine the functional roles of each individual ionic channel current in modulating the AP heterogeneity at the rabbit Purkinje-ventricular junction, and to identify specific currents responsible for the differential response of PFs and ventricular cells to pharmacological interventions.

Optimal velocity and safety of discontinuous conduction through the heterogeneous Purkinje-ventricular junction

Slow and discontinuous wave conduction through nonuniform junctions in cardiac tissues is generally considered unsafe and proarrythmogenic. However, the relationships between tissue structure, wave conduction velocity, and safety at such junctions are unknown. We have developed a structurally and electrophysiologically detailed model of the canine Purkinje-ventricular junction (PVJ) and varied its heterogeneity parameters to determine such relationships. We show that neither very fast nor very slow conduction is safe, and there exists an optimal velocity that provides the maximum safety factor for conduction through the junction. The resultant conduction time delay across the PVJ is a natural consequence of the electrophysiological and morphological differences between the Purkinje fiber and ventricular tissue. The delay allows the PVJ to accumulate and pass sufficient charge to excite the adjacent ventricular tissue, but is not long enough for the source-to-load mismatch at the junction to be enhanced over time. The observed relationships between the conduction velocity and safety factor can provide new insights into optimal conditions for wave propagation through nonuniform junctions between various cardiac tissues.

Role of subendocardial Purkinje network in triggering torsade de pointes arrhythmia in experimental long QT syndrome

AIMS

The present study addresses the controversy regarding the 'primary' role of the subendocardial Purkinje network in triggering torsade de pointes (TdP) ventricular tachyarrhythmia (VAs) in the long QT syndrome (LQTS).

METHODS AND RESULTS

We investigated the well-established canine anthopleurin-A (AP-A) surrogate model of LQT3 to study the role of the subendocardial Purkinje network in triggering VAs. Three-dimensional activation and repolarization patterns were analysed from unipolar extracellular electrograms utilizing 64 plunge needle electrodes. In 6 dogs, the animals were placed on cardiopulmonary bypass and chemical ablation of the endocardial Purkinje network was obtained using Lugol's solution. Spontaneous VAs consistently developed in response to AP-A infusion and were triggered by a subendocardial focal activity acting on a substrate of spatial three-dimensional dispersion of repolarization. Endocardial ablation was considered successful by the development of complete atrioventricular block in the absence of ventricular escape rhythm. Following endocardial ablation spontaneous VAs were no longer observed. However, an appropriately coupled premature stimulus consistently induced re-entrant VAs.

CONCLUSION

The present study strongly suggests that in the LQTS, focal activity generated in subendocardial Purkinje tissue is the primary, if not the only, trigger for TdP VAs by acting on a substrate of three-dimensional dispersion of myocardial repolarization to induce re-entrant excitation.

Construction of a computer model to investigate sawtooth effects in the Purkinje system

The sawtooth effect refers to how one end of a cardiac cell is depolarized, while the opposite end is hyperpolarized, upon exposure to an exogenous electric field. Although hypothesized, it has not been observed in tissue. The Purkinje system is a one-dimensional (1-D) cable-like system residing on the endocardial surface of the heart and is the most obvious candidate for the manifestation of this phenomenon. This paper describes a computer modeling study of the effect of electric fields on the Purkinje system. Starting with a three-dimensional geometrically realistic, finite element, ventricular description, a Purkinje system is constructed which adheres to general physiological principles. Electrical activity in the Purkinje is described by use of 1-D cubic Hermite finite elements. Such a formulation allows for accurate modeling of loading effects at the Purkinje-myocyte junctions, and for preserving the discrete nature of the system. The response of a strand of Purkinje cells to defibrillation strength shocks is computed under several conditions. Also, the response of the isolated Purkinje network is illustrated. Results indicate that the geometry of the Purkinje system is the greatest determinant for far field excitation of the system. Given parameters within the plausible physiological range, the 1-D nature of the Purkinje system may lead to sawtooth potentials which are large enough to affect excitation. Thus, the Purkinje system is capable of affecting the defibrillation process, and warrants further experimentation to elucidate its role.

Propagated endothelial Ca2+ waves and arteriolar dilation in vivo: measurements in Cx40BAC GCaMP2 transgenic mice

To study endothelial cell (EC)- specific Ca(2+) signaling in vivo we engineered transgenic mice in which the Ca(2+) sensor GCaMP2 is placed under control of endogenous connexin40 (Cx40) transcription regulatory elements within a bacterial artificial chromosome (BAC), resulting in high sensor expression in arterial ECs, atrial myocytes, and cardiac Purkinje fibers. High signal/noise Ca(2+) signals were obtained in Cx40(BAC)-GCaMP2 mice within the ventricular Purkinje cell network in vitro and in ECs of cremaster muscle arterioles in vivo. Microiontophoresis of acetylcholine (ACh) onto arterioles triggered a transient increase in EC Ca(2+) fluorescence that propagated along the arteriole with an initial velocity of approximately 116 microm/s (n=28) and decayed over distances up to 974 microm. The local rise in EC Ca(2+) was followed (delay, 830+/-60 ms; n=8) by vasodilation that conducted rapidly (mm/s), bidirectionally, and into branches for distances exceeding 1 mm. At intermediate distances (300 to 600 microm), rapidly-conducted vasodilation occurred without changing EC Ca(2+), and additional dilation occurred after arrival of a Ca(2+) wave. In contrast, focal delivery of sodium nitroprusside evoked similar local dilations without Ca(2+) signaling or conduction. We conclude that in vivo responses to ACh in arterioles consists of 2 phases: (1) a rapidly-conducted vasodilation initiated by a local rise in EC Ca(2+) but independent of EC Ca(2+) signaling at remote sites; and (2) a slower complementary dilation associated with a Ca(2+) wave that propagates along the endothelium.

Our search for the porcine mother rotor

A single stationary mother rotor, located in the fastest activating region and giving rise to activation fronts that propagate throughout the remainder of the myocardium, has been hypothesized to be responsible for the maintenance of ventricular fibrillation (VF). Others have reported a mother rotor in guinea pigs and rabbits. We wanted to see if a mother rotor exists in a larger heart, that is, pigs. Epicardial mapping studies have demonstrated that VF wavefronts in pigs tend to propagate from the posterior basal LV to the anterior LV and on to the anterior RV, raising the possibility of a mother rotor in the posterior LV. However, no sustained reentry consistent with a mother rotor was found on the posterior LV epicardium, even though an intramural mapping study showed that the fastest activating transmural layer was near the epicardium. Many wavefronts in the posterior LV entered the mapped region from the posterior boundary of the mapping array, adjacent to the posterior descending coronary artery, raising the possibility that a mother rotor is located in the right ventricle or septum. Since a previous study has shown that the RV activates more slowly than the LV during VF, the more likely site for a mother rotor was the septum. However, we then performed a study in which we recorded from the right side of the septum and found that reentry was uncommon there also and that the activation rate was slower than the posterobasal LV. Many of the VF wavefronts in the septum passed from the posterior septum toward the anterior septum. This fact coupled with the fact that many wavefronts passed from the posterior LV free wall toward the anterior LV free wall point to the region where the posterior free wall intersects with the septum, the region where the posterior papillary muscle is located, as the possible site of a mother rotor. Indeed, a recent abstract by others reports that, after propranolol, a stable reentrant circuit is present on the endocardium at the insertion of the posterior papillary muscle into the LV free wall in pigs.

In situ Ca2+ dynamics of Purkinje fibers and its interconnection with subjacent ventricular myocytes

Purkinje fibers play essential roles in impulse propagation to the ventricles, and their functional impairment can become arrhythmogenic. However, little is known about precise spatiotemporal pattern(s) of interconnection between Purkinje-fiber network and the underlying ventricular myocardium within the heart. To address this issue, we simultaneously visualized intracellular Ca(2+) dynamics at Purkinje fibers and subjacent ventricular myocytes in Langendorff-perfused rat hearts using multi-pinhole type, rapid-scanning confocal microscopy. Under recording of electrocardiogram at room temperature spatiotemporal changes in fluo3-fluorescence intensity were visualized on the subendocardial region of the right-ventricular septum. Staining of the heart with either fluo3, acetylthiocholine iodide (ATCHI), or di-4-ANEPPS revealed characteristic structures of Purkinje fibers. During sinus rhythm (about 60 bpm) or atrial pacing (up to 3 Hz) each Purkinje-fiber exhibited spatiotemporally synchronous Ca(2+) transients nearly simultaneously to ventricular excitation. Ca(2+) transients in individual fibers were still synchronized within the Purkinje-fiber network not only under high-K(+) (8 mM) perfusion-induced Purkinje-to-ventricular (P-V) conduction delay, but also under unidirectional, orthodromic P-V block produced by 10-mM K(+) perfusion. While spontaneous, asynchronous intracellular Ca(2+) waves were identified in injured fibers of Purkinje network locally, surrounding fibers still exhibited Ca(2+) transients synchronously to ventricular excitation. In summary, these results are the first demonstration of intracellular Ca(2+) dynamics in the Purkinje-fiber network in situ. The synchronous Ca(2+) transients, preserved even under P-V conduction disturbances or under emergence of Ca(2+) waves, imply a syncytial role of Purkinje fibers as a specialized conduction system, whereas unidirectional block at P-V junctions indicates a substrate for reentrant arrhythmias.

Triggered activity due to delayed afterdepolarizations in sites of focal origin of ischemic ventricular tachycardia

This study for the first time systematically evaluated the site of origin of focal ventricular tachycardia (VT) induced 1–3 h after acute coronary artery ligation in dogs. We determined whether delayed afterdepolarizations (DADs) and triggered activity (TA) are more often recorded from ischemic endocardium excised from focal sites of VT origin. A total of 145 α-chloralose-anesthetized dogs were studied: in 54 dogs without inducible VT, normal or ischemic endocardium was investigated in vitro; in 91 dogs, inducible VT was studied by three-dimensional activation mapping, with in vitro study of 51 endocardial foci compared with 40 endocardial ischemic sites not of VT origin. Incidence of DADs (71% vs. 33%, P < 0.05) and TA (32% vs. 11%, P < 0.05) was greater in ischemic than in normal Purkinje tissues. Purkinje sites of origin of focal VT demonstrated the greatest frequency of DADs (92%, P < 0.05) and TA (75%, P < 0.05), with repetitive TA predominating. Similar results were obtained in endocardial sites of origin. Action potentials were mildly depolarized and prolonged in the focal sites of origin. These abnormalities were stable up to 2.5 h of recording. This study demonstrated that DADs and TA may underlie a majority of focal VTs in ischemic endocardium and Purkinje tissue.

Earliest cardiac toxicity induced by iron overload selectively inhibits electrical conduction

Female guinea pigs were injected intraperitoneally with 0.083 g/kg iron dextran (Fe-D) to achieve progressively increasing levels of iron load; controls received dextran. Delayed and blocked cardiac conductivity at the Purkinje fiber-papillary muscle junction was initially observed with Fe-D loads of 0.33 g/kg. Serial magnetic resonance relaxation time measurements obtained from livers of live animals showed a decrease (8.1 +/- 0.86 vs. 14.8 +/- 1.03 ms in controls, P < 0.001) that was first observed in animals loaded with 0.25 g/kg Fe-D. Iron concentrations in hearts and livers were significantly increased (P < 0.001). Left ventricular pressure measurements on 1.5 g/kg Fe-D animals failed to demonstrate a defect in contractility, but 27% (9/33) (P < 0.050) of the animals died without warning signs. We conclude that 1) initial decreases in liver magnetic resonance-relaxation time occur in the same range of iron excess as the threshold of iron load that induces delay or blockade of cardiac conduction and 2) a high incidence of sudden death, presumably from cardiac arrhythmias, was observed with large doses of iron that did not decrease left ventricular contractility.

Purkinje and ventricular contributions to endocardial activation sequence in perfused rabbit right ventricle

Interactions between peripheral conduction system and myocardial wave fronts control the ventricular endocardial activation sequence. To assess those interactions during sinus and paced ventricular beats, we recorded unipolar electrograms from 528 electrodes spaced 0.5 mm apart and placed over most of the perfused rabbit right ventricular free wall endocardium. Left ventricular contributions to electrograms were eliminated by cryoablating that tissue. Electrograms were systematically processed to identify fast (P) deflections separated by >2 ms from slow (V) deflections to measure P-V latencies. By using this criterion during sinus mapping (n = 5), we found P deflections in 22% of electrograms. They preceded V deflections at 91% of sites. Peripheral conduction system wave fronts preceded myocardial wave fronts by an overall P-V latency magnitude that measured 6.7 +/- 3.9 ms. During endocardial pacing (n = 8) at 500 ms cycle length, P deflections were identified on 15% of electrodes and preceded V deflections at only 38% of sites, and wave fronts were separated by a P-V latency magnitude of 5.6 +/- 2.3 ms. The findings were independent of apical, basal, or septal drive site. Modest changes in P-V latency accompanied cycle length accommodation to 125-ms pacing (6.8 +/- 2.6 ms), although more pronounced separation between wave fronts followed premature stimulation (11.7 +/- 10.4 ms). These results suggested peripheral conduction system and myocardial wave fronts became functionally more dissociated after premature stimulation. Furthermore, our analysis of the first ectopic beats that followed 12 of 24 premature stimuli revealed comparable separation between wave fronts (10.7 +/- 5.5 ms), suggesting the dissociation observed during the premature cycles persisted during the initiating cycles of the resulting arrhythmias.

Effects of [K(+)](o) on electrical restitution and activation dynamics during ventricular fibrillation

To test whether hyperkalemia suppresses ventricular fibrillation (VF) by reducing the slope of the action potential duration (APD) restitution relation, we determined the effects of the extracellular K(+) concentration ([K(+)](o)) ([KCl] = 2.7-12 mM) on the restitution of APD and maximum upstroke velocity (V(max)) the magnitude of APD alternans and spatiotemporal organization during VF in isolated canine ventricle. As [KCl] was increased incrementally from 2.7 to 12 mM, V(max) was reduced progressively. Increasing [KCl] from 2.7 to 10 mM decreased the slope of the APD restitution relation at long, but not short, diastolic intervals (DI), decreased the range of DI over which the slope was >/=1, and reduced the maximum amplitude of APD alternans. At [KCl] = 12 mM, the range of DI over which the APD restitution slope was >/=1 increased, and the maximum amplitude of APD alternans increased. For [KCl] = 4-8 mM, the persistence of APD alternans at short DI was associated with maintenance of VF. For [KCl] = 10-12 mM, the spontaneous frequency during VF was reduced, and activation occurred predominantly at longer DI. The lack of APD alternans at longer DI was associated with conversion of VF to a periodic rhythm. These results provide additional evidence for the importance of APD restitution kinetics in the development of VF.

Dynamics of intramural scroll waves in three-dimensional continuous myocardium with rotational anisotropy

It has been suggested that reentrant activity in three-dimensional cardiac muscle may be organized as a scroll wave rotating around a singularity line called the filament. Experimental studies indicate that filaments are often concealed inside the ventricular wall and consequently, scroll waves do not manifest reentrant activity on the surface. Here we analyse how such concealed scroll waves are affected by a twisted anisotropy resulting from rotation of layers of muscle fibers inside the ventricular wall. We used a computer model of a ventricular slab (15x15x15 mm(3)) with a fiber twist of 120 degrees from endocardium to epicardium. The action potential was simulated using FitzHugh-Nagumo equations. Scroll waves with rectilinear filaments were initiated at various depths of the slab and at different angles with respect to fiber orientation. The analysis shows that independent of initial conditions, after a certain transitional period, the filament aligns with the local fiber orientation. The alignment of the filament is determined by the directional variations in cell coupling due to fiber rotation and by boundary conditions. Our findings provide a mechanistic explanation for the prevalence of intramural reentry over transmural reentry during polymorphic ventricular tachycardia and fibrillation.

Electrical restitution and spatiotemporal organization during ventricular fibrillation

Despite recent advances in our understanding of the mechanism for ventricular fibrillation (VF), important electrophysiological aspects of the development of VF still are poorly defined. It has been suggested that the onset of VF involves the disintegration of a single spiral wave into many self-perpetuating waves. It has been further suggested that such a process requires that the slope of the electrical restitution relation be >/=1. The same theory anticipates that a single spiral wave will be stable (not disintegrate) if the maximum slope of the restitution relation is <1. We have shown previously that the slope of the restitution relation during rapid pacing and during VF is >/=1 in canine ventricle. We now show that drugs that reduce the slope of the restitution relation (diacetyl monoxime and verapamil) prevent the induction of VF and convert existing VF into a periodic rhythm. In contrast, a drug that does not reduce the slope of the restitution relation (procainamide) does not prevent the induction of VF, nor does it regularize VF. These results indicate that the kinetics of electrical restitution is a key determinant of VF. Moreover, they suggest novel approaches to preventing the induction or maintenance of VF.

Modulation of repolarization in rabbit Purkinje and ventricular myocytes coupled by a variable resistance

Purkinje-ventricular junctions (PVJs) have been implicated as potential sites of arrhythmogenesis, in part because of the dispersion of action potential duration (APD) between Purkinje (P) and ventricular (V) myocytes. To characterize electrotonic modulation of APD as a function of junctional resistance (Rj), we coupled single isolated rabbit P and V myocytes with an electronic circuit. In seven of eight PV myocyte pairs, both APDs shortened on coupling at Rj = 50 MOmega. This was in contrast to modulation of APD in paired ventricular myocytes, which demonstrated APD shortening of the intrinsically longer action potential and APD prolongation of the intrinsically shorter action potential. Companion computer simulations, performed to suggest possible mechanisms for the paradoxical shortening of the V action potential in paired P and V myocytes, showed that the difference in intrinsic peak plateau potentials (Vpp) of the P and V myocytes determined whether the V action potential shortened or prolonged on coupling. This difference in Vpp caused a large, repolarizing coupling current to flow to the V myocyte, contributing to early inactivation of the L-type calcium current and early activation of the inward rectifier current. These results suggest that intrinsic differences in phase 1 repolarization could yield differing patterns of APD shortening or prolongation in the network of subendocardial PVJs, leaving some PVJs vulnerable to conduction of premature stimuli while other PVJs remain refractory.

Purkinje-muscle reentry as a mechanism of polymorphic ventricular arrhythmias in a 3-dimensional model of the ventricles

Multiple electrode mapping of the ventricles during complex tachyarrhythmias has revealed focal subendocardial activation whose mechanism remains unexplained. We hypothesized that reentry involving the Purkinje-muscle junctions (PMJs) may be a mechanism for such focal excitations. We have constructed an anatomically appropriate computerized 3-dimensional model of the mammalian ventricles that includes the Purkinje conduction system and 214 PMJs distributed throughout the endocardium. Isochronal maps during normal excitation, as well as during right or left bundle branch block, resembled experimental measurements and compared well with isochronal maps of propagation in the human heart. Activity observed at both sides of a PMJ in the model showed that propagation from Purkinje fibers to muscle was slower than in the opposite direction. Under these realistic and normal conditions, the evolution of reentrant activity involving muscle and the Purkinje network was simulated. The reentry pattern was independent of the initiation site. It evolved with drifting epicardial breakthroughs and transformed on the endocardium from focal activity to figure-of-8 reentry. In addition, the ECG amplitude undulated during the evolution, and decrease in the cycle period, apparent wavelength, and propagation velocity were observed. Finally, the reentry was terminated if the Purkinje system was disconnected from the muscle before it reached a relative steady state. The simulation results suggest the following: (1) Epicardial breakthroughs and endocardial focal activity may originate at the PMJs. (2) The ECG amplitude may decrease as the reentry stabilizes and the excitation wavelength decreases. (3) The Purkinje system may have a double role in the evolution of reentry: first, it is essential to the reentry at the initial stage; second, it may lead to the establishment of intramyocardial reentry, at which time the Purkinje system becomes irrelevant.

Conduction between isolated rabbit Purkinje and ventricular myocytes coupled by a variable resistance

Conduction at the Purkinje-ventricular junction (PVJ) demonstrates unidirectional block under both physiological and pathophysiological conditions. Although this block is typically attributed to multidimensional electrotonic interactions, we examined possible membrane-level contributions using single, isolated rabbit Purkinje (P) and ventricular (V) myocytes coupled by an electronic circuit. When we varied the junctional resistance (Rj) between paired V myocytes, conduction block occurred at lower Rj values during conduction from the smaller to larger myocyte (115 +/- 59 M omega) than from the larger to smaller myocyte (201 +/- 51 M omega). In Purkinje-ventricular myocyte pairs, however, block occurred at lower Rj values during P-to-V conduction (85 +/- 39 M omega) than during V-to-P conduction (912 +/- 175 M omega), although there was little difference in the mean cell size. Companion computer simulations, performed to examine how the early platea currents affected conduction, showed that P-to-V block occurred at lower Rj values when the transient outward current was increased or the calcium current was decreased in the model P cell. These results suggest that intrinsic differences in phase 1 repolarization can contribute to unidirectional block at the PVJ.

Human ventricular action potential duration during short and long cycles. Rapid modulation by ischemia

BACKGROUND

Mechanisms underlying the initiation of ventricular arrhythmias in ischemia by a premature beat or after a pause remain unclear. The kinetics of electrical restitution, which is the modulation of action potential duration (APD) by an abrupt alteration in cycle length, may be important.

METHODS AND RESULTS

We recorded one or two simultaneous monophasic action potentials (MAPs) from the right ventricular septum during balloon occlusion of the left anterior descending coronary artery (LAD) (14 patients), which is expected to induce ischemia at the recording site, and during occlusion of the right coronary artery (RCA) (7 patients), which is not expected to induce ischemia at the recording area. The latter acted as a control. A test pulse sequence was incorporated whereby during steady-state pacing, test beats of altered cycle length were interpose. During LAD occlusion, APD for basic beats shortened from 260 +/- 4 to 236 +/- 4 ms (P < .0001), whereas the control group (RCA occlusion) showed no significant change (251 +/- 7 to 249 +/- 9 ms; P = NS). LAD occlusion resulted in flattening of the slope relating APD of test beats to diastolic interval (P = .001), whereas in the control group (RCA occlusion) the slope remained unchanged. Similar results were obtained during a second occlusion.

CONCLUSIONS

LAD occlusion in patients during balloon angioplasty shortened MAP duration of basic beats and minimized, abolished, or reversed the normal APD/diastolic-interval relation of test beats of altered cycle length at sites served by the occluded vessel. The results suggest that ischemia flattens the electrical restitution curve in the human endocardium. These findings may have important implications in arrhythmogenesis.

Triggered activity as a mechanism for inherited ventricular arrhythmias in German shepherd Dogs

OBJECTIVES

This study sought to determine whether early afterdepolarization-induced triggered activity is responsible for the initiation of ventricular arrhythmias in dogs with an inherited predisposition to sudden death.

BACKGROUND

We have identified a colony of German shepherd dogs that display inherited ventricular ectopic activity and sudden cardiac death. The arrhythmias in these animals are pause dependent but are not associated with a prolonged QT interval, suggesting that they might be initiated by early afterdepolarization-induced triggered activity in Purkinje fibers.

METHODS

Cardiac Purkinje fibers obtained from dogs that either did or did not exhibit ventricular tachyarrhythmias at the time of study were superfused in vitro with normal Tyrode solution (extracellular potassium ion concentration 4 mmol/liter) and were studied using standard microelectrode techniques.

RESULTS

Early afterdepolarizations and triggered activity occurred spontaneously in Purkinje fibers obtained from affected dogs (n = 7) but not in fibers obtained from unaffected dogs (n = 13). Exit conduction block of triggered responses occurred to varying degrees within the Purkinje fiber but not at the Purkinje-muscle junction. Overdrive pacing suppressed triggered activity. The reemergence of triggered activity after cessation of pacing was both time and rate dependent. Triggered activity in fibers obtained from affected dogs was potentiated by phenylephrine and epinephrine and was suppressed by isoproterenol. Triggered activity was not induced by phenylephrine or epinephrine in fibers obtained from unaffected dogs.

CONCLUSIONS

These results support the hypothesis that early afterdepolarization-induced triggered activity in Purkinje fibers is responsible for the initiation of ventricular arrhythmias in this canine model of inherited sudden death.

Biphasic restitution of action potential duration and complex dynamics in ventricular myocardium

The purpose of this study was to determine whether biphasic restitution of action potential duration (APD) in ventricular muscle permits the development of complex dynamic behavior. Such behavior is expected because of the steep ascending slope of restitution and the presence of a maximum. Action potentials recorded from strips of epicardial muscle in which biphasic APD restitution occurred demonstrated a characteristic pattern of phase locking during progressive shortening of the pacing cycle length. 1:1 locking was replaced by irregular dynamics, which in turn was replaced by higher order periodic behavior (eg, 8:8 locking), then by 2:2 locking, and finally by 2:1 locking. Similar patterns of dynamic behavior were produced in a computer model by using a piecewise linear approximation of biphasic APD restitution. Features of APD restitution that were critical determinants of irregular dynamics included the slopes of the ascending and the nonmonotonic regions. These results suggest that rate-related alterations of APD and refractoriness may be affected significantly by small nonmonotonicities in APD restitution.