Interaction between strong electrical stimulation and reentrant wavefronts in canine ventricular fibrillation

Effect of anisotropy on ventricular vulnerability to unidirectional block and reentry by single premature stimulation during normal sinus rhythm in rat heart

Single high-intensity premature stimuli when applied to the ventricles during ventricular drive of an ectopic site, as in Winfree's "pinwheel experiment," usually induce reentry arrhythmias in the normal heart, while single low-intensity stimuli barely do. Yet ventricular arrhythmia vulnerability during normal sinus rhythm remains largely unexplored. With a view to define the role of anisotropy on ventricular vulnerability to unidirectional conduction block and reentry, we revisited the pinwheel experiment with reduced constraints in the in situ rat heart. New features included single premature stimulation during normal sinus rhythm, stimulation and unipolar potential mapping from the same high-resolution epicardial electrode array, and progressive increase in stimulation strength and prematurity from diastolic threshold until arrhythmia induction. Measurements were performed with 1-ms cathodal stimuli at multiple test sites (n = 26) in seven rats. Stimulus-induced virtual electrode polarization during sinus beat recovery phase influenced premature ventricular responses. Specifically, gradual increase in stimulus strength and prematurity progressively induced make, break, and graded-response stimulation mechanisms. Hence unidirectional conduction block occurred as follows: 1) along fiber direction, on right and left ventricular free walls (n = 23), initiating figure-eight reentry (n = 17) and tachycardia (n = 12), and 2) across fiber direction, on lower interventricular septum (n = 3), initiating spiral wave reentry (n = 2) and tachycardia (n = 1). Critical time window (55.1 ± 4.7 ms, 68.2 ± 6.0 ms) and stimulus strength lower limit (4.9 ± 0.6 mA) defined vulnerability to reentry. A novel finding of this study was that ventricular tachycardia evolves and is maintained by episodes of scroll-like wave and focal activation couplets. We also found that single low-intensity premature stimuli can induce repetitive ventricular response (n = 13) characterized by focal activations.NEW & NOTEWORTHY We performed ventricular cathodal point stimulation during sinus rhythm by progressively increasing stimulus strength and prematurity. Virtual electrode polarization and recovery gradient progressively induced make, break, and graded-response stimulation mechanisms. Unidirectional conduction block occurred along or across fiber direction, initiating figure-eight or spiral wave reentry, respectively, and tachycardia sustained by scroll wave and focal activations.

Calcium transient dynamics and the mechanisms of ventricular vulnerability to single premature electrical stimulation in Langendorff-perfused rabbit ventricles

BACKGROUND

Single strong premature electrical stimulation (S(2)) may induce figure-eight reentry. We hypothesize that Ca current-mediated slow-response action potentials (APs) play a key role in the propagation in the central common pathway (CCP) of the reentry.

METHODS

We simultaneously mapped optical membrane potential (V(m)) and intracellular Ca (Ca(i)) transients in isolated Langendorff-perfused rabbit ventricles. Baseline pacing (S(1)) and a cathodal S(2) (40-80 mA) were given at different epicardial sites with a coupling interval of 135 +/- 20 ms.

RESULTS

In all 6 hearts, S(2) induced graded responses around the S(2) site. These graded responses propagated locally toward the S(1) site and initiated fast APs from recovered tissues. The wavefront then circled around the refractory tissue near the site of S(2). At the side of S(2) opposite to the S(1), the graded responses prolonged AP duration while the Ca(i) continued to decline, resulting in a Ca(i) sinkhole (an area of low Ca(i)). The Ca(i) in the sinkhole then spontaneously increased, followed by a slow V(m) depolarization with a take-off potential of -40 +/- 3.9 mV, which was confirmed with microelectrode recordings in 3 hearts. These slow-response APs then propagated through CCP to complete a figure-eight reentry.

CONCLUSION

We conclude that a strong premature stimulus can induce a Ca(i) sinkhole at the entrance of the CCP. Spontaneous Ca(i) elevation in the Ca(i) sinkhole precedes the V(m) depolarization, leading to Ca current-mediated slow propagation in the CCP. The slow propagation allows more time for tissues at the other side of CCP to recover and be excited to complete figure-eight reentry.

Post-shock synchronized pacing in isolated rabbit left ventricle: evaluation of a novel defibrillation strategy

INTRODUCTION

A failed near-threshold defibrillation shock is followed by an isoelectric window (IEW) and rapid repetitive responses that reinitiate ventricular fibrillation (VF). We hypothesized that properly timed (synchronized) postshock pacing stimuli (SyncP) may capture the recovered tissues during the repetitive responses and prevent postshock reinitiation of VF, resulting in improved defibrillation efficacy.

METHODS AND RESULTS

We explored the effect of postshock SyncP on defibrillation efficacy in isolated rabbit hearts (n = 12). Optical recording-guided real-time detection and electrical stimulation (5 mA) of recovered tissues in anterior/posterior left ventricle (LV) were performed following IEW. The IEW duration was found to be 69 +/- 13 ms. With the same shock strength, successful and failed defibrillation episodes were associated with 50% and 15% of the myocardium, respectively, captured by the SyncP (P < 0.001). Electrical stimulation from the posterior LV resulted in 75% of episodes capturing myocardium, as compared with anterior LV stimulation (55%; P < 0.01) and higher successful defibrillation rate (14%, posterior vs. 3%, anterior LV). The overall success in terminating VF by postshock SyncP was approximately 10%. The causes for failed myocardium capture by postshock SyncP included lack of IEW after low-strength shock (42.9%), incorrect locations of reference site (25.7%) and pacing electrodes (17.9%), and others, such as wave breakthroughs (13.5%).

CONCLUSION

Postshock SyncP was feasible and the larger the myocardium captured area, the more likely was the successful defibrillation. Postshock SyncP delivered to the posterior LV was more effective than anterior LV to terminate VF.

Quatrefoil Reentry Caused by Burst Pacing

BACKGROUND

Experiments and clinical studies have shown that high-frequency (burst) pacing can induce reentry and fibrillation without a strong shock. We hypothesize that a train of weak stimuli induces quatrefoil reentry, and investigate the mechanism and threshold for this mode of reentry induction.

METHODS

We apply a train of weak stimuli at different pacing rates to determine the threshold necessary to induce quatrefoil reentry. Numerical calculations are used to simulate cardiac tissue, based on the bidomain model with unequal anisotropy ratios. We consider both anodal and cathodal stimuli.

RESULTS

Quatrefoil reentry is initiated using much smaller currents during burst pacing (0.9 mA) compared to a single premature pulse (8.6 mA). As we varied the pacing rate, we observed reentry at the border between different modes of phase locking, such as between 1:1 and 2:1 responses.

CONCLUSION

Burst pacing can significantly reduce the threshold for reentry. However, the extreme sensitivity of reentry induction to the exact number of stimuli in the pulse train makes the method difficult to use as a consistent, reproducible way to induce reentry.

Ultrasonically Actuated Silicon Microprobes for Cardiac Signal Recording

In this paper, we report on ultrasonically actuated silicon thin microprobes that successfully penetrated canine cardiac tissue in vitro, and recorded the electrophysiological signals from multiple sites simultaneously within the heart wall. The penetration force--maximum force encountered by the probe during penetration--is found to reduce with increasing ultrasonic driving voltage, on both excised canine right ventricular muscle and chicken breast muscle. The rate of force decrease varies with tissue type and microprobe dimension. With ultrasonic actuation, the silicon microprobes are inserted into isolated perfused canine heart without breakage or significant buckling, under 10Vpp actuating voltage. Recordings were obtained from isolated perfused canine heart during pacing, following the induction of ventricular tachycardia, and during the transition from ventricular tachycardia to ventricular fibrillation. Local conduction velocity of 0.60 +/- 0.03 m/s was observed from the multichannel recordings from the canine right ventricular wall under epicardial pacing. The application of the ultrasonic microprobes in cardiac electrophysiology study can provide information for reconstruction of electrical wave propagation within the heart, which is important to understanding the mechanisms of cardiac arrhythmias.

New model of ventricular fibrillation

The purpose of this study was to develop a more efficient and stable model of ventricular fibrillation (VF) in the isolated rabbit heart, because there is not a satisfactory model with this animal. We also observed the effects of increasing extracellular calcium in the stability and reversibility of the arrhythmia. After suspending the hearts in a classical Langendorff preparation, VF was induced by burst stimulation (current = 2.0 mA, pulse duration = 3 milliseconds, frequency = 50 Hz, voltage = 10 V, duration of stimulation = 5 minutes). The hearts were then divided into 2 groups, A and B. The hearts in group B were perfused with a modified Krebs-Henseleit solution, which contained twice as much calcium as the solution used in the other group. The rate of success with this model was 100% for both groups. The hearts fibrillated up to 30 minutes in group A and more than 40 minutes in group B, longer then all studies ever published in rabbit hearts. Ventricular fibrillation reverted to sinus rhythm in 100% of the hearts of group A when treated with an antifibrillatory drug, whereas no reversion at all was observed in the hearts of group B. We conclude that high extracellular calcium makes the reversion to sinus rhythm more difficult in this model. Our high rate of success and the exceptionally stable and long-lasting VF turn our model very effective for the study of antiarrhythmic interventions in the isolated rabbit heart.

Improvement of Defibrillation Efficacy with Preshock Synchronized Pacing

INTRODUCTION

We previously demonstrated that wavefront synchronization by spatiotemporal excitable gap pacing (Sync P) is effective at facilitating spontaneous termination of ventricular fibrillation (VF). Therefore, we hypothesized that a spatiotemporally controlled defibrillation (STCD) strategy using defibrillation shocks preceded by Sync P can improve defibrillation efficacy.

METHOD AND RESULTS

We explored the STCD effects in 13 isolated rabbit hearts. During VF, a low-voltage gradient (LVG) area was synchronized by Sync P for 0.92 second. For Sync P, optical action potentials (OAPs) adjacent to four pacing electrodes (10 mm apart) were monitored. When one of the electrodes was in the excitable gap, a 5-mA current was administered from all electrodes. A shock was delivered 23 ms after the excitable gap when the LVG area was unexcitable. The effects of STCD was compared to random shocks (C) by evaluating the defibrillation threshold 50% (DFT(50); n = 35 for each) and preshock coupling intervals (n = 208 for STCD, n = 172 for C). Results were as follows. (1) Sync P caused wavefront synchronization as indicated by a decreased number of phase singularity points (P < 0.0001) and reduced spatial dispersion of VF cycle length (P < 0.01). (2) STCD decreased DFT(50) by 10.3% (P < 0.05). (3) The successful shocks showed shorter preshock coupling intervals (CI; P < 0.05) and a higher proportion of unexcitable shock at the LVG area (P < 0.001) than failed shocks. STCD showed shorter CIs (P < 0.05) and a higher unexcitable shock rate at LVG area (P < 0.05) than C.

CONCLUSION

STCD improves defibrillation efficacy by synchronizing VF activations and increasing probability of shock delivery to the unexcitable LVG area.

Nonreentrant focal activations in pulmonary veins in canine model of sustained atrial fibrillation

Repetitive rapid activities are present in the pulmonary veins (PVs) in dogs with pacing-induced sustained atrial fibrillation (AF). The mechanisms are unclear. We induced sustained (>48 h) AF by rapidly pacing the left atrium (LA) in six dogs. High-density computerized mapping was done in the PVs and atria. Results show repetitive focal activations in all dogs and in 12 of 18 mapped PVs. Activation originated from the middle of the PV and then propagated to the LA and distal PV with conduction blocks. The right atrium (RA) was usually activated by a single large wavefront. Mean AF cycle length in the PVs (left superior, 82 +/- 6 ms; left inferior, 83 +/- 6 ms; right inferior, 83 +/- 4 ms) and LA posterior wall (87 +/- 5 ms) were significantly (P < 0.05) shorter than those in the LA anterior wall (92 +/- 4 ms) and RA (107 +/- 5 ms). PVs in normal dogs did not have focal activations during induced AF. No reentrant wavefronts were demonstrated in the PVs. We conclude that nonreentrant focal activations are present in the PVs in a canine model of pacing-induced sustained AF.

Obstacle-induced transition from ventricular fibrillation to tachycardia in isolated swine right ventricles: insights into the transition dynamics and implications for the critical mass

OBJECTIVES

The study was done to test the hypothesis that an artificial anatomical obstacle prevents the maintenance of ventricular fibrillation (VF) by stabilizing reentrant wavefronts (RWF) and increases the critical mass (CM) of myocardium required to sustain VF.

BACKGROUND

Artificial obstacles can anchor RWF in simulated models of VF. Whether an artificial obstacle affects multiple-wavelet VF in real tissue is unclear.

METHODS

The endocardial surfaces of seven isolated, perfused swine right ventricles were mapped using a plaque of 477 bipolar electrodes with 1.6-mm resolution. An 8-mm hole was punched in the tissue. The CM was reached by tissue mass reductions, at which VF converted to periodic activity (ventricular tachycardia, VT).

RESULTS

After the creation of the obstacle, the VF cycle length increased from 71.6+/-18.4 ms to 87.5+/-13.0 ms (p<0.05). The obstacle, together with the papillary muscle, facilitated the transition from VF to VT by serving as attachment sites for the RWF. When one RWF attaches to the obstacle and another attaches to the papillary muscle, it may result in stable VT with figure-eight patterns. The CM for VF in the presence of an 8-mm hole (28.7+/-3.8 g) was higher than in the control group (swine right ventricles without holes, 24.0+/-3.4 g, p<0.05).

CONCLUSIONS

An artificial anatomical obstacle induces slowing and regularization of VF, impairs the persistence of VF as judged by an increase of the CM, and can convert VF to VT by serving as an attachment site to reentrant excitation.

Influence of wavefront dynamics on transmembrane potential characteristics during atrial fibrillation

INTRODUCTION

Although computerized mapping studies have demonstrated the presence of multiple wavelets during atrial fibrillation (AF) and that action potential amplitude and duration in AF vary significantly from beat to beat, no study has correlated the single cell action potential changes with the patterns of activation during AF.

METHODS AND RESULTS

We studied wavefront dynamics and single cell transmembrane potential (TMP) characteristics in 12 isolated perfused canine right atria. The endocardial surface was mapped using 477 bipolar electrodes while TMP was recorded with a standard glass microelectrode from an epicardial cell. AF was induced in the presence of acetylcholine. Successful simultaneous TMP recordings and activation maps were made during six episodes of AF and for a total of 141 activations. Large variations of TMP amplitude and duration were observed frequently; 34% of them have a low amplitude (<50% of the amplitude recorded during pacing). Low-amplitude potentials were recorded when the impaled cell was (1) in an area of random reentry (67%, n = 36); (2) within 3.2 mm of the core of organized functional reentry (22%, n = 12); (3) in the middle of two merging wavefronts (9%, n = 5); and (4) at the point of spontaneous wavebreak (2%, n = 1).

CONCLUSION

Large variations of TMP are observed frequently during in vitro AF. Low-amplitude TMPs are associated with specific patterns of AF activation wavefronts.

Nicotine increases ventricular vulnerability to fibrillation in hearts with healed myocardial infarction

The vulnerability of the infarcted hearts to ventricular fibrillation (VF) was tested in in situ canine hearts during nicotine infusion. The activation pattern was mapped with 477 bipolar electrodes in open-chest anesthetized dogs (n = 8) 5-6 wk after permanent occlusion of the left anterior descending coronary artery. Nicotine (129 +/- 76 ng/ml) lengthened (P < 0.01) the pacing cycle length at which VF was induced from 171 +/- 8.9 to 210 +/- 14. 7 ms. Nicotine selectively amplified the magnitude of conduction time and monophasic action potential (MAP) amplitude and duration (MAPA and MAPD, respectively) alternans in the epicardial border zone (EBZ) but not in the normal zone. With critical reduction of the MAPA and MAPD in the EBZ, conduction block occurred across the long axis of the EBZ cells. Block led immediately to reentry formation in the EBZ with a mean period of 105 +/- 10 ms, which, after one to two rotations, degenerated to VF. Nicotine widened the range of diastolic intervals over which the dynamic MAPD restitution curve had a slope >1. We conclude that nicotine facilitates conduction block, reentry, and VF in hearts with healed myocardial infarction by increasing the magnitude of depolarization and repolarization alternans consistent with the restitution hypothesis of vulnerability to VF.

Graded response and restitution hypotheses of ventricular vulnerability to fibrillation: insights into the mechanism of initiation of fibrillation

According to the upper limit of vulnerability (ULV), failed defibrillation (DF) shocks reinitiate ventricular fibrillation (VF) by falling on the vulnerable period of one or more of the fibrillation wavefronts. The failed shock first induces reentry (stage I VF), which within few cycles degenerate to stage II VF. We developed 2 hypotheses of vulnerability that explain DF failure using isolated and intact in situ ventricles. Activation maps were constructed with high-resolution electrodes and action potential (AP) recorded with microelectrodes. According to the graded response (GR) hypothesis, reentry is formed when a critical shock strength induces a GR that transiently increases local refractoriness. The GR propagates and initiates distal regenerative activity that propagates around the site of block to reenter through it as it recovers. Ultrastrong shocks prevent reentry by converting unidirectional block to bidirectional block by excessive increase in refractoriness, a finding that supports the ULV hypothesis. In situ ventricle stimulus-induced termination of reentry and stage I VF (protective zone) could be explained by the GR hypothesis. The induced functional reentry with periods of 100 to 160 ms engages the steep (unstable) portion of the AP duration restitution curves (slope >1) that promotes meandering and breakup. This leads to transition from stage I to stage II VF (the restitution hypothesis). We conclude that the GR and restitution hypotheses provide an insight into the mechanism of ventricular vulnerability to fibrillation induced by a stimulus. These hypotheses provide a new paradigm for effective antifibrillatory strategies.

Experimental study of the effects of multi-site sequential ventricular pacing on the prophylaxis of ventricular fibrillation

Previous studies report a significant prophylactic effect on the occurrence of atrial fibrillation by simultaneous multi-site atrial pacing. We investigated the effects of multi-site sequential ventricular pacing (MSVP), which may be preferable to simultaneous multi-site pacing in terms of the prophylaxis of the occurrence of ventricular fibrillation (VF). Needle electrodes were inserted at ten different epicardial sites on both ventricles for MSVP in 12 adult beagle dogs. Four premature ventricular extrastimuli (PVE) were introduced to provoke VF reproducibly from a separate electrode in the left ventricle. The 4 PVE were applied to try to provoke VF during MSVP in a comparable fashion to the activation sequence during sinus rhythm. We compared the prophylactic effects of MSVP on the inducibility of VF by changing the number of stimulation sites to either 1, 3, 5, or 10 epicardial sites. We performed a total of 363 trials of induction and suppression of VF. The occurrence rates of VF by the 4 PVE for the various number of epicardial stimulation sites of MSVP, i.e., at 1, 3, 5, and 10 sites, were 0.8263, 0.4286, 0.4450, and 0.2857, respectively (p < 0.05). There was a significant prophylactic effect of MSVP on the inducibility of VF, and this effect became stronger as the number of MSVP sites was increased from 3 to 10. The hemodynamic state was relatively stable during MSVP. MSVP seems to be a promising method with which to reduce the occurrence of VF, and a larger number of stimulation sites would be more effective in terms of the prophylaxis of VF.

Relation between cellular repolarization characteristics and critical mass for human ventricular fibrillation

INTRODUCTION

The critical mass for human ventricular fibrillation (VF) and its electrical determinants are unclear. The goal of this study was to evaluate the relationship between repolarization characteristics and critical mass for VF in diseased human cardiac tissues.

METHODS AND RESULTS

Eight native hearts from transplant recipients were studied. The right ventricle was immediately excised, then perfused (n = 6) or superfused (n = 2) with Tyrode's solution at 36 degrees C. The action potential duration (APD) restitution curve was determined by an S1-S2 method. Programmed stimulation and burst pacing were used to induce VF. In 3 of 8 tissues, 10 microM cromakalim, an ATP-sensitive potassium channel opener, was added to the perfusate and the stimulation protocol repeated. Results show that, at baseline, VF did not occur either spontaneously or during rewarming, and it could not be induced by aggressive electrical stimulation in any tissue. The mean APD at 90% depolarization (APD90) at a cycle length of 600 msec was 227+/-49 msec, and the mean slope of the APD restitution curve was 0.22+/-0.08. Among the six tissues perfused, five were not treated with any antiarrhythmic agent. The weight of these five heart samples averaged 111+/-23 g (range 85 to 138). However, after cromakalim infusion, sustained VF (> 30 min in duration) was consistently induced. As compared with baseline in the same tissues, cromakalim shortened the APD90 from 243+/-32 msec to 55+/-18 msec (P < 0.001) and increased the maximum slope of the APD restitution curve from 0.24+/-0.11 to 1.43+/-0.10 (P < 0.01).

CONCLUSION

At baseline, the critical mass for VF in diseased human hearts in vitro is > 111 g. However, the critical mass for VF can vary, as it can be reduced by shortening APD and increasing the slope of the APD restitution curve.

Nicotine Increases Spatiotemporal Complexity of Ventricular Fibrillation Wavefront on the Epicardial Border Zone of Healed Canine Infarcts

BACKGROUND: The influence of a pharmacologic agent on wavefront dynamics during ventricular fibrillation (VF) in a setting of remodeled and healed myocardial infarction (MI) remains poor explored. We hypothesized that nicotine, by virtue of its complex direct and indirect cardiovascular effects, increases wavefront complexity during VF. Specifically, we sought to determine whether nicotine increases the number and complexity (approximate entropy) of wavelets during stage II VF in hearts with healed MI. METHODS AND RESULTS: The left anterior descending coronary artery was permanently occluded in five mongrel dogs and wavefront dynamics during VF studied 5 to 6 weeks after occlusion in the open-chest anesthetized state. VF was induced by rapid pacing and the activation pattern mapped on the surviving epicardial border zone (EBZ) of the left ventricle with a plaque (3.2 x 3.8 cm) having 477 bipolar electrodes 1.6 mm apart. VF was mapped before and 20 minutes after 5 µg/kg/min nicotine infusion. Nicotine with a mean arterial plasma concentration of 127 +/- 76 ng/mL (range 57 to 240 ng/mL) significantly (P <.01) increased the number of wavelents from 3.8 +/- 0.4 to 5 +/- 0.41. The increased number of wavelets was caused by an increase (P <.01) in the spontaneous breakup of wavefronts from 4.1 +/- 0.9 times/s to 6.9 +/- 1.1 times/s. Wavebreak over the EBZ was functional in nature as no breakup occurred during normal sinus rhythm. Approximate entropy, a measure of complexity, significantly (P <.01) increased after nicotine administration from 0.23 +/- 0.02 to 0.28 +/- 0.01. CONCLUSIONS: Nicotine increases the number of wavelets and their complexity during VF by promoting spontaneous wavebreak over the EBZ of healed MI.

Cardiac electrical restitution properties and stability of reentrant spiral waves: a simulation study

Spiral wave breakup is a proposed mechanism underlying the transition from ventricular tachycardia to fibrillation. We examined the importance of the restitution of action potential duration (APD) and of conduction velocity (CV) to the stability of spiral wave reentry in a two-dimensional sheet of simulated cardiac tissue. The Luo-Rudy ventricular action potential model was modified to eliminate its restitution properties, which are caused by deactivation or recovery from inactivation of K+, Ca2+, and Na+ currents (IK, ICa, and INa, respectively). In this model, we find that 1) restitution of ICa and INa are the main determinants of the steepness of APD restitution; 2) for promoting spiral breakup, the range of diastolic intervals over which the APD restitution slope is steep is more important than the maximum steepness; 3) CV restitution promotes spiral wave breakup independently of APD restitution; and 4) "defibrillation" of multiple spiral wave reentry is most effectively achieved by combining an antifibrillatory intervention based on altering restitution with an antitachycardia intervention. These findings suggest a novel paradigm for developing effective antiarrhythmic drugs.

Transmembrane potential properties of atrial cells at different sites of a spiral wave reentry: cellular evidence for an excitable but nonexcited core

Transmembrane action potentials (TAPs) were recorded during simultaneous mapping of a reentrant wavefront induced in canine isolated atria. The activation pattern was visualized dynamically using a high resolution electrode catheter mapping system. During functional reentry (spiral wave), cells in the core of the spiral wave remained quiescent near their resting membrane potential. Cells away from the core progressively gained TAP amplitude and duration, and at the periphery of the spiral wave the cells generated TAPs with full height and duration. During anatomical reentry, when the tip of the wavefront remained attached to the obstacle (a condition of high source-to-sink ratio), the TAP near the obstacle had normal amplitude and duration. However, when the tip of the wavefront detached from the obstacle (condition of lowered source-to-sink ratio) the TAP lost amplitude and duration. These results are consistent with the theory that the source-to-sink ratio determines the safety factor for wave propagation and wave block near the core. With decreasing source-to-sink ratio, TAP progressively decreases in amplitude and duration. In the center of the core, the cells, while excitable, remain quiescent near their resting potential. This decrease reflects a progressive decrease in the source-to-sink ratio. TAP vanishes in the core where cells remain quiescent near their resting potential. Functional and meandering reentrant wavefronts are compatible with the spiral mechanism of reentry where block at the rotating point is provided by the steep curvature of the wave tip.

The effects of ventricular fibrillation duration and site of initiation on the defibrillation threshold during early ventricular fibrillation

OBJECTIVES

The purpose of this study was to determine if the defibrillation threshold (DFT) is lower during the first few cycles of ventricular fibrillation (VF) than after 10 s of VF and, if so, if the effect is caused by local or global factors.

BACKGROUND

The DFT may be low very early during VF because: (1) for the first few cycles VF arises from a localized region close to a defibrillation electrode where the shock field is strong (local factors), or (2) during early VF the effects of ischemia and sympathetic discharge have not yet fully developed and the heart has not yet completely dilated (global factors).

METHODS

Protocol 1 included seven pigs in which a defibrillation electrode and a pacing catheter were both placed in the right ventricular apex. VF was induced by delivering a high current premature stimulus from the pacing catheter that should have caused reentry confined to the right ventricular apex for the first few cycles of VF. A bipolar electrogram was recorded from the tip of the defibrillation catheter. Using a three reversal up-down protocol, the DFT was determined for biphasic shocks delivered after 1, 2, 3, 4, 5, 7, 10, 15, 20 and 25 activations in this electrogram and after 10 s (control). Protocol 2 included seven pigs undergoing the same procedure as in protocol 1 except that an additional pacing catheter was placed in the left ventricle. Defibrillation thresholds were determined after 1, 2, 3, 4 and 5 VF activations following VF induction from the right ventricle (RV) or the left ventricle (LV) and after 10 s (control).

RESULTS

In protocol 1, the mean +/- SD DFrs were lower during the first three cycles than after 10 s of VF (3.0 +/- 4.1 J for the first VF cycle vs 15.8 +/- 6.6 J after 10 s of VF, p < 0.05). In protocol 2, the DFF for the first few cycles of VF induced away from the defibrillation electrode in the LV (6.9 +/- 1.4 J for the first VF cycle) was significantly lower than that after 10 s of VF (16.0 +/- 2.2 J), whereas the DFF for the first few cycles induced near the defibrillation electrode in the right ventricular apex was significantly lower (2.3 +/- 2.7 J for the first VF cycle) than that induced from the LV.

CONCLUSIONS

This study demonstrates that the DFT is significantly lower during the first few VF cycles of VF than after 10 s of VF and that this decrease may be caused by both local factors and global factors. These results provide an impetus for exploring earlier shock delivery in implantable devices.

Characteristics of wave fronts during ventricular fibrillation in human hearts with dilated cardiomyopathy: role of increased fibrosis in the generation of reentry

OBJECTIVES

We sought to evaluate the characteristics of wave fronts during ventricular fibrillation (VF) in human hearts with dilated cardiomyopathy (DCM) and to determine the role of increased fibrosis in the generation of reentry during VF.

BACKGROUND

The role of increased fibrosis in reentry formation during human VF is unclear.

METHODS

Five hearts from transplant recipients with DCM were supported by Langendorff perfusion and were mapped during VF. A plaque electrode array with 477 bipolar electrodes (1.6-mm resolution) was used for epicardial mapping. In heart no. 5, we also used 440 transmural bipolar recordings. Each mapped area was analyzed histologically.

RESULTS

Fifteen runs of VF (8 s/run) recorded from the epicardium were analyzed, and 55 episodes of reentry were observed. The life span of reentry was short (one to four cycles), and the mean cycle length was 172 +/- 24 ms. In heart no. 5, transmural scroll waves were demonstrated. The most common mode of initiation of reentry was epicardial breakthrough, followed by a line of conduction block parallel to the epicardial fiber orientation (34 [62%] of 55 episodes). In the areas with lines of block, histologic examination showed significant fibrosis separating the epicardial muscle fibers and bundles along the longitudinal axis of fiber orientation. The mean percent fibrous tissue in these areas (n = 20) was significantly higher than that in the areas without block (n = 28) (24 +/- 7.5% vs. 10 +/- 3.8%, p < 0.0001).

CONCLUSIONS

In human hearts with DCM, epicardial reentrant wave fronts and transmural scroll waves were present during VF. Increased fibrosis provides a site for conduction block, leading to the continuous generation of reentry.

Current concepts of ventricular defibrillation

The aim of this article is to review the current concepts of ventricular defibrillation. We studied the interaction between strong electrical stimulus and cardiac responses in both animal models and in humans. We found that a premature stimulus (S2) of appropriate strength results in figure-eight reentry in vitro by inducing propagated graded responses. The same stimulation protocol induces figure-eight reentry and ventricular fibrillation (VF) in vivo. When the S2 strength and the magnitude of graded responses increase beyond a critical level, the increase in refractoriness at the site of the stimulus becomes so long that the unidirectional block becomes bidirectional block, preventing the formation of reentry (upper limit of vulnerability [ULV]). In other studies, we found that the effects of an electrical stimulation on reentry is in part determined by the timing of the stimulus. A protective zone is present after the induction of VF and after an unsuccessful defibrillation shock during which an electrical stimulus can terminate reentry and protect the heart from VF. These results indicate that the effects of a defibrillation shock is dependent on both the strength and the timing of the shock. Timing is not important in areas where the shock field strength is > or = ULV because the shock terminates all reentry but cannot reinitiate new ones. However, in areas where shock field strength is < ULV, the effects of the shock are determined by the timing of the shock relative to local VF activations. This ULV hypothesis of defibrillation explains the probabilistic nature of ventricular defibrillation. It also indicates that, to achieve a high probability of successful defibrillation, a shock must result in a shock field strength of > or = ULV throughout the ventricles.

High-dose lidocaine does not affect defibrillation efficacy: implications for defibrillation mechanisms

This study assessed the effect of low (10 mg.kg-1.h-1) and very high (18 mg.kg-1.h-1) doses of lidocaine on defibrillation energy requirements (DER) to relate changes in indexes of sodium-channel blockade with changes in DER values using a dose-response study design. In group 1 (control; n = 6 pigs), DER values were determined at baseline and during treatment with 5% dextrose in water (D5W) and with D5W added to D5W. In group 2 (n = 7), DER values were determined at baseline and during treatment with low-dose lidocaine followed by high-dose lidocaine. In group 3 (n = 3), DER values were determined at baseline and high-dose lidocaine. Group 3 controlled for the order of lidocaine treatment with the addition of high-dose lidocaine after baseline. DER values in group 1 did not change during D5W. In group 2, low-dose lidocaine increased DER values by 51% (P = 0.01), whereas high-dose lidocaine added to low-dose lidocaine reduced DER values back to within 6% of baseline values (P = 0.02, low dose vs. high dose). DER values during high-dose lidocaine in group 3 also remained near baseline values (16.2 +/- 2.7 to 12.9 +/- 2.7 J), demonstrating that treatment order had no impact on group 2. Progressive sodium-channel blockade was evident as incremental reduction in ventricular conduction velocity as the lidocaine dose increased. Lidocaine also significantly increased ventricular fibrillation cycle length as the lidocaine dose increased. However, the greatest increase in DER occurred when ventricular fibrillation cycle length was minimally affected, demonstrating a negative correlation (P = 0.04). In summary, lidocaine has an inverted U-shaped DER dose-response curve. At very high lidocaine doses, DER values are similar to baseline and tend to decrease rather than increase. Increased refractoriness during ventricular fibrillation may be the electrophysiological mechanism by which high-dose lidocaine limits the adverse effects that low-dose lidocaine has on DER values. However, there is a possibility that an unidentified action of lidocaine is responsible for these effects.

Evolving perspectives during 12 years of electrical turbulence

This Focus issue describes a problem in electrical dynamics which has fascinated generations of physiologists. There are today so many views of fibrillation that only the rarest generalization can embrace all of them. Fifty-two prominent investigators collaborate here to present aspects of the problem in these eighteen articles (including this introduction) tailored for readers whose principal expertise lies elsewhere. In "The High One's Lay" (Norse Runes, ca. 800) Odin remarks, "Much too early I came to many places: the beer was not yet ready, or was already drunk em leader " but to this one we come at very nearly the right time in 1998. This introduction attempts to guide newcomers by noting the changed or multiple meanings of novel technical terms while sorting the key facts and ideas into an order that facilitates comparison and contrast with those of a dozen years ago. This Focus issue is authored by some of the foremost innovators of both theory and experiment in this area. By assimilating their presentations the readers of Chaos can become well poised to appreciate and evaluate the definitive evidence expected in the next few years. (c) 1998 American Institute of Physics.

Computerized mapping of fibrillation in normal ventricular myocardium

It is well known that the ability to fibrillate is intrinsic to a normal ventricle that exceeds a critical mass. The questions we address are how is ventricular fibrillation (VF) initiated and perpetuated in normal myocardium, and why is VF not seen more often in the general population if all ventricles have the ability to fibrillate. To study the mechanisms of VF, we used computerized mapping techniques with up to 512 channels of simultaneous multisite recordings for data acquisition. The data were then processed for dynamic display of the activation patterns and for mathematical analyses of the activation intervals. The results show that in normal ventricles, VF can be initiated by a single strong premature stimulus given during the vulnerable period of the cardiac cycle. The initial activations form a figure-eight pattern. Afterward, VF will perpetuate itself without any outside help. The self-perpetuation itself is due to at least two factors. One is that single wave fronts spontaneously break up into two or more wavelets. The second is that when two wavelets intersect perpendicular to each other, the second wavelet is broken by the residual refractoriness left over from the first wavelet. Mathematical analyses of the patterns of activation during VF revealed that VF is a form of chaos, and that transition from ventricular tachycardia (VT) to VF occurs via the quasiperiodic route. In separate experiments, we found that we can convert VF to VT by tissue size reduction. The physiological mechanism associated with the latter transition appears to be the reduction of the number of reentrant wave fronts and wandering wavelets. Based on these findings, we propose that the reentrant wave fronts and the wandering wavelets serve as the physiological equivalent of coupled oscillators. A minimal number of oscillators is needed for VF to perpetuate itself, and to generate chaotic dynamics; hence a critical mass is required to perpetuate VF. We conclude that VF in normal myocardium is a form of reentrant cardiac arrhythmia. A strong electrical stimulus initiates single or dual reentrant wave fronts that break up into multiple wavelets. Sometimes short-lived reentry is also generated during the course of VF. These organized reentrant and broken wavelets serve as coupled oscillators that perpetuate VF and maintain chaos. Although the ability to support these oscillators exists in a normal ventricle, the triggers required to generate them are nonexistent in the normal heart. Therefore, VF and sudden death do not happen to most people with normal ventricular myocardium. (c) 1998 American Institute of Physics.

Patterns of spiral tip motion in cardiac tissues

In support of the spiral wave theory of reentry, simulation studies and animal models have been utilized to show various patterns of spiral wave tip motion such as meandering and drifting. However, the demonstration of these or any other patterns in cardiac tissues have been limited. Whether such patterns of spiral tip motion are commonly observed in fibrillating cardiac tissues is unknown, and whether such patterns form the basis of ventricular tachycardia or fibrillation remain debatable. Using a computerized dynamic activation display, 108 episodes of atrial and ventricular tachycardia and fibrillation in isolated and intact canine cardiac tissues, as well as in vitro swine and myopathic human cardiac tissues, were analyzed for patterns of nonstationary, spiral wave tip motion. Among them, 46 episodes were from normal animal myocardium without pharmacological perturbations, 50 samples were from normal animal myocardium, either treated with drugs or had chemical ablation of the subendocardium, and 12 samples were from diseased human hearts. Among the total episodes, 11 of them had obvious nonstationary spiral tip motion with a life span of >2 cycles and with consecutive reentrant paths distinct from each other. Four patterns were observed: (1) meandering with an inward petal flower in 2; (2) meandering with outward petals in 5; (3) irregularly concentric in 3 (core moving about a common center); and (4) drift in 1 (linear core movement). The life span of a single nonstationary spiral wave lasted no more than 7 complete cycles with a mean of 4.6+/-4.3, and a median of 4.5 cycles in our samples. Conclusion: (1) Patently evident nonstationary spiral waves with long life spans were uncommon in our sample of mostly normal cardiac tissues, thus making a single meandering spiral wave an unlikely major mechanism of fibrillation in normal ventricular myocardium. (2) A tendency toward four patterns of nonstationary spiral tip motion was observed. (c) 1998 American Institute of Physics.

Spatiotemporal complexity of ventricular fibrillation revealed by tissue mass reduction in isolated swine right ventricle. Further evidence for the quasiperiodic route to chaos hypothesis

We have presented evidence that ventricular fibrillation is deterministic chaos arising from quasiperiodicity. The purpose of this study was to determine whether the transition from chaos (ventricular fibrillation, VF) to periodicity (ventricular tachycardia) through quasiperiodicity could be produced by the progressive reduction of tissue mass. In isolated and perfused swine right ventricular free wall, recording of single cell transmembrane potentials and simultaneous mapping (477 bipolar electrodes, 1.6 mm resolution) were performed. The tissue mass was then progressively reduced by sequential cutting. All isolated tissues fibrillated spontaneously. The critical mass to sustain VF was 19.9 +/- 4.2 g. As tissue mass was decreased, the number of wave fronts decreased, the life-span of reentrant wave fronts increased, and the cycle length, the diastolic interval, and the duration of action potential lengthened. There was a parallel decrease in the dynamical complexity of VF as measured by Kolmogorov entropy and Poincaré plots. A period of quasiperiodicity became more evident before the conversion from VF (chaos) to a more regular arrhythmia (periodicity). In conclusion, a decrease in the number of wave fronts in ventricular fibrillation by tissue mass reduction causes a transition from chaotic to periodic dynamics via the quasiperiodic route.

Can catheter ablation in cardiac arrest survivors prevent ventricular fibrillation recurrence?

Ventricular tachyarrhythmias are the most common cause for sudden cardiac death. The success of catheter ablation for supraventricular tachycardias led to the supposition that ablation could also be used in the treatment of ventricular tachycardias. Despite the promising results in bundle branch reentry and some forms of idiopathic ventricular tachycardia, the success rate in patients with coronary artery disease is still low. There is hope that new approaches to reliably localize the critical region of the tachycardia and new ablation techniques to create larger areas of injury may lead to a wider application of ablation therapy in the treatment of ventricular tachycardia. Survivors of cardiac arrest typically have more rapid and unstable arrhythmias than patients with sustained ventricular tachycardia, and these rapid arrhythmias frequently degenerate into ventricular fibrillation. The instability of the arrhythmia makes it impossible to localize the arrhythmia origin with current mapping techniques. Experimental and clinical data, however, suggest that these arrhythmias also frequently start from a localized area of electrical activation. With developments in mapping techniques and energy delivery, catheter ablation may soon become a feasible therapeutic approach in some patients with unstable arrhythmias. The article discusses the prerequisites for this approach and suggests the patients who may be appropriate candidates for this technique.

Quasiperiodicity and chaos in cardiac fibrillation

In cardiac fibrillation, disorganized waves of electrical activity meander through the heart, and coherent contractile function is lost. We studied fibrillation in three stationary forms: in human chronic atrial fibrillation, in a stabilized form of canine ventricular fibrillation, and in fibrillation-like activity in thin sheets of canine and human ventricular tissue in vitro. We also created a computer model of fibrillation. In all four studies, evidence indicated that fibrillation arose through a quasiperiodic stage of period and amplitude modulation, thus exemplifying the "quasiperiodic transition to chaos" first suggested by Ruelle and Takens. This suggests that fibrillation is a form of spatio-temporal chaos, a finding that implies new therapeutic approaches.

Mechanism of antiarrhythmic drug-induced changes in defibrillation threshold: role of potassium and sodium channel conductance

OBJECTIVES

We sought to determine which ion current predominantly affects defibrillation outcomes by using specific pharmacologic probes (lidocaine [a sodium channel blocking agent] and cesium [an outward potassium channel blocking agent]) in 26 swine.

BACKGROUND

The effect of a drug on sodium or potassium channel conductance, or both, may affect defibrillation threshold values. However, it is unknown which ion channel predominates.

METHODS

Each pig was randomly assigned to one of four treatment groups with two treatment phases: group 1 = placebo (D5W) in treatment phase I followed by placebo plus cesium in treatment phase II (n = 6); group 2 = lidocaine followed by lidocaine plus placebo (n = 7); group 3 = lidocaine followed by lidocaine plus cesium (n = 7); group 4 = placebo followed by placebo plus placebo (n = 6). Defibrillation threshold values and electrocardiographic measurements were obtained at baseline and at treatment phases I and II.

RESULTS

Lidocaine increased defibrillation threshold values from baseline by 71% in group 2 (p = 0.02) and by 92% in group 3 (p < 0.01). There were no changes in defibrillation threshold values from baseline to D5W in groups 1 and 4. When D5W was added to lidocaine in group 2 and D5W in group 4, there were no significant changes in defibrillation threshold values. However, when cesium was added to lidocaine in group 3, the elevated defibrillation threshold values (mean +/- SD) returned to baseline values (from 15.7 +/- 3.46 to 7.55 +/- 3.19 J, p < 0.01). Cesium added to D5W in group 1 also significantly reduced defibrillation threshold values from 7.10 +/- 1.27 to 4.14 +/- 1.75 J (p < 0.01). The effect of cesium on defibrillation threshold values was similar between groups 1 and 3, regardless of lidocaine, such that these values were reduced by 40 +/- 14% and 51 +/- 18%, respectively (p = 0.28).

CONCLUSIONS

Cesium, through potassium blockade, reverses lidocaine-induced elevation in defibrillation threshold values. The magnitude of defibrillation threshold reduction when cesium was added to lidocaine was similar to the defibrillation threshold reduction when cesium was added to placebo. Thus, inhibiting outward potassium conductance and prolonging repolarization decreases defibrillation threshold values independent of sodium channel blockade.