Progressive action potential duration shortening and the conversion from atrial flutter to atrial fibrillation in the isolated canine right atrium

OBJECTIVES

We sought to evaluate the effects of progressive shortening of the action potential duration (APD) on atrial wave front stability.

BACKGROUND

The mechanisms of conversion from atrial flutter to atrial fibrillation (AF) are unclear.

METHODS

Isolated canine right atria were perfused with 1 to 5 micromol/l of acetylcholine (ACh). We mapped the endocardium by using 477 bipolar electrodes and simultaneously recorded transmembrane potentials from the epicardium. The APD(90) was measured during regular pacing (S(1)) with cycle lengths of 300 ms. Atrial arrhythmia was induced by a premature stimulus (S(2)).

RESULTS

At baseline, only short runs of repetitive beats (<10 cycles) were induced. After shortening the APD(90) from 124 +/- 15 ms to 72 +/- 9 ms (p < 0.01) with 1 to 2.5 micromol/l of ACh, S(2) pacing induced single, stable and stationary re-entrant wave fronts (307 +/- 277 cycles). They either anchored to pectinate muscles (5 tissues) or used pectinate muscles as part of the re-entry (4 tissues). When ACh was raised to 2.5 to 5 micromol/l, the APD(90) was further shortened to 40 +/- 12 ms (p < 0.01); S(2) pacing induced in vitro AF by two different mechanisms. In most episodes (n = 13), AF was characterized by rapid, nonstationary re-entry and multiple wave breaks. In three episodes with APD(90) <30 ms, AF was characterized by rapid, multiple, asynchronous, but stationary wave fronts.

CONCLUSIONS

Progressive APD shortening modulates atrial wave front stability and converts atrial flutter to AF by two mechanisms: 1) detachment of stationary re-entry from the pectinate muscle and the generation of multiple wave breaks; and 2) formation of multiple, isolated, stationary wave fronts with different activation cycle lengths.

Modulation of QT interval by cardiac sympathetic nerve sprouting and the mechanisms of ventricular arrhythmia in a canine model of sudden cardiac death

INTRODUCTION

We previously reported that there is a high incidence of sudden cardiac death (SCD) in dogs with myocardial infarction (MI), complete AV block (CAVB), and nerve growth factor (NGF) infusion to the left stellate ganglion (LSG). Whether or not QT interval prolongation underlines the mechanism of SCD was unclear.

METHODS AND RESULTS

We analyzed QT intervals in three groups of dogs. All dogs had CAVB and MI. The LSG group (n = 9) and right stellate ganglion (RSG) group (n = 6) received NGF infusion via the osmotic pumps over a 5-week period to LSG and RSG, respectively. The control group (n = 6) received no NGF. The dogs either died suddenly or were sacrificed within 2 to 3 months after MI. Heart rhythm and QT and RR intervals were monitored using implantable cardioverter defibrillator ECG recordings. There was a time-dependent increase of QTc intervals in the LSG group and a time-dependent decrease of QTc intervals in the RSG group. At the end of NGF infusion, QTc intervals in the LSG group (408 +/- 41 msec) were significantly longer than those in the control (350 +/- 41 msec; P < 0.05) and RSG groups (294 +/- 23 msec; P < 0.01). In the LSG group, 4 of 9 dogs died of SCD. There was no SCD in either the RSG or control group. Immunocytochemical staining showed NGF infusion to LSG and RSG resulted in left and right ventricular sympathetic nerve sprouting and hyperinnervation, respectively.

CONCLUSION

NGF infusion to the LSG in dogs with MI and CAVB resulted in increased QT interval and incidence of ventricular tachycardia, ventricular fibrillation, and SCD, whereas NGF infusion to the RSG shortened QT interval and reduced the incidence of ventricular tachycardia. These findings indicate that QT interval prolongation is causally related to the occurrence of ventricular arrhythmia in dogs with nerve sprouting, MI, and CAVB.

Optical mapping of ventricular defibrillation in isolated swine right ventricles: demonstration of a postshock isoelectric window after near-threshold defibrillation shocks

BACKGROUND

Investigators who studied ventricular defibrillation by use of optical mapping techniques failed to observe an initial defibrillation event (isoelectric window or quiescent period) shown by electrode mapping studies. This discrepancy has important implications for the mechanisms of defibrillation. The purpose of the present study was to demonstrate an optical equivalent of an isoelectric window after a near-threshold defibrillation shock. Methods and Results-- We studied 10 isolated, perfused swine right ventricles. Upper limit of vulnerability was determined by shocks on T waves. A 50% probability of successful defibrillation (DFT50) was determined with an up-down algorithm. Immediately after unsuccessful defibrillation shock, new wavefronts were generated. When the shock strength was low, immediate reinitiation of reentry and ventricular fibrillation might occur without a postshock isoelectric window. However, if the shock strength was within 50 V of DFT50 (near-threshold), a synchronized activation occurred, followed by organized repolarization that ended 64+/-18 ms after shock. After a period of quiescence (18+/-24 ms), activation recurred 83+/-33 ms after shock and reinitiated ventricular fibrillation. Similar patterns of activation, including a quiescent period, were observed after shock was applied on the T wave of the paced beat that induced ventricular fibrillation. Upper limit of vulnerability correlated well with DFT50.

CONCLUSIONS

In isolated swine right ventricles, an optical equivalent of an isoelectric window exists after near-threshold defibrillation shocks. These findings support the idea that a near-threshold defibrillation shock terminates all activation wavefronts but fails to halt ventricular fibrillation because the same shock reinitiates ventricular fibrillation after an isoelectric window.

Patterns of wave break during ventricular fibrillation in isolated swine right ventricle

Several different patterns of wave break have been described by mapping of the tissue surface during fibrillation. However, it is not clear whether these surface patterns are caused by multiple distinct mechanisms or by a single mechanism. To determine the mechanism by which wave breaks are generated during ventricular fibrillation, we conducted optical mapping studies and single cell transmembrane potential recording in six isolated swine right ventricles (RV). Among 763 episodes of wave break (0.75 times x s(-1) x cm(-2)), optical maps showed three patterns: 80% due to a wave front encountering the refractory wave back of another wave, 11.5% due to wave fronts passing perpendicular to each other, and 8.5% due to a new (target) wave arising just beyond the refractory tail of a previous wave. Computer simulations of scroll waves in three-dimensional tissue showed that these surface patterns could be attributed to two fundamental mechanisms: head-tail interactions and filament break. We conclude that during sustained ventricular fibrillation in swine RV, surface patterns of wave break are produced by two fundamental mechanisms: head-tail interaction between waves and filament break.

Effects of diacetyl monoxime and cytochalasin D on ventricular fibrillation in swine right ventricles

Whether or not the excitation-contraction (E-C) uncoupler diacetyl monoxime (DAM) and cytochalacin D (Cyto D) alter the ventricular fibrillation (VF) activation patterns is unclear. We recorded single cell action potentials and performed optical mapping in isolated perfused swine right ventricles (RV) at different concentrations of DAM and Cyto D. Increasing the concentration of DAM results in progressively shortened action potential duration (APD) measured to 90% repolarization, reduced the slope of the APD restitition curve, decreased Kolmogorov-Sinai entropy, and reduced the number of VF wave fronts. In all RVs, 15-20 mmol/l DAM converted VF to ventricular tachycardia (VT). The VF could be reinduced after the DAM was washed out. In comparison, Cyto D (10-40 micromol/l) has no effects on APD restitution curve or the dynamics of VF. The effects of DAM on VF are associated with a reduced number of wave fronts and dynamic complexities in VF. These results are compatible with the restitution hypothesis of VF and suggest that DAM may be unsuitable as an E-C uncoupler for optical mapping studies of VF in the swine RVs.

Sympathetic nerve sprouting, electrical remodeling and the mechanisms of sudden cardiac death

The purpose of this article is to review the nerve sprouting hypothesis of sudden cardiac death. It is known that sympathetic stimulation is important in the generation of sudden cardiac death. For example, there is a diurnal variation of sudden death rate in patients with myocardial infarction. Beta blockers, or drugs with beta blocking effects, are known to prevent sudden cardiac death. It was unclear if the cardiac nerves in the heart play only a passive role in the mechanisms of sudden death. To determine if nerve sprouting and neural remodeling occur after myocardial infarction, we performed immunocytochemical studies of cardiac nerves in explanted native hearts of transplant recipients. We found that there was a positive correlation between nerve density and a clinical history of ventricular arrhythmia. Encouraged by these results, we performed a study in dogs to determine whether or not nerve growth factor (NGF) infusion to the left stellate ganglion can facilitate the development of ventricular tachycardia (VT), ventricular fibrillation (VF), and sudden cardiac death (SCD). The results showed that augmented myocardial sympathetic nerve sprouting through NGF infusion plus atrioventricular (AV) block and MI result in a 44% incidence (four of nine dogs) of SCD and a high incidence of VT in the chronic phase of MI. In contrast, none of the six dogs (with AV block and MI) without NGF infusion died suddenly or had frequent VT episodes. Based on these findings, we propose the nerve sprouting hypothesis of ventricular arrhythmia and SCD. The hypothesis states that MI results in nerve injury, followed by sympathetic nerve sprouting and regional (heterogeneous) myocardial hyperinnervation. The coupling between augmented sympathetic nerve sprouting with electrically remodeled myocardium results in VT, VF and SCD. Modification of nerve sprouting after MI may provide a novel opportunity for arrhythmia control.

Cellular mechanism of reentry induced by a strong electrical stimulus: implications for fibrillation and defibrillation

The objective of this review article is to describe the graded response hypothesis of reentry induced by a strong single electrical stimulus in the normal canine ventricular myocardium. It is shown that the graded responses (subthreshold depolarization during phase 3 of the action potential) induced at a site distant (S2) from the regular S1--S1 pacing site, propagate slowly over short distances (approximately 5 mm) and initiate a regenerative action potentials in recovered cells near the S1 site. Activation wave then blocks near the S2 site (unidirectional block) but reenters when the S2 site recovers it excitability. Super strong S2 currents do not induce reentry (upper limit of vulnerability). Since similar activation sequence and properties are shown to exist in intact canine hearts during induction of ventricular fibrillation with a similar S2 stimulus, the graded response hypothesis may have relevance to vulnerability to fibrillation. Furthermore, since the upper limit of vulnerability is closely related to defibrillation threshold, the graded response hypothesis might also be relevant to defibrillation mechanism. Other proposed mechanisms of fibrillation and defibrillation (critical point hypothesis, the progressive depolarization hypothesis and the hypothesis of phase singularity of defibrillation failure) are also discussed in this review paper and compared to the graded response hypothesis.

Dynamics of intramural and transmural reentry during ventricular fibrillation in isolated swine ventricles

The intramural dynamics of ventricular fibrillation (VF) remain poorly understood. Recent investigations have suggested that stable intramural reentry may underlie the mechanisms of VF. We performed optical mapping studies of VF in isolated swine right ventricles (RVs) and left ventricles (LVs). Nine RV walls were cut obliquely in their distal edge exposing the transmural surface. Six LV wedge preparations were also studied. Results showed that intramural reentry was present. In RV, 28 of 44 VF episodes showed reentry; 15% of the activation pathways were reentrant. Except for 4 episodes, reentry was transmural, involving subendocardial structures as the papillary muscle (PM) or trabeculae. In LV, reentry was observed in 27 of 27 VF episodes; 23% of the activations were part of reentrant pathways (P<0.05 compared with RV). All LV reentrant pathways were truly intramural (confined to the wall) and were frequently located at the PM insertion. In both ventricles, reentry was spatially and temporally unstable. Histological studies showed abrupt changes in fiber orientation at sites of reentry and wave splitting. Connexin 40 immunostaining demonstrated intramyocardial Purkinje fibers at sites of reentry in the PM root and around endocardial trabeculae. Our results confirm that reentry is frequent-but unstable-in the myocardial wall during VF. In RV, reentry is mostly transmural and requires participation of subendocardial structures. The LV has a greater incidence of reentry and is intramural. Anisotropic anatomic structures played key roles in the generation of wave splitting and in the maintenance of reentry.

Increased wave break during ventricular fibrillation in the epicardial border zone of hearts with healed myocardial infarction

BACKGROUND

The action potential duration (APD) restitution hypothesis of wave break during ventricular fibrillation (VF) in the epicardial border zone (EBZ) of hearts with chronic myocardial infarction is unknown.

METHODS AND RESULTS

VF was induced by rapid pacing, and the EBZ with the two adjoining sites (right ventricle and lateral left ventricle) were sequentially mapped in random order in 7 open-chest anesthetized dogs 6 to 8 weeks after left anterior descending artery occlusion and in 4 control dogs. At each site, 3 seconds of VF was mapped with 477 bipolar electrodes 1.6 mm apart. The number of wave fronts and approximate entropy were significantly (P:<0.01) higher in the EBZ than all other sites in both groups independent of the rate of invasion of new wave fronts and epicardial breakthroughs. The higher wavelet density in the EBZ was caused by increased (P:<0.01) incidence of spontaneous wave breaks. There was no difference between the two groups in either reentry period (80 episodes) or VF cycle length. Reentry in the EBZ had a smaller core perimeter, slower rotational speed, and a small or no excitable gap (P:<0.01), often causing termination after one rotation. The dynamic monophasic action potential duration restitution curve in the EBZ had longer (P:<0.01) diastolic intervals, over which the slope was >1. Connexin43-positive staining was significantly (P:<0.01) and selectively reduced in the EBZ.

CONCLUSIONS

A selective increase in wave break and alteration of reentry occur in the EBZ during VF in hearts with healed myocardial infarction. Increased wave break in the EBZ is compatible with the action potential duration restitution hypothesis.

Pulmonary veins and ligament of Marshall as sources of rapid activations in a canine model of sustained atrial fibrillation

BACKGROUND

In dogs, chronic rapid pacing may result in sustained atrial fibrillation (AF). However, activation patterns in pacing-induced sustained AF are unclear.

METHODS AND RESULTS

We induced sustained AF (>48 hours) in 6 dogs by rapid pacing for 139+/-84 days. We then performed computerized atrial epicardial mappings and recorded the activations in the ligament of Marshall (LOM) and the pulmonary veins (PVs). During AF, mean activation cycle length in the right atrial free wall (126+/-17 ms) was significantly longer than that in the left atrial free wall (96+/-5 ms, P:=0.006). In addition, mean activation cycle length in the left atrial free wall was significantly longer than that in the LOM (84+/-5 ms, P:<0.001), the left inferior PV (81+/-4 ms, P:=0.001), and the left superior PV (85+/-7 ms, P:=0.003). Similarly, the dominant frequency was highest in the LOM and the PVs (range 11.2 to 13.3 Hz), followed by the left and right atria (P:<0.001). In all dogs studied, rapid and complicated electrograms were consistently observed at the LOM and the PVs. During AF, both wandering wavelets and organized reentry were present. There were more wave fronts in the left atrium than in the right atrium (P:<0.001).

CONCLUSIONS

In chronic pacing-induced sustained AF, the LOM and the PVs are the sources of rapid activations. The mechanism by which the left atrium activates faster and has more wave fronts than the right atrium may relate to the fact that the left atrium is closer to the sources of rapid activations.

Nerve sprouting and sympathetic hyperinnervation in a canine model of atrial fibrillation produced by prolonged right atrial pacing

BACKGROUND

Long-term rapid atrial pacing may result in atrial fibrillation (AF) in dogs. Whether there is histological evidence for neural remodeling is unclear.

METHOD AND RESULTS

We performed rapid right atrial pacing in 6 dogs for 111+/-76 days to induce sustained AF. Tissues from 6 healthy dogs were used as controls. Immunocytochemical staining of cardiac nerves was performed using anti-growth-associated protein 43 (GAP43) and anti-tyrosine hydroxylase (TH) antibodies. In dogs with AF, the density of GAP43-positive and TH-positive nerves in the right atrium was 470+/-406 and 231+/-126 per mm(2), respectively, which was significantly (P:<0.001) higher than the nerve density in control tissues (25+/-32 and 88+/-40 per mm(2), respectively). The density of GAP43-positive and TH-positive nerves in the atrial septum was 317+/-36 and 155+/-85 per mm(2), respectively, and was significantly (P:<0.001) higher than the nerve density in control tissues (9+/-13 and 30+/-7 per mm(2), respectively). Similarly, the density of GAP43-positive and TH-positive nerves in the left atrium of dogs with AF was 119+/-61 and 91+/-40 per mm(2), respectively, which was significantly (P:<0.001) higher than the nerve density in control tissues (10+/-15 and 38+/-39 per mm(2), respectively). Furthermore, in dogs with AF, the right atrium had a significantly higher nerve density than the left atrium. Microscopic examinations revealed an inhomogeneous distribution of cardiac nerves within each sampling site.

CONCLUSIONS

Significant nerve sprouting and sympathetic hyperinnervation are present in a canine model of sustained AF produced by prolonged right atrial pacing. The magnitude of nerve sprouting and hyperinnervation was higher in the right atrium than in the left atrium.

Ventricular fibrillation: how do we stop the waves from breaking?

Combined experimental and theoretical developments have demonstrated that in addition to preexisting electrophysiological heterogeneities, cardiac electrical restitution properties contribute to breakup of reentrant wavefronts during cardiac fibrillation. Developing therapies that favorably alter electrical restitution properties have promise as a new paradigm for preventing fibrillation.

Comparing cardiac action potentials recorded with metal and glass microelectrodes

Machine-pulled high-impedance glass capillary microelectrode is standard for transmembrane potential (TMP) recordings. However, it is fragile and difficult to impale, especially in beating myocardial tissues. We hypothesize that a high-impedance pure iridium metal electrode can be used as an alternative to the glass microelectrode for TMP recording. The TMPs were simultaneously recorded from isolated perfused swine right ventricles with a metal microelectrode and a standard glass microelectrode during pacing and during ventricular fibrillation. The basic morphology of TMP recorded with these electrodes was comparable. The action potential duration (APD) at 90% repolarization was 241 +/- 29 ms for the metal microelectrode and 236 +/- 31 ms for the glass microelectrode with a good correlation (r = 0.99, P < 0.0001). The maximum slope value of the APD restitution curves during pacing was also significantly correlated. One metal microelectrode and >20 glass microelectrodes were needed per study. We conclude that, in isolated perfused swine right ventricles, the TMP recorded by the metal microelectrode is comparable with that recorded by the glass microelectrode. Because the metal microelectrode is more durable than the glass microelectrode, it can serve as an alternative for APD recording and for restitution analyses.

Colocalization of tenascin and sympathetic nerves in a canine model of nerve sprouting and sudden cardiac death

INTRODUCTION

Sympathetic nerve sprouting after myocardial infarction (MI) may contribute significantly to the occurrence of ventricular arrhythmia and sudden cardiac death. Tenascin-X (TnX), a matrix protein known to be associated with nerve growth in central and peripheral nerves, also may play a role in cardiac nerve sprouting after MI.

METHODS AND RESULTS

Immunocytochemical staining techniques were used to identify nerves in 5-microm serial sections from 6 normal dogs and 11 dogs with MI. Among the dogs with MI, 4 also received nerve growth factor infusion to the left stellate ganglion. The time between MI to tissue harvest averaged 35.7 +/- 14.4 days. Tyrosine hydroxylase (TH) stain was used to identify sympathetic nerves, and growth-associated protein-43 (GAP-43) was used to identify growing nerves. Polyclonal antibody was obtained for use in identifying TnX. Nerves were evident in both the infarcted and noninfarcted areas. Many nerves were found around blood vessels. A total of 181 nerves in 69 slides were examined: 89 were from noninfarcted myocardium, 4 from infarct, 13 from infarct border zone, and 75 from perivascular regions. Except in normal dogs, all nerves stained positive for TH also stained positive for GAP-43, indicating sympathetic nerve sprouting after MI. In all dogs, the nerves that stained positive for TH also stained positive for TnX.

CONCLUSION

There is a colocalization of TnX, GAP-43, and TH in sprouted cardiac nerves. These results suggest that TnX is important not only in the existing normal myocardial nerve cells but also in cardiac sympathetic nerve sprouting after MI.

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.

The ligament of Marshall: a structural analysis in human hearts with implications for atrial arrhythmias

OBJECTIVES

We sought to study the anatomy of human ligament of Marshall (LOM).

BACKGROUND

Although the LOM has been implicated in the genesis of focal atrial tachyarrhythmias, its gross anatomic and microscopic features in humans hearts have not been completely defined.

METHODS

We studied seven postmortem human hearts from five men and two women with a mean age of 52 +/- 26 years. Four did not have any heart disease. One woman had dilated cardiomyopathy, and two men had chronic atrial fibrillation. A block of tissue encompassing the LOM from the coronary sinus (CS) cephalad, between the atrial appendage and left pulmonary veins, was dissected. Serial sections from this tissue were then stained with hematoxylin and eosin, trichrome, and/or tyrosine hydroxylase.

RESULTS

The LOM consists of multiple sympathetic nerve fibers, ganglia, blood vessels and multiple myocardial tracts (Marshall Bundles) insulated by fibrofatty tissue. One or more myocardial tracts was inserted directly into the left atrial free wall and CS. The distance between insertion sites was 7.8 +/- 2.5 mm. Nerve fibers, some tyrosine hydroxylase positive, were present within the fibrofatty matrix and within the myocardial tracts.

CONCLUSIONS

Human LOM 1) is innervated by sympathetic nerve fibers; 2) is more complex than the LOM in canine hearts; and 3) has multiple myocardial tract insertions into the left atrial free wall and CS, forming a substrate of reentry. Radiofrequency catheter ablation from the CS may fail to reach the free wall insertion.

Mechanisms of ventricular fibrillation induction by 60-Hz alternating current in isolated swine right ventricle

BACKGROUND

The mechanisms by which 60-Hz alternating current (AC) can induce ventricular fibrillation (VF) are unknown.

METHODS AND RESULTS

We studied 7 isolated perfused swine right ventricles in vitro. The action potential duration restitution curve was determined. Optical mapping techniques were used to determine the patterns of activation on the epicardium during 5-second 60-Hz AC stimulation (10 to 999 microA). AC captured the right ventricles at 100+/-65 microA, which is significantly lower than the direct current pacing threshold (0.77+/-0.45 mA, P:<0.05). AC induced ventricular tachycardia or VF at 477+/-266 microA, when the stimulated responses to AC had (1) short activation CLs (128+/-14 ms), (2) short diastolic intervals (16+/-9 ms), and (3) short diastolic intervals associated with a steep action potential duration restitution curve. Optical mapping studies showed that during rapid ventricular stimulation by AC, a wave front might encounter the refractory tail of an earlier wave front, resulting in the formation of a wave break and VF. Computer simulations reproduced these results.

CONCLUSIONS

AC at strengths less than the regular pacing threshold can capture the ventricle at fast rates. Accidental AC leak to the ventricles could precipitate VF and sudden death if AC results in a fast ventricular rate coupled with a steep restitution curve and a nonuniform recovery of excitability of the myocardium.

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.

Preventing ventricular fibrillation by flattening cardiac restitution

Ventricular fibrillation is the leading cause of sudden cardiac death. In fibrillation, fragmented electrical waves meander erratically through the heart muscle, creating disordered and ineffective contraction. Theoretical and computer studies, as well as recent experimental evidence, have suggested that fibrillation is created and sustained by the property of restitution of the cardiac action potential duration (that is, its dependence on the previous diastolic interval). The restitution hypothesis states that steeply sloped restitution curves create unstable wave propagation that results in wave break, the event that is necessary for fibrillation. Here we present experimental evidence supporting this idea. In particular, we identify the action of the drug bretylium as a prototype for the future development of effective restitution-based antifibrillatory agents. We show that bretylium acts in accord with the restitution hypothesis: by flattening restitution curves, it prevents wave break and thus prevents fibrillation. It even converts existing fibrillation, either to a periodic state (ventricular tachycardia, which is much more easily controlled) or to quiescent healthy tissue.

Nerve sprouting and sudden cardiac death

The factors that contribute to the occurrence of sudden cardiac death (SCD) in patients with chronic myocardial infarction (MI) are not entirely clear. The present study tests the hypothesis that augmented sympathetic nerve regeneration (nerve sprouting) increases the probability of ventricular tachycardia (VT), ventricular fibrillation (VF), and SCD in chronic MI. In dogs with MI and complete atrioventricular (AV) block, we induced cardiac sympathetic nerve sprouting by infusing nerve growth factor (NGF) to the left stellate ganglion (experimental group, n=9). Another 6 dogs with MI and complete AV block but without NGF infusion served as controls (n=6). Immunocytochemical staining revealed a greater magnitude of sympathetic nerve sprouting in the experimental group than in the control group. After MI, all dogs showed spontaneous VT that persisted for 5.8+/-2.0 days (phase 1 VT). Spontaneous VT reappeared 13.1+/-6.0 days after surgery (phase 2 VT). The frequency of phase 2 VT was 10-fold higher in the experimental group (2.0+/-2.0/d) than in the control group (0.2+/-0.2/d, P<0.05). Four dogs in the experimental group but none in the control group died suddenly of spontaneous VF. We conclude that MI results in sympathetic nerve sprouting. NGF infusion to the left stellate ganglion in dogs with chronic MI and AV block augments sympathetic nerve sprouting and creates a high-yield model of spontaneous VT, VF, and SCD. The magnitude of sympathetic nerve sprouting may be an important determinant of SCD in chronic MI.

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.

Mechanisms of atrial fibrillation and flutter and implications for management

Both electrophysiologic and anatomical substrates are important in the generation and maintenance of atrial fibrillation. This review discusses the nature of re-entrant wavefronts in atrial fibrillation and the importance of anatomical structures, such as the pectinate muscles, in the generation and maintenance of re-entry. The involvement of the pectinate muscle structure on intra-atrial re-entry may have significant implications for both ablation and pharmacologic management of patients with atrial fibrillation.

Role of papillary muscle in the generation and maintenance of reentry during ventricular tachycardia and fibrillation in isolated swine right ventricle

BACKGROUND

The role of papillary muscle (PM) in the generation and maintenance of reentry is unclear.

METHODS AND RESULTS

Computerized mapping (477 bipolar electrodes, 1.6-mm resolution) was performed in fibrillating right ventricles (RVs) of swine in vitro. During ventricular fibrillation (VF), reentrant wave fronts often transiently anchored to the PM. Tissue mass reduction was then performed in 10 RVs until VF converted to ventricular tachycardia (VT). In an additional 6 RVs, procainamide infusion converted VF to VT. Maps showed that 77% (34 of 44) of all VT episodes were associated with a single reentrant wave front anchored to the PM. Purkinje fiber potentials preceded the local myocardial activation, and these potentials were recorded mostly around the PM. When PM was trimmed to the level of endocardium (n = 4), sustained VT was no longer inducible. Transmembrane potential recordings (n = 5) at the PM revealed full action potential during pacing, without evidence of ischemia. Computer simulation studies confirmed the role of PM as a spiral wave anchoring site that stabilized wave conduction.

CONCLUSIONS

We conclude that PM is important in the generation and maintenance of reentry during VT and VF.

Relation between ligament of Marshall and adrenergic atrial tachyarrhythmia

BACKGROUND

The mechanism of the adrenergic atrial tachyarrhythmia is unclear. We hypothesize that the ligament of Marshall (LOM) is sensitive to adrenergic stimulation and may serve as a source of the adrenergic atrial tachyarrhythmia.

METHODS AND RESULTS

We performed computerized mapping studies in isolated-perfused canine left atrial tissues from normal dogs (n=9) and from dogs with chronic atrial fibrillation (AF) induced by 10 to 41 weeks of rapid pacing (n=3). Before isoproterenol, spontaneous activity occurred in only one normal tissue (cycle length, CL >1300 ms). During isoproterenol infusion, automatic rhythm was induced in both normal tissues (CL=578+/-172 ms) and AF tissues (CL=255+/-29 ms, P<0.05). The origin of spontaneous activity was mapped to the LOM. In the AF tissues, but not the normal tissues, we observed the transition from rapid automatic activity to multiple wavelet AF. Ablation of the LOM terminated the spontaneous activity and prevented AF. Immunocytochemical studies of the LOM revealed muscle tracts surrounded by tyrosine hydroxylase-positive (sympathetic) nerves.

CONCLUSIONS

We conclude that the LOM is richly innervated by sympathetic nerves and serves as a source of isoproterenol-sensitive focal automatic activity in normal canine atrium. The sensitivity to isoproterenol is upregulated after long-term rapid pacing and may contribute to the development of AF in this model.

Mechanism of procainamide-induced prevention of spontaneous wave break during ventricular fibrillation. Insight into the maintenance of fibrillation wave fronts

BACKGROUND

Ventricular fibrillation (VF) is maintained by 2 mechanisms: first by reentry formation and second by spontaneous wave break or wave splitting. We hypothesized that spontaneous wave break results from a critical shortening of the action potential duration (APD) during VF and that its prevention by procainamide eliminates spontaneous wave break.

METHODS AND RESULTS

The endocardial surfaces of 7 isolated, perfused swine right ventricles were mapped with a 3.2x3.8 cm plaque with 477 bipolar electrodes. Activation pattern during VF was visualized dynamically while simultaneously recording epicardial action potentials with a glass microelectrode. APD restitution curves were constructed during VF (dynamic) and during S(1)S(2) protocols. At baseline, VF was maintained by 5.3+/-1 wavelets. Procainamide (PA) at 10 microgram/mL decreased the number of wavelets to 3.5+/-1 (P<0.05). At baseline VF was maintained by spontaneous wave break and by new reentrant wave front formation. PA eliminated spontaneous wave break during VF while having no effect on reentry formation. PA increased the cycle length of the VF (148.5+/-41.2 ms vs 81+/-10 ms, P<0.01) and the core area of the reentry from 5.8 to 14.5 mm(2) (P<0.05). Dynamic APD restitution curve during VF showed that PA eliminated the initiation of activation with APDs shorter than 30 ms. The effects of PA on cellular properties and wave front dynamics were reversed during 60 minutes of drug-free perfusion.

CONCLUSIONS

Critically short APDs during VF promote spontaneous wave break. Their elimination with PA, however, maintains VF by generating new reentrant wave front.

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.

Spatiotemporal heterogeneity in the induction of ventricular fibrillation by rapid pacing: importance of cardiac restitution properties

The mechanism by which rapid pacing induces ventricular fibrillation (VF) is unclear. We performed computerized epicardial mapping studies in 10 dogs, using 19-beat pacing trains. The pacing interval (PI) of the first train was 300 ms and then was progressively shortened until VF was induced. For each PI, we constructed restitution curves for the effective refractory period (ERP). When the PI was long, the activation cycle length (CL) was constant throughout the mapped region. However, as the PI shortened, there was an increase in the spatiotemporal complexity of the CL variations and an increase in the slope of the ERP restitution curve. In 5 dogs, we documented the initiation of VF by wavebreak at the site of long-short CL variations. Computer simulation studies using the Luo-Rudy I ventricular action potential model in simulated 2-dimensional tissue reproduced the experimental results when normal ERP and conduction velocity (CV) restitution properties were intact. By altering CV and ERP restitutions in this model, we found that CV restitution creates spatial CL variations, whereas ERP restitution underlies temporal, beat-to-beat variations in refractoriness during rapid pacing. Together, the interaction of CV and ERP restitutions produces spatiotemporal oscillations in cardiac activation that increase in amplitude as the PI decreases, ultimately causing wavebreak at the site of intrinsic heterogeneity. This initial wavebreak then leads to the formation of spiral waves and VF. These findings support a key role for both CV and ERP restitutions in the initiation of VF by rapid pacing.

Chaos and the transition to ventricular fibrillation: a new approach to antiarrhythmic drug evaluation

Sudden cardiac death resulting from ventricular fibrillation can be separated into 2 components: initiation of tachycardia and degeneration of tachycardia to fibrillation. Clinical drug studies such as CAST and SWORD demonstrated that focusing exclusively on the first component is inadequate as a therapeutic modality. The hope for developing effective pharmacological therapy rests on a comprehensive understanding of the second component, the transition from tachycardia to fibrillation. We summarize evidence that the transition from tachycardia to fibrillation is a transition to spatiotemporal chaos, with similarities to the quasiperiodic transition to chaos seen in fluid turbulence. In this scenario, chaos results from the interaction of multiple causally independent oscillatory motions. Simulations in 2-dimensional cardiac tissue suggest that the destabilizing oscillatory motions during spiral-wave reentry arise from restitution properties of action potential duration and conduction velocity. The process of spiral-wave breakup in simulated cardiac tissue predicts remarkably well the sequence by which tachycardia degenerates to fibrillation in real cardiac tissue. Modifying action potential duration and conduction velocity restitution characteristics can prevent spiral-wave breakup in simulated cardiac tissue, suggesting that drugs with similar effects in real cardiac tissue may have antifibrillatory efficacy (the Restitution Hypothesis). If valid for the real heart, the Restitution Hypothesis will support a new paradigm for antiarrhythmic drug classification, incorporating an antifibrillatory profile based on effects on cardiac restitution and the traditional antitachycardia profile (classes 1 through 4).

Idiopathic paroxysmal atrial fibrillation induced by a focal discharge mechanism in the left superior pulmonary vein: possible roles of the ligament of Marshall

INTRODUCTION

The origin of double potentials inside the left superior pulmonary vein and their relation to the mechanisms of idiopathic paroxysmal focal atrial fibrillation (AF) are unclear.

METHODS AND RESULTS

A total of 40 patients were studied. Group I included 15 patients who underwent radiofrequency catheter ablation of accessory pathway. Double potentials were found inside the left superior pulmonary vein during sinus rhythm in 10 patients and during premature atrial contractions in the remaining five patients. Group II included 25 patients with idiopathic paroxysmal AF. Double potentials were also identified in the left superior pulmonary vein. In 15 patients (Group IIA), the earliest automatic discharge during premature atrial contractions and at the onset of AF was within the left superior pulmonary vein. AF was ablated by radiofrequency energy application at the site registering double potentials. Radiofrequency ablation in the remaining 10 patients failed to terminate AF (Group IIB). The patients in Group IIA had significantly more male patients and more frequent premature atrial contractions and atrial tachycardia on 24-hour Holter recordings prior to the procedure than patients in Group IIB.

CONCLUSIONS

Double potentials are present at the left superior pulmonary veins in patients with and without a history of AF. The first potential is due to the activation of atrial myocardium and the second is due to the activation of a different muscular structure. Rapid discharge of this structure triggers episodes of paroxysmal AF. Patients with focal AF originating from the left superior pulmonary vein can be identified by Holter recordings.

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.

Mechanism of acceleration of functional reentry in the ventricle: effects of ATP-sensitive potassium channel opener

BACKGROUND

The effect of effective refractory period (ERP) shortening on the vulnerability and characteristics of induced functional reentry in the ventricle remain poorly defined. We hypothesized that ERP shortening increases ventricular vulnerability to reentry and accelerates its rate, as is the case in the atrium.

METHODS AND RESULTS

The epicardial surfaces of 19 isolated and superfused canine right ventricular slices (4x4 cm and <2 mm thick) were mapped with 480 bipolar electrodes 1.6 mm apart. Vulnerability was tested during pacing at a cycle length (CL) of 600 ms and with a single premature stimulus of 5-ms duration at increasing current strength of 1 to 100 mA. Cromakalim (10 micromol/L), an ATP-sensitive potassium channel opener, caused a significant (P<0. 001) shortening of the ERP but had no effect on conduction velocity. Cromakalim increased (P<0.01) the vulnerability (product of current and the stimulus coupling interval) for reentry induction. Reentry had a significantly shorter CL and lasted for a longer duration (P<0. 001). The central core around which the wave front rotated became smaller, which caused shortening of the CL of reentry. A significant (P<0.001) linear correlation was found between core size and reentry CL. These effects of cromakalim were reversible. Two-dimensional simulation studies using the modified Luo-Rudy I model of cardiac action potential, in which the refractory period was variably shortened by a progressive increase of the time-independent potassium conductance, reproduced the experimental findings.

CONCLUSIONS

ERP shortening by an ATP-sensitive potassium channel opener increases ventricular vulnerability to reentry and accelerates its rate by decreasing the core size around which the wave front rotates.

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.

Transmembrane potential properties at the core of functional reentrant wave fronts in isolated canine right atria

BACKGROUND

The characteristics of transmembrane potential (TMP) at the core of functional reentry in the atrium are not well understood.

METHODS AND RESULTS

In protocol 1 (11 dogs), isolated perfused canine right atria were mapped from the endocardial surface while simultaneous TMPs were recorded from the epicardial surface. Episodes of reentry (n=64) were induced in the presence of 1 to 5 micromol/L acetylcholine. Successful simultaneous TMP recordings and activation maps were made in 8 episodes. The TMP was "near the core" if it was within 3.2 mm of the core; otherwise, it was considered to be "in the periphery." The mean cycle length of reentry was 110+/-35 ms. The TMP amplitude, duration (90% repolarization), and (dV/dt)max near the core (n=106) were 58+/-22 mV, 46+/-14 ms, and 33+/-20 V/s, respectively, significantly less than those in the periphery (n=241): 70+/-8 mV, 94+/-32 ms, and 55+/-10 V/s (P<0.001 for all). In 2 episodes of reentry, the cell at the core remained unexcited at its resting membrane potential. In protocol 2 (2 dogs), we performed simultaneous high-density mapping in 4 episodes of reentry and showed synchronous activation patterns on both surfaces with similar locations of the core.

CONCLUSIONS

During meandering functional reentry in isolated canine right atria, (1) TMPs of cells near the core have a reduced amplitude, duration, and (dV/dt)max, and (2) cells at the core may remain unexcited at their resting membrane potential. These findings are compatible with the spiral wave concept of functional reentry in the atrium.

Role of pectinate muscle bundles in the generation and maintenance of intra-atrial reentry: potential implications for the mechanism of conversion between atrial fibrillation and atrial flutter

To determine the role of pectinate muscle (PM) bundles in the formation of intra-atrial reentry, 10 isolated canine right atrial tissues were perfused with Tyrode's solution containing 1 to 2.5 micromol/L acetylcholine (ACh). The endocardium was mapped using 477 bipolar electrodes with 1.6-mm resolution. Reentry was induced by a premature stimulus (S2). Computer simulation studies were used to investigate the importance of regional myocardial thickness in reentry formation. A total of 40 episodes of reentry were induced; 28 episodes were stationary, and the remaining 12 were nonstationary. The stationary reentry was induced either immediately after the S2 stimuli (n=9) or after an initial period of irregular activations that lasted 1460+/-1077 ms (n= 19). Of 28 episodes, 20 were initiated by conduction block along large PM ridges, leading to wave break and the initiation of reentry. The reentrant wave fronts remained stationary and rotated around these ridges as anchoring sites. During the transition from the initial irregular activations to stationary reentry, the electrogram morphology converted from "fibrillation-like" to "flutter-like" activity. In 8 episodes, initially stationary reentry converted to irregular activations because of interference with outside wave fronts (n=5) or spontaneous separation of waves from the ridges (n=3). Compared with stationary reentry, nonstationary reentry always occurred over an area without large PMs, and the mean life span was much shorter (102+/-151 versus 3.8+/-1.1 rotations, P<0.001). Computer simulation studies showed that a critical ridge thickness is needed for reentry to anchor, thereby converting fibrillation to flutter. We conclude that PM ridge forms an area where wave break occurs, allowing the initiation of reentry. It also provides a natural anchor to the reentrant wave front, lengthening the life span of reentry. The attachment and detachment of the reentrant wave front to and from the ridge determine "flutter-like" or "fibrillation-like" activity.

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.

Effects of procainamide on wave-front dynamics during ventricular fibrillation in open-chest dogs

BACKGROUND

There is increasing evidence that both functional reentrant wave fronts and multiple wavelets are present during ventricular fibrillation (VF). However, the effects of procainamide on the characteristics of activation waves during VF are poorly understood.

METHODS AND RESULTS

Seven dogs were studied; six underwent subendocardial chemical ablation procedures. A plaque with 317 to 480 bipolar electrodes was sutured to the right ventricular free wall, and the patterns of activation were registered with a computerized mapping system. VF was electrically induced, and the patterns of activation were registered at baseline and during procainamide infusion (serum concentration, 9.3+/-1.9 microg/mL). Among the six dogs that had their subendocardium ablated, reentrant wave fronts were present in 6 of the 108 runs of VF at baseline and in 6 of the 100 runs of VF during procainamide infusion. By analyzing the wave fronts, we found that the cycle length, refractory period, conduction velocity, and wavelength at baseline were 101+/-9 ms, 54+/-5 ms, 0.93+/-0.21 mm/ms, and 51+/-16 mm, respectively, and during procainamide infusion, values became 125+/-11 ms (P<.001), 119+/-7 ms (P<.001), 0.42+/-0.02 mm/ms (P<.001), and 50+/-4 mm (P=.8), respectively. The vast majority of the activation waves do not form organized reentry. These activation waves broke up more frequently at baseline than during procainamide administration. The number of activation waves was 7.25+/-1.39 s(-1) x cm(-2) at baseline and 4.45+/-1.80 s(-1) x cm(-2) during procainamide administration (P<.001). The dog without subendocardial ablation had similar results.

CONCLUSIONS

Procainamide decreases the number of wavelets during VF by preventing spontaneous wave breaks. This represents a novel mechanism of antiarrhythmic drug action.

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.

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.

Induction of meandering functional reentrant wave front in isolated human atrial tissues

BACKGROUND

The purpose of this study was to test the hypothesis that a single meandering functional reentrant wave front can result in rapid and irregular electrogram activity in human atrial tissues.

METHODS AND RESULTS

The study used the explanted hearts of five human cardiac transplant recipients. Three right and two left atrial tissue samples, 3.4+/-0.3 mm thick, were excised and trimmed to 3.5x3.0 cm. The isolated atrium was placed endocardial surface down in a chamber with a 477 bipolar recording electrode array built into the bottom of the tissue bath. The interelectrode distance was 1.6 mm. The tissue was constantly superfused with 36.5 degrees C oxygenated Tyrode's solution at a rate of 10 mL/min. After eight baseline stimuli (S1) delivered at 400- or 600-ms cycle length from the edge of the tissue, a single premature stimulus (S2) was given at the center of the tissue to induce reentry. A total of nine episodes of reentry were induced with S1-S2 coupling intervals of 232+/-29 ms (range, 190 to 290 ms) and an S2 strength of 10+/-3 mA (range, 5 to 15 mA). In all samples, a single meandering reentrant wave front was induced, causing irregular and rapid bipolar electrogram activity. These wave fronts had a mean cycle length of 229+/-45 ms (160 to 290 ms) and persisted for 1.1+/-0.3 seconds (0.6 seconds to 2.5 seconds), or 5.2+/-1.4 (3 to 9) cycles, before spontaneous termination.

CONCLUSIONS

A single meandering functional reentrant wave front can be induced in human atrial tissues and produce rapid and irregular electrical activity.

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.

Importance of location and timing of electrical stimuli in terminating sustained functional reentry in isolated swine ventricular tissues: evidence in support of a small reentrant circuit

BACKGROUND

In excitable chemical media, a spiral wave is formed by reentrant excitation around a core and normal propagation away from the core. Whether or not this applies to cardiac muscle is unknown.

METHODS AND RESULTS

In six isolated swine ventricular slices, we induced sustained episodes of functional reentry with a stationary core. A train of stimuli applied away from the core (7- to 8-mm distance) and near the core (within 1.6 mm) terminated 5 of 24 and 14 of 17 episodes of reentry, respectively (P<.001). When the stimulus was applied away from the core, successful terminations occurred when the line connecting the stimulus and the core was along the myocardial fiber orientation and when the coupling interval was 54+/-11% of the reentrant cycle length. Stimulation near the core terminated reentry primarily by propagation of the stimulus-induced wave fronts that closed up the excitable gap. However, in two episodes, the application of a stimulus near the core changed the electrogram morphology in only four bipolar pairs. This was sufficient to cause abrupt termination of reentry.

CONCLUSIONS

(1) A thin layer of activation near the core is responsible for the maintenance of functional reentry. (2) Access to the tissue near the core is essential for the termination of functional reentry by a point stimulus. (3) To terminate reentry with a stimulus away from the core, the stimulus must occur at certain critical coupling intervals, and the line connecting the stimulus and the core must be roughly parallel to the fiber orientation.

Meandering and unstable reentrant wave fronts induced by acetylcholine in isolated canine right atrium

The mechanism(s) by which acetylcholine (ACh) increases atrial vulnerability to reentry and maintains its activity for longer durations remains poorly defined. In the present study we used high-resolution activation maps to test the hypothesis that ACh promotes meandering of atrial reentrant wave fronts, resulting in breakup and the generation of new wave fronts that sustain the activity. Reentry was induced in 11 isolated canine right atrial tissues (3.8 x 3.2 cm) by a premature point stimulus (S2) before and after superfusion with ACh (15 x 10(-6) M). Endocardial isochronal activation maps were constructed with the use of 509 bipolar electrodes (1.6-mm spatial resolution), and the dynamics of the activation wave fronts were visualized with animation. A vulnerable period was found during which an S2 current strength > 4.4 +/- 2.5 mA [lower limit of vulnerability (LLV)] and < 26 +/- 13 mA [upper limit of vulnerability (ULV)] induced a single stationary reentrant wave front that lasted 3 +/- 2.5 s with a period of 159 +/- 17 ms (16 episodes). AC shortened the refractory period from 100 +/- 12 to 59 +/- 9 ms (P < 0.001) and increased vulnerability to reentry induction by simultaneous decrease in the LLV (0.7 +/- 0.2 mA, P < 0.001) and an increase in the ULV (82 +/- 24 mA, P < 0.01). ACh accelerated the rate (period of 110 +/- 16 ms, P < 0.001) and converted the stationary reentrant wave front to a nonstationary (meandering) reentrant wave front showing polymorphic electrograms, i.e., "fibrillation-like" activity (22 episodes). Rapid meandering of the reentry tip led to wave front breakup (18 episodes) and the generation of new wave fronts (19 episodes). These wave front dynamics also led to sustained (76 +/- 224 s, P < 0.001) fibrillation-like electrograms. We conclude that ACh increases the ULV and promotes meandering of a single reentrant wave front, leading to breakup and the generation of new wave fronts. Single meandering and complex wave front dynamics cause fibrillation-like activity and sustain the activity for longer duration.

Cellular graded responses and ventricular vulnerability to reentry by a premature stimulus in isolated canine ventricle

BACKGROUND

The cellular mechanism by which a point strong premature stimulus (S2) induces reentry is unknown. We hypothesized that cellular graded responses induced by an S2 mediate and control tissue vulnerability to reentry.

METHODS AND RESULTS

Reentry is induced in normal canine ventricular epicardial slices (30x38x2 mm, n=30) by an S2 at intervals shorter than the effective refractory period. The S1 is applied at the edge and the S2 at the center of the tissue. The line connecting the S1-S2 sites is parallel to the long axis of the fiber orientation. Isochronal activation maps were constructed with 56 to 480 bipolar electrodes, and the activation pattern was visualized dynamically. Reentry induced by an S2 is mediated by the graded responses as follows: The induced graded responses propagate with decrement toward recovered cells. When the amplitude of the propagated depolarizing graded responses reaches threshold relative to the recovering cells, an action potential is initiated along the fiber 2 to 3 mm away from the cathode of the S2. The distally initiated activation wave front blocks near the S2 site because the same S2-induced graded response prolongs the refractory period. The "broken" wave front then circulates around both sides of the block and reenters when the site of block recovers its excitability, completing the first figure-eight reentry cycle. Reentry cannot be induced when the S2 strength is >72+/-21 mA (upper limit of vulnerability) because these strong S2-induced graded responses convert the unidirectional block to bidirectional block by excess prolongation of the refractoriness.

CONCLUSIONS

We conclude that the magnitude and the propagation of S2-induced cellular graded responses mediate and control vulnerability to reentry in the ventricular epicardium.