Entrainment of ventricular tachycardia

Etiology and management of sustained ventricular tachycardia

Ventricular tachyarrhythmias secondary to a variety of underlying cardiovascular problems pose a therapeutic challenge to the clinician. The initial presentation may be as sudden cardiac death, which underlies its public health problem. The underlying conditions predisposing to this arrhythmia include ischemic heart disease, dilated cardiomyopathy, hypertrophic cardiomyopathy, arrhythmiogenic right ventricle dysplasia and certain postoperative states including corrective surgery for tetralogy of Fallot and valve replacement. Other causes include prolonged QT syndrome, idiopathic right and left ventricle tachycardia and bundle branch re-entry tachycardia. Ischaemic heart disease is the most common cause of ventricular tachycardia and therapy has evolved considerably over the past two decades. The development of and refinements in the implantable cardioverter-defibrillator (ICD) have introduced a new dimension in therapeutic options and markedly improved survival in these patients. Insights in the dichotomy between arrhythmia suppression and total mortality have reoriented drug therapy with a decrease in the use of sodium channel blockers. beta-blockers have emerged as antiarrhythmic drugs in their own right and their synergistic effects with amiodarone have strengthened the antiarrhythmic drug arm. The role of these drugs in patients with hemodynamically stable ventricular tachycardia, especially in relatively preserved ventricles needs to be explored. Catheter ablation techniques have provided curative therapy in patients with idiopathic and bundle branch reentry tachycardia. Further advances in radiofrequency ablation, including use of newer mapping techniques, promise a greater role for ablation of ischemic ventricular tachycardia in the future. A hybrid approach consisting of drugs, catheter ablation and/ or ICD may provide effective therapeutic approach in some situations. Further innovations and technologic developments promise a further reorientation in therapy towards identification and treatment of the underlying arrhythmogenic substrate.

Ablation of ventricular tachycardia in the setting of coronary artery disease

Ablation of reentrant ventricular tachycardia (VT) is an accepted therapy for certain patients with VT caused by coronary artery disease (CAD). Its use is currently limited to patients with sustained, monomorphic, hemodynamically tolerated VT. The use of entrainment in mapping reentrant VT has made possible increasingly accurate localization of critical sites on the reentrant pathway that are amenable to ablation. Recent work has examined the accuracy with which various mapping criteria are able to predict successful ablation of reentrant VT in patients with CAD. Other recent studies have investigated attempted ablation of all inducible VTs in patients with multiple VT morphologies. In the future, substrate mapping may make possible ablation of VT in patients with nonsustained or fast, hemodynamically unstable VTs, thus allowing VT ablation to become a first-line therapy for many patients with VT in the setting of CAD.

First postpacing interval variability during right ventricular stimulation: a single algorithm for the differential diagnosis of regular tachycardias

BACKGROUND

Failure to differentiate supraventricular from ventricular arrhythmias is the most frequent cause of inappropriate implantable cardioverter-defibrillator therapies. Although a sudden-onset criterion is available to differentiate sustained monomorphic ventricular tachycardias (SMVTs) and sinus tachycardias (STs), SMVTs arising during ST and SMVTs gradually accelerating above the cutoff rate can remain undetected. Regular paroxysmal atrial tachycardias (ATs) also can be undetected by onset and stability algorithms. We hypothesized that the first postpacing interval (FPPI) variability after overdrive right ventricular pacing may differentiate SMVTs from STs and ATs.

METHODS AND RESULTS

FPPI variability was measured in 23 SMVTs (cycle length [CL] 366+/-50 ms [VT group]), 27 supraventricular tachycardias, 15 episodes of induced or simulated ATs (CL 376+/-29 ms [AT group]), and 12 exercise-related STs (CL 381+/-24 [ST group]). Sequences of trains of 5, 10, and 15 beats were delivered with a CL 40 ms shorter than the tachycardia CL. An FPPI absolute mean difference between consecutive trains of 5 and 10 beats (deltaFPPI) < or =25 ms identified all VTs (mean difference 5+/-7 ms). In the AT group, the deltaFPPI was >25 ms in all sequences (mean difference 129+/-60 ms, P<0.01). In the ST group, the deltaFPPI was >50 ms in all STs (mean difference 118+/-47 ms, P<0.01).

CONCLUSIONS

FPPI variability may differentiate SMVT from AT and ST. This criterion is potentially useful in implantable devices that use a single ventricular lead.

Acceleration of typical atrial flutter due to double-wave reentry induced by programmed electrical stimulation

BACKGROUND

Acceleration of reentrant tachycardia induced by programmed electrical stimulation is a well-documented phenomenon, but the mechanisms remain poorly understood.

METHODS AND RESULTS

Twelve patients with typical atrial flutter were studied. Activation sequence of the underlying reentrant circuit was recorded by multiple multipolar electrodes placed in the right atrium. In five patients, 27 episodes of atrial flutter acceleration were induced by single extrastimuli delivered in the isthmus between the tricuspid annulus and eustachian ridge (TA-ER isthmus) and one by rapid overdrive atrial pacing. Analyses of the activation sequences, intracardiac electrograms, and 12-lead surface ECG P-wave morphology indicated that the acceleration was caused by two successive activation wave fronts circulating in the same direction along the same reentrant circuit (double-wave reentry, DWR). DWR was induced only within a narrow range of coupling interval, from 2 to 45 ms beyond the effective refractory period, and was associated with unidirectional antidromic block of the paced impulse. Patients with DWR had a shorter effective refractory period (138.8+/-13.4 versus 163.8+/-12.2 ms, P<.015) and larger excitable gap (124.0+/-22.6 versus 83.2+/-13.2 ms, P<.009) compared with patients without inducible DWR. All of the DWR episodes were transient. Most (78.6%) terminated after one of the double wave fronts was blocked in the TA-ER isthmus.

CONCLUSIONS

DWR is one of the mechanisms responsible for programmed electrical stimulation-induced atrial flutter acceleration in human subjects. Its induction requires a sufficient excitable gap and antidromic unidirectional block of the paced impulse in the TA-ER isthmus. In addition, the TA-ER isthmus is the usual site of DWR termination.

Analysis of the degree of QRS fusion necessary for its visual detection: importance for the recognition of transient entrainment

BACKGROUND

Fixed fusion is the hallmark for the demonstration of transient entrainment. However, the degree of accuracy of its recognition on the surface ECG is unknown. The purpose of the present study was to evaluate the ability to detect fusion in the QRS complex.

METHODS AND RESULTS

While pacing the ventricles at a fixed rate, a model of ventricular fusion was created by introducing late extra stimuli at a second site. In this model, the presence and degree of fusion are known. Pacing sites were the RV apex, outflow tract, and left ventricle in various configurations. We analyzed 433 QRS complexes with different degrees of fusion (or no fusion) in 21 patients. Each QRS was "read" by three investigators blinded to intracardiac recordings but having a reference QRS with no fusion. There was a statistically significant correlation between the degree of fusion and its recognition. Fusion was detected with a sensitivity of 75% and a specificity of 87%. Fusion was accurately detected in all configurations only when >22% of the QRS was fused. In patients with organic left ventricular disease, fusion was better recognized when the driving pacing site was the left ventricle than when it was a right ventricular site. The interobserver agreement was moderate between two pairs of observers and only fair between the remaining pair.

CONCLUSIONS

Our results suggest that an accurate detection of ventricular fusion can only be accomplished when fusion occurs during a significant proportion of the QRS duration. The potential lack of recognition of minor degrees of fusion may produce underdetection of transient entrainment.

Atrial flutter: entrainment characteristics

Entrainment was first described based on observations during rapid (overdrive) pacing of type I atrial flutter. Entrainment is capture of the reentrant circuit of a tachycardia without interrupting the tachycardia, so that with cessation of pacing, the spontaneous reentrant tachycardia is still present. During entrainment, the orthodromic wavefront from the pacing impulse resets the tachycardia to the pacing rate, while the antidromic wavefront either collides with the orthodromic wavefront of the previous beat (usual case) or is blocked by some other mechanism (refractoriness or another cause of block). Entrainment may be either manifest or concealed. The principles of entrainment during type I atrial flutter have permitted identification of targets for successful ablation, of mapping sites within or outside the reentrant circuit, and of appropriate pacing rates to successfully interrupt atrial flutter and restore sinus rhythm.

Transient entrainment: the evolution of a medical concept from description to prescription

Entrainment is a phenomenon that has come to have considerable utility in cardiac electro-physiology diagnosis and treatment; specifically, to identify a zone of slow conduction in a reentrant circuit, a zone hypothetically vulnerable to intervention from the application of RF energy. The observation of entrainment has gone through an evolutionary sequence in the literature, from the initial simple observations of the phenomenon to the present stage of relatively fixed criteria of identification. This article follows the evolution of the specific features of the criteria of entrainment to their current crystallization into features that are suggested to prescribe sites for attempted ablation. This examination of the evolutionary course of the development of the conception of entrainment is of interest not only to cardiac electrophysiology, but also to philosophers of science, by illustrating how scientists emphasize and develop certain observations with the ultimate aim of applying the observations for successful intervention in pathological entities.

Spontaneous sustained ventricular tachycardia in the Electrophysiologic Study Versus Electrocardiographic Monitoring (ESVEM) Trial

OBJECTIVES

We compared the QRS waveforms of the initial and subsequent complexes of spontaneous sustained monomorphic ventricular tachycardia and the rhythm induced at electrophysiologic study to test the theory that premature ventricular complexes "trigger" spontaneous ventricular tachycardia and that a stable substrate exists such that the spontaneous arrhythmia can be reproduced at electrophysiologic study.

BACKGROUND

Failure rates have been high in several recent studies in which prevention of ventricular tachyarrhythmias was guided by suppression of premature ventricular complexes or induced ventricular tachycardias.

METHODS

Digital waveform analysis was used to distinguish events of ventricular tachycardia initiated by configurationally distinct, possibly triggering, complexes (type 1) from events in which the initial QRS waveforms were identical to subsequent complexes, suggesting no requirement for premature ventricular beats (type 2).

RESULTS

Of 1,102 episodes of spontaneous ventricular tachycardia, 73 (6.6%) were type 1; 1,012 were type 2 (91.8%); and 17 (1.5%) were uncertain. Of 59 patients only 14 (24%) had only type 1 episodes (group 1), whereas 37 patients (63%) had predominantly type 2 events (group 2) (p < 0.0001). Sustained ventricular tachycardia was inducible in all group 1 patients, and in most (57%) the induced rhythm was similar to the spontaneous rhythm. Ventricular tachycardia could not be induced in 7 patients from group 2 (19%), and in 18 patients (49%) the induced and spontaneous rhythms were dissimilar. Recurrence of arrhythmia rates differed according to the guidance method in group 2.

CONCLUSIONS

Discrepancies between observed and predicted modes of initiation of ventricular tachycardia and between spontaneous and induced rhythms could result in inappropriate guidance and subsequent failure of antiarrhythmic treatment.

Mechanisms in the interruption of reentrant tachycardia by pacing

The mechanisms by which pacing interrupts reentrant tachycardia associated with a structural obstacle were investigated using a computer model of propagated excitation. The model simulated cycle length-dependent refractoriness and slow propagation during incomplete recovery of excitability. Previously established features of the mechanism consisting of collision of reentrant with paced antidromic propagation and block of orthodromic propagation were demonstrated in the model, and factors affecting the mechanism were defined. Arrival time of paced orthodromic excitation at a potential block site and the duration of refractoriness at that site were major factors. Arrival time was determined by pacing stimulus time and propagation velocity. Slow propagation of a particular response acted to prevent the required block during that response, but enhanced the likelihood of a block of a subsequent response by affects on the onset time and duration of refractoriness at the block site at fast rates. In some conditions, responses to later stimuli resulted in block and interruption of tachycardia, while earlier stimuli with slower propagation during the same cycle failed to. Tachycardia rate affected its interruption by pacing by means of the shorter refractory period of the potential block site at fast rates, so that a paced response with a particular arrival time might fail to block. A greater number of successive paced responses were then required to terminate rapid tachycardia.

Characterization of return cycle responses predictive of successful pacing-mediated termination of ventricular tachycardia

OBJECTIVES

The purpose of this study was to characterize response patterns during overdrive pacing that predict successful termination of ventricular tachycardia.

BACKGROUND

Overdrive pacing during ventricular tachycardia typically results in entrainment at slow pacing rates and in termination or acceleration at faster rates. The factors that determine the critical paced cycle length that results in tachycardia termination have not been extensively studied.

METHODS

Ventricular tachycardias in 14 patients with coronary artery disease were studied with overdrive pacing at several cycle lengths. Return cycles were measured after each additional paced beat at each paced cycle length. The return cycle responses during pacing trials that resulted in tachycardia termination and those that resulted in entrainment were compared.

RESULTS

Three return cycle responses were identified: flat, plateau and increasing. Twenty trials of overdrive pacing resulted in tachycardia termination; all were characterized by an increase in the return cycle with the delivery of each successive beat in the pacing drive until the tachycardia terminated (increasing response). Thirty-four pacing trials resulted in entrainment and not termination; these were characterized either by a constant return cycle (flat response) or an initial increase in return cycle followed by a longer, constant return cycle (plateau response) with the delivery of additional paced beats. The longest paced cycle length that resulted in tachycardia termination correlated with the relative refractory period of the circuit, defined as the tachycardia cycle length minus the fully excitable gap (r2 = 0.764, p = 0.0001). Tachycardia termination was not observed unless the paced cycle length was shorter than the relative refractory period of the circuit.

CONCLUSIONS

The critical paced cycle length that causes termination of ventricular tachycardia depends on the relative refractory period of the circuit because this factor determines whether the nth + 1 beat of the pacing drive will encounter partially recovered tissue. These data provide insights into the mechanism of pacing-mediated tachycardia termination and entrainment and are applicable to the development of improved antitachycardia pacing algorithms.

Insights into the electrophysiology of ventricular tachycardia gained by the catheter ablation experience: "learning while burning"

The success of catheter ablation has significantly improved the treatment of patients with cardiac arrhythmias and has established electrophysiology as an increasingly interventional subspecialty. Some members of the electrophysiology community have expressed concern that this success has been purchased at the cost of undermining what had been our primary concern: understanding the anatomic and physiologic basis of arrhythmia syndromes. In many laboratories, endpoints such as case load and primary success have eclipsed physiologic investigation. Despite these trends, however, catheter ablation is not inherently at odds with investigation and education. On the contrary, because the lesions delivered with current techniques are much more discrete than the effects of antiarrhythmic agents or surgical ablation, catheter ablation can be used as a research tool directed toward a more precise understanding of arrhythmia substrates. Conscious attempts at "learning while burning" have already provided important and unique information about arrhythmia pathogenesis.

Entrainment of reentrant tachycardia in a computer model

Capture of the cardiac rate by pacing followed by an immediate return to the original rate after pacing has been proposed as characteristic of reentrant rhythms. In this study, such entrainment has been demonstrated using computer-model simulations of propagated excitation and of reentry associated with structural and functional obstacles. With structural obstacles, the mechanism of entrainment was bidirectional propagation of paced excitation in reentry circuits, with collision of the reentrant and paced excitation in one direction and continued propagation of paced excitation in the other direction. The time of pacing onset, rate, and location all affected the QRS waveform during entrainment. With a particular time of onset and rate of pacing, the duration of time during which the QRS waveform underwent dynamic change was directly related to the distance between the pacing site and reentrant circuit. The location of reentry associated with functional obstacles moved so that the relationship between pacing-induced and reentrant excitation varied. In some cycles, pacing did not alter reentrant circuits, that is, entrainment did not occur, while other cycles were entrained, but by a different mechanism than that with structural obstacles. Leading circle reentry circuits, consisting of propagation away from and returning to reentry sites, did not have an excitable gap and paced excitation did not enter those circuits. Paced excitation did, however, enter the propagation paths between leading circle reentry circuits and modified the circuits by affecting the recovery of excitability.

Conductive property of the zone of slow conduction of reentrant ventricular tachycardia and its relation to pacing induced terminability

In order to assess the functional characteristics of the zone of slow conduction of reentrant VT, rapid pacing was performed to entrain VT. The orthodromic conduction time was measured as the interval between the stimulus and the orthodromically captured electrogram recorded distal to the zone of slow conduction, but not precisely at the exit point, and its response to rapid pacing was evaluated. In 32 of 33 consecutive patients, rapid pacing was performed to entrain VT. Of these, rapid pacing was repeated in 28 patients at 3-10 cycle lengths in steps of 10 msec before VT was terminated, or rapid pacing produced an acceleration of the rate. A pacing induced prolongation of the orthodromic conduction time (slowed conduction) was observed in 16 (57.1%) patients and in another 12 (42.9%) patients, the conduction time was constant. The pacing induced termination was observed in 93.8% of VT with slowed conduction and in 50% of VT with constant conduction, and the difference was significant (P < 0.05). There was no difference in the cycle length of VT or the shortest paced cycle length between VT with and without slowed conduction. The zone of slow conduction in human VT showed different conductive properties and VT with slowed conduction was associated with an easier and safer terminability with rapid pacing. The fact might be useful in selecting patients for antitachycardia pacing.

Frequency of presumed reentry with an excitable gap in sustained ventricular tachycardia unassociated with coronary artery disease

In sustained ventricular tachycardia (VT) unrelated to coronary artery disease, the incidence of reentry with an excitable gap was examined, and rapid pacing was performed to entrain VT in 48 episodes in 42 consecutive patients. Coronary artery disease was excluded by coronary arteriography. The underlying heart diseases were postoperative congenital heart diseases (n = 5), dilated (n = 7) or hypertrophic (n = 4) cardiomyopathy, arrhythmogenic right ventricular dysplasia (n = 6) and miscellaneous heart diseases (n = 5), as well as no demonstrable heart disease (n = 15) in which 8 patients had verapamil-responsive VT. Except for 1 patient with hypertrophic cardiomyopathy, 48 morphologically distinct monomorphic sustained VTs were induced. Twenty-five VTs showed right bundle branch block morphology and 23 left bundle branch block morphology, and VT was entrained in 84 and 96%, respectively. The overall incidence of the entrainment was 89.6% (43 of 48 monomorphic VTs), and the frequency of the ability to entrain VT ranged between 33.3 and 100% in the subgroups. The lowest frequency was found in hypertrophic cardiomyopathy. In conclusion, most inducible monomorphic sustained VT unassociated with coronary artery disease was presumed to be reentry with an excitable gap.

Comparison of resetting and entrainment of uniform sustained ventricular tachycardia. Further insights into the characteristics of the excitable gap

BACKGROUND

Resetting and entrainment have both been used to characterize the electrophysiological properties of the reentrant circuit in ventricular tachycardia. Several entrainment studies have suggested that the circuit has decremental properties, because the return cycle increases at faster pacing rates. Resetting, however, demonstrates a fully excitable gap in the majority of tachycardias.

METHODS AND RESULTS

The response to resetting and overdrive pacing was analyzed in 18 ventricular tachycardias. Resetting demonstrated some duration of a fully excitable gap in 14 of 18 tachycardias. Overdrive pacing was performed at several cycle lengths with an incremental number of stimuli (1-15 beats) such that the first beat that interacted with the tachycardia (the nth beat) could be identified. The return cycles measured during resetting and the nth beat of pacing were identical (r = 0.99). At relatively long paced cycle lengths, paced beats after the nth beat resulted in a constant return cycle in most tachycardias with a fully excitable gap. At rapid paced cycle lengths, an increase in the return cycle from the nth to the nth + 1 beat was associated with progressive prolongation in the return cycle with each incremental paced beat until a longer equilibrium return cycle was reached or the tachycardia terminated in response to pacing.

CONCLUSIONS

We propose that the responses to resetting and overdrive pacing with or without entrainment appear to provide conflicting information about the characteristics of the circuit because they in fact measure entirely different electrophysiological parameters. The nth beat of pacing foreshortens the excitable gap to the extent that it arrives prematurely. Subsequent paced beats interact with an altered tachycardia circuit that has had less time to recover excitability. Resetting is the interaction of a single paced beat with the tachycardia and, as such, provides a more accurate assessment of the characteristics of the unaltered tachycardia circuit.

Overdrive stimulation of functional reentrant circuits causing ventricular tachycardia in the infarcted canine heart. Resetting and entrainment

BACKGROUND

Clinical electrophysiology studies have used, for the most part, models of anatomic reentrant circuits to explain entrainment of ventricular tachycardia. Our studies use activation maps to directly determine mechanisms of entrainment of functional circuits that cause tachycardia.

METHODS AND RESULTS

Electrograms were recorded from 192 sites on reentrant circuits in the epicardial border zone of canine myocardial infarcts during sustained ventricular tachycardia. Overdrive stimulation from different sites and at different cycle lengths was investigated. The reentrant circuits were shown to be functional, yet stimulated impulses could enter and repetitively reset the circuits (entrainment), demonstrating the presence of an excitable gap. Entrainment could occur from different stimulation sites with the stimulated impulses from each site activating the circuit with a different pattern. Entrainment, however, did not occur when the stimulated wave fronts obliterated the lines of functional block in the circuit. Fusion on the ECG occurred during entrainment when the stimulated impulses activated the ventricles concurrently with a previous stimulated impulse leaving the reentrant circuit at a different site. The first postpacing QRS was captured but not fused because it was caused by the last stimulated impulse emerging from the circuit. The first postpacing cycle length on the ECG was either equal to or longer than the overdrive cycle length depending on whether there was a fusion QRS during overdrive. The first postpacing cycle length at sites in the reentrant circuit equaled the pacing cycle length. At an appropriately short overdrive cycle length, stimulated impulses blocked in the circuit to terminate reentry.

CONCLUSIONS

Functional reentrant circuits causing ventricular tachycardia can be reset and entrained. Activation maps directly show the mechanisms.

Factors for transient entrainment of ventricular tachycardias by rapid atrial pacing

Thirteen patients with sustained ventricular tachycardia (VT) were studied to elucidate predisposing factors for the development of constant and progressive fusion by rapid atrial pacing. All patients demonstrated transient entrainment by rapid ventricular pacing during VT. Constant and progressive fusion were observed in 7 patients (positive group) during rapid atrial pacing, but not in 6 (negative group). In the positive group, VT was induced by atrial pacing in 2 patients. The demonstration of constant and progressive fusion by atrial pacing was not dependent on QRS morphology or ventriculoatrial conduction during VT. VT cycle length in the positive group (363 +/- 59 ms) was longer than in the negative group (297 +/- 31 ms; p = 0.033). The maximal atrial pacing rate producing 1:1 atrioventricular (AV) conduction in the positive group was 171 +/- 18 beats/min compared with 125 +/- 22 beats/min in the negative group (p = 0.002). There were distinct differences between the positive and negative groups in the ratio of VT cycle length to minimal atrial cycle length causing 1:1 AV conduction (1.02 +/- 0.12 vs 0.61 +/- 0.12; p = 0.0001). It is concluded that AV conduction, VT cycle length and especially their ratio are important factors for the development of transient entrainment by rapid atrial pacing during VT. Therefore, atrial pacing can be used as an easy and useful method to examine transient entrainment during VT.

Localization of breakthrough site of canine monomorphic ventricular tachycardia by pacemapping. A vectorial approach

BACKGROUND

The precision and limitations of ventricular pacemapping as a method to localize the site of earliest breakthrough of ventricular tachycardia (VT) were investigated in a canine model of experimental myocardial infarction.

METHODS AND RESULTS

Forty-one episodes of VT induced in 10 animals were mapped using a standard grid of 64 endocardial and epicardial bipolar electrodes to determine the site of earliest endocardial or epicardial breakthrough of activation during VT. Each of these 64 recording sites was also used for ventricular pacing during sinus rhythm at cycle lengths comparable to those of the VTs. Orthogonal X, Y, and Z Frank electrocardiographic (ECG) leads were recorded during all episodes of VT and ventricular pacing from all sites after the chest was closed in all animals. Surface ECG waveforms corresponding to each VT and each ventricular pacing were compared pairwise by measuring the Euclidean metric difference between the VT and ventricular pacing vectors with the orthogonal ECG leads as their X, Y, and Z components. The pacing site that generated the vector most similar to VT vector (smallest vectorial difference) was defined as the predicted breakthrough site. This predicted site of breakthrough was identical to the actual site of breakthrough determined by activation sequence mapping during VT for only nine VTs (22%). However, for an additional 27 VTs (66%), the observed and predicted breakthrough locations were at adjacent (1 cm or less apart) recording sites. For five VTs (12%), the two sites were remote, the distance between them exceeding 1 cm.

CONCLUSIONS

In this model, locating the breakthrough site by pacemapping is exact in only a small minority of VTs. However, when orthogonal surface ECG leads are used for comparison, pacemapping can predict the site of earliest breakthrough during VT with a 1-cm resolution in the majority of VTs.

Termination of ventricular tachycardia by nonpropagated local depolarization: further observations on entrainment of ventricular tachycardia from an area of slow conduction

A 60-year-old woman with a large left ventricular apical aneurysm underwent preoperative catheter mapping of ventricular tachycardia. A zone of slow conduction with marked decremental conductive properties was identified between the left ventricular aneurysmal pouch and the right ventricular septum. Pacing from the right ventricular septum produced a QRS on the surface electrocardiogram of the same morphology as that of spontaneous ventricular tachycardia, while pacing from the left ventricular aneurysm caused tachycardia entrainment without fusion. Termination of ventricular tachycardia invariably occurred in association with an unpropagated left ventricular capture, followed by a change in ventricular activation to an opposite direction. This case provides a direct demonstration of reentrant ventricular tachycardia termination by orthodromic block in a zone of slow conduction.

Comparison of QRS morphologies of spontaneous premature ventricular complexes and ventricular tachycardia induced by programmed stimulation

We addressed the hypothesis that premature ventricular complexes (PVCs) and sustained ventricular tachycardia (VT) have identical QRS morphologies in 20 patients with recurrent sustained VT. Continuous six-lead ECGs of PVCs and sustained VT induced with programmed stimulation were recorded. A computer program divided the PVCs and VT beats of each patient into distinct morphologic groups and a representative waveform was obtained by averaging the PVC and VT beats of the group members. A correlation coefficient was then derived between the QRS complexes of each PVC and VT morphology. The mean number of PVC morphologies was 12 +/- 8 per patient (range 1 to 26), the mean number of VT morphologies was 2 +/- 1 per patient (range 1 to 5). The correlation coefficient between the dominant PVC morphology and a VT morphology was greater than 0.7 in only three patients. The combined percent contribution of all PVCs with morphologies that were at least possibly identical to those of VT averaged only 13%. Thus PVCs with morphologies identical to those of VT are present in some patients with sustained VT, but these constitute a small proportion of all PVCs.

Characterization of double potentials in human atrial flutter: studies during transient entrainment

Double potentials, defined as atrial electrograms with two discrete deflections per beat separated by an isoelectric interval or a low amplitude baseline, have been observed during right atrial endocardial mapping of human atrial flutter. In this study, bipolar atrial electrograms were recorded during atrial flutter (mean cycle length 235 +/- 27 ms [+/- SEM]) from the high right atrium, the His bundle region, the coronary sinus and at least 30 right atrial endocardial mapping sites in 10 patients. Double potentials were recorded from the right atrium in all patients during atrial flutter. Double potentials were evaluated during transient entrainment of atrial flutter by rapid high right atrial pacing in 5 of the 10 patients. In four of these five patients during such transient entrainment 1) one deflection of the double potential was captured with a relatively short activation time (mean interval 89 +/- 45 ms) and the other deflection was captured with a relatively long activation time (mean interval 233 +/- 24 ms), producing a paradoxical decrease in the short interdeflection interval from a mean of 75 +/- 20 ms to a mean of 59 +/- 24 ms; and 2) the configuration of the double potential remained similar to that observed during spontaneous atrial flutter. On pacing termination 1) the two double potential deflections were found to be associated with two different atrial flutter complexes in the electrocardiogram (ECG); 2) the previous double potential deflection relation resumed; and 3) when sinus rhythm was present, the double potentials were replaced by a broad, low amplitude electrogram recording at the same site.(ABSTRACT TRUNCATED AT 250 WORDS)

Programmed electrical stimulation at potential ventricular reentry circuit sites. Comparison of observations in humans with predictions from computer simulations

The purpose of this study was to define specific types of resetting responses to programmed electrical stimulation during human ventricular tachycardia and to use computer simulations of reentry circuits to assess the possible mechanisms and pacing site location relative to the reentry circuit for each type of response. The effects of scanning single stimuli at 35 left ventricular endocardial sites during sustained monomorphic ventricular tachycardia in 12 patients were studied. In considering alterations in QRS configuration and the delay between the stimulus and the advanced QRS, we identified three types of resetting responses to scanning stimuli consistent with stimulation at sites in or near the reentry circuit at 12 abnormal endocardial sites in eight patients. Type 1: all capturing stimuli were followed after a delay by early QRS complexes that had the same configuration as the tachycardia complexes. Type 2: late stimuli reset tachycardia as in type 1 but early stimuli reset the tachycardia after altering the QRS configuration. Type 3: late stimuli reset tachycardia as in type 1, but early stimuli advanced tachycardia with a short stimulus to QRS delay without altering the QRS configuration. In the simulations, premature depolarization of sites in the circuit produced orthodromic and antidromic wavefronts. The orthodromic wavefront propagated through the circuit and exited the circuit at the same site as did the previous tachycardia wavefronts and advanced the tachycardia without altering the configuration of the advanced QRS. The antidromic wavefront of relatively late stimuli was confined within or near the circuit by collision with the orthodromic wavefront of the preceding tachycardia beat and failed to alter ventricular activation distant from the circuit. Therefore, the QRS configuration after the stimulus was unchanged. A type 1 response occurred when all capturing stimuli produced this effect. However, with increasing stimulus prematurity, the antidromic wavefront propagated farther before colliding with an orthodromic wavefront, and under some conditions, it exited the circuit from a site other than the original circuit "exit," and altered the ventricular activation sequence distant from the circuit and, therefore, the QRS configuration, producing a type 2 pattern. The type 3 pattern occurred when the antidromic wavefront of early premature beats captured the original circuit exit. The effect of a stimulus was dependent on the stimulus prematurity, the relative conduction times from the stimulation site to the potential sites of "exit" from the circuit, and the timing of the excitable gap at the stimulation site.(ABSTRACT TRUNCATED AT 400 WORDS)

Entrainment of ventricular tachycardia by sinus rhythm

Entrainment of ventricular tachycardia (VT) may be manifest as fixed and progressive QRS fusion with ventricular and, rarely, atrial pacing. Only a single example of spontaneous VT entrainment by another rhythm, rapid atrioventricular nodal tachycardia, has been reported. This article describes an example of fixed and progressive QRS fusion between conducted sinus rhythm and VT consistent with entrainment. In contrast to entrainment with pacing, entrainment of VT by sinus rhythm occurred with drug-mediated arrhythmia slowing and demonstrated progressive QRS fusion at a constant cycle length. However, it did not demonstrate unfused but entrained QRS complexes. The resulting short PR interval and wide QRS mimicked a preexcited rhythm.

The effect of an entrainment protocol on ouabain-induced ventricular tachycardia

Overdrive stimulation of reentrant ventricular tachycardias (VT) may result in entrainment and/or termination of these arrhythmias. We investigated whether surface ECG criteria of entrainment can also be observed in nonreentrant VT. For this purpose ouabain-induced tachycardias were used that are considered to be based on delayed afterdepolarizations. In nine conscious dogs, having surgically induced complete AV block, pacing was performed using trains of 20 stimuli from a site distant to the origin of the VT. The pacing intervals were shortened in steps of 5-10 msec, until complete capture from the pacing site or termination of the VT was obtained. During stimulation variable fusion was seen and complete capture of the ventricles from the pacing site occurred just after a slight decrease in pacing cycle length (20 +/- 10 msec). Overdrive stimulation resulted only in 1 out of 58 stimulation trains in termination of VT. Following stimulation it was observed that: (1) The length of the first postpacing interval was significantly longer (P less than 0.001) than both the mean prepacing VT cycle length and interstimulus interval; (2) A change in QRS configuration occurred after 57% of the stimulation trains; (3) The VT accelerated slightly in comparison to the prepacing rate (P less than 0.05); and (4) The length of the first postpacing interval and the mean R-R interval of the VT postpacing were directly related to the interstimulus interval (r = 0.82 and 0.97, respectively). In conclusion, overdrive stimulation of ouabain-induced arrhythmias did not result in entrainment or in termination of the tachycardia. Instead, other responses were seen that may be of help in differentiating between arrhythmias caused by delayed afterdepolarizations and reentry.

Preferential effect of procainamide on the reentrant circuit of ventricular tachycardia

Transient entrainment was used to test the hypotheses that 1) procainamide prolongs the cycle length of ventricular tachycardia in patients with coronary artery disease because it has a preferential effect on the reentrant tachycardia circuit, and 2) regions of slow conduction in the reentrant circuit are more susceptible to the effect of procainamide than are other areas of the ventricles. In five patients with prior myocardial infarction, sustained ventricular tachycardia with identical QRS configuration was inducible before and after intravenous infusion of procainamide. Transient entrainment of ventricular tachycardia was demonstrated at two or more cycle lengths by rapid pacing in the baseline state and after procainamide. Rapid pacing was performed from the same site during sinus rhythm at the cycle lengths that demonstrated transient entrainment of ventricular tachycardia. The conduction interval to the transiently entrained site during ventricular tachycardia (orthodromic interval) was compared with the conduction interval to the same site during pacing in sinus rhythm (antidromic interval). The mean tachycardia cycle length increased by 27% after procainamide administration (p = 0.002). The antidromic conduction intervals were prolonged by 9% (p = 0.06) compared with a 28% increase in the mean orthodromic conduction interval (p = 0.002). The difference between the orthodromic and antidromic conduction intervals increased by 40% (p = 0.003). Prolongation of the tachycardia cycle length after procainamide administration correlated positively with increases in the orthodromic conduction intervals (r = 0.94, p = 0.02) but not with changes in the antidromic intervals (r = -0.08, p = NS). The effect of procainamide on the difference between correlated strongly with changes in the cycle length of ventricular tachycardia (r = 0.97, p = 0.006).(ABSTRACT TRUNCATED AT 250 WORDS)

Entrainment of ventricular tachycardia by AV nodal reentrant tachycardia

Endocardial recordings from a patient with both sustained ventricular tachycardia and AV nodal reentrant tachycardia are presented that demonstrate spontaneous transient entrainment of ventricular tachycardia by AV nodal reentrant tachycardia. During electrophysiological catheterization, there were repeated episodes of spontaneous conversion from a wide to a narrow QRS morphology following the induction of ventricular tachycardia. With conversion from the wide to the narrow QRS, the ventricular deflection in the coronary sinus electrograms demonstrated an abrupt change in morphology, indicating a change in activation sequence at this site from the wavefront of depolarization emerging from the ventricular tachycardia circuit to a wavefront conducting over the His-Purkinje system. However, the right ventricular apex electrogram demonstrated a constant morphology with a decrease in cycle length equal to that of the other intracardiac electrograms, indicating a constant direction of activation from the ventricular tachycardia circuit, and that ventricular tachycardia had been transiently entrained by AV nodal reentrant tachycardia. In addition, rapid atrial pacing during ventricular tachycardia narrowed the QRS and demonstrated transient entrainment of the right ventricular apex electrogram. Although transient entrainment of a tachycardia is evidence supporting reentry with an excitable gap as the probable mechanism, its demonstration has required the use of rapid pacing techniques. This case is a spontaneously occurring example of transient entrainment of one tachycardia circuit by another, a phenomenon that has not been previously described.

Use of transient entrainment during ventricular tachycardia to localize a critical area in the reentry circuit for ablation

We have previously shown that demonstration of any of the criteria for transient entrainment is possible only when pacing is performed orthodromically proximal to the area of slow conduction in a reentrant circuit with an excitable gap. Pacing orthodromically distal to the area of slow conduction will not permit demonstration of the transient entrainment criteria (concealed entrainment). Additionally, the demonstration of one form of concealed entrainment, namely pacing during a ventricular tachycardia from a site which increases the tachycardia to the pacing rate but does not change the morphology of the QRS complexes, we suggest also identifies the area of slow conduction is a keystone for maintenance of the reentrant circuit, ablation of this area should be expected to provide effective therapy of the tachycardia. Thus, we propose that using the principles of transient entrainment, one should be able to localize a critical area of slow conduction in the reentrant circuit of a ventricular tachycardia, ablate it effectively, and thereby successfully treat the ventricular tachycardia.

Entrainment of idiopathic ventricular tachycardia of left ventricular origin with evidence for reentry with an area of slow conduction and effect of verapamil

Recurrent sustained ventricular tachycardia (VT) with QRS morphology of the right bundle branch block and left axis deviation was studied in 4 patients without any underlying heart diseases. The mean VT rate was 155 beats/min and the endocardial catheter mapping during VT showed the earliest activation site at the left ventricular lateral wall near the apex. In all patients, rapid pacing from the right ventricular outflow tract during VT resulted in constant fusion beats except for the last entrained beat (thus VT was entrained), while pacing from the right ventricular apex and from the earliest activation site failed to demonstrate entrainment. During entrainment from the right ventricular outflow tract (mean pacing rate 168 beats/min), conduction intervals from the pacing site to the earliest activation site (St-A interval) and to the right ventricular apex (St-B interval) were measured in 3 patients. The St-A intervals were 400, 410 and 440 ms and the St-B intervals were 80, 70 and 90 ms, respectively. A small dose of verapamil (1.0 mg) was administered during VT, which resulted in a decrease of VT rate by a mean of 23 beats/min. During entrainment from the right ventricular outflow tract the St-A interval was prolonged in all 3 patients while the St-B interval remained the same. In conclusion, the mechanism of this VT was best explained by reentry with an area of slow conduction. Verapamil slowed the rate of VT by prolonging conduction within the area of slow conduction. Tachycardia entrainment makes possible a selective examination of antiarrhythmic drug effect on the area of slow conduction within the reentry circuit of VT.

Entrainment of ventricular tachycardia in postoperative tetralogy of Fallot

The mechanism of ventricular tachycardia (VT) in postoperative tetralogy of Fallot has been ascribed to both reentry and triggered automaticity. We performed electrophysiologic studies on a patient with this condition and induced sustained uniform ventricular tachycardia by programmed extrastimulation. Pacing during the tachycardia at multiple cycle lengths from the right ventricular apex (RVA) and outflow tract (RVOT) produced constant but progressive fusion between the paced and tachycardia QRS. With termination of pacing, the last captured complex was unfused but coupled at the paced cycle length and then the tachycardia resumed at its intrinsic rate. Therefore, the VT was entrained. In addition, an area of slow conduction between the RVOT and RVA was demonstrated. These findings support a reentrant mechanism of this arrhythmia.

Shift of atrial reentrant tachycardia with transient entrainment to an uncommon and a common type of atrial flutter

Atrial reentrant tachycardia (ART) which demonstrated transient entrainment shifted to an uncommon type of atrial flutter (AF) with premature atrial stimulation, and then returned to ART spontaneously. Subsequently, this ART shifted to a common type of AF by rapid atrial pacing, which was further transformed into an uncommon type of AF and finally terminated by rapid atrial pacing. The mechanism of AF in clinical cases is still controversial, but in this case, AF, both uncommon and common types, is considered due to macro-reentry within the atria. To explain the shift of ART to AF and mutual transformation between common and uncommon type of AF, we made a schematic figure of reentry loop within the atria of ART and AF.

A fourth criterion for transient entrainment: the electrogram equivalent of progressive fusion

Prior data pertaining to transient entrainment and associated phenomena have been best explained by pacing capture of a reentrant circuit. On this basis, we hypothesized that rapid pacing from a single site of two different constant pacing rates could constantly capture an appropriately selected bipolar electrogram recording site from one direction with a constant stimulus-to-electrogram interval during pacing at one rate, yet be constantly captured from another direction with a different constant stimulus-to-electrogram interval when pacing at a different constant pacing rate. To test this hypothesis, we studied a group of patients, each with a representative tachycardia (ventricular tachycardia, circus-movement tachycardia involving an atrioventricular bypass pathway, atrial tachycardia, and atrial flutter). For each tachycardia, pacing was performed from a single site for at least two different constant rates faster than the spontaneous rate of the tachycardia. We observed in these patients that a local bipolar recording site was constantly captured from different directions at two different pacing rates without interrupting the tachycardia at pacing termination. The evidence that the same site was captured from a different direction at two different pacing rates was supported by demonstrating a change in conduction time to that site associated with a change in the bipolar electrogram morphology at that site when comparing pacing at each rate. The mean conduction time (stimulus-to-recording site electrogram interval) was 319 +/- 69 msec while pacing at a mean cycle length of 265 +/- 50 msec, yet only 81 +/- 38 msec while pacing at a second mean cycle length of 233 +/- 51 msec, a mean change in conduction time of 238 +/- 56 msec. Remarkably, the faster pacing rate resulted in a shorter conduction time. The fact that the same electrode recording site was activated from different directions without interruption of the spontaneous tachycardia at pacing termination is difficult to explain on any mechanistic basis other than reentry. Also, these changes in conduction time and electrogram morphology occurred in parallel with the demonstration of progressive fusion beats on the electrocardiogram, the latter being an established criterion for transient entrainment.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification and catheter ablation of a zone of slow conduction in the reentrant circuit of ventricular tachycardia in humans

Three patients who had incessant ventricular tachycardia and in whom a zone of slow conduction was identified are presented. Each patient's tachycardia was refractory to multiple antiarrhythmic drugs and was being treated with amiodarone at the time of the electrophysiologic study. The ventricular tachycardia cycle length was 500 to 580 ms. In Patients 1 and 2, a single site at the posterolateral wall or low septum in the left ventricle was identified at which overdrive pacing during ventricular tachycardia resulted in ventricular capture with a stimulus to QRS interval of 280 to 400 ms and with little or no change in the configuration of the QRS complexes during pacing as compared with during ventricular tachycardia. In Patient 3, the same phenomenon was observed at two areas in the left ventricle: at the inferior wall, overdrive pacing during ventricular tachycardia resulted in a stimulus to QRS interval of 440 to 470 ms, whereas at the posterolateral wall, the stimulus to QRS interval was 320 to 360 ms. Transcatheter shocks of 100 to 240 J delivered at the pacing sites have been successful in preventing recurrences of ventricular tachycardia over a follow-up period of 10 to 11 months. These observations may be explained by the pacing site being located within a reentrant circuit in a zone of slow conduction bounded by inexcitable tissue between the pacing site and the exit site of the reentrant circuit. In Patient 3, the variable stimulus to QRS intervals are explained by variable proximity of the pacing sites within the slow conduction zone to the exit site of the reentrant circuit.(ABSTRACT TRUNCATED AT 250 WORDS)

Coupling intervals of ventricular extrastimuli causing resetting of sustained ventricular tachycardia secondary to coronary artery disease: relation to subsequent termination

Single and double ventricular extrastimuli (VE) delivered during sustained, uniform ventricular tachycardia (VT) are able to reset or terminate the tachycardia. The relation between the coupling intervals of single and double VE resetting VT and those terminating it was examined in 80 uniform, morphologically distinct VT occurring in 52 patients. Of the 80 tachycardias receiving single VE, 41 were reset and 8 terminated. The corrected coupling interval of single VE first causing resetting was 0.81 +/- 0.08 compared with 0.66 +/- 0.06 for termination (p less than 0.001). Forty-two tachycardias received double VE with 33 being reset and 13 terminating. The corrected coupling interval of double VE at which resetting was first seen was 0.86 +/- 0.08 compared with 0.73 +/- 0.05 for termination (p less than 0.001). If the longest corrected coupling interval causing resetting was greater than or equal to 0.75, then 7 of 34 tachycardias terminated with single VE and 13 of 31 terminated with double VE compared with only 1 of 46 terminating with single VE and 0 of 10 with double VE if resetting was not observed by a corrected coupling interval of 0.75 (p less than 0.01 and p less than 0.02, respectively). If the longest corrected coupling interval at which resetting occurred was greater than or equal to 0.75, the predictive value for VT termination was 21% with single VE and 42% with double VE compared with only 2% with single VE and none with double VE if resetting was not observed by this corrected coupling interval.(ABSTRACT TRUNCATED AT 250 WORDS)

Entrainment of ventricular tachycardia: explanation for surface electrocardiographic phenomena by analysis of electrograms recorded within the tachycardia circuit

Transient entrainment was demonstrated during 59 pacing events in 18 episodes of sustained uniform ventricular tachycardia (VT) while recording electrograms from the site of origin of tachycardia (LE-SOO). During entrainment, the morphology of the initial component of the LE-SOO was identical to the morphology observed during the tachycardia in 13 VTs (group I), but in five VTs (group II), the initial component changed at a "critical" paced cycle length. The presence of the proposed surface electrocardiographic criteria for entrainment--fixed fusion and a first postpacing complex without fusion but occurring at the paced cycle length--were integrally dependent on the morphologic changes in the local presystolic electrogram. Fixed fusion of the surface electrocardiogram at one or more paced cycle lengths was detected during entrainment at 35 of 59 paced cycle lengths in 12 of 18 tachycardias, 10 of which were group I and two of which were group II VTs. Fixed fusion demonstrated by analysis of the LE-SOO was observed at one or more pacing cycle lengths in 17 of 18 VTs. In five tachycardias in which surface electrocardiographic fusion was not observed, fixed fusion was evident on analysis of the left ventricular LE-SOO during right ventricular pacing. The first postpacing interval, as measured at the surface electrocardiogram, was consistently equal to the paced cycle length in only one of 18 tachycardias and was greater than the VT cycle length in eight of 17 tachycardias. A pathway with a long conduction time was demonstrated during entrainment. However, in those 12 VTs in patients in whom pacing was performed at more than one cycle length and there was preservation of the LE-SOO morphology, the conduction time between the stimulus and presystolic electrogram remained constant. Thus, no evidence for "atrioventricular nodal-like" decremental conduction was observed over a wide range of pacing cycle lengths. We conclude that: (1) two of the previously proposed criteria for diagnosis of entrainment (fixed fusion on the surface electrocardiogram and a first postpacing interval equal to the paced cycle length) are overly restrictive criteria for definition of "entrainment" of VT, (2) analysis of endocardial recordings from the site of origin of tachycardia during attempted entrainment of VT is useful for documenting the presence of entrainment, and (3) such analysis provides a basis for the understanding of surface electrocardiographic phenomenon associated with entrainment.

Resetting of ventricular tachycardia with electrocardiographic fusion: incidence and significance

The incidence and significance of fusion of the QRS complex during resetting of sustained ventricular tachycardias (VTs) was determined in 53 VTs induced by programmed stimulation in 46 patients with prior myocardial infarction. All 53 VTs were reset with one or two extrastimuli delivered at the right ventricular apex (RVA); 29 (54.7%) demonstrated fusion of the VT QRS complex coincident with the extrastimulus resetting the VT. Activation time at the RVA during VT (measured from the onset of the VT QRS complex to the first rapid deflection of the RVA electrogram) was longer in VT reset with fusion compared with those without fusion (91 +/- 30 vs 33 +/- 32 msec; p less than .001). A right bundle branch block VT QRS morphology and a rightward and inferior axis were more common in VT reset with electrocardiographic (ECG) fusion. Additionally, the shortest return cycle following the extrastimulus resetting the VT was shorter in VT reset with ECG fusion compared with those without (327 +/- 66 vs 423 +/- 84 msec; p less than .001). Fusion of the endocardial electrogram recorded at the site of VT origin was noted in 11 of 15 VTs that were reset while a recording catheter was positioned at this site, including all eight VTs with evidence of surface ECG fusion and three of seven VTs without fusion. Seventeen VTs were reset from the right ventricular outflow tract as well as the RVA; eight demonstrated QRS fusion at both sites, five from the right ventricular outflow tract only, and four from neither site.(ABSTRACT TRUNCATED AT 250 WORDS)

Incidence of reentry with an excitable gap in ventricular tachycardia: a prospective evaluation utilizing transient entrainment

The demonstration of transient entrainment has been proposed as evidence of reentry, with an excitable gap as the probable mechanism of tachycardia. A prospective series of 27 consecutive patients with sustained ventricular tachycardia induced by programmed electrical stimulation was studied to determine the frequency with which transient entrainment can be demonstrated and to define the optimal location of pacing and recording electrodes. In all patients, electrodes for pacing and recording were placed in both the left and right ventricles during electrophysiologic study. Among the 19 patients in whom the response to rapid pacing could be evaluated (25 episodes of ventricular tachycardia), transient entrainment was demonstrated in 79% (76% of episodes). Ten of 12 episodes of ventricular tachycardia with a left bundle branch block QRS configuration in lead V1 and 9 of 13 episodes with a right bundle branch block QRS configuration could be transiently entrained (p = NS). Transient entrainment was demonstrated for 8 of 11 episodes of ventricular tachycardia with a left bundle branch block configuration during pacing from the left ventricle, but for only 2 of 10 episodes during pacing from the right ventricular apex (p less than 0.05). Conversely, 9 of 13 episodes of ventricular tachycardia with a right bundle branch block configuration were transiently entrained during pacing from the right ventricular apex, but 0 of 10 episodes were transiently entrained by left ventricular pacing (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Resetting of ventricular tachycardia: implications for localizing the area of slow conduction

Analysis of local endocardial electrograms recorded during reentrant ventricular tachycardia does not provide direct information as to the participation of the recording site in the tachycardia circuit. To determine if programmed electrical stimulation at the recording site can assist in localizing areas of slow conduction that are participating in the tachycardia circuit, seven patients with sustained monomorphic ventricular tachycardia were studied. The cardiac cycle was scanned with single stimuli delivered during ventricular tachycardia at multiple endocardial sites. In four patients, an endocardial site was identified at which stimuli advanced the tachycardia with marked conduction delay and without alteration of the ventricular activation sequence, as indicated by a lack of change in the configuration of the QRS complex and endocardial electrograms distant from the stimulation site. This finding was seen only during stimulation at sites displaying abnormal electrograms and is consistent with premature depolarization of an area of slow conduction within the tachycardia focus by stimuli delivered at or near that area. Attempted endocardial catheter ablation at or adjacent to these sites in three patients was followed by persistent noninducibility of ventricular tachycardia in one patient, marked modification of the configuration and cycle length of inducible tachycardia in one patient and transient noninducibility of tachycardia in one patient. Programmed electrical stimulation during ventricular tachycardia at sites with abnormal electrograms may provide information about the proximity of the stimulation site to the tachycardia circuit.

Electrical devices for treatment of arrhythmias

Electrical devices can be used for preventing and terminating tachycardia and for achieving hemodynamic improvement during a continuing tachycardia. Conventional approaches to tachycardia prevention include pacing at physiologic rates to prevent brady-cardia-related tachycardia or tachycardias associated with prolonged QT-interval syndromes. More exotic techniques, such as those involving stimulation during the refractory period, are undergoing investigation. Some tachycardias cannot be easily terminated or recur incessantly. Hemodynamics can be improved by pacing methods that result in a narrower QRS complex by coupled pacing and, in supraventricular tachycardias, by pacing rapidly enough to create atrioventricular block. Most clinical tachycardias are caused by reentry. Careful analysis of the timing of individual stimuli that successfully terminate tachycardias indicate that critical relations exist in the conduction velocity, refractoriness and physical properties and dimensions of the reentry circuit and the remaining myocardium. Elucidating these relations has permitted inferences into the mechanisms by which pacing terminates or accelerates tachycardias. A vast number of pacing patterns have evolved for use in tachycardia termination. None of these appear to be foolproof. There is widespread and justified concern about the risk of acceleration of tachycardia when antitachycardia pacing is used in the ventricle. Experience indicates that only a few patients are suitable for termination of ventricular tachycardia by pacing, but these carefully selected patients may do well. Both the results and the potential for widespread use may be better with pacing for termination of supraventricular tachycardia. Life-threatening tachycardias or fibrillation can be terminated by direct-current countershock. Although many technical problems remain, implantable cardioverter-defibrillators, possibly combined with antitachycardia pacemakers, will play an increasing role in the management or serious arrhythmias.

Region of slow conduction in sustained ventricular tachycardia: direct endocardial recordings and functional characterization in humans

Direct endocardial recording from a discrete region of slow conduction in the left ventricle was performed in a patient during sustained ventricular tachycardia. The tachycardia had a right bundle branch block and superior axis configuration with the earliest site of endocardial activation in the posterolateral left ventricle. At this site, the left ventricular electrogram during the tachycardia displayed two deflections with distinctly different responses to rapid pacing. During rapid pacing from the right ventricular apex, one deflection was transiently entrained, representing activation of the ventricle distal to a region of slow conduction in the reentrant circuit. However, the other deflection was not entrained and arose from activation proximal to this region. At a critically rapid pacing rate, interruption of the tachycardia was associated with conduction block in the region of slow conduction, as demonstrated by dissociation of the two deflections on the posterolateral left ventricular electrogram. At pacing rates that transiently entrained but did not terminate the tachycardia, decremental properties of conduction were demonstrated in the region of slow conduction, but not in the rest of the reentrant circuit. These responses of the posterolateral left ventricular electrogram to pacing during ventricular tachycardia strongly suggest that the recordings bracketed a discrete region of slow conduction in the left ventricle that was critical for the maintenance of ventricular tachycardia. Furthermore, these data demonstrate that this region of slow conduction in the left ventricle had decremental conduction properties and was the site of block during rapid pacing at rates sufficient to interrupt the tachycardia.

Electrophysiologic study in the management of cardiac arrest survivors: a critical review

This review discusses the value and limitations of EPS in the management of cardiac arrest survivors. Uncertainties associated with EPS include a lack of consensus with respect to stimulation protocol, end points for VT suppression during drug testing, significance of induced polymorphic VT or VF, and timing of EPS after myocardial infarction. Despite methodologic shortcomings in most clinical studies, a useful body of knowledge has emerged. In cardiac arrest survivors, incidence of inducible sustained VT ranged from 35% to 75%. Where induced VT (sustained or nonsustained) was successfully suppressed, recurrent arrhythmic events occurred in 0% to 33% of patients over a 1- to 5-year follow-up period. Failed regimens correlated with a high risk of arrhythmic recurrence. EPS also helps to select patients for the implantable defibrillator or electrocardiac surgery. In conclusion, EPS appears empirically useful in the management of cardiac arrest survivors with coronary artery disease; its value in other disease entities is uncertain.

The resetting response of ventricular tachycardia to single and double extrastimuli: implications for an excitable gap

To evaluate the influence of local tissue refractoriness and delay in intervening tissue on the ability of single ventricular extrastimuli to reset and characterize a resetting response pattern in ventricular tachycardia (VT), single ventricular extrastimuli were delivered during 81 VTs and double ventricular extrastimuli in 45 of the 81 VTs. Resetting of VT was recognized as a less than fully compensatory pause after stimulation and was seen in 43 of 81 VTs (53%) with single ventricular extrastimuli and 35 of 45 (78%) with double ventricular extrastimuli. Double ventricular extrastimuli reset 16 VTs not reset by single ventricular extrastimuli. The return cycle, the interval from the extrastimulus to the first VT beat after extrastimuli, has 1 of 3 distinct response patterns: flat, increasing, and flat plus increasing. In 19 VTs, resetting was seen with both single ventricular extrastimuli and double ventricular extrastimuli; 4 flat responses with single ventricular extrastimuli became flat plus increasing with double ventricular extrastimuli. All other patterns were unchanged. In the 19 VTs reset by both single and double ventricular extrastimuli, the estimate of both the total reset zone (94 +/- 36 vs 56 +/- 32 ms) and the flat portion of the reset zone (52 +/- 42 vs 42 +/- 28 ms) was significantly longer with double ventricular extrastimuli (p less than 0.001 and p less than 0.02, respectively).

IN CONCLUSION

(1) when single ventricular extrastimuli failed to reset a VT, double ventricular extrastimuli from the same site may reset the VT.

Transient entrainment and interruption of atrioventricular node tachycardia

The possibility of transiently entraining and interrupting the common type of atrioventricular (AV) node tachycardia (anterograde slow, retrograde fast AV node pathway) was studied using atrial and ventricular pacing in 18 patients with paroxysmal AV node tachycardia. Transient entrainment occurred in all patients. During atrial pacing, localized block in the AV node for one beat followed by anterograde conduction over the fast pathway was observed in three patients. During ventricular pacing, localized block for one beat followed by retrograde conduction over the slow pathway was not observed in any patient. Neither atrial nor ventricular fusion beats were observed during entrainment. These observations indicate in a way not previously shown that reentry involving two functionally dissociated pathways in the AV node is the underlying mechanism of paroxysmal AV node tachycardia. The inability to demonstrate atrial or ventricular fusion beats during entrainment suggests a true intranodal location of the reentrant circuit. Finally, the ability to transiently entrain intranodal tachycardia demonstrates that this electrophysiologic phenomenon is not exclusively limited to macroreentrant circuits.

Mechanisms responsible for countershock-induced ventricular tachycardia in the intact canine heart

To determine the mechanism for postcountershock ventricular ectopy, internal and external shocks were delivered to 20 anesthetized dogs. Shock energies of 25 and 50 joules were employed internally while 100 and 200 joules were delivered externally. Experiments were performed in both the presence and absence of a nearly toxic dose of ouabain. All shocks resulted in the occurrence of nonsustained (less than 15 seconds) ventricular tachycardia. When bursts of rapid ventricular pacing were synchronized with a shock, the pacing stimuli invariably captured the ventricles and overdrove the shock-induced ventricular tachycardia. However, the burst pacing never appeared to break a tachycardia, since the termination of pacing was followed immediately by the resumption of the shock-induced ventricular tachycardia. The presence of ouabain did not alter the response of the ventricles to postshock burst pacing. Administration of verapamil (0.5 mg/kg) had no effect on the duration of shock-induced ventricular arrhythmia. Elevation of the serum potassium level to 8.5 +/- 0.6 mEq/L drastically reduced the duration of postshock ventricular tachycardia in both the presence and absence of ouabain. The results suggest that postshock ventricular ectopy results from an abnormality of impulse initiation rather than reentry.

Demonstration of the presence of slow conduction during sustained ventricular tachycardia in man: use of transient entrainment of the tachycardia

To test the hypothesis that an area of slow conduction is present during reentrant ventricular tachycardia in man, and that the earliest activation site during ventricular tachycardia is within or orthodromically just distal to the area of slow conduction in the reentry loop, we studied 12 episodes of ventricular tachycardia (mean rate 185 +/- 32 beats/min) that were induced in nine patients with ischemic heart disease. Rapid ventricular pacing was performed at selected sites during ventricular tachycardia while recording electrograms from an early activation site relative to the onset of the QRS complex (site A) and from a site close to the pacing site (site B). Rapid pacing from the right ventricular apex during ventricular tachycardia with a right bundle branch block pattern and from selected left ventricular sites during ventricular tachycardia with a left bundle branch block pattern (mean pacing rate 202 +/- 38 beats/min) resulted in constant ventricular fusion beats on the electrocardiogram except for the last captured beat (i.e., the ventricular tachycardia was entrained) in 11 of 12 episodes. During entrainment: sites A and B were activated at the pacing rate, conduction time from the last pacing impulse to the last captured ventricular electrogram at site A (St-A interval) was 359 +/- 69 msec and spanned the diastolic interval, while that at site B (St-B interval) was only 28 +/- 13 msec, site A had the same ventricular electrogram morphology as that during ventricular tachycardia, while site B had a different electrogram morphology, indicating that site A was activated in the same direction during entrainment as during ventricular tachycardia. Eight episodes of ventricular tachycardia were entrained at two or more different pacing rates. The St-A interval increased during pacing at the faster rate(s) in four of eight episodes, while the St-B interval remained unchanged. Rapid ventricular pacing performed from the same site during sinus rhythm (mean pacing rate 201 +/- 37 beats/min) resulted in an St-A interval of 103 +/- 37 msec (p less than .001 vs the value during entrainment) and an St-B interval of 31 +/- 15 msec (p = NS vs the value during entrainment). It is concluded that an area of slow conduction not demonstrable during sinus rhythm exists during ventricular tachycardia, and that the earliest activation site during ventricular tachycardia is at or orthodromically distal to this area of slow conduction.