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

A constant postpacing interval in response to overdrive pacing with variable number of beats: An aid in the identification of entrainment for a particular pacing train during macroreentrant atrial tachycardias

BACKGROUND

When pacing trains with a constant cycle length (CL) but increasing number of beats are introduced during a macroreentrant atrial tachycardia (MAT), the postpacing interval (PPI) is expected to increase if entrainment does not occur but could be stable if entrainment occurs. We tested the ability of PPI analysis to detect entrainment.

METHODS

Synchronized pacing trains with increasing number of beats (1-20) were delivered from the coronary sinus (CS) and lateral right atrium (RA) at a CL 20 ms shorter than the MAT CL. Pacing trains were grouped in pairs differing by one-paced beat, and the ∆PPI measured.

RESULTS

In an initial prospective cohort of 21 patients (48% had left atrial flutter) the mean ∆PPI was 21.3 ± 5.6 ms for pairs of pacing trains in which neither entrained the MAT and 2.8 ± 1.4 ms for those in which both entrained the MAT (P < .0001). Results were similar for common vs atypical flutter, PPI-TCL ≤30 ms vs PPI-TCL >30 ms, presence vs absence of antiarrhythmic drugs and faster vs slower MAT. When an index pacing train was compared to one with two-paced beats less, a PPI difference of <19 ms identified entrainment with 95% sensitivity and 98% specificity. In a validation cohort of 16 patients, this cut-off value resulted in sensitivity and specificity of 90% and 94%.

CONCLUSIONS

A relatively constant ∆PPI in response to overdrive pacing with identical CL but different number of beats allows accurate discrimination between trains that entrained vs those which did not entrain a MAT.

Usefulness of entrainment mapping using the activation sequence of the last captured excitation in complex dual-loop atrial tachycardia

BACKGROUND

Electroanatomical mapping is useful for locating the atrial reentrant circuit, but analysis of the dynamic relation of the reentrant circuit is sometimes difficult. This article describes three cases of complex dual-loop reentrant atrial tachycardia analyzed by entrainment mapping using not only the postpacing interval (PPI) but also the activation sequence of the last captured beats.

METHODS

Case 1 was dual-loop reentry consisting of the tricuspid annulus (TA) and a localized atrial reentry at the coronary sinus (CS) ostium with different exit sites to the right and the left atrium that was cured by catheter ablation at the CS ostium showing fractionated potential. Case 2 was dual-loop reentry around the TA and the superior trans-septal incision line. Case 3 was dual-loop reentry around the TA and longitudinal dissociation along the cavo-tricuspid isthmus.

RESULTS

In Cases 1 and 2, entrainment with a shorter pacing cycle length demonstrated antidromic penetration to the circuit and changed the activation sequence of the last captured beat depending on the anatomical relation of the reentrant circuit. In Cases 1-3 with dual-loop reentry, the excitation wavefront induced by stimulation entered one circuit after going around the other; thus, the penetration to the other reentry circuit became the second beat after the stimulus (one lap behind).

CONCLUSIONS

The PPI is obtained from the pacing site only, but the last captured beat could be obtained from all electrodes. It is advantageous to use the information from all available electrode recordings to determine the dynamic relation between complex dual-loop reentrant circuits.

Effect of the restitution properties of cardiac tissue on the repeatability of entrainment mapping response

BACKGROUND

The difference between the postpacing interval (PPI) and the tachycardia cycle length (TCL; PPI-TCL) is a useful tool in mapping macro-reentrant tachycardias. However, entrainment pacing causes some perturbation of the conduction velocity within the tachycardia circuit, which may affect the repeatability and consequently the accuracy of the measurement of PPI-TCL. The aim of this study was to assess PPI-TCL repeatability both in vivo and in silico.

METHODS AND RESULTS

In the experimental part, entrainment pacing was performed twice at each of the 124 tested sites for 30 patients undergoing radiofrequency ablation of atrial and ventricular re-entrant arrhythmias. A similar protocol was used in a simplified computer model of the cardiac tachycardia circuit in a 2-dimensional tissue strip using a Fenton-Karma model of cardiac tissue. In vivo, in the case of fast tachycardias (<350 ms), PPI-TCL variability observed was doubled compared with slow tachycardias (>350 ms; 95% Limits of Agreement ranged from -21.4 to 21.6 ms for TCL<350 ms and from -10.8 to 11.5 ms for TCL>350 ms). Simulations show that this increase of variability may be because of the oscillations of the conduction velocity inside the tachycardia circuits. The effect of the restitution properties of cardiac tissue on the outcome of entrainment pacing is discussed.

CONCLUSIONS

PPI-TCL is characterized by a high repeatability with the differences between the results for individual stimulations of ≤20 ms. The variability of this parameter is significantly lower in the case of slow tachycardias.

Resetting and entrainment of reentrant arrhythmias: part I: concepts, recognition, and protocol for evaluation: surface ECG versus intracardiac recordings

In this paper, we review the information accumulated over the years regarding the phenomena of resetting and entrainment of reentrant arrhythmias. Over three decades of research and clinical applications, these phenomena have demonstrated that they stay as a main tool for an intellectual understanding of reentry and to base strategies for localization of critical areas for ablative therapies. This review will be divided into two parts. This first part deals with the bases for the concept development, the means for the detection of these phenomena, and their mechanistic implications. Resetting is described as a particular response of a given rhythm to an external perturbation, indicating interaction between them. Entrainment indicates continuous reset of the rhythm when the perturbation is repetitive. The mechanisms that explain these responses in reentrant rhythms are presented. Fusion, both at the surface electrocardiogram and at the level of intracardiac recordings, is discussed in detail, with its value and limitations as a key concept to recognize entrainment and reentry. Computer simulations are used as an aid to a better understanding. Differences between resetting and entrainment are considered, and a pacing protocol to study these phenomena described.

From bedside to bench: entrainment and other stories

The concepts of transient entrainment of reentrant rhythms started with studies of overdrive pacing of atrial flutter (AFL) in patients in the immediate period after open heart surgery. Initial studies demonstrated the need to achieve a critical pacing rate and a critical duration of pacing at the critical pacing rate to interrupt AFL. Further pacing studies of AFL, ventricular tachycardia, atrioventricular (AV) reentrant tachycardia, AV nodal reentrant tachycardia, and atrial tachycardia refined the understanding of what occurs during overdrive pacing of reentrant tachycardias, and permitted a mechanistic understanding of transient entrainment as continuous resetting of a reentrant tachycardia to a pacing rate that is faster than the rate of the tachycardia, but which fails to interrupt it. The demonstration of transient entrainment of a tachycardia provides a reliable clinical tool to establish the presence of a reentrant rhythm. Moreover, the principles of entrainment have also been applied clinically to assist in effective application of antitachycardia pacing and catheter ablation techniques.

Reentrant tachycardia in pulmonary veins of patients with paroxysmal atrial fibrillation

INTRODUCTION

Although slow automatic pulmonary vein (PV) activity dissociated from the atrium after achievement of PV isolation in patients with atrial fibrillation (AF) has been reported, little is known about dissociated PV tachycardia. The aim of this study was to investigate the inducibility and the mechanism of sustained PV tachycardia (SPVT).

METHODS AND RESULTS

One hundred thirty-two patients with drug-refractory paroxysmal AF underwent PV isolation by radiofrequency catheter ablation. Programmed stimulation was performed in 269 PVs of 110 of these patients after achievement of PV isolation. In 7 PVs (2.6%; left superior PV: n = 2, right superior PV: n = 4, right inferior PV: n = 1) of 7 (6.4%) of 110 patients, 18 SPVTs were induced. Fifteen regular SPVTs (mean cycle length 152 +/- 34 msec) were induced in 6 of 7 PVs, and 3 irregular SPVTs (cycle length range: 94-276 msec) were induced in 3 of 7 PVs. In 2 PVs, both regular and irregular SPVTs were induced. SPVT was terminated by burst pacing in 4 PVs, and entrainment was observed during regular SPVT in 5 PVs. Slow PV automatic activity dissociated from the atrium and decremental conduction properties were shown in all 7 PVs. The shortest pacing cycle length with 1:1 capture was < or =150 msec in 6 of 7 PVs.

CONCLUSION

Reentrant tachycardia can occur in some isolated PVs with both decremental conduction properties and short refractory periods, which suggests that reentry may be one of the mechanisms of PV arrhythmogenicity.

Early versus late atrial fibrillation after atrial flutter ablation

INTRODUCTION

Radiofrequency catheter ablation of atrial flutter (AFl) has high initial success with a 10-15% recurrence. Atrial fibrillation (AFib) after radiofrequency catheter ablation of AFl can occur but may be transient (lasting no more than four weeks).

METHODS

Of one hundred seventeen consecutive patients studied, one hundred and four consecutive patients with sustained, symptomatic AFl, as the predominant rhythm disturbance (some of whom had transient pre-ablation AFib), referred for radiofrequency catheter ablation, had clinical follow-up. All had evidence for successful AFl ablation. Patients were followed prospectively.

RESULTS

Over a mean follow-up of 28 months, 28 patients developed AFib after ablation of AFl [12 early AFib (<2 months) and 16 late AFib (>2 months)]. Seven of 12 (58%) patients in the early onset group reverted to normal sinus rhythm; none required long-term antiarrhythmic therapy. Only one (8%) developed permanent AFib. No patient in the late onset group remained in sinus rhythm without an antiarrhythmic drug. Three (19%) developed permanent AFib despite therapy among those with late onset AFib. Two (17%) patients with early onset AFib reverted to normal sinus rhythm with treatment versus 5 (31%) in the late onset group. Finally, only 2 patients (17%) with paroxysmal/persistent episodes of Afib from the early onset group stayed in normal sinus rhythm despite therapy, while 8 patients (50%) with paroxysmal/persistent AFib episodes from the late onset group required therapy to maintain normal sinus rhythm.

CONCLUSION

Early onset AFib after ablation of AFl is likely to be transient and self-limited. Late onset AFib after ablation of AFl can persist and require chronic therapy.

Sensitivity and specificity of concealed entrainment for the identification of a critical isthmus in the atrium: relationship to rate, anatomic location and antidromic penetration

OBJECTIVES

This study was designed to determine the sensitivity and specificity of concealed entrainment (CE) for the identification of a critical isthmus in the atrium.

BACKGROUND

Isthmus identification during entrainment mapping of macro-reentrant atrial tachycardia (MRAT) relies on the demonstration of CE.

METHODS

Using the model of typical atrial flutter, entrainment was performed in 10 patients at four rates (flutter cycle length [FCL] minus 10/20/30/40 ms) from seven sites: isthmus entrance/exit, low lateral/high lateral/high septal right atrium and proximal/distal coronary sinus. Surface 12-lead electrocardiogram fusion was evaluated by three observers blind to patient status. The extent of antidromic penetration (AP) was measured off the pacing catheter positioned around the tricuspid annulus.

RESULTS

The sensitivity for CE identifying any isthmus site was greatest at FCL-10 (100%), but the specificity was poor (54%). Conversely, specificity was greatest at FCL-40 (98%), but the sensitivity was poor (65%), with manifest entrainment (ME) observed from the isthmus entrance in 70% of episodes. At FCL-30, sensitivity (85%) and specificity (90%) were "balanced," but CE still resulted during entrainment from a non-isthmus site in five of 10 patients. Antidromic penetration increased with pacing CL shortening (p < 0.001) and correlated with the development of ME (p < 0.001). Antidromic penetration was significantly blunted from the isthmus exit compared to all other sites (p = 0.003).

CONCLUSIONS

The sensitivity and specificity of CE for identifying an isthmus in the atrium are critically dependent on the pacing rate and the precise anatomic pacing site within the isthmus. These findings may have implications for the use of entrainment in the mapping of unknown MRAT circuits.

[Electrophysiological and clinical characterization of left atrial macroreentrant tachycardia]

BACKGROUND AND OBJECTIVE

We are reporting the characteristics of 9 patients with left atrial macroreentrant tachycardia, an arrhythmia not well studied in man.

PATIENTS AND METHOD

Mean age was 60 years and 7 were men. Tachycardia was spontaneous in 6 and induced in 3. Two had no heart disease, 2 sick sinus syndrome, 3 aortic prosthesis, 2 hypertension, 1 cardiomyopathy and 1 chronic bronchitis. Simultaneous recordings from right atrial, coronary sinus and right pulmonary artery were obtained at baseline and with atrial pacing. Macroreentrant tachycardia was diagnosed when entrainment with fusion was documented.

RESULTS

Cycle length was 230-440 ms (287 67). The ECG showed atypical flutter in 3 patients and P waves with flat baseline in 6. Coronary sinus activation was distal to proximal in 7. Right atrial activation was circular in 3 with previous typical flutter ablation. Entrainment from the right atrium produced long return cycles in the right atrial recordings, but equal to basal tachycardic cycle in coronary sinus recordings. Entrainment from the coronary sinus produced local return cycles equal to basal cycle in 8 and prolonged in 1. After stimulation, 4 recovered sinus rhythm, 4 went to atrial fibrillation and 1 had no change. After a follow-up of 9-19 months 5 remain in sinus rhythm treated with antiarrhythmic drugs and/or atrial pacing.

CONCLUSIONS

Left atrial macroreentrant tachycardia is associated with organic heart disease. The ECG most frequent pattern tends to show P waves with flat baseline at a relatively slow rate. Most circuits turn clockwise in anterior view. Atrial stimulation is not very effective for cardioversion to sinus rhythm. The prognosis of long term rhythm is uncertain.

Apparent bidirectional conduction block following radiofrequency catheter ablation of typical atrial flutter

OBJECTIVE

The purpose of this study is to determine the reliability of activation sequence mapping in assessing the presence of bidirectional conduction block (BCB) in typical atrial flutter (AFL) ablation.

INTRODUCTION

Radiofrequency ablation (RFA) can cure typical AFL by creating BCB across the right atrial isthmus. Effective conduction block across this region can prevent AFL recurrence, but accurate assessment of isthmus conduction may be flawed.

METHODS

BCB was measured before and after RFA by pacing at multiple rates on both sides of the isthmus during sinus rhythm. Pacing was performed from a low lateral tricuspid annulus site (proximal to the isthmus) and a coronary sinus Os site (distal to the isthmus), while recording simultaneously from 8-10 right atrial sites bordering the isthmus (4-5 free wall sites; 4-5 septal sites) as well as from an isthmus site. After ablation reinduction of atrial flutter was attempted from both sides of the block with rapid atrial pacing after BCB was established in all patients. In some patients lines of conduction block were evident at the isthmus (using the ablation catheter to map).

RESULTS

Of 65 patients undergoing RFA of AFL, 59 had typical AFL. In all 59 patients, BCB was demonstrated at all pacing cycle lengths 30 min after RFA applications. In 6 of these 59, AFL was inducible with atrial pacing despite apparent BCB. Further RFA resulted in non inducibility in all 6 patients. In the remaining 53/59 patients, BCB was associated with noninducibility at 30 min. A total of 8 recurrences were seen during a mean 19.3 +/- 8.3 (SD) month follow-up.

CONCLUSION

Apparent BCB as determined by activation sequence mapping outside of the isthmus is an excellent marker, but, as measured, may be a misleading method of assessing the presence or absence of conduction through the isthmus. It is necessary to attempt reinduction of AFL after apparent success. Elimination of typical AFL does not preclude other AFLs.

Diagnostic significance of the morphological change in the atrial electrogram during Para-Hisian pacing

Para-Hisian pacing (PHP), a pacing method to differentiate between conduction occurring over an accessory pathway (AP) from that over the atrioventricular node (AVN), is assessed essentially by comparing the timing in the atrial electrogams. Morphological change in the atrial electrograms is often observed during PHP, but its significance has not been investigated. Prior to the catheter ablation procedure, PHP was performed in 52 patients with an AP and in 36 patients with AV nodal reentrant tachycardia (AVNRT). The morphological change in the atrial electrograms, which was retrospectively assessed between the His bundle and proximal right bundle branch (HB-RB) captured and non-captured beats, was identified in 15 of 52 patients with an AP and in 26 of 36 patients with AVNRT. The atrial electrogram in the 6 of these 15 AP patients changed its morphology without overlapping the ventricular electrogram. All 6 AP patients exhibited a PHP pattern with the presence of 2 retrograde conduction routes, an AP and the AVN. In the patients demonstrating no morphological change in the atrial electrogram, 33 of 37 AP patients and all 10 AVNRT patients had only one retrograde conduction route. Morphological change in the atrial electrogram without overlapping the ventricular electrogram seems to have diagnostic significance indicating the presence of both AP and AVN conduction.

Helical representation of atrial reentry: a teaching aid for electrophysiology

Cardiac clinical electrophysiology is difficult to teach because it requires mental integration of complex information on cardiac activation, which includes tridimensional spatial orientation of the cardiac structures involved and variables such as refractory period, conduction velocity, and cycle length. Commonly used representations of cardiac arrhythmias include snapshot sequences or ladder diagrams. The former lack time dimension, and the latter lack spatial dimension. We propose a schematic tridimensional representation of reentry as a helicoidal shape that is the result of adding a time dimension to a circular representation. This presentation, which also can be called a "loop diagram," allows better integration of spatial phenomena with recorded electrogram sequences and can help the teaching of basic clinical electrophysiology.

Verification of linear lesions using a noncontact multielectrode array catheter versus conventional contact mapping techniques

INTRODUCTION

Creation of linear lesions is an established ablation goal. Verification of complete conduction block at the ablation line is required to determine ablation success. Conventional mapping techniques are sequential endocardial activation mapping and documentation of double potentials. Recently, a noncontact multielectrode array catheter was developed that allows instantaneous three-dimensional mapping by simultaneous reconstruction of > 3,000 electrograms. In this study, we prospectively compared the accuracy of noncontact mapping to identify discontinuities in linear lesions and to verify a conduction block with that of conventional mapping techniques.

METHODS AND RESULTS

In 12 patients with atrial flutter, radiofrequency pulses were applied between the tricuspid annulus and either the inferior vena cava or the eustachian ridge. Following each application, pulse propagation at the ablation line was determined during pacing by conventional mapping techniques. The findings were compared to high-density isopotential mapping using the noncontact multielectrode array catheter. It was found that noncontact mapping reliably distinguished conduction delays from a conduction block as defined by contact mapping. In addition, noncontact mapping instantaneously identified the area where a discontinuity in the line of block was present. In these patients, complete conduction block was achieved by radiofrequency pulses guided by the noncontact mapping system.

CONCLUSION

Noncontact mapping is highly accurate in distinguishing conduction delays from a complete conduction block. By providing an instantaneous high-density propagation vector at all sites along the ablation line, three-dimensional isopotential mapping is helpful in localizing discontinuities of linear lesions and, thus, may facilitate the creation of a complete conduction block.

Role of bipolar electrogram polarity mapping in localizing recurrent conduction in the isthmus early and late after ablation of atrial flutter

OBJECTIVES

Bipolar electrogram polarity was analyzed to localize the recurrent conduction site in the isthmus between the tricuspid annulus (TA) and inferior vena cava (IVC) in recurrent atrial flutter (AF).

BACKGROUND

Despite the initial successful linear isthmus ablation, recurrence of transisthmus conduction and AF is not uncommon. It is unclear how the recurrent conduction site can be identified.

METHODS

Fourteen patients with recurrent AF were studied: four with late recurrence remote from the first ablation and 10 with early recurrence within 60 minutes after the initial successful ablation. Bipolar electrogram polarity mapping was performed during low lateral right atrium (LLRA) pacing during sinus rhythm while recording bipolar electrograms from the septal portion of the isthmus along the previously ablated line. The septal side of the isthmus from TA to IVC was arbitrarily divided into five sites, and the bipolar electrodes with cathode at the tip and anode at the second was placed at each site. The recurrent conduction site was localized by analyzing the polarity of the bipolar electrogram recorded at each site.

RESULTS

All recurrent AF was due to reentry around TA. During pacing from LLRA, as the mapping electrode was moved from TA to IVC side, the major polarity of the electrogram changed from negative to positive in all patients. A transitional electrogram with the equal amplitudes in positive and negative components was recorded between the sites showing mainly negative and positive electrograms, indicating electrogram polarity reversal at this site. Application of radiofrequency energy to this single site resulted in the elimination of transisthmus conduction in all patients with a single application in 11 patients and 2 or 3 in the remaining 3.

CONCLUSIONS

Bipolar electrogram polarity mapping with attention to the polarity reversal point is useful for identifying and ablating the recurrent conduction site.

Coronary sinus pacing initiates counterclockwise atrial flutter while pacing from the low lateral right atrium initiates clockwise atrial flutter. Analysis of episodes of direct initiation of atrial flutter

INTRODUCTION

Rapid atrial pacing in sinus rhythm may directly induce atrial flutter without provoking intervening atrial fibrillation, or initiate atrial flutter indirectly, by a conversion from an episode of transient atrial fibrillation provoked by rapid atrial pacing. The present study was performed to examine whether or not the direct induction of clockwise or counterclockwise atrial flutter was pacing-site (right or left atrium) dependent.

METHODS AND RESULTS

We analyzed the mode of direct induction of atrial flutter by rapid atrial pacing. In 46 patients with a history of atrial flutter, rapid atrial pacing with 3 to 20 stimuli (cycle length = 500 - 170 ms) was performed in sinus rhythm to induce atrial flutter from 3 atrial sites, including the high right atrium, the low lateral right atrium, and the proximal coronary sinus, while recording multiple intracardiac electrograms of the atria. Direct induction of atrial flutter by rapid atrial pacing was a rare phenomenon and was documented only 22 times in 15 patients: 3, 11, and 8 times during stimulation, respectively, from the high right atrium, low lateral right atrium, and the proximal coronary sinus. Counterclockwise atrial flutter (12 times) was more frequently induced with stimulation from the proximal coronary sinus than from the low lateral right atrium (8 vs 1, P = .0001); clockwise atrial flutter (10 times) was induced exclusively from the low lateral right atrium (P = .0001 for low lateral right atrium vs proximal coronary sinus, P = .011 for low lateral right atrium vs high right atrium).

CONCLUSIONS

Direct induction of either counterclockwise or clockwise atrial flutter was definitively pacing-site dependent; low lateral right atrial pacing induced clockwise, while proximal coronary sinus pacing induced counterclockwise atrial flutter. Anatomic correlation between the flutter circuit and the atrial pacing site may play an important role in the inducibility of counterclockwise or clockwise atrial flutter.

Atypical form of the fourth criterion for transient entrainment

The typical fourth criterion for transient entrainment is defined when both a sudden shortening in conduction interval to and a distinct change in electrogram morphology at a bipolar recording site are demonstrated while performing overdrive pacing of a reentrant tachycardia from a single pacing site at two different constant rates. The purpose of this article was to test the hypothesis that if an intracardiac recording site showing both orthodromic and antidromic capture with entrainment pacing is located suitably distant from the circuit, sudden shortening in conduction interval to that site may occur without any significant change in the bipolar electrogram morphology (i.e., atypical form of the fourth criterion). Atrial overdrive pacing of orthodromic tachycardia was performed in 20 patients with either left anterior (12 patients) or left posterior (8 patients) accessory pathways. We investigated the effects of overdrive pacing from the proximal or distal coronary sinus, specifically effects on the electrogram interval and the electrogram morphology at the right atrial appendage. Overdrive pacing of orthodromic tachycardia from the proximal coronary sinus was performed in 10 of the 12 patients with left anterior accessory pathways; those 10 patients demonstrated the first entrainment criterion at the right atrial appendage site. Overdrive pacing of orthodromic tachycardia at still shorter cycle lengths demonstrated a sudden shortening in conduction interval to the right atrial appendage site. Despite shortening in conduction interval the morphology of the right atrial appendage electrogram was completely or almost identical to that during orthodromic tachycardia, indicating an atypical form of the fourth criterion. This criterion was not demonstrated in patients with left posterior accessory pathways. Thus, atypical fourth entrainment criterion was demonstrated during overdrive pacing of orthodromic tachycardia from the proximal coronary sinus only in patients with left anterior accessory pathways. Demonstration of atypical fourth criterion seems largely dependent on the location of the accessory pathway, the pacing, and the recording sites.

Acute and long-term effects of consecutive radiofrequency applications on conduction properties of the subeustachian isthmus in type I atrial flutter

INTRODUCTION

Bidirectional conduction block at the subeustachian isthmus predicts long-term efficacy of atrial flutter ablation. Limited data are available on the incidence and outcome of minor conduction changes such as unidirectional or incomplete block. This prospective study sought to systematically assess discrete acute and long-term alterations of bidirectional conduction prior to a complete conduction block.

METHODS AND RESULTS

In 41 patients with type I atrial flutter, pulse propagation through the subeustachian isthmus during low lateral and proximal coronary sinus pacing was documented and analyzed following each consecutive radiofrequency (RF) application. In cases of altered conduction properties and noninducibility of atrial flutter, patients were followed-up for 12 months. Three sets of results were found. First, following RF application, 23 patients presented a progressive conduction delay prior to a complete conduction block. Second, RF application did not always affect counterclockwise and clockwise conduction simultaneously or to the same extent. In 13 patients, an initial alteration of counterclockwise conduction was present before an alteration of clockwise conduction; in 5 patients, clockwise conduction was primarily affected. Third, the recurrence rate of typical atrial flutter was 9% (2/22) in patients with a complete bidirectional conduction block, 54% (7/13) in patients with unidirectional conduction block, and 100% (6/6) in patients with sole bidirectional conduction delay.

CONCLUSION

In 50% of the patients, consecutive RF applications resulted primarily in a progressive conduction delay rather than a sudden conduction block. Since counterclockwise and clockwise conduction were not always affected simultaneously or to the same extent, lateral as well as septal pacing is recommended for improvement of bidirectional conduction block. Normalization of primarily altered conduction and, therefore, recurrence of atrial flutter are high in all patients without bidirectional block.

Conduction properties of the inferior vena cava-tricuspid annular isthmus in patients with typical atrial flutter

INTRODUCTION

A functional region of slow conduction located in the inferior right atrium has been postulated to be critical to the induction and maintenance of typical human atrial flutter. We reexamined the potential role of functional conduction delay in the annular isthmus between the tricuspid valve and the inferior vena cava; it is within this region that such delays have been postulated to occur, and where interruption of conduction by radiofrequency energy application has been shown to eliminate typical flutter.

METHODS AND RESULTS

Thirty patients with type I atrial flutter (30 counterclockwise, 14 clockwise) were studied. Counterclockwise and clockwise isthmus activation times adjacent and parallel to the tricuspid valve were measured during three conditions: (1) atrial pacing in sinus rhythm, (2) atrial flutter, and (3) entrainment of atrial flutter. During pacing in sinus rhythm at progressively shorter cycle lengths, both counterclockwise and clockwise isthmus activation times remained unchanged; decremental conduction prior to flutter induction or loss of capture was not observed. Counterclockwise isthmus activation time did not significantly differ during flutter (68 +/- 23 msec), inferolateral tricuspid annulus pacing (71 +/- 23 msec), or entrainment of flutter (72 +/- 23 msec). Similarly, clockwise isthmus activation times did not significantly differ between flutter (65 +/- 22 msec), proximal coronary sinus pacing (73 +/- 21 msec), or entrainment of flutter (64 +/- 15 msec).

CONCLUSION

Decremental conduction is not characteristic of activation through the isthmus when activation is assessed parallel and adjacent to the tricuspid annulus. Functional slowing or conduction delay does not develop in this region during typical atrial flutter.

Exploring postinfarction reentrant ventricular tachycardia with entrainment mapping

Ventricular tachycardia late after myocardial infarction is usually due to reentry in the infarct region. These reentry circuits can be large, complex and difficult to define, impeding study in the electrophysiology laboratory and making catheter ablation difficult. Pacing through the electrodes of the mapping catheter provides a new approach to mapping. When pacing stimuli capture the effects on the tachycardia depend on the location of the pacing site relative to the reentry circuit. The effects observed allow identification of various portions of the reentry circuit, without the need for locating the entire circuit. Isthmuses where relatively small lesions produced by radiofrequency catheter ablation can interrupt reentry can often be identified. A classification that divides reentry circuits into one or more functional components helps to conceptualize the reentry circuit and predicts the likelihood that heating with radiofrequency current will terminate tachycardia. These methods are helping to define human reentry circuits.

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.

[Radiofrequency catheter ablation of ventricular tachycardias in patients with postinfarction scars]

Radiofrequency catheter ablation has recently emerged as a therapeutic option for ventricular tachycardia in postinfarction patients. However, the indications for its use and the mapping procedure remain controversial. The most common arrhythmogenic circuit found fits an "8" shape model. This model incorporates a slow conducting central area, separated from the surrounding myocardium by conduction blocking areas and with entrance and exit sites. This circuit has classically been confined in the left ventricle. However, recently successful radiofrequency catheter ablation of ventricular tachycardia has been reported from the right ventricle. Several markers for adequate positioning of the ablation catheter have been reported: local presystolic activity, isolated mid diastolic potential, transient entrainment with concealed fusion, match between electrogram-QRS and stimulus-QRS intervals, match between first postpacing interval and tachycardia cycle length and tachycardia electrocardiographic reproduction by pace-mapping. Procedure related complications are rare and the success rate is around 70%. Nevertheless, currently this technique should be limited to postinfarction patients with ventricular tachycardia meeting certain requisites.

Ventricular tachycardia: pathophysiology and radiofrequency catheter ablation

Limitations of pharmacological therapy for VT have led to great interest in alternative nonpharmacological therapies. The appeal of a curative therapy for VT initially led to the search for operative techniques to identify and destroy the underlying substrate, and more recently, has resulted in the development of catheter techniques to achieve the same goal in the electrophysiology laboratory. Investigations into the pathophysiology of VT have resulted in the recognition that this arrhythmia reflects a mechanistically and anatomically heterogeneous set of disorders. Recent growth in our understanding of these distinctions has both led to, and resulted from, simultaneous advances in catheter ablation techniques. The clinical electrophysiology laboratory has served as a testing ground for theories derived from in vitro and animal experiments while also providing its own set of human experimental data regarding the pathophysiology and treatment of VT. As a result of this process, several distinct forms of VT that are amenable to catheter ablation have been characterized. This article will summarize current knowledge of the pathophysiology of various VT subtypes and of techniques for catheter mapping and ablation.

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.

Atrial flutter mapping and ablation. I. Studying atrial flutter mechanisms by mapping and entrainment

Endocardial mapping has led to a detailed knowledge of reentry mechanisms in atrial flutter. Multipolar and deflecting tip catheters allow recording local electrograms from multiple areas of the right atrium, and from the coronary sinus. In common flutter, with the typical "sawtooth" pattern, there is circular activation of the right atrium in a "counterclockwise" direction, descending in the anterior and lateral walls, and ascending in the septum and posterior wall. Superior and inferior vena cava, linked by a "line" of functional block in the posterolateral wall, make the central obstacle for circular activation. The cranial and caudal turning points are the atrial "roof," and the isthmus between the inferior vena cava and the tricuspid valve. Complex conduction patterns, probably including slow conduction are detectable in the low septal area, around the coronary sinus. Atypical flutter, without the sharp negative deflections of common flutter, sometimes shows circular activation in the right atrium, rotating in the opposite direction of common flutter (clockwise). Other atypical flutters show no circular right atrial activation, and only partial data from coronary sinus activation, combined with the response to atrial stimulation (entrainment) allow the diagnosis of left atrial reentry, without a precise delimitation of the circuits. In patients having undergone cardiac surgery, atypical flutter may be based on reentry around surgical scars. To our knowledge, the mechanism of type II flutter has not been disclosed in humans.

Atypical antidromic resetting during programmed extrastimulation of reentrant ventricular tachycardia

A patient with reentrant ventricular tachycardia exhibited both the orthodromic and antidromic resetting responses at a single intracardiac recording site during programmed extrastimulation of ventricular tachycardia. The transition from orthodromic to antidromic resetting with extrastimulation demonstrated a sudden shortening in conduction interval to an electrogram recording site and unexpected identical morphology of the spontaneous and captured electrograms at that site, indicating atypical antidromic resetting. This newly observed resetting phenomenon with programmed extrastimulation suggests that the fourth entrainment criterion with overdrive pacing may likely be demonstrated in an atypical form; that is, a sudden shortening in conduction interval to an electrogram recording site may occur without any significant change in the bipolar electrogram morphology at that site when overdrive pacing is performed during tachycardia from a single pacing site at two different constant rates.

Characteristics of slow conduction zone demonstrated during entrainment of idiopathic ventricular tachycardia of left ventricular origin

Idiopathic ventricular tachycardia (VT) with the right bundle branch block pattern and left-axis deviation has been shown to be due to reentry, but the property of the slow conduction zone within the reentry circuit is little understood. In 7 patients (mean VT cycle length [CL]: 361 +/- 49 ms), rapid pacing from the right ventricular outflow tract was performed during VT while recording electrograms at the early activation site in the left ventricle and at the right ventricular apex; also, conduction times from the pacing site to these recording sits (St-A and St-B intervals, respectively) were measured. Both constant fusion (except for the last paced beat) and progressive fusion were seen in all patients, indicating VT entrainment. The left ventricular site was captured orthodromically with an St-A of 394 +/- 57 ms at the pacing CL of 351 +/- 47 ms during entrainment, while the right ventricular apex was captured directly with an St-B interval of 63 +/- 19 ms. The St-A interval was gradually prolonged with the shortening of the pacing CL, whereas the St-B interval remained unchanged. VT was interrupted in all patients at the pacing CL of 279 +/- 39 ms. The effects of intravenous lidocaine (1 mg/kg) and verapamil (1 mg) were examined in 5 and 7 patients, respectively. Neither drug terminated VT but the VT-CL was increased to 369 +/- 57 ms after lidocaine (p <0.05) and to 413 +/- 69 ms after verapamil (p <0.05) (p <0.05 vs after lidocaine). The St-A interval was significantly increased after lidocaine (p <0.05) and after verapamil (p <0.05), while the St-B interval remained unchanged. A significant correlation between changes in St-A interval and VT-CL after verapamil was noted (p <0.001). In conclusion, the slow conduction zone of this VT shows tachycardia-dependent conduction delay, and the mechanism of this slow conduction involves mainly calcium channel-dependent conduction and partly depressed sodium channel-dependent conduction.

Entrainment techniques for mapping atrial and ventricular tachycardias

Many atrial tachycardias, atrial flutter, and postmyocardial infarction ventricular tachycardias are due to reentry through large "macroreentrant" circuits. These circuits can be difficult to define by catheter mapping of the activation sequence. Entrainment techniques allow the relation of a mapping site to the reentrant circuit to be assessed on a site-by-site basis during catheter mapping. Regions of abnormal conduction that are in the reentrant circuit can be distinguished from bystander sites outside the circuit. A mapping site classification to guide catheter ablation is reviewed.

Catheter ablation of ventricular tachycardia in 136 patients with coronary artery disease: results and long-term follow-up

OBJECTIVES

This study attempted to determine the feasibility and long-term efficacy of catheter ablation by means of either radiofrequency or direct current energy in a selected group of patients with coronary artery disease.

BACKGROUND

Catheter ablation of ventricular tachycardia has proved to be highly effective in patients with idiopathic and bundle branch reentrant ventricular tachycardia. In patients with coronary artery disease and recurrent sustained ventricular tachycardia resistant to medical antiarrhythmic management, the value of catheter ablation has not yet been established.

METHODS

One hundred thirty-six patients with coronary artery disease and one configuration of monomorphic sustained ventricular tachycardia underwent radiofrequency (72 patients) or direct current catheter ablation (64 patients). The mapping procedure to localize an adequate site for ablation included pace mapping during sinus rhythm, endocardial activation mapping, identification of isolated mid-diastolic potentials and pacing interventions during ventricular tachycardia.

RESULTS

Primary success was achieved in 102 (75%) of 136 patients (74% of 72 undergoing radiofrequency and 77% of 64 with direct current ablation). Complications were noted in 12% of patients. During a mean (+/- SD) follow-up period of 24 +/- 13 months (range 3 to 68), ventricular tachycardia recurred in 16% of patients.

CONCLUSIONS

Catheter ablation of ventricular tachycardia in coronary artery disease is feasible in patients with one configuration of monomorphic sustained ventricular tachycardia. There is no significant difference with respect to the type of energy applied. The follow-up data show that in a selected group of patients with coronary artery disease, catheter ablation offers a therapy alternative.