(Circulation. 1999;100:1784-1790.)
© 1999 American Heart Association, Inc.
Clinical Investigation and Reports |
Transient Entrainment of Bundle-Branch Reentry by Atrial and Ventricular Stimulation
Elucidation of the Tachycardia Mechanism Through Analysis of the Surface ECG
From the Arrhythmia Unit, Department of Cardiology, Hospital La Paz, Universidad Autónoma, Madrid, Spain.
Correspondence to Dr Jose L. Merino, Laboratorio Electrofisiología Cardíaca, Hospital General La Paz, P. Castellana 261, 28046 Madrid, Spain. E-mail jlmerino{at}jet.es
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Abstract |
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Background—Different responses to entrainment have been reported in relation to the pacing site of a variety of tachycardias. However, transient entrainment of bundle-branch reentrant tachycardia (BBRT) has not been investigated systematically.
Methods and Results—We attempted entrainment of 13 BBRTs in 9 patients by pacing first the right ventricle and then the right atrium. The initial pacing cycle length (CL) was 10 ms faster than the tachycardia CL. Subsequent pacing sequences were performed with 5- to 10-ms CL decrements until tachycardia termination or loss of postatropine 1:1 AV conduction. Both full ventricular-paced and AV-conducted QRS complex references were obtained during sinus rhythm pacing from the same sites and with similar CL as during entrainment. Transient entrainment was achieved by ventricular and atrial stimulation in 11 and 8 tachycardias, respectively. Constant fusion was always present during entrainment by ventricular stimulation. There was no change in the QRS complex (orthodromically concealed fusion) during entrainment by atrial stimulation in 6 of 6 tachycardias with left bundle-branch block morphology and in 1 of 2 tachycardias with right bundle-branch block morphology.
Conclusions—BBRT, especially if it has a left bundle-branch block morphology, can be differentiated from other wide-QRS-complex tachycardia mechanisms through analysis of the ECGs recorded during tachycardia entrainment by atrial and ventricular stimulation. This diagnostic approach may be especially useful when it is difficult to record a stable or sufficiently sized His bundle electrogram or when spontaneous changes in the ventricular CL precede similar changes in the His bundle CL.
Key Words: ablation • bundle-branch block • electrophysiology • entrainment • tachycardia
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Introduction |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Bundle-branch reentry (BBR) is an uncommon but clinically relevant ventricular tachycardia (VT) mechanism, because catheter ablation can abolish the condition. It was previously thought to occur typically in patients with dilated cardiomyopathy but rarely in other clinical settings.1 However, more recently, BBR has commonly been found to be responsible for VT among patients with myotonic dystrophy2 3 or those who have undergone valve replacement.4 In addition, several reports have shown that BBR may be responsible for VT despite the observation of ventricular cycle length (CL) oscillations preceding rather than following His bundle CL oscillations,2 5 which is considered the most important diagnostic criterion. These findings suggest that BBR is often unrecognized and underline the need for additional diagnostic criteria.
Different responses have been described during transient
entrainment of a variety of
supraventricular6 7 8 and
ventricular9 10 11 12 13 14 tachycardias.
However, no systematic studies have been made of the responses to
entrainment of BBR tachycardia (BBRT). The purpose of the
present work was to study the responses to transient entrainment of
BBRT by atrial and ventricular stimulation in an attempt to
find new criteria to differentiate BBR from other wide-QRS-complex
tachycardia mechanisms. The following hypotheses were
tested (Figure 1
): first, entrainment of
BBRT by atrial pacing may result in the concealment of the
antidromic wave front within the His-Purkinje system and in a QRS
complex that displays neither fusion nor change; and second,
entrainment of BBRT by ventricular pacing may result in the
collision of the antidromic and orthodromic wave fronts within
unprotected ventricular myocardium and in a QRS
complex that displays manifest fusion.
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Methods |
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Patients
Nine consecutive patients with spontaneous wide-QRS-complex tachycardia were included in the study. All of them had at least 1 inducible wide-QRS-complex tachycardia that met the diagnostic criteria of BBRT (see below). The patients' characteristics are shown in Table 1
. A more detailed description of
5 of these patients has been given previously.2
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Electrophysiological Study
Cardiac invasive
electrophysiological studies were performed
in accordance with the institutional guidelines concerning informed
consent and after all antiarrhythmic drugs had been withdrawn. Three
quadripolar catheters were introduced percutaneously
through the right femoral vein and placed in the high right atrium
(HRA), His bundle area, and right ventricle. Three or 4 surface ECG
traces and 3 or 4 bipolar intracardiac recordings, filtered
between 30 and 500 Hz, were displayed simultaneously on a
digital multichannel oscilloscope (LabSystem, Bard Electrophysiology or
Midas, PPG Biomedical Systems). All 12 ECG traces and intracardiac
electrograms were stored on an optical disk for later
reproduction at 25, 50, 100, or 200 mm/s. Programmed
ventricular extrastimulation was performed at not less than
2 constant basic CLs from the right ventricular apex (RVA)
and the right ventricular outflow tract (RVOT) with 
3
extrastimuli. The stimulation protocol was repeated under isoproterenol
infusion when no tachycardia was induced at baseline.
BBRT diagnosis was established according to previously published criteria (criteria A)1 15 16 : (1) QRS complex morphology with typical bundle-branch–block pattern consistent with ventricular depolarization through the appropriate bundle branch; (2) AV dissociation during tachycardia; (3) exclusion of a tachycardia from supraventricular origin by established criteria; (4) prolonged HV interval during sinus rhythm; (5) a stable His or bundle-branch electrogram preceding each ventricular activation during tachycardia with an HV interval longer than, equal to, or <10 ms shorter than that recorded during sinus rhythm; (6) spontaneous changes in the bundle potential CL preceding similar changes in the ventricular CL; and (7) suppression of inducibility after right or left bundle-branch ablation, both at baseline and during isoproterenol infusion.
Because BBR has been found to be the tachycardia mechanism
despite criterion 6 not being demonstrated,2 5 BBRT
diagnosis was also established when all the following criteria
(criteria B) were fulfilled: (1) all criteria outlined in criteria A
were fulfilled except for criterion 6, that is, spontaneous changes in
the bundle potential CL followed rather than preceded similar changes
in the ventricular CL; (2) 
1 additional BBRT morphology
was also inducible and fulfilled all criteria given in criteria A; (3)
the difference in tachycardia CL was 30 ms compared with
those of the other induced BBRTs; (4) no myocardial VT, either
sustained or nonsustained, was inducible; (5) the patient had no
structural heart disease; and (6) the inducibility of all
tachycardias was suppressed after bundle-branch
ablation.
Entrainment Pacing Protocol
We attempted tachycardia transient entrainment by
pacing first from the RVA and later from the HRA. Pacing was performed
continuously during tachycardia for 5 seconds with a CL
10 ms shorter than that of the tachycardia. Subsequent
entrainment sequences were performed with 5- to 10-ms decreases in the
pacing CL until tachycardia termination or loss of 1:1 AV
conduction. Entrainment was also attempted by pacing from the RVOT when
there were similar QRS-complex configurations during
tachycardia and during pacing from the RVA during sinus
rhythm. Two milligrams of atropine, 2 µg/min isoproterenol, or both
were infused when there was no 1:1 AV conduction at the
tachycardia CL.
After tachycardia termination, full ventricular-paced and full AV-conducted QRS complex references were obtained during sinus rhythm, by ventricular and atrial pacing, respectively, from the same sites and with similar CLs (±25 ms) as during entrainment.
Twelve-lead ECG traces were printed at 50 mm/s and 10 mm/mV, and QRS-complex differences were analyzed by 2 independent evaluators.
Definitions
Transient entrainment was defined as transient and
constant acceleration to the pacing rate of all components necessary
for the tachycardia continuation (ventricular
and His-Purkinje activations in the case of BBRT) and with resumption
of the tachycardia at its intrinsic rate once pacing is
stopped.8 9 13 17 There is a fixed relationship
between the pacing CL and the first return cycle.
Transient entrainment with fusion was defined as transient entrainment in which there is evidence of ECG6 17 or intraventricular electrogram fusion.17 18
Transient entrainment with manifest fusion (manifest entrainment) was defined as entrainment demonstrating QRS-complex constant fusion at a constant pacing CL or different degrees of fusion at different pacing CLs, except for the last captured QRS complex, which is entrained but does not show fusion.6 17
Transient entrainment with concealed fusion (concealed entrainment) was defined as entrainment showing a constant QRS morphology with no evidence of surface ECG fusion. Concealed fusion is suspected when either (1) there is evidence of intraventricular electrogram fusion6 17 18 ; (2) manifest fusion during entrainment of the same tachycardia is demonstrated by pacing from a different site with the same CL11 or from the same site with a different CL; or (3) the QRS-complex morphology recorded during entrainment is the same as that recorded during spontaneous tachycardia and different from that recorded during sinus rhythm pacing from the same site and with the same CL as during entrainment.
Transient entrainment with orthodromically concealed fusion (orthodromically concealed entrainment) was defined as entrainment with concealed fusion of the QRS complex that displays the same morphology as that recorded during spontaneous tachycardia. The antidromic wave front is concealed within the ECG nonexpressive area by the orthodromic wave front, which is the only one apparent on the ECG.
Transient entrainment with antidromically concealed fusion (antidromically concealed entrainment) was defined as entrainment with concealed fusion of the QRS complex that displays the same configuration as that recorded during sinus rhythm pacing from the same site and with the same CL as during entrainment. The orthodromic wave front is concealed within the ECG nonexpressive area by the antidromic wave front, which is the only one apparent on the ECG.
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Results |
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Inducible Tachycardias
Eleven sustained tachycardias were inducible (Table 2
). Except for tachycardia 3
of patient 8 and tachycardia 2 of patient 9, these
tachycardias fulfilled all criteria A of BBRT. The former
tachycardias displayed oscillations in the VV
interval preceding those of the HH interval and fulfilled all criteria
B of BBRT.
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Achievement of Transient Entrainment
Entrainment of all 11 sustained tachycardias was
achieved by ventricular stimulation and, except for the
tachycardias of patients 6 and 7 and
tachycardia 3 of patient 8, also by atrial stimulation
(Figures 2 through 5). In patients
6 and 7, tachycardia was not entrained by atrial
stimulation because no 1:1 AV conduction was obtained at the
tachycardia CL. No attempts were made to entrain
tachycardia 3 of patient 8 by atrial stimulation because
the tachycardia was not reinduced after termination by
ventricular stimulation. The return cycle after entrainment
by ventricular stimulation showed a flat pattern in all
entrained tachycardias with an entrainment zone of 30 ms
except for the tachycardia of patient 7,
tachycardias 1 and 3 of patient 8, and
tachycardia 1 of patient 9, which showed a mixed
flat/increasing pattern (31.0±5.5 ms return-cycle difference).
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The infusion of atropine (1 to 2 mg) was required to obtain 1:1 AV
conduction and tachycardia entrainment by atrial
stimulation in all tachycardias except
tachycardia 1 of patient 8 and the tachycardia
of patient 5, who was undergoing isoproterenol infusion when
entrainment was attempted (Table 2
). There were no complications
related to the administration of atropine or isoproterenol.
Surface ECG Analysis
The QRS complex exhibited manifest fusion during entrainment by
ventricular stimulation of all sustained
tachycardias (Figures 2 and 3). Differences between spontaneous
tachycardia, entrained, and fully paced QRS complexes were
readily visible in all cases when pacing was performed from the RVA.
However, in patient 2, these differences were much better appreciated
when pacing was performed from the RVOT (Figure 3).
Orthodromically concealed QRS complex fusion was observed during
entrainment by atrial stimulation (Figures 2 through 4) of all entrained
tachycardias except for tachycardia 2 of
patient 8 (Figure 5). In this
latter tachycardia, the QRS complex displayed manifest
fusion during entrainment by atrial pacing. All entrained
tachycardias, except for tachycardia 1 of
patient 9, showed differences between the QRS complex recorded
during tachycardia and that recorded during pacing from
the HRA atrium with the same CL as during entrainment and while the
patient was in sinus rhythm.
Catheter Ablation
BBRT was successfully treated by bundle-branch ablation guided by
the recording of a bundle-branch potential in all
patients except for patients 3 and 9, in whom no right
bundle-branch potential recording was achieved but successful
ablation was accomplished in the right bundle-branch area.
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Discussion |
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Diagnosis of BBRT is most commonly established by the observation of a His bundle electrogram preceding each QRS complex by an interval equal to or longer than that observed during sinus rhythm and that shows spontaneous changes in its CL preceding those of the ventricular activation.1 Other criteria, such as tachycardia initiation or termination with a critical conduction delay or block within the His-Purkinje system, are also dependent on the recording of a His or bundle-branch electrogram. However, there are limitations to this approach2 5 19 that could lead to the failure to recognize BBRT. First, and as in our patients 6 and 7, catheter displacement during tachycardia often makes the recording of a quality, stable His bundle electrogram difficult.1 Second, and as in our patients 3 and 4, BBRT typically occurs in patients with a severely diseased conducting system, which is associated with the recording of a low-amplitude or smooth His bundle electrogram, particularly at short CLs (Figure 4
). Third, spontaneous changes
in the His electrogram CL preceding those of the
ventricular activation are typically seen in AV nodal or
hisian tachycardias and therefore are not useful for the
differentiation of these tachycardias from BBRT. Finally,
spontaneous changes in the His electrogram CL following rather than
preceding those of the ventricular activation do not
exclude BBR,2 5 because changes in the HV interval (due to
conduction variations in the antegrade conducting bundle branch)
result in VV-interval oscillations driving those of the HH
interval, as in tachycardia 3 of our patient 8 and
tachycardia 2 of our patient 9. The present study
introduces new criteria for the diagnosis of BBRT without the need for
a His bundle or bundle-branch electrogram.
Differential Diagnosis
Several mechanisms of wide-QRS-complex
tachycardia have to be ruled out before a BBRT diagnosis
can be established. Myocardial VT and AV nodal tachycardias
should be particularly considered, because VA dissociation is almost
always present in BBRT.1
Myocardial VT With Secondary His-Purkinje Activation
In the present study, BBRT entrainment by
ventricular stimulation exhibited manifest QRS-complex
fusion in all. Therefore, the observation of this ECG response does not
distinguish between BBRT and myocardial reentrant VT, because manifest
fusion during entrainment by RVA stimulation is demonstrated in 
67%
of postinfarction VTs.13 On the other hand, concealed
fusion, either antidromic or orthodromic, during
tachycardia entrainment by RVA stimulation makes BBR
unlikely and favors myocardial reentry. In this case, failure to
recognize fusion, due to a similar QRS complex during
tachycardia and during pacing from the RVA while the
patient is in sinus rhythm, should be excluded, and, if this is the
case, entrainment by RVOT stimulation is warranted.
Manifest fusion during entrainment by atrial stimulation has been reported in up to 54% of myocardial VT.10 12 Conversion to a supraventricular QRS-complex morphology or no 1:1 AV conduction at the tachycardia CL were seen in the remainder. In those studies, neither atropine or isoproterenol infusions were made to achieve 1:1 AV conduction at the shortest atrial pacing CL. In the present study, BBRT entrainment by atrial stimulation exhibited orthodromically concealed fusion in all tachycardias except for tachycardia 2 of patient 8, 1 of the only 2 tachycardias with a right bundle-branch block morphology. Therefore, the observation of orthodromically concealed fusion during entrainment of a wide-QRS-complex tachycardia by atrial stimulation rules out a pure myocardial reentry mechanism and favors BBR.
AV Nodal and Hisian Tachycardias
The presentation of AV nodal
tachycardias, either reentrant or automatic, with VA
dissociation has been reported.20 21 These
tachycardias, although uncommon, should be differentiated
from BBRT, particularly if the QRS complex recorded during
tachycardia is similar to that recorded during sinus
rhythm, as in tachycardia 1 of our patient 3, the
tachycardia of our patient 6, and tachycardia 1
of our patient 9.
Entrainment of AV nodal reentrant tachycardia by atrial7 and ventricular7 8 stimulation has been studied, and in theory, the same responses should be observed during entrainment of hisian tachycardias, either of microreentrant or focal mechanism. Orthodromically and antidromically concealed fusions of the QRS complex are typically seen during entrainment by atrial and ventricular stimulation, respectively. Therefore, entrainment by atrial stimulation is not useful to distinguish between BBRT and AV nodal or hisian tachycardias. On the other hand, the observation of manifest fusion during entrainment of a wide-QRS-complex tachycardia by ventricular stimulation rules out a pure AV nodal or hisian mechanism in favor of BBR.
Interfascicular Reentrant Tachycardia
There are few reports of interfascicular reentrant
tachycardia.15 19 None of them studied
entrainment in this particular setting, and therefore, the responses to
entrainment are speculative. However, from a theoretical point of view
and similar to that shown in Figure 5, the impulse should
initiate conduction in the right bundle branch earlier than in either
of the 2 left hemifascicles during entrainment by atrial stimulation,
and therefore, in the absence of right bundle-branch block at the
tachycardia CL, a change of the tachycardia QRS
complex should be expected. Nevertheless, this tachycardia
may be distinguished from BBRT because it usually displays a markedly
shorter HV interval than that recorded during sinus rhythm.
Limitations
There are several limitations with regard to the use of
entrainment techniques to differentiate BBR from other mechanisms of
wide-QRS-complex tachycardia. First, the
tachycardia should be sustained and tolerated to allow
pacing maneuvers. Second, the coexistence of chronic atrial
fibrillation prevents entrainment by atrial pacing. Finally, a poor 1:1
AV conduction, as in our patients 6 and 7, may preclude the achievement
of tachycardia entrainment by atrial stimulation even after
atropine infusion. Nevertheless, entrainment was achieved by
ventricular and atrial stimulation in the majority of BBRTs
in the present study despite these potential limitations.
Conclusions
Analysis of the ECG during entrainment of BBR by
nonsimultaneous atrial and ventricular
stimulation can be used to differentiate this mechanism from other
types of wide-QRS-complex tachycardia. Observation of
manifest fusion during entrainment by ventricular pacing
rules out an AV nodal or hisian mechanism and should be expected in
BBRT. Observation of orthodromically concealed fusion during
entrainment by atrial pacing rules out VT due to pure myocardial
reentry and should be expected in BBRT. However, observation of
manifest fusion during entrainment by atrial stimulation does not rule
out BBRT when it has a right bundle-branch morphology, and an
alternative diagnostic approach is warranted. Finally, this
diagnostic approach should be considered especially useful
when it is difficult to record a stable or sufficiently sized His
bundle or bundle-branch electrogram during tachycardia,
when AV nodal or hisian tachycardias are considered, or
when spontaneous changes in the ventricular CL precede
rather than follow similar changes in the His bundle or bundle-branch
CL.
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Acknowledgments |
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This study was honored with the PES award at the 8th International Madrid Arrhythmia Meeting, Spain, April 1997. The authors wish to thank Dr Jesús Almendral for his critical review of the manuscript, Elena Morala, RN, for her technical assistance, and Martin Hadley-Adams for his assistance with the English language.
Received December 31, 1998; revision received July 9, 1999; accepted July 13, 1999.
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