2.VA interval VA>70ms aAVNRT,AVRT,A T
VA≤70ms tAVNRT,AT
VA>AV aAVNRT,AT,AVRT using slowly
conducting AP
3.Atrial activation sequence High to low AT
Concentric AVNRT,AVRT,AT
Eccentric AVRT,A T*
4.Spontaneous termination Ends with an“A”AVNRT,AVRT
Ends with a“V”AVNRT,AVRT,AT
5.HH changes precede and predict AA changes Y es
No
AVNRT,AVRT
AVNRT,AVRT,AT
6.VA increase>30ms with functional BBB Y es AVRT with free wall AP ipsilateral to
BBB
No AVNRT,AVRT,A T
aAVNRT=atypical AVNRT;tAVNRT=typical AVNRT;ONVRT=orthodromic nodoventricular reciprocating tachycardia;ONFRT= orthodromic nodofascicular reciprocating tachycardia.*AVNRT with a Leftward atrionodal exit is uncommon but still possible.AT is most likely,but AVNRT and AVRT are theoretically still possible.
variations in tachycardia cycle length(TCL),His-His(HH)or interventricular(VV)interval changes precede and predict interatrial(AA)interval changes(i.e.,the His-Atrial[HA]or VA interval is constant despite HH or VV interval changes),(e) termination of SVT on a nonpremature terminal atrial electrogram with the same atrial activation sequence as SVT,and(f)changes in the VA interval with the appearance or disappearance of functional bundle branch block.It is noteworthy that,after studying these features of the SVT,an ablation catheter rather than a pacing maneuver may be what is required next.For example,if SVT has a septal VA interval<70ms(excluding AVRT)and HH interval changes precede and predict AA interval changes,or the SVT stops with a nonpremature terminal atrial electrogram (excluding AT,particularly if the latter happens more than once so that this is not a mere coincidence),a diagnosis of typical AVNRT can be made and the slow atrioventricular(AV)nodal pathway can be targeted for ablation.On the other hand,it should not be surprising that differentiating among tachycardias with a1:1V-A relationship,septal V-A interval>70ms,and a central atrial activation sequence has received considerable attention in the literature,since each of the usual SVT mechanisms may be operative.One would want to be very sure of the correct SVT mechanism in these cases before choosing an ablation target because of the variable risk of AV block associated with ablation at different sites in the septum.For example,it would be unwise to mistake AVNRT for a septal AT or AVRT employing a septal accessory pathway(AP),since both of the latter mechanisms require mapping to the earliest atrial electrogram,which,in the case of AVNRT,could well lead one to ablate in a region where the risk of AV block is considerably higher than targeting the slow AVN pathway in the region between the coronary sinus os and the tricuspid valve annulus.It is also noteworthy that often the correct diagnostic pacing maneuver must be chosen after some combination of the features in Table I permits the SVT mechanism to be narrowed down to only two possibilities.
Quickly Ruling AT In or Out
Since AT can have any atrial activation sequence and any VA interval,it is in the differential diagnosis of12(80%)of the15 diagnostic categories found in the extreme right column of Table I.Overdrive pacing from the right ventricle(RV)at a cycle length(CL)that is 10–40ms shorter than the TCL provides a rapid tool to rule AT in or out.1If,during overdrive
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Figure1.Panel A:response after cessation of overdrive ventricular pacing(340ms)in an atrial
tachycardia(cycle length360ms).The atrial cycle length(CL)was accelerated to the ventricular
pacing CL and then slowed immediately after pacing was stopped.The last atrial electrogram
that was accelerated to the pacing CL is the?rst atrial electrogram labeled“A.”The response
after pacing is stopped is A-A-V,which indicates a diagnosis of AT.Also note that the septal
atrial electrograms(pHIS,CS9,10)precede the high right atrial(HRA)electrogram during pacing
with1:1ventriculoatrial(VA)conduction(low-to-high atrial activation),while that activation is
reversed during the AT(high-to-low atrial activation).A change in the atrial activation sequence
during overdrive ventricular pacing with1:1VA conduction is also consistent with a diagnosis
of AT(In fact,a descending pattern of atrial activation during the tachycardia alone is suf?cient
to indicate a diagnosis of AT).Panel B:response after cessation of right ventricular overdrive
pacing in an orthodromic atrioventricular reciprocating tachycardia(AVRT)using a concealed
left-sided accessory pathway.The atrial CL was accelerated to the ventricular pacing CL and
then slowed to the prepacing tachycardia atrial CL immediately after pacing was stopped.The
atrial activation sequence during VOP is the same as the atrial activation sequence during SVT.
The last atrial electrogram that was accelerated to the pacing CL is the atrial electrogram labeled
“A.”The response after pacing is stopped is A-V,which excludes a diagnosis of AT.(stimulus-
to-atrial[SA]=285ms;VA=210ms;SA-VA=75ms;PPI=520ms;TCL=380ms;PPI-TCL=
140ms;cPPI-TCL=120ms.The cPPI-TCL and SA-VA differences are“borderline”because of
the distance of the pacing site from the left sided AVRT circuit.)
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ventricular pacing,the atrial CL is accelerated to the pacing CL,and the tachycardia continues after pacing is stopped,then a post-VOP response that is atrial-atrial-ventricular(A-A-V)rules in AT while a post-VOP response that is atrial-ventricular(A-V)rules out AT(effectively ruling in AVRT or AVNRT).The last atrial electrogram accelerated to the pacing CL is the?rst atrial electrogram counted in the interpretation of this response(Fig.1). The main shortcoming of this pacing maneuver is that,in50–80%of cases of AT,the atria are not accelerated to the pacing CL(the ventricles are dissociated from the tachycardia),so the response is technically not interpretable(though this particular response still excludes AVRT).2–4When this is the case,the diagnosis is usually AT,2,4 though further information would still be required to prove a diagnosis of AT rather than AVNRT.
It is important to understand why VOP can rule AT in or out.First consider AT:during VOP, as long as the VA block CL is not longer than the TCL,there will eventually be1:1VA conduction over the normal AV conduction system,and each retrogradely conduced atrial wavefront will overdrive,suppress,or entrain(if reentrant)the AT.The last paced retrogradely conducted atrial wavefront(which will be responsible for the ?rst atrial electrogram counted in the post-VOP response)cannot echo back to the ventricle be-cause the AV conduction system is still refractory from having just conducted that wavefront to the atrium.It seems that even when either dual AVN physiology or a bystander AP happens also to be present,none of these routes are available for the last paced retrogradely conducted atrial wavefront to echo to the ventricle as all are penetrated by the pacing wavefront and remain refractory to the antegrade conduction that would lead to such an echo beat.1If the AT has not terminated, the next atrial electrogram(i.e.,the second atrial electrogram counted in the post-VOP response) will result from the continuation of the AT,and by then,the AV conduction system will usually have recovered so that the next electrogram will be a His electrogram followed by a ventricular electrogram, hence,an A-A-V response(Fig.2A).
In orthodromic AVRT,as long as the refrac-tory periods of the participating ventricle,AP, and atrium do not exceed the TCL,eventually, during VOP,there will be1:1VA conduction of the stimulated orthodromic wavefront(so called because it travels in the same direction as the tachycardia circuit)via the AP(so the atrial activation sequence during VOP ought to be identical to that of the SVT).A portion of that stimulated wavefront,called the stimulated antidromic wavefront(because it travels in the opposite direction of the tachycardia circuit),will collide with the orthodromic wavefront from the preceding beat either in ventricular myocardium or in the AV conduction system(Fig.2B).At this point,each stimulated orthodromic wavefront is resetting the tachycardia to the pacing CL, and each stimulated antidromic wavefront is colliding with,or fusing with,the orthodromic wavefront from the previous beat.Thus,the tachycardia is entrained(i.e.,continually reset by the pacing train).If there is fusion in the QRS complex morphology during VOP(i.e.,the QRS morphology is a fusion beat combining some aspects of the QRS complex morphology of a fully paced beat with some aspects of the QRS complex morphology of the SVT),there is proof that the SVT is entrained(manifest entrainment).In the absence of evidence of fusion,as long as AVRT continues after pacing is stopped,it is reasonable to assume that AVRT was entrained(concealed entrainment).The last paced retrogradely conducted atrial wavefront (producing the?rst atrial electrogram counted in the post-VOP response)can now revolve through the AVRT circuit and,because there is no new stimulated antidromic wavefront for it to collide with,conduct back to the ventricles over the normal AV conduction system,producing a His electrogram and a ventricular electrogram,hence, an A-V response.
Exactly the same thing happens when typical AVNRT is entrained by VOP,except that the stimulated wavefront must travel up the His-Purkinje system to reach the AV node where the stimulated orthodromic wavefront resets AVNRT via the fast AV node pathway(and also accelerates the atria to the pacing CL),and the stimulated an-tidromic wavefront collides with the orthodromic wavefront from the previous beat somewhere in the slow AV node pathway(Fig.2C).The last retrogradely conducted stimulated wavefront produces the?rst atrial electrogram that is counted in the post-VOP response,and also revolves around the AV node circuit to conduct back down the slow AV node pathway to reach the lower common pathway,His-Purkinje network, and ventricles because there is no new stimulated antidromic wavefront for it to collide with,hence, an A-V response.Note that,during entrainment of typical AVNRT,the collision between the stimu-lated antidromic wavefront and the orthodromic wavefront from the previous beat occurs in the AVN and cannot possibly occur in ventricular myocardium.Accordingly,QRS complex mor-phology fusion cannot occur,and entrainment of AVNRT cannot be proven.(See later.)
Thus,a post-VOP response that is A-V indicates entrainment of AVRT or AVNRT and excludes AT(though proof of entrainment of
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Figure2.Responses to ventricular overdrive pacing(VOP)for three common SVT mechanisms.
In each panel,square wave=pacing site,solid arrows=antidromic paced wavefront,dashed
arrows=orthodromic wavefront,dotted arrows=orthodromic wavefront from the previous
beat.Panel A:in atrial tachycardia(AT),VOP causes overdrive suppression(if focal mechanism,
faded star),or entrainment(if reentrant mechanism)of the SVT.The last paced impulse conducts
retrogradely to the atrium and is followed by a beat of AT(star),which then conducts antegradely
to the ventricle—hence an“A-A-V”or“A-A-H”response.Panel B:in orthodromic AVRT(shown
in this case using a right free-wall pathway),the last impulse of VOP conducts via the accessory
pathway to the atrium,then continues around the circuit through the AV node and conduction
system to the ventricle—hence an“A-V”or“A-H”response.The antidromic paced impulse
collides with the orthodromic impulse from the previous beat either in the conduction system
(as shown here,black bar)or in ventricular myocardium.Panel C:In typical AVNRT,the last
paced impulse of VOP conducts retrogradely via the conduction system,enters the excitable gap
in the AV nodal circuit,activates the atrium via the fast pathway,then conducts via the slow
pathway back to the His-Purkinje system to activate the ventricle—hence an“A-V”or“A-H”
response.Collision of the stimulated antidromic wavefront during VOP occurs in the AV nodal
slow pathway(black bar).
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Figure 3.A “pseudo A-A-V response.”The atria are accelerated to the overdrive pacing cycle length (CL =360ms)and the tachycardia resumes after pacing is stopped (380ms).Note that the second atrial electrogram after the last paced beat is the last atrial electrogram that is accelerated to the pacing CL,so this is an A-V response.Failure to recognize this could lead to counting the ?rst atrial electrogram after the last paced beat (*)in the post ventricular pacing response,leading to an erroneous conclusion of an A-A-V response.This was,in fact,a case of fast-slow AVNRT (PPI =565ms;cPPI-TCL =185ms;SA =415ms;VA =265ms;SA-VA =150ms).The recordings labeled pABL and dABL are from the right atrium.The coronary sinus catheter electrodes are labeled 9,10as proximal and 1,2as distal.
AVRT is only available when there is evidence of fusion),while a post-VOP response of A-A-V indicates that the SVT mechanism is not capable of echoing the last stimulated retrogradely conducted atrial wavefront back to the ventricles,as is the case in AT.Note that the features of,and criteria for,entrainment can be studied in the context of familiar SVT circuits.5
Pitfalls in the interpretation of the post-VOP response include the following:
1.Incorrectly identifying the last atrial electrogram that is accelerated to the pacing CL .
As mentioned above,the last atrial electro-gram accelerated to the pacing CL is the ?rst response that ought to be annotated after pacing is stopped.When the SVT mechanism is AVRT or AVNRT,but the retrograde limb of the circuit conducts slowly,as it might in atypical AVNRT (aAVNRT)(such as in so-called “slow-slow”or “fast-slow”AVNRT)or AVRT employing a slowly conducting AP,the VA interval after the last paced beat may be longer than the pacing CL such that the second atrial electrogram after the last paced ventricular beat may in fact be the last atrial electrogram that was accelerated to the pacing CL (Fig.3).When this is the case,the interval between the ?rst and second atrial electrograms after the last paced ventricular beat will be the same as the pacing CL.If this is not recognized,then a “pseudo-A-A-V”response may be incorrectly interpreted as indicating a diagnosis of AT.It is possible that the ?rst beat of an AT may occur after the last atrial electrogram resulting from VA conduction at a time interval that is equal to the pacing CL by pure coincidence.6Accordingly,it is worthwhile to repeat VOP several times and at decrementally shorter CLs whenever possible to show that the A-A-V response is reproducible.It is also necessary to examine whether the atrial activation sequence during 1:1V-A conduction is the same as during SVT,6as a different atrial activation sequence in this setting could be the clue that AT is present (Fig.1).
2.When the pacing CL is not short enough,or when the TCL shortens just before or during pacing,so that 1:1VA conduction during pacing is not present and the tachycardia and the pacing train are just isorhythmically dissociated from each other.
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Figure 4.Overdrive pacing at a CL of 340ms during typical AVNRT (CL 380ms).The electrogram labeled “A”is the last atrial electrogram accelerated to the pacing CL (340ms).Note that on the ?rst beat after pacing stops,the HV interval exceeds the HA interval.The HA interval on the ?rst beat after pacing can be shorter than during stable SVT because of the preceding decrement in slow pathway conduction induced by VOP,as in this case.One could be tempted to include the subsequent atrial electrogram (*)in the post overdrive pacing response,but this would be incorrect.Although it does precede the following ventricular electrogram,it occurs after the His bundle electrogram indicating that it could not possibly have conducted antegradely through the His-Purkinje system to produce the ventricular electrogram.Only atrial electrograms that can conduct antegradely to produce the ventricular electrogram are counted in the post ventricular pacing response.For cases such as these,where the HV interval exceeds the HA interval,the error of considering this to represent an A-A-V response can be avoided by considering the response as A-H instead.The recordings labeled pABL and dABL are from the right atrium.The coronary sinus catheter electrodes are labeled 9,10as proximal and 1,2as distal.
Two ways to avoid this problem include 1)performing the maneuver repeatedly and decrementing the pacing CL by 10–20ms after each apparently successful attempt to accelerate the atria to the pacing CL and 2)checking to see that after pacing has stopped,the TCL immediately returns to the longer pre-pacing TCL,or at least a CL that is longer than that to which the atria were accelerated during pacing (Fig.1and 3).
3.When the HV interval exceeds the HA interval in AVNRT.
In AVNRT,if the HV interval exceeds the HA interval (which can happen when the HV interval is long or when the HA interval is very short or even a negative value as can occur in AVNRT with a long lower common pathway),the last entrained atrial electrogram will be followed by a ventricular electrogram but that ventricular electrogram will be preceded by a second atrial electrogram resulting from the ongoing AVNRT circuit (and not from pacing)(Fig.4).This “pseudo-A-A-V”response could lead to an error that can easily be avoided by considering the post-VOP response as A-A-H or A-H rather than as A-A-V or A-V.7Doing so should also avoid the potential for a pseudo-A-A-V response in the rare case where the ?rst return beat of AVNRT blocks below the His-bundle.Accordingly,for the rest of this article,we will refer to the post-VOP response as A-H or A-A-H.
4.AVNRT with block below the lower common pathway on the ?rst return beat.
Theoretically,the ?rst return beat after entrainment of AVNRT could block below the lower common pathway,but above the His bundle (so that a His potential would not be recorded),resulting in an A-A-H response in AVNRT.Accordingly,the post-VOP response may be unreliable in cases where there is spontaneous AV block during SVT.We are not aware of a
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case where this has happened,and so we believe that the chances of block occurring below the lower common pathway on only the?rst return beat after overdrive ventricular pacing are low enough that such a response should prompt strong consideration of a diagnosis of AT.Other features or pacing maneuvers may need to be considered if AVNRT is otherwise suggested.
5.Coexistence of AVNRT or AVRT with an AT
An A-H response indicates that either AVNRT or AVRT are present,but does not exclude the possibility that an AT may also be present.It is always prudent to test for the inducibility of other forms of SVT after one substrate has been eliminated.
What Next?
If AT is ruled in by VOP,then the next step would be mapping and ablation of the AT.If AT is ruled out by VOP,the diagnosis of AVNRT or AVRT may be clear,based on the features in Table I.When the atrial activation sequence is concentric,and the VA interval is>70ms, more information will be required to distinguish between AVRT employing a septal AP and aAVNRT.Thankfully,that information is often already present in other features of the response to VOP.For the remainder of the discussion, unless otherwise stated,only tachycardias with concentric atrial activation(aAVNRT with a VA >70ms and AVRT employing a septal AP)are considered.
Fusion during Entrainment
As mentioned above,a post-VOP response of A-H indicates entrainment of either AVNRT or AVRT and,in the case of entrainment of AVRT,there is an opportunity to observe QRS complex fusion due to collision of the stimu-lated antidromic wavefront with the orthodromic wavefront from the previous beat occurring in ventricular myocardium(Fig.5).Because QRS complex fusion is impossible during entrainment of AVNRT,the presence of QRS complex fusion distinguishes AVRT from AVNRT.Unfortunately, like most diagnostic features for SVT,QRS complex fusion during entrainment is speci?c for AVRT,but not sensitive.That is,during VOP, entrainment of AVRT is usually concealed:the QRS complex morphology is that of a paced beat because the orthodromic wavefront from the previous beat collides with the stimulated antidromic wavefront in the AV conduction system,and fusion(and therefore proof of entrainment)is not present(e.g.,as depicted in Fig.2B).This is called“concealed entrainment”(the tachycardia is entrained,but entrainment cannot be proven,since proof of entrainment requires the demonstration of fusion).
To maximize the opportunity to detect fusion during entrainment of AVRT by VOP(i.e.,to increase the sensitivity of fusion for AVRT), one can also include evidence of fusion in the intracardiac tracings rather than just in the surface QRS complex morphology.The presence of an orthodromically captured His or right bundle potential during VOP indicates that the orthodromic wavefront from the previous beat has reached the AV conduction system and will surely collide with the stimulated antidromic wavefront (Fig.5).This collision point may occur within the distal AV conduction system or within ventricular myocardium before the orthodromic wavefront from the previous beat has depolarized a suf?cient amount of ventricular myocardium to affect the paced QRS complex morphology.It is worth emphasizing this point:in the latter two cases the QRS complex morphology will either be identical to,or virtually indistinguishable from,the QRS complex morphology of a fully paced beat,yet the presence of an orthodromically captured His po-tential is intracardiac evidence that fusion is tak-ing place in the circuit somewhere distal to the His bundle,thus indicating that the circuit is AVRT.
The Importance of the Pacing Site in Permitting Fusion during Entrainment of AVRT by VOP
Another way to increase the sensitivity of QRS complex morphology fusion during entrainment of AVRT is to permit the orthodromic wavefront from the previous beat to depolarize as much ventricular myocardium as possible before col-liding with the stimulated antidromic wavefront. Because the orthodromic wavefront from the previous beat begins to depolarize ventricular myocardium as it exits the His-Purkinje network, the pacing location that would permit that wavefront to depolarize as much ventricular myocardium as possible before colliding with the stimulated antidromic wavefront is the one farthest from the interface of the His-Purkinje network and ventricular myocardium,while still close to the AVRT circuit.That is,as close as possible to the ventricular insertion of the AP, on the ventricular side of the AV groove opposite the earliest atrial electrogram in SVT(Fig.6). The closer the pacing site is to the ventricular insertion of the AP,the more likely fusion becomes,perhaps to the point of concealed fusion (where the paced QRS complex morphology is the same as the QRS complex morphology of the tachycardia)(Fig.7).Accordingly,when VOP is performed from the right ventricular apex, manifest entrainment(QRS complex fusion during entrainment)is appreciable in the majority of
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Figure 5.Manifest entrainment of orthodromic AVRT employing a left free wall AP (CL =340ms)with fusion.Panel A:The atria are accelerated to the pacing CL (320ms)and the tachycardia continues after pacing is stopped.Note that the pacing site is the basal LV via a branch of the CS.The post-VOP response is A-H.There is an orthodromically captured His potential during pacing that is visible just after the pacing stimulus,indicating entrainment with intracardiac electrogram evidence of fusion.The PPI-TCL difference is 90ms (430–340ms).The ?rst return AH interval is 240ms,which is 60ms longer than the AH interval during tachycardia (due to decremental conduction slowing through the AV node at the shorter pacing CL),so the corrected PPI-TCL difference is only 30ms (90–60ms).The SA-VA interval difference is ?20ms.Panel B:QRS complex morphology of purely paced beats from the same pacing site that was used for VOP.Note that the QRS complexes during VOP (left side of Panel A)are narrower than those of a purely paced beat,and have an intermediate morphology between fully paced beats and beats of the https://www.doczj.com/doc/182986049.html, =left atrial;LV =left ventricular.(The PPI is measured from the pacing stimulus to the ?rst return ventricular electrogram recorded by the pacing channel.The SA interval is measured from the pacing stimulus to a consistent atrial electrogram,usually the earliest atrial electrogram.The VA interval is measured from the beginning of the earliest QRS complex in SVT to the same atrial electrogram that was used to measure the SA interval.)
AVRTs employing septal or right-sided APs,but is rarely evident for AVRTs employing a left-sided AP.4Similarly,when VOP is performed from the left ventricle (LV),manifest entrainment is appreciable in the majority of AVRTs employing left-sided APs.4
Note that when VOP is performed from the basal septum,inadvertent His bundle capture (or proximal right or left bundle capture)could produce a narrow QRS complex that could mimic fusion.This can be avoided by pacing superior or inferior to the His and right bundles.
Entrainment,Fusion,and “Bystander”APs This discussion has indicated that transient entrainment by VOP that results in manifest
fusion proves that the SVT is reentrant,that an AP is participating,and that the ventricle is a required component of the circuit,excluding both AVNRT and AT.Theoretically,a simultaneous AT originating close to the atrial aspect of an AP,or a simultaneous AVNRT with an atrial exit close to an AP,are not excluded by these ?ndings.Such an exceptional circumstance would require a double tachycardia or a double loop tachycardia where one of the tachycardias is orthodromic AVRT.Accordingly,such an AP could not strictly be considered a “bystander.”Ablation of the AP would ultimately be required both clinically and to unmask the second tachycardia mechanism.Thus,entrainment of SVT with fusion indicates that orthodromic AVRT is present;
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Figure6.Importance of pacing site to demonstration of QRS complex fusion.In panel A,with VOP from an apical site,collision of the stimulated antidromic wavefront(solid arrow)with the orthodromic impulse from the previous beat(dotted arrow)occurs within the conduction system(black bar).Thus the ventricle is activated entirely by the paced wavefront and QRS complex fusion will not be seen.In panel B,the pacing site has been moved to the base,close to the site of earliest atrial activation.In this case,the impulse from the previous beat(dotted arrow)has the greatest chance to exit the His-Purkinje system and activate most of the ventricle.The collision of the antidromic impulse and the prior orthodromic impulse occurs in ventricular myocardium.Manifest QRS fusion,or even concealed fusion(where the QRS morphology matches that of the tachycardia),will be apparent.The latter must be distinguished from isorrhythmic dissociation of the pacing stimulus from the tachycardia(i.e.,when the tachycardia is not actually accelerated to the pacing CL, but rather,the pacing CL is the same as the tachycardia CL because an inappropriately long pacing CL was chosen or the tachycardia accelerated as pacing was initiated).
nevertheless,other SVT mechanisms could also be present.
Beyond Fusion:The Need for Quantitative
Features of Entrainment of SVT by VOP
Despite considering evidence of fusion in intracardiac electrograms and pacing from ventric-ular sites close to the ventricular insertion of the operative AP,fusion during entrainment of AVRT is not always appreciable(i.e.,only concealed entrainment may be possible).Nevertheless, AVRT and AVNRT can reliably be distinguished by studying certain quantitative features of their entrainment,including the difference between the PPI and the TCL(the PPI-TCL difference), and the difference between the stimulus to atrial electrogram(SA)interval and the tachycardia VA interval(the SA-VA interval difference).
The PPI-TCL Difference
The PPI is the time required for the last stimulated orthodromic wavefront to propagate to an excitable gap in a reentrant circuit,make one revolution around that circuit,and return to the pacing site.Accordingly,if the pacing site is in the circuit,the PPI should approximate the TCL,and the PPI-TCL difference should only be0–30ms.The PPI-TCL difference should increase as the distance of the pacing site from the circuit increases.A portion of ventricular myocardium is“in circuit”for AVRT,while ventricular myocardium is relatively far from AVNRT circuits,separated from them by the intervening His-Purkinje network that must be traversed twice during entrainment of AVNRT by VOP:once to reach the AVN circuit,and once to get back to ventricular myocardium.Accordingly, the PPI-TCL difference should be considerably longer after entrainment of AVNRT than after entrainment of AVRT by VOP8(Fig.8).
The Corrected PPI-TCL Difference While the PPI will increase as the distance of the pacing site from the circuit increases,it may also increase if overdrive pacing causes decremen-tal(i.e.,rate dependent)conduction slowing.This is most likely to occur in the AV node because the AV node typically displays decremental conduction properties.During entrainment of AVRT by VOP,the atria are accelerated to the pacing CL.Therefore,the input to the AV node is also accelerated to the pacing CL and the AV node conduction time will increase in keeping with its decremental conduction properties.Thus,when AVRT is entrained by VOP,the?rst Atrio-His(AH) interval(or,assuming that the His-ventricular interval remains more or less constant,the?rst AV interval)after entrainment is often prolonged compared to the AH(or AV)interval during AVRT. This increase in the subsequent PPI is unrelated to the distance of the pacing site to the circuit.The PPI-TCL difference can be corrected for the degree of decremental conduction slowing by subtracting the magnitude of the increase in the AH(or AV)interval on the?rst return beat compared to the AH(or AV)interval during spontaneous AVRT from the PPI-TCL difference9(Fig.5).The overlap between PPI-TCL differences in patients with AVNRT and patients with AVRT employing a septal AP disappears when the corrected PPI-TCL difference(cPPI-TCL)is considered:a cPPI-TCL difference<110ms is consistent with a diagnosis of AVRT employing a septal AP while a cPPI-TCL difference>110ms is consistent with a diagnosis of AVNRT(Fig.3).9The cPPI-TCL difference may be>110ms in a case of AVRT if the pacing site
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Figure 7.Manifest entrainment of orthodromic AVRT employing a left free wall AP with near concealed fusion.The atria are accelerated to the pacing CL (340ms)and the tachycardia resumes at a longer CL (355ms)immediately after pacing stops.The post-VOP response is AV.Note that the QRS complexes during VOP are almost identical to the QRS complexes during tachycardia.There is only a slight difference in the end of the QRS complexes in leads III and V1during entrainment.Note that the pacing site is the posterobasal LV.
is far from the circuit (as is the case when AVRT using a left-sided AP is entrained by VOP from the right ventricular apex—Fig.1B)or if the AP has decremental conduction properties (as might be encountered during a long RP interval SVT).
Correction of the PPI-TCL difference as described above will also avoid a similar problem with the uncorrected PPI-TCL difference that can arise in patients with both dual AV node physiology and AVRT employing a septal AP.During entrainment of AVRT that uses the fast AVN pathway as the antegrade limb,the pacing CL may encroach upon a fast AV node pathway refractoriness so that the stimulated orthodromic wavefront may be forced to use a slow AV node pathway.The AH interval on the ?rst return beat would be considerably prolonged in such a case.This would prolong the PPI-TCL difference not because of decremental conduction in the AV node but because of a “jump”to the slow AVN pathway during entrainment.10
The SA-VA Difference
During AVNRT the ventricle and atrium are activated in parallel,while during entrainment of AVNRT by VOP,their activation is in series.In contrast,both during orthodromic AVRT and during entrainment of orthodromic AVRT by VOP,the ventricle and atrium are activated in series.Accordingly,the difference between the VA interval during entrainment and SVT should be longer for AVNRT than for AVRT (Fig.8).The VA interval during entrainment is measured from the pacing stimulus,so it is called the SA interval.SA-VA differences <85ms are consistent with AVRT (Fig.5)while SA-VA differences >85ms are consistent with AVNRT.8
SA-VA differences have tended to di-chotomize patients with AVNRT and AVRT less well than PPI-TCL and cPPI-TCL differences when VOP is performed from or near the RV apex.8,9While the SA-VA difference is not subject to decremental conduction slowing through the AV node during the A-H response,the SA-VA difference could be relatively long (close to or >85ms)if the pacing site is far from the operative AP (for instance,in the case of entrainment of orthodromic AVRT using a left-sided AP by pacing from the RVA)or if the AP has decremental conduction properties (as might be encountered during a long RP interval SVT).
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Figure8.Quantitative features of entrainment of SVT by VOP.A,left panel:tachycardia circuit in orthodromic AVRT(shown here using a right-sided accessory pathway).The VA interval will be the time from initial ventricular activation(breakout from the His-Purkinje system,marked“V”)to earliest atrial activation(atrial breakout from the accessory pathway,marked“A”).A, right panel:last beat of VOP,with an apical pacing site close to the circuit.The stimulus-to-atrial(SA)interval will be similar to the VA interval since the impulse is travelling over the same route in each.Note that during SVT and VOP,the atrium and ventricle are activated in series.The postpacing interval(PPI,time from the last paced stimulus to the return electrogram at that site)will also approximate the tachycardia cycle length (TCL).Thus the PPI-TCL and SA-VA differences will be relatively short.B,left panel:tachycardia circuit in typical AVNRT.The VA interval during SVT is short since the two chambers are activated in parallel.B, right panel:last beat of VOP.The SA interval will be longer than the VA interval during tachycardia since activation of V and A is forced to occur in series.The PPI will also be longer than the TCL since the impulse must travel retrogradely up the conduction system,complete one revolution of the AVNRT circuit,then conduct antegradely again to the ventricular pacing site.Thus, PPI-TCL and SA-VA will be relatively long.See text for details.
The Importance of the Pacing Site with Respect to the cPPI-TCL and SA-VA Differences
Compared to apical ventricular pacing sites, pacing sites close to the AV groove should be closer to an AP that operates in a conventional AVRT circuit(if close to the region with early atrial activation),yet farther from AV nodal
circuits Figure9.In?uence of pacing site on PPI-TCL and SA-VA differences.Panel A:in orthodromic AVRT(shown here using a left free-wall accessory pathway),an impulse from an RV apical pacing site(square wave 1)may have a larger distance to travel to enter the tachycardia circuit(solid arrow),compared to a basal site close to the site of earliest atrial activation(square wave2).The latter site should thus give shorter PPI-TCL and SA-VA differences.Panel B:in contrast,in AVNRT the apical site is“electrically closer”to the circuit than a basal site,since in the latter case the impulse must ?rst travel through ventricular myocardium(grey arrow) before entering the distal arborizations of the Purkinje system.In the case of the PPI,it must also travel this distance a second time to get back to the pacing site. Thus,in AVNRT a basal pacing site will cause PPI-TCL and SA-VA differences to be greater.
(because of the extra distance required for a basal pacing stimulus to reach the more apical arborization that appears to be the usual input to the His-Purkinje network)(Fig.9).Accordingly, compared to apical pacing sites,basal pacing sites would be expected to(1)increase the cPPI-TCL and SA-VA differences for a given AVNRT circuit and(2)decrease the SA-VA difference for a given AVRT circuit.Basal pacing sites should also decrease the cPPI-TCL difference for a given AVRT circuit to the extent that the basal site,being closer to the operative AP,may be closer to the circuit (Fig.9).These hypotheses have been born out by a prospective study where,interestingly,the discriminatory value for the cPPI-TCL difference (110ms)and the SA-VA difference(85ms) remained essentially unchanged(110ms and 80ms,respectively),but the spread between the highest values in AVRT and the lowest values in AVNRT increased signi?cantly.In addition, there was no overlap between these values in cases of AVNRT and these values in cases of AVRT,regardless of the location of the AP, when basal VOP was performed.These results indicate that cPPI-TCL and SA-VA differences obtained from entrainment by VOP from basal
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SVT
Figure10.Onset of ventricular overdrive pacing at a CL of280ms during SVT with a CL of300ms.The ?rst four paced beats show progressive QRS complex morphology fusion.The?fth paced beat is the?rst beat to have?xed QRS complex morphology as determined by studying all12leads at the onset of pacing(not shown here).Thus,the transition zone begins with the ?rst pacing stimulus and ends with the?fth paced beat. Note that the atrial CL is accelerated to the pacing CL in the transition zone after the second paced beat, which is fused.The arrows indicate a?xed stimulus-atrial(SA)interval,which is also established in the transition zone.If the diagnosis were AVNRT,one would not expect the atrial CL to be perturbed or a ?xed SA interval to be established until one or more beats after the transition zone.As we will discuss in Part two of this review,the second pacing stimulus results in a His-refractory ventricular premature beat(it must be His-refractory since the QRS complex is fused, indicating that the stimulated antidromic wavefront collided with the orthodromic wavefront from the previous beat in ventricular myocardium after it had to exit the His-Purkinje network)that preexcites the atrium by20ms without a change in the atrial acti-vation sequence,indicating a diagnosis of orthodromic AVRT.sites better dichotomize AVNRT circuits from AVRT circuits.4As discussed earlier,basal pacing sites close to the earliest atrial activation are also most likely to permit the identi?cation of fusion(Fig.6).Compared to RV apical pacing sites,basal ventricular pacing sites are not as easy to access with stability and without inadvertent atrial or His bundle capture.Accordingly,we reserve VOP from basal sites for those instances when VOP from the RV apex results in an A-H response with concealed entrainment and borderline or questionable cPPI-TCL and/or SA-VA differences.
Differential Entrainment
Based on the results of apex versus base pacing(to be discussed in detail in Part two of this review)by Martinez-Alday et al.,11we hypoth-esized that the SA-VA and PPI-TCL differences after entrainment of AVNRT by basal VOP ought to be at least10and20ms longer,respectively, than after apical VOP.These differences should be speci?c for AVNRT provided that long and similar pacing CLs are employed at both sites to avoid any increases that might be due to decremental conduction slowing during VOP.We coined the term“differential entrainment”12to describe this phenomenon.
Our colleagues at the University of Western Ontario provided further proof of this concept.13 Apical VOP was performed with the RV catheter advanced as far out to the RV apex as possible. Basal VOP was performed by advancing a steerable ablation catheter just beyond and superior to the His bundle where inadvertent atrial and His bundle capture could be reliably avoided. The differential cPPI-TCL values and differential SA-VA(which they called the differential VA) interval values were de?ned as those values obtained after VOP from the RV apex subtracted from those values obtained after VOP from the RV base.The differential VA interval could be calculated even in cases where VOP consistently terminated the SVT if VOP did accelerate the atria to the pacing CL prior to termination(see number2below).All patients with differential VA and differential cPPI-TCL values>20and 30ms,respectively,had AVNRT,while all those who did not had AVRT.Differential entrainment performed in this way has the strength of not requiring knowledge of the site of earliest atrial activation,which might be particularly valuable if a coronary sinus(CS)catheter is not routinely used,or if the CS cannot be cannulated.
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Figure 11.A supraventricular tachycardia with a long RP interval and earliest atrial activation along the mitral annulus.Overdrive posterobasal left ventricular pacing is initiated about half way through the tracing.The second pacing stimulus captures enough ventricular myocardium to result in a fusion beat.Note that the QRS complex morphology associated with the second pacing stimulus is partly that of the QRS complex morphology of the native tachycardia (compare to the QRS complex morphology of the preceding beats)and partly that of the QRS complex morphology of a fully paced beat (compare to the QRS complex morphology of the subsequent paced beats).Importantly,the second pacing stimulus is followed by abrupt VA block,which terminates the SVT.Ventricular pacing with 1:1VA conduction ensues.The atrial CL was perturbed (termination with abrupt VA block)in the transition zone.As we will discuss in Part two of this review,the second pacing stimulus results in a His-refractory ventricular premature beat (it must be His-refractory since the QRS complex is fused indicating that the stimulated antidromic wavefront collided with the orthodromic wavefront from the previous beat in ventricular myocardium after it had to exit the His-Purkinje network)that terminates the SVT without conduction to the atrium.If one only studied the end of this overdrive pacing train and found that SVT had stopped,one would miss this information,indicating a diagnosis of orthodromic AVRT (in this case,employing a slowly conducting left free-wall accessory pathway).
What if the Response to VOP Is Not
Interpretable?
Three responses to VOP,while often consid-ered uninterpretable,may still provide important diagnostic information.
1.The atria are not accelerated to the pacing CL:As already discussed,repeated failure to accelerate atrial activation to the ventricular pacing CL suggests,but does not prove,that the SVT is an AT.
2.The atria are accelerated to the pacing CL,but SVT fails to continue after VOP stops.In this situation,the reason that the SVT stops is because the VOP CL has encroached upon the antegrade refractory period of the AVN,so that
the last retrogradely conducted atrial wavefront cannot echo back to the ventricles to produce an AH response after VOP.Often,this problem can be overcome by shortening the refractory period of the AVN by intravenous isoproterenol.If this problem cannot be overcome,the response can still be helpful diagnostically,particularly when it is part of differential entrainment as discussed above.(In this instance,the term “differential entrainment”is inappropriate,since the SVT cannot be said to be entrained if VOP results in its termination.)
Recently,three retrospective studies have reported that resetting of the timing of atrial activation (usually advancement;less commonly
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Figure 12.A proposed ?ow chart for the use of ventricular overdrive pacing in the diagnosis of SVT.*=closest to the site of earliest atrial activation.
delay or tachycardia termination without atrial activation)during the early portion of VOP can differentiate AVNRT (typical or atypical)from AVRT (regardless of AP location)with a very high degree of sensitivity and speci?city.14–16When VOP is initiated,there is usually a gradual change from the tachycardia QRS morphology,through a transition zone of varying degrees of fusion,to a ?nal stable paced QRS complex morphology (which may be either fully paced or represent stable fusion)(Figs.10and 11).De?nition of the ?rst stable QRS morphology beat is central to this discriminator and should use all 12surface electrocardiogram leads.In these studies,the positive predictive value of resetting of the timing of atrial activation either before or at the time of the ?rst beat showing ?nal paced QRS morphology for AVRT and the positive predictive value of resetting of the timing of atrial activation after the ?rst beat showing ?nal paced QRS morphology for AVNRT both exceed 90%.In AVRT,VOP impulses will reach the atria as soon as the ventricular paced impulses reach the AP and traverse it.In AVNRT,VOP impulses will not reach the atria until the ventricular paced impulses reach the input to the distal His-Purkinje system and traverse the His-Purkinje system and the AVN retrogradely.This discriminator
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between AVRT and AVNRT is attractive be-cause it does not require that the tachycardia continue after termination of VOP.Accordingly, prospective studies of this discriminator seem warranted.
Potential pitfalls should be kept in mind, including variable TCLs,pacing at a CL shorter than40ms less than TCL(where the paced wavefront could penetrate the His bundle and AV node and perturb the atrial CL during AT or AVNRT in fewer beats),decremental APs (where the paced wavefront could take more beats to perturb the atrial CL in AVRT),the presence of bystander APs(which could provide a route to readily perturb the atrial CL during AT or AVNRT),and dif?culties in identifying the?rst paced beat with a stable QRS complex morphology,for which interobserver agreement appears to be around80%.16For the same reasons discussed above,one would expect the diagnostic yield of?ndings at the beginning of VOP to be improved by basal VOP close to the earliest atrial activation in challenging or borderline cases.That is,one would expect that the atrial CL would be perturbed earlier in the transition zone for cases of AVRT,and even longer after the transition zone for cases of AVNRT,if VOP were performed at basal ventricular sites,especially ones close to the earliest atrial activation.
A proposed?ow chart employing VOP for SVT diagnosis is presented in Fig.12.
When SVT repeatedly terminates during VOP, it may also be useful to burst pace the ventricle for 3–6beats at a CL of200–250ms.Around60%of the time,the ventricles will be dissociated from the SVT mechanism(excluding AVRT)or the SVT will terminate without conduction to the atrium (excluding AT).2
Conclusion
VOP during sustained,stable SVT is quick and easy to perform.Because it is qualitatively and quantitatively information rich,it can provide a diagnosis of SVT mechanism in a majority of cases.Basal VOP and differential entrainment can be useful in cases where the results of VOP from the RVA are borderline.A good understanding of VOP as a diagnostic tool will provide a solid foundation for understanding SVT mechanisms, the principles of entrainment,and other diagnostic pacing maneuvers,which will be discussed in Part two of this review.
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