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International Journal of Arrhythmia 2011;12(2): 7-13.
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Introduction
The goals of electrophysiologic (EP) testing in
patients with supraventricular tachycardias (SVTs)
include the following: ① evaluation of baseline
cardiac electrophysiology; ② induction of SVT;
③ evaluation of the mode of initiation of the SVT;
④ definition of atrial activation sequence during
the SVT; ⑤ definition of the relationship of the P
wave to the QRS at the onset and during the SVT; ⑥
evaluation of the effect of BBB on the tachy cardia
cycle length and ventriculoatrial (VA) interval; ⑦
evaluation of the SVT circuit and requirement of the
atria, His bundle (HB), and/or ventricles in the
initiation and maintenance of the SVT; ⑧ evaluation
of SVT response to programmed electrical
stimulation and overdrive pacing from the atrium
and ventricle; and ⑨ evaluation of the effects of
drugs and physiological maneuvers on the SVT.
Discussion in this section will focus on differential
diagnosis of narrow QRS complex paroxysmal SVTs,
including AT, ORT, and AVNRT divided into
Baseline findings, Tachycardia characteristics, and
Maneuvers during tachycardias.
1. Baseline Findings
There are several baseline findings to differentiate
these three SVTs. We have to confirm three
evidences of baseline findings i.e. evidence before
the induction of SVT. First, we have to find
evidence of an accessory pathway (AP), second,
evidence of a dual AV nodal pathway, and third,
evidence of intra atrial conduction delays or scars.
First, evidence of an accessory pathway (AP)
1) Ventricular preexcitation
The definition of preexcitation is below 35 msec
of an HV interval it can be measured using the
earliest intracardiac ECG. The degree of
preexcitation depends on AV nodal conduction time
and the location of the accessory pathway.
Although the prevalence is low, the positive
predictive value may be 86%.
Second, evidence of a dual AV nodal pathway
2) Dual AV nodal physiology
The evidence of a dual AV nodal physiology
during atrial extra stimuli and decremental
ventriculoatrial (VA) conduction are the substrate
for AVNRT. The definition of AH jump (atrio-his
jump) is prolongation of the AH interval ≥50 msec
with a 10 msec decrease in extra-atrial stimulus
interval (A1A2).
3) Decremental VA conduction
Decremental VA conduction makes an accessory
pathway unlikely, but does not exclude a retrograde
block in the pathway with conduction up the AV
node and a decremental accessory pathway.
Third, evidence of intra atrial conduction delays
or scars with the information of past medical
history and endocardial electrograms.
Other baseline findings
4) VA block cycle length ＞600 msec
Absence of VA conduction makes AVRT unlikely.
However, rare accessory pathways are catecholamine
dependent. Consequently, there is about 5% chance
of AVRT if the VA block cycle length is over 600
msec at baseline.
5) VA block with adenosine
VA block with adenosine during ventricular
pacing suggests the absence of an AP. Absence of
VA block is always not diagnostic of an AP because
retrograde conduction couldn't be blocked in some
fast AV nodal pathways with adenosine. Retrograde
fast pathway conduction is not blocked by 12 mg of
adenosine in 38% of patients with typical AVNRT.1
6) Para-Hisian pacing2
Accessory pathway response during para-Hisian
pacing favors ORT with high predictive value (Figure 1).
(1) Methods
Para-Hisian pacing uses right ventricular pacing
close to the His bundle or proximal right bundle
branch. The pacing output is altered to produce
ventricular capture with an intermittent His bundle
or right bundle branch capture to selectively alter
the timing of His-bundle activation without
changing the timing of local ventricular activation.
With retrograde AV nodal conduction, the delay in
timing of retrograde His bundle activation should
produce an equal delay in the timing of retrogradeatrial activation without changing the retrograde
atrial activation sequence (Figure 1A). With
retrograde conduction over an AP, the loss of His
bundle capture should not change the timing of
retrograde atrial activation or the retrograde atrial
activation sequence (Figure 1B). A mixture of these
two responses (delay in the timing of atrial
activation in the His bundle electrogram with a
change in the retrograde atrial activation sequence)
would be expected with retrograde conduction over
both an AP and the AV node (Figure 1C). The degree
of contribution to atrial activation by the AP and
the AV node (atrial fusion) should depend on the
distance of the AP from the para-Hisian pacing site
and the retrograde conduction times over the AP
and AV node.
(2) Measures
The response to para-Hisian pacing is determined
by the change in the following variables between
HB-RB capture and HB-RB non-capture and
measures the ① atrial activation sequence; ② the S-A
interval in each electrogram, including an electrogram
recorded close to the site of the earliest retrograde
atrial activation during tachycardia; and ③ H-A
interval in the His bundle electrogram. These
variables are examined before and after AP ablation.
2. Tachycardia Characteristics
1) Septal ventriculoatrial (VA) interval
If the VA interval during tachycardia is below 70 msec, it favors AVNRT with more than 99% positive
predictive value.
2) Eccentric atrial activation
Early atrial activity in the proximal coronary
sinus electrodes suggests a posteroseptal pathway
or AV node slow pathway conduction. Eccentric
atrial activation is any atrial activation that does
not activate the AV node and the area around the
AV node first. This is frequently seen with
retrograde ventricular stimulation, when the
retrograde impulse finds the AV node refractory.
The site of earliest atrial activation is then the
distal CS and not in the proximal area closer to the
AV node/His bundle.3,4
3) SVT termination with VA block
Spontaneous termination of an atrial tachycardia
(without an atrial premature depolarization) would
not be expected to be followed by AV block.
4) Coumel's law (increase VA time ＞20 ms with
bundle branch block [BBB])
An increase in the VA time ＞20 ms with BBB is
diagnostic of ORT using an AP that is ipsilateral to
the side of BBB. Beware that a compensatory
decrease in the AH or HV may not cause a change
in the tachycardia cycle length.
5) LBBB aberration during PSVT
The mere development of LBBB is more common
during ORT regardless of change in the VA interval.
There are two reasons for this. First, the ORT tends
to be faster (however, LBBB is more common
during ORT association independent of rate) and
second, LBBB facilitates induction of ORT using a left-sided accessory pathway by allowing more
time for the pathway to recover
6) Induction dependent on a critical AH
AVNRT is more likely when the induction is
dependent on a critically long AH interval (i.e.
anterograde block in the fast pathway). Similarly,
AVNRT is more likely when the termination is
dependent on a critically short AH interval. The
same principal applies to ORT, which is the most
likely mechanism when the induction depends on
an anterograde block in an accessory pathway.
7) SVT cycle length = 500 msec
AVNRT tends to be slower than ORT, but there is
considerable overlap. However, very slow tachycardias
are more likely to be AVNRT
8) AV block during SVT
AV block during SVT excludes ORT. Although AV
block is more common during AT than AVNRT,
AVNRT is so common that when AV block occurs,
the tachycardia is still more likely to be AVNRT. AV
block during AVNRT is usually 2:1, is infranodal
and functional, and resolves during ventricular
pacing.
3. Maneuvers during Tachycardias
* Atrial pacing
1. Pace the A during SVT at a CL 10~40 ms < SVT CL
2. Pace the A during SVT at the AV block CL
3. Scan diastole with a premature atrial extrastimulus
* Ventricular pacing
1. Pace the V during SVT at a CL 10~40 ms < SVT CL
2. Pace the V during SVT at a CL 200~250 ms for 3~6 beats
3. Scan diastole with a premature ventricular extrastimulus
1) Atrial overdrive pacing maneuvers
Dependence of tachycardia termination or
continuance on last AH interval
(1) Pacing Maneuver
Pace atrium during SVT at longest cycle length
(CL) resulting in AV block
(2) Observation
Is SVT termination dependent on last AH
interval?
(3) Interpretation
AVNRT usually terminates after pacing when the
last AH interval is short relative to the AH of the
tachycardia, and continues after pacing when the
last AH interval is long.
2) Ventricular overdrive pacing maneuvers
Response to ventricular pacing just faster than
the tachycardia
(1) Pacing Maneuver
Pace the V during SVT at a CL 10~40 ms ＜ SVT CL
(2) Observation
Can the atrial 'A' rate be accelerated to the
ventricular 'V' pacing rate? (Entrainment)
Entrainment can be defined when the atrial cycle
length accelerated to the pacing cycle length,
without change in the atrial activation sequence,
and the tachycardia continued after pacing was
discontinued.
A. If not entrained,
① can SVT be terminated with a PVC during His
bundle refractoriness?
② can atrial activation be advanced with a PVC
during His bundle refractoriness?
③ can SVT be terminated with a PVC without
depolarizing the atrium?
# Interpretation
Tachycardia termination with a PVC delivered
during His refractoriness is diagnostic of ORT.
To conclude that the His bundle was refractory,
the ventricular extrastimulus must result in
ventricular capture and not precede the expected
time of the His bundle depolarization by more than
the baseline HV interval.
① Advancement of atrial activation with a PVC
delivered during His refractoriness is diagnostic of
an AP, but not of an ORT. If atrial activation has
been advanced but caused a different activation,
then the AP may not participate in the tachycardia.
Inability to advance A with a PVC does not exclude
an AP. You may not see advancement if an
extrastimulus is delivered far from the AP.
② If SVT terminated with a PVC without
depolarizing the atrium, there is no possibility of
atrial tachycardia.
B. If entrained,
① is the response or cessation of pacing 'A-A-V' or 'A-V'?
② is the PPI (post-pacing interval) - TCL (tachycardia
cycle length) > or < 115 msec?
# Interpretation
① If the response or cessation of pacing during
tachycardia is 'A-A-V', it favors AT and if 'AV',
it favors AVNRT or ORT. If the tachycardia
mechanisms are atypical AVNRT, pseudo' A-AV'patterns
may be observed because of longer
retrograde atrial conduction times through slow
pathways of AV node.
②The S-A (stimulus-atrial interval) VA (ventriculoatrial
interval) and PPI-TCL are useful in
distinguishing atypical AVNRT from ORT using a
septal accessory pathway. Consequently, patients
with QRT using a septal accessory pathway have
an S-A-VA < 85 msec and PPI-TCL < 115 msec5
(Figure 2).
4. Differential Diagnosis of PSVT in the EP
Laboratory
In most cases of SVT, a combination of baseline
findings, tachycardia characteristics, and pacing
maneuvers are needed to make a diagnosis (Figure 3).
The combination of atrial activation sequence,
septal VA interval, and response after entrainment
from the ventricle provides a diagnosis in 65% of
cases and excludes one mechanism in an additional
27%.
References
- Souza JJ, Zivin A, Flemming M, Pelosi F, Oral H, Knight BP, Goyal
R, Man KC, Strickberger SA, Morady F. Differential effect of
adenosine on anterograde and retrograde fast pathwayconduction in patients with atrioventricular nodal reentrant
tachycardia. J Cardiovasc Electrophysiol. 1998;9:820-824.
- Nakagawa H, Jackman WM. Para-Hisian pacing: Useful clinical
technique to differentiate retrograde conduction between
accessory atrioventricular pathways and atrioventricular nodal
pathways. Heart Rhythm. 2005;2:667-672.
- Nam GB, Rhee KS, Kim J, Choi KJ, Kim YH. Left atrionodal
connections in typical and atypical atrioventricular nodal reentrant
tachycardias. J Cardiovasc Electrophysiol. 2006;17:171-177.
- Hwang Chun, Martin DJ, Goodman JS, Gang ES, Mandle WJ,
Swerdlow CD, Peter CT, Chen PS. Atypical atrioventricular
node reciprocating tachycardia masquerading as tachycardia
using a left-sided accessory pathway. J Am Coll Cardiol.
1997;30:218-225.
- Michaud GF, Tada H, Chough S, Baker R, Wasmer K, Sticherling
C, Oral H, Pelosi F, Knight BP, Strickberger SA, Morady F.
Differentiation of atypical atrioventricular node re-entrant
tachycardia from orthodromic reciprocating tachycardia using a
septal accessory pathway by the response to ventricular pacing.
J Am Coll Cardiol. 2001;38:1163-1167.
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