KEY TEACHING POINTS
KEY TEACHING POINTS
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A proportion of atrial tachycardias post–atrial fibrillation ablation have a microreentrant
mechanism.
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These microreentrant circuits usually occur at sites of gaps in ablation lines.
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High-resolution mapping systems allow small reentrant circuits to be defined and characterized
and aid the targeting of ablation therapy for microreentrant tachycardias.
Introduction
Microreentrant tachycardias are well described and are thought to be responsible for
a small proportion of atrial tachycardias post–atrial fibrillation (AF) ablation.
However, because of the small size of these reentrant circuits and the poor spatial
resolution of conventional mapping tools, they have not previously been mapped accurately
in vivo in humans and have therefore been difficult to distinguish from nonreentrant
focal tachycardias. The newly developed Rhythmia electroanatomic mapping system allows
for the rapid creation of activation maps of ultra-high resolution. In this case report,
we provide the first images of a microreentrant atrial tachycardia circuit in a post-AF
setting, mapped with the high-resolution Rhythmia mapping system.
Case report
A 71-year-old woman with paroxysmal AF, who had an AF ablation procedure 8 years ago,
attended for repeat ablation after a recurrence of AF in the past 12 months, which
was not controlled despite flecainide. At her last ablation, she underwent pulmonary
vein isolation with wide area circumferential ablation.
She was in sinus rhythm at the start of the procedure, and her procedure was performed
under general anesthesia. After transseptal puncture, left atrial geometry was created
using the Rhythmia mapping system (Boston Scientific, Marlborough, MA). Pacing from
the coronary sinus demonstrated reconnection of the left pulmonary veins, with activation
seen to cross the line of the previous wide area circumferential ablation into the
left pulmonary veins (Figure 1).
While mapping was performed, the patient developed an atrial tachycardia (cycle length
220 ms) with distal-to-proximal activation on the coronary sinus catheter. Further
mapping anterior to the left pulmonary veins demonstrated a localized microreentrant
circuit at the site of the gap on the previous ablation line (Figure 2 and Online
Supplemental Video 1). The colors on the activation maps represent different timings
throughout the tachycardia cycle length, and the wavefront can be tracked by following
the regions where “early meets late,” that is, where red meets purple.
1
The 4 sequential activation maps show activation progressing in an anticlockwise fashion,
at this site anterior to the left-sided pulmonary veins. A corresponding bipolar voltage
map during the tachycardia is also shown in Figure 2. In this case, the voltage map
was not helpful in identifying the specific location of microreentrant circuit as
the majority of bipolar electrograms in this region had amplitudes greater than 0.3
mV.
Interrogation of the bipolar electrograms in that region demonstrated significantly
fractionated electrograms, with electrogram timings spanning the entire cycle length
of the tachycardia within a localized region with an area of 0.32 cm2 (Figures 3A–3C).
Entrainment mapping was performed close to the site of the microreentrant circuit
using the Orion catheter, as shown in Figure 3D. Because of amplifier saturation,
the postpacing interval was measured to the second tachycardia beat after entrainment
(PPI(n+2)) and the difference between that value and twice the tachycardia cycle length
was 50 ms. Mechanical termination of the tachycardia occurred during mapping at that
location, and therefore no further activation mapping or entrainment mapping was performed.
A cluster of ablation lesions were delivered at this location (Figure 3C), and no
further atrial tachycardias were induced after ablation with burst pacing. The patient
also proceeded to have reisolation of her pulmonary veins during that procedure.
Discussion
In this case, we provide classical images of a microreentrant left atrial tachycardia
localized to the site of ablation gaps from previous wide area circumferential ablation,
mapped with the ultra-high-resolution Rhythmia mapping system. Atrial tachycardias
occur in up to 30%–50% of patients with previous AF ablation, depending on the initial
ablation strategy.
2
The majority (75%) of post-AF ablation atrial tachycardias are macroreentrant in nature,
3
the most common forms of which are the mitral isthmus–dependent and left atrial roof–dependent
atrial tachycardias. The remaining 25% are composed of focal atrial tachycardias,
resulting from triggered activity or enhanced automaticity, and microreentrant tachycardias,
which have diameters of reentrant circuits <3 cm.2, 3
Focal or microreentrant atrial tachycardias post-AF ablation have been well described
and are known to occur at sites of gaps of ablation, especially anterior to the left
superior pulmonary vein.
4
The differentiation between true focal and microreentrant tachycardias has utility
during the targeting of ablation therapy, as the entrainment mapping can be used to
localize the circuit if the tachycardia mechanism is known to be reentrant in nature.
Furthermore, knowledge of the tachycardia mechanism can inform pharmacotherapy in
the event of tachycardia recurrence, with focal tachycardias due to triggered activity
more responsive to calcium channel blockers, while microreentrant tachycardias may
be more suitably treated with antiarrhythmic drugs that modify refractoriness.
However, it had previously been difficult to differentiate true focal, that is, triggered
activity or enhanced automaticity, from microreentrant tachycardias because existing
mapping systems lacked adequate resolution to accurately define the paths of microreentrant
circuits. The recently developed Rhythmia high-density, high-resolution electroanatomic
mapping system, which uses a small basket array of 64 electrodes (Orion catheter),
can rapidly create high-density activation maps with little or no manual annotation
of activation.
5
Using this system, we were able to characterize in detail a microreentrant circuit
at the gap of previous wide area circumferential ablation. Importantly, using the
Rhythmia algorithm, this was done rapidly without the need for manual annotation or
verification of activation times. The automated mapping feature is a key advantage
of the Rhythmia system with significantly reduced mapping times. In a recent study
using the Rhythmia system, an average of 2753–3566 data points was collected with
continuous mapping, taking an average of only 5.2–9.5 minutes.
6
This compares favorably with other lower-density 3-dimensional mapping systems. For
example, in a study using the NavX mapping system, even with the multipolar pentarray
catheter, only 365 ± 108 points were collected during an average mapping time of 8
± 3 minutes.
7
High-resolution electroanatomic mapping systems such as the Rhythmia system enable
more accurate and rapid mapping of localized reentrant atrial tachycardia circuits,
which may improve success rates in the targeting of post-AF ablation atrial tachycardias.
Conclusion
We provide the first human in vivo evidence of a microreentrant circuit post-AF ablation,
with an area of 0.32 cm2. High-resolution mapping systems such as the Rhythmia system
allow for detailed characterization of small reentrant circuits and allow better targeting
of ablation therapy for microreentrant tachycardias.