Focal Arrhythmia Ablation With Multipolar Mapping: Does it Still Make
Sense to Stay Off-Grid?
Jay Montgomery, MD
The history of invasive arrhythmia therapy is paved with technological
advances that improve safety and efficiency. Open surgical intervention
for accessory pathways was replaced with catheter-based treatments.
Direct current fulguration was replaced with radiofrequency ablation.
Fluoroscopic imaging was largely supplanted by electroanatomic mapping
(EAM). Initially, creation of anatomic maps was performed with the same
catheter used for ablation, and this is still sometimes the case. A
further refinement of EAM has been the advent of multipolar mapping
(MPM) catheters with small electrodes and narrow bipoles which serve the
purposes of defining activation patterns and creation of a detailed map
without the ability to deliver therapy. These types of catheters give
more granular detail of low voltage regions in both atrial and
ventricular applications.1,2
In most published MPM studies, the MPM catheter was used to define a
complex substrate or re-entrant circuit. Multipolar activation mapping
of focal arrhythmias is less well-described. In these scenarios, careful
delineation of substrate is typically less important, and the area
benefitting from detailed activation mapping is often small. MPM is
likely helpful in these scenarios, but a more difficult question might
be whether it is warranted. For easily-inducible and sustained focal
arrhythmias, both point-by-point (PbyP) and MPM should identify the
earliest-activating site within a reasonable amount of time, provided
the site is accessible and the signal amplitude is large enough to be
detected. For more fleeting or unstable arrhythmias, MPM is probably
more likely to adequately map the arrhythmia despite a limited total
arrhythmia time. Additionally, very low-amplitude signals could
potentially be missed by an ablation catheter but detected with MPM.
Therefore, it is plausible that a subset of focal arrhythmias would have
inadequate mapping unless MPM were used.
In this setting, Chieng et al. used a Case-Control design to compare MPM
using the AdvisorTM HD Grid catheter (Abbott Medical,
Abbott Park, IL) to PbyP with a contact force-sensing ablation catheter
for mapping and ablation of focal atrial and ventricular
tachycardias.(CITE THE JCE PAPER HERE) The study is limited somewhat by
the non-randomized design and relatively low numbers, though the
operators were the same and the cases all relatively recent. The primary
findings were that acute and medium term outcomes were similar (acute
success 85% vs 81% MPM vs PbyP; recurrence at 9.4 months 8.7% MPM vs
9.1% PbyP, all p=NS) but that mapping and procedure times were shorter
in the MPM group (mapping 35 mins vs 53 mins; p= 0.03; procedure
duration 126 mins vs 153 mins in; p = 0.02). Additionally, the earliest
recorded electrogram as compared to the P wave or QRS complex was
earlier in the MPM group (39ms pre-QRS) than with the ablation catheter
in the PbyP group (33ms pre-QRS, p=0.02). When compared in the same
patients (in only the MPM group), the association seemed to remain (39
ms by MPM catheter vs 35 ms by ablation catheter), though it was not
statistically significant. Despite case-control matching for age and
case type (and chamber), overall numbers of atrial and ventricular
arrhythmias were not the same in the two groups (more patients with
multiple ventricular arrhythmias in the MPM group). Additionally, while
a single mapping system was used in the MPM group (Precision, Abbott
Medical, Abbott Park, IL), some of the PbyP cohort were mapped with the
Carto system (Biosense Webster, Diamond Bar, CA).
It is, of course, not clear how operators decided on multipolar vs PbyP.
Given that we are told that the operators were the same, it seems
plausible that the patients or arrhythmias were then systematically
different (at least to a small degree) if a different approach was used.
MPM may have been preferentially chosen in patients with
difficult-to-induce or non-sustained arrhythmias or those thought to
have a more complex substrate. Given this, the fact that mapping time
was 18 minutes less and procedures were 27 minutes shorter in the MPM
group seems convincing, as the most obvious confounders would bias
toward the opposite. Further, these cases were performed prior to the
availability of the LiveView module (Abbott Medical, Abbott Park, IL)
which may further truncate mapping times in focal arrhythmias using the
HD Grid catheter.
The disparity between the earliest detected electrograms between the two
mapping modalities could be due to either failure to detect the same
signals despite measuring at the same site or failure of PbyP mapping to
find the true earliest sites. The earliest electrograms detected by MPM
may have been inherently too low amplitude to be detected by the
ablation catheter (with its larger electrode) or could have gone
undetected because the wavefront vector was nearly orthogonal to the
ablation bipole. Supporting this latter theory, 9 of 27 tachycardias
mapped with the HD Grid were found to have a difference of 6 or more
milliseconds in the earliest electrogram timing when comparing bipoles
along vs across splines. To the extent that wavefront directionality
accounts for this disparity in earliest electrograms, the findings from
this study utilizing the HD Grid catheter do not necessarily generalize
to other MPM catheters without orthogonal bipoles. Further, while this
small, non-randomized study showed no difference in clinical outcomes,
it is certainly plausible that these earlier-detected signals on the HD
Grid catheter may sometimes be anatomically distant enough from the
earliest signal seen by an ablation catheter as to reduce the likelihood
of arrhythmia recurrence.
It may be tempting to say that a reduction in procedure time translates
to a reduction in risk. However, it is within the realm of possibility
that some MPM catheter designs could increase risk due to enhanced
thrombogenicity or other mechanical factors.3Additionally, some data suggests that exchanging mapping and ablation
catheters in a single sheath during ablation of atrial fibrillation
increases the risk of silent cerebral microemboli.4Therefore, there is insufficient data to say what, if any, positive or
negative safety effect may exist for the use of MPM catheters generally
or the HD Grid catheter specifically for focal arrhythmias.
Each currently-available MPM catheter design has likely advantages and
disadvantages for various applications. The HD Grid catheter has a
relatively fixed form which may not be best-suited for rapidly creating
an anatomic shell (such as prior to pulmonary vein isolation) but is
unique in its ability to record and integrate information from
orthogonal bipoles due to fixed 3-3-3 mm inter-spline and along-spline
spacing. In addition to increasing the likelihood that a bipole is
aligned with the wavefront vector, the electrograms from two adjacent
and orthogonal bipoles can be used to create a voltage loop assigned to
the center of the triangle formed by the two bipoles. The resulting
omnipole can be rotated 360 degrees to match the unipolar signal for
best fit. The resulting electrogram amplitude is independent of catheter
orientation.5-7 This novel mapping technique, not yet
commercially available in the United States or during the study
discussed above, should serve to negate the effects of wavefront
directionality on EGM amplitude.
If these non-randomized results reflect a true effect and MPM with the
HD Grid catheter shortens mapping and procedure times but has no effect
on success for focal arrhythmias, what should guide practice? Should
electrophysiologists weigh the incremental costs of the HD Grid catheter
against an additional 27 minutes per procedure? The current, most
prudent course may be to carefully consider whether to use MPM for each
focal arrhythmia based on the location, ease of inducibility, and
anticipated presence or absence of abnormal substrate. However, given
the inevitable march of technological progress, MPM will continue to
improve, and operators will become accustomed to having it. It seems
inevitable that MPM will be increasingly used across most mapping case
types as was the case with EAM in general over the past two decades. The
mapping system will sense wavefront directionality within a single beat
and guide the operator toward the focus in real-time, further decreasing
mapping time. It may even be able to triangulate the focus in three
dimensions, giving an estimate of the depth within the myocardium or
likely focus in the opposite chamber. The real question for the future
is: will that same multipolar catheter then be used to deliver therapy,
and, if so, what ablation modality will it deliver?
References
1. Anter E, Tschabrunn CM, Josephson ME. High-resolution mapping of
scar-related atrial arrhythmias using smaller electrodes with closer
interelectrode spacing. Circ Arrhythm Electrophysiol 2015;8:537-45.
2. Tschabrunn CM, Roujol S, Dorman NC, Nezafat R, Josephson ME, Anter E.
High-Resolution Mapping of Ventricular Scar: Comparison Between Single
and Multielectrode Catheters. Circ Arrhythm Electrophysiol 2016;9.
3. Nakamura K, Sasaki T, Take Y, et al. Impact of the type of
electroanatomic mapping system on the incidence of cerebral embolism
after radiofrequency catheter ablation of left atrial tachycardias.
Heart Rhythm 2020;17:250-7.
4. Deneke T, Nentwich K, Schmitt R, et al. Exchanging Catheters Over a
Single Transseptal Sheath During Left Atrial Ablation is Associated with
a Higher Risk for Silent Cerebral Events. Indian Pacing Electrophysiol J
2014;14:240-9.
5. Masse S, Magtibay K, Jackson N, et al. Resolving Myocardial
Activation With Novel Omnipolar Electrograms. Circ Arrhythm
Electrophysiol 2016;9:e004107.
6. Porta-Sanchez A, Magtibay K, Nayyar S, et al. Omnipolarity applied to
equi-spaced electrode array for ventricular tachycardia substrate
mapping. Europace 2019;21:813-21.
7. Deno DC, Bhaskaran A, Morgan DJ, et al. High Resolution, Live,
Directional Mapping. Heart Rhythm 2020.