<div>Leadless Pacing with Mechanical Atrial Sensing and Variable AV
Conduction</div><div>Jason Cook, MD</div><div>Travis D. Richardson, MD</div><div>From Vanderbilt University Medical Center. Nashville, Tennessee</div><div>Corresponding author:</div><div>Travis D. Richardson, MD</div><div>Assistant Professor Cardiac Electrophysiology</div><div>Vanderbilt Heart and Vascular Institute</div><div>1215 21st Ave S. Nashville, TN</div><div>Medical Center East, South Tower, Suite 5209</div><div>ph (615) 936-7537</div><div>fax (615) 936-5064</div><div>travis.d.richardson@vanderbilt.edu</div><div>Word Count:1331</div><div>Disclosures: The authors report no relevant financial disclosures.</div><div>Funding: None</div><div>The Micra<sup>TM</sup> leadless transcatheter pacing system
(Medtronic Inc., Mounds View, MN) has been shown to be an effective
alternative to transvenous pacing with excellent implantation success
rates and durable long-term performance.<sup>1–3</sup> The first
generation device provided single chamber right ventricular pacing with
rate responsiveness enabled by a 3-axis accelerometer.</div><div>Recently, the MARVEL 2 study (Micra Atrial tRacking using a Ventricular
accELerometer 2) reported the ability of software enhancements to allow
a leadless pacemaker to deliver single chamber atrioventricular (AV)
synchronized pacing.<sup>4</sup> In contrast to dual-chamber
transvenous pacemakers which sense atrial electrograms, the MARVEL 2
algorithm adjudicates atrial events using mechanically sensed atrial
activity from the 3-axis accelerometer. During initial programming, the
relative timing of mechanical events to the ventricular electrogram
allows for identification of A3 (passive ventricular filling) and A4
(atrial contraction). Atrial-sensed events are then defined by the A4
signal, and tracking may occur. MARVEL 2 reported VDD pacing was
achieved at rest in an impressive 89.2% of patients.</div><div>The Micra AV<sup>TM</sup> system’s unique programming includes
three basic pacing modes: VDD, VVI and VDIR (Figure 1). Additionally,
two mode switch algorithms are available and by default programmed on:
the AV conduction mode switch and the activity mode switch. Unlike mode
switch algorithms in dual chamber pacing systems, which are intended to
avoid inappropriate tracking of atrial arrhythmias, these algorithms are
intended to 1) minimize ventricular pacing, and 2) to improve rate
support during patient activity respectively.</div><div>When the AV conduction mode switch algorithm is enabled, the device
periodically switches from VDD to VVI at 40 bpm to allow for intrinsic
AV conduction. If ventricular sensing occurs above a rate of 40 bpm, in
order to reduce right ventricular pacing, VVI 40 programming will
continue regardless of the programmed lower rate limit. However, if two
of any window of four beats are paced at VVI 40, the device reverts to
VDD. Thereafter, reassessments of AV conduction are performed at
increasing intervals starting at 2 minutes until either AV conduction is
detected or 8 hours is reached at which point subsequent testing occurs
at regular 8 hour intervals.</div><div>The activity mode switch algorithm utilizes the sensor indicated rate in
an attempt to ensure adequate ventricular rate support during patient
activity regardless of AV conduction. The sensor in the
Micra<sup>TM</sup> is always running. If at any time 1) the
sensor indicated rate is above the device programmed ADL rate, and 2)
the current ventricular rate is &gt;20 BPM below the sensor
rate, the activity mode switch will change the device to VDIR mode with
heart rates determined by the sensor. This switch may occur from either
the VDD mode or VVI in the setting of AV conduction. The device will
revert to VDD mode when the sensor rate drops below the ADL rate.</div><div>With the added functionality of atrial sensing and the incorporation of
the MARVEL 2 algorithms described above, in this issue of the Journal of
Cardiovascular Electrophysiology, Garweg et al. examined the pacing
behavior of the Micra AV<sup>TM</sup> in the presence of variable
AV conduction, atrial arrhythmias, sinus bradycardia (&lt; 40
bpm), sinus arrhythmia, and periods of atrial and ventricular ectopy
(Reference). During the data collection period in MARVEL 2, ECG,
electrogram, accelerometer waveforms, and device marker data were
obtained; this was collected either after initial implant and follow-up
or, for patients with previously placed devices, during a single
encounter. The average monitoring period was 153 minutes. The study
included 73 patients with normal sinus node function and varying degrees
of AV block.</div><div>While the number of patients with variable AV conduction was small (5),
the investigators found that the rhythm checks allowed for appropriate
mode adjustments during the study period. During periods of AV block, as
expected, 99.9% ventricular pacing was observed while during 1:1 AV
conduction only 0.2% pacing was observed. Ventricular pacing was
monitored in patients with 1:1 AV conduction using conventional VVI
pacing and MARVEL 2 programming. MARVEL 2 programming using the AV
conduction mode switch algorithm resulted in a reduction in ventricular
pacing from 22.8% to 0.2% (n=18). Reducing the burden of ventricular
pacing is an important enhancement to the system with the potential to
minimize pacing-induced cardiomyopathy.<sup>5</sup></div><div>One potential pitfall of atrial sensing addressed by this study is
tracking of atrial arrhythmias. While the sample size was small (n=7),
tracking of atrial fibrillation resulting in pacing at the upper
tracking rate was not observed in any of the patients. In one patient
with atrial flutter, intermittent atrial tracking did occur but did not
result in tachycardia. In contrast to atrial rate based mode switching
used in conventional dual-chamber pacemakers, the behavior of the MARVEL
2 algorithm during atrial fibrillation is dictated by the sensed
ventricular rate. With the AV conduction mode switch enabled, if the
ventricular rate is above 40 bpm, the pacing mode will be VVI at 40 bpm.
If rates are less than 40 bpm, the pacing mode will be VDD. In the
context of atrial fibrillation, reduced atrial contractility results in
lack of mechanical sensing, and pacing at the lower rate is observed. In
this small sample size, atrial arrhythmias did not result in device
tracking resulting in tachycardia. Further investigation in a larger
number of patients is warranted to better characterize these findings
and to assess pacing behavior during more organized atrial arrhythmias
which could result in mechanical sensing (atrial tachycardia and atrial
flutter, for example).</div><div>While the MARVEL 2 programming seems to perform well in the setting of
atrial fibrillation or intermittent complete AV block, there are some
potential pitfalls. AV conduction mode switch behavior is based on
sensed ventricular rates with a threshold of 40 bpm; this cutoff is not
currently programmable. Any ventricular sensed rhythm with a rate
greater than 40 bpm will result in the device continuing at VVI 40. For
example, in a patient with sinus rhythm at 90 bpm and 2:1 AV conduction,
the device would not track the atrium and pace at 90 bpm, but rather
remain VVI 40 because the ventricular sensed rate is above 40 bpm. The
same would be observed in patients with junctional or ventricular escape
rhythms &gt;40 bpm. In this sense, pacing could be
inappropriately inhibited during a potentially hemodynamically
significant rhythm. For this reason, in our opinion, the AV conduction
mode switch algorithm should be disabled in the majority of patients
with AV block as this physiology is dynamic and sudden loss of rate
support can have deleterious consequences. While the activity mode
switch algorithm may address some of these concerns real world data are
needed for validation.</div><div>There is no question that the functionality and indications for leadless
pacemakers will continue to expand. In current guidelines, which predate
the development of the Micra AV<sup>TM</sup>, single chamber
ventricular pacing is only recommended in patients with AV block and
permanent atrial fibrillation, a low burden of anticipated pacing, or
substantial comorbidities.<sup>6</sup> Given the potential for
lower complication rates compared with transvenous systems, Micra AV may
be a superior option in some patients with complete heart block and
preserved ventricular function. However, with the advent of conduction
system pacing, the decreased risks of a leadless system have to be
balanced with the relative risk of long term right ventricular pacing.
Although the results will need to be validated with larger, longer-term
studies, which are underway (Clinical trials.gov NCT04245345), these
data indicate that Micra AV<sup>TM</sup> is likely to perform
well in the setting of atrial arrhythmias. In patients with variable AV
conduction, there are certainly pitfalls to the AV conduction mode
switch algorithm, many of which could be avoided by the ability to
program the mode switch VVI rate. While leadless pacing is often
considered in patients with multiple comorbidities at high risk of
complications from a transvenous system, we may be on the cusp of a
dramatic paradigm shift. The technological developments and success of
leadless pacing to date prompt the question of when, and not if,
leadless dual chamber pacing and potentially even cardiac
resynchronization will be available.</div><div>References:</div><div>1. Reynolds D, Duray GZ, Omar R, et al. A Leadless Intracardiac
Transcatheter Pacing System. https://doi.org/10.1056/NEJMoa1511643.
doi:10.1056/NEJMoa1511643</div><div>2. El-Chami MF, Al-Samadi F, Clementy N, et al. Updated performance of
the Micra transcatheter pacemaker in the real-world setting: A
comparison to the investigational study and a transvenous historical
control. <i>Heart Rhythm</i> . 2018;15(12):1800-1807.
doi:10.1016/j.hrthm.2018.08.005</div><div>3. Duray GZ, Ritter P, El-Chami M, et al. Long-term performance of a
transcatheter pacing system: 12-Month results from the Micra
Transcatheter Pacing Study. <i>Heart Rhythm</i> . 2017;14(5):702-709.
doi:10.1016/j.hrthm.2017.01.035</div><div>4. Steinwender C, Khelae SK, Garweg C, et al. Atrioventricular
Synchronous Pacing Using a Leadless Ventricular Pacemaker: Results From
the MARVEL 2 Study. <i>JACC Clin Electrophysiol</i> . 2020;6(1):94-106.
doi:10.1016/j.jacep.2019.10.017</div><div>5. Merchant FM, Mittal S. Pacing induced cardiomyopathy. <i>J
Cardiovasc Electrophysiol</i> . 2020;31(1):286-292. doi:10.1111/jce.14277</div><div>6. Kusumoto Fred M., Schoenfeld Mark H., Barrett Coletta, et al. 2018
ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With
Bradycardia and Cardiac Conduction Delay: A Report of the American
College of Cardiology/American Heart Association Task Force on Clinical
Practice Guidelines and the Heart Rhythm Society. <i>Circulation</i> .
2019;140(8):e382-e482. doi:10.1161/CIR.0000000000000628</div>