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\begin{document}

\title{Unusual Response to Magnet application: What is the Mechanism?}



\author[1]{Anindya Ghosh}%
\author[2]{Chenni S. Sriram}%
\author[1]{Ulhas M. Pandurangi}%
\affil[1]{Madras Medical Mission Institute of Cardio Vascular Diseases}%
\affil[2]{Children's Hospital of Michigan}%


\vspace{-1em}



  
  \date{May 29, 2024}


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\selectlanguage{english}
\begin{abstract}
An asymptomatic 48-year-old gentleman presented for routine
interrogation of his Medtronic dual chamber transvenous pacemaker. The
device was implanted 4 years ago for post-operative intermittent high
grade atrio-ventricular block following mechanical aortic valve
replacement. As per our protocol, we performed a 12-lead
electrocardiogram with and without magnet application. The unusual
response elicited after application of magnet is mechanistically
explained.%
\end{abstract}%



\sloppy


\textbf{Unusual Response to Magnet application: What is the Mechanism?}

\emph{Anindya Ghosh, MD, Chenni S. Sriram, MD, FHRS, and Ulhas M.
Pandurangi, MD, DM}

Anindya Ghosh, MD\textsuperscript{1} | Chenni S. Sriram,
MD\textsuperscript{2} | Ulhas M. Pandurangi, MD\textsuperscript{1}

\textsuperscript{1}Department of Cardiac Electrophysiology and Pacing,
Arrhythmia Heart Failure Academy, The Madras Medical Mission, Chennai,
Tamil Nadu, India

\textsuperscript{2}Division of Cardiology Sub-section of
Electrophysiology, Children's Hospital of Michigan and Detroit Medical
Center, Detroit, MI, USA

\textbf{Correspondence}

Ulhas M. Pandurangi, MD, DM

Chief - Cardiac Electrophysiology \& Pacing

Arrhythmia Heart Failure Academy, The Madras Medical Mission

4-A, Dr. JJ Nagar, Mogappair,

Chennai, Tamil Nadu India, 600037

Phone:~+9144-26568000, +9144-26565943;~Extension:~2237

Email: arrhythmiaheartfailureacademy@gmail.com

\textbf{Abstract}

An asymptomatic 48-year-old gentleman presented for routine
interrogation of his Medtronic dual chamber transvenous pacemaker. The
device was implanted 4 years ago for post-operative intermittent high
grade atrio-ventricular block following mechanical aortic valve
replacement. As per our protocol, we performed a 12-lead
electrocardiogram with and without magnet application. The unusual
response elicited after application of magnet is mechanistically
explained.

KEYWORDS

magnet response, concealed conduction, asynchronous pacing

\textbf{1 | CASE PRESENTATION}

An asymptomatic 48- year old gentleman presented to our device clinic
for a routine interrogation of his pacemaker. He had undergone a
coronary artery bypass grafting and mitral valve replacement 5 years
ago. Unfortunately, he developed infective endocarditis of his aortic
valve within a year which necessitated mechanical aortic valve
replacement. This was complicated by persistent post-operative
intermittent high grade atrio-ventricular (AV) block. Therefore, we
implanted a Medtronic (ADVISA MRI DR) dual chamber transvenous pacemaker
with right atrial and right ventricular apical leads. Previous device
interrogations had not detected any sensing/pacing abnormalities. The
pacemaker was programmed to a DDDR mode with a lower rate of 70 beats
per minute (bpm), upper tracking rate of 120 bpm, upper sensor rate of
130 bpm, sensed and paced AV delay of 150 and 180 milliseconds (ms)
respectively.

As per our protocol, we performed a 12 lead electrocardiogram (ECG) at
baseline (Figure 1A) followed by magnet application (rhythm strip is
shown Figure 1B). Figure 1A shows a predominantly wide QRS rhythm (70
bpm) interspersed with shorter coupled relatively narrower QRS complexes
(140 ms) of a different morphology. These were initially suspected to be
premature ventricular complexes. When magnet is applied, there is
transition from the slower wide QRS rhythm (70 bpm) to the relatively
narrower QRS rhythm at a slightly faster rate of 85 bpm (Figure 1B). A
previous ECG showing intrinsic rhythm without pacing is also compared to
the current one obtained after magnet application (Figure 1C). What is
the most likely mechanism for the transition from wide to relatively
narrower QRS rhythm pursuant to magnet application?

\textbf{2 | DISCUSSION}

Analysis of baseline ECG along with the magnified rhythm strip (Figure
1A) is compatible with sequential AV pacing at the lower rate of 70 bpm.
Atrial and ventricular pacing spikes are associated with consistent
capture, albeit with low amplitude paced P waves. The atypical left
bundle branch morphology of the wide QRS rhythm with its late precordial
transition and left-superior axis is also compatible with right
ventricular apical pacing. The narrower QRS complexes are followed by a
delay in resumption of sequential AV pacing. This suggests that they are
adequately sensed by the ventricular lead with appropriate reset of the
pacemaker timing cycle. Also, the faster and narrower QRS rhythm after
magnet application (Figure 1 B) resembles the isolated beats seen in
figure 1A.

A thorough scrutiny of the magnet response (Figure 1B) reveals that the
first four wide QRS beats (4\textsuperscript{th} beat denoted by arrow)
occur at the lower pacemaker rate of 70 bpm and are preceded by tiny
pacing spikes. This is consistent with DDD pacing, but the low amplitude
P waves are not easily discernable. This is followed by acceleration of
the paced QRS complexes to 85 bpm. This is infact the expected magnet
rate response for a normally functioning newer generation Medtronic
pacemaker (asynchronous DOO pacing in this case). The first narrower QRS
complex is coupled to the last paced QRS complex at a short cycle length
of 600 ms (100 bpm). Subsequently the rate of the narrower QRS rhythm is
also 85 bpm. While a ventricular origin for the narrower QRS rhythm is
possible, the exact rate of 85 bpm alludes to the possibility of a
conducted rhythm.

Figure 1C is quite helpful in further clarifying the mechanism. The
upper left hand panel demonstrates a slower intrinsic rhythm with
shorter and longer coupled RR intervals. There is a suggestion of low
amplitude P waves preceding some of the QRS complexes. However it is
difficulty to confidently annotate a P wave in front of every QRS. We
can still infer that at-least the shorter coupled RR intervals may be
associated with sinus/atrial capture (conducted QRS) with prolonged AV
conduction (better discernable in aVF).

The relatively narrower QRS complexes noted during magnet response
(Figure 1C; upper right hand panel) exactly replicate (12/12 lead ECG
match) the intrinsic QRS morphology (left panel). Magnified lower panel
of figure 1C shows tiny atrial pacing spikes (black arrows; at a rate of
85 bpm) preceding the narrower QRS complexes (also at a rate of 85 bpm)
during the magnet response. The second black arrow is relatively shorter
coupled (85 bpm) to the first black arrow and represents the onset of
asynchronous DOO magnet response (blue arrow). The most likely inference
is that there is 1:1 AV conduction with very prolonged PR interval
during magnet response. The captured P waves are likely masked by the
conducted QRS complexes.

A mechanistic explanation is provided in subsequent figures 2 and 3.
Figure 2A clearly depicts onset and termination of magnet response. The
conducted QRS complexes are not sensed during asynchronous DOO pacing.
Ventricular pacing (VP) with functional non-capture occurs at the end of
each conducted QRS. When magnet test ends, the last atrial paced beat
(rate of 85 bpm) conducts with a very long PR interval resulting in a
sensed QRS (annotated by the device as VS). Subsequently there is
resumption of DDD pacing (70 bpm) with sequential atrial and ventricular
capture. Atrial pacing at faster rates (from 40 bpm to 90 bpm) confirms
decremental AV conduction with progressive prolongation of the PR
interval and eventually early onset of AV Wenckebach (at and above
atrial pacing rate of 100 bpm) (Figure 2B).

A ladder diagram (Figure 3) is furnished to explicate the proposed
electrophysiological mechanisms involved. During sequential AV pacing at
baseline (Figure 3A) there is likely antegrade and retrograde concealed
conduction into the AV conduction system. Decremental retrograde
concealed conduction with retrograde block for every 4th paced beat
(Wenckebach pattern) is postulated. \textsuperscript{1} This allows the
preceding atrial paced beat to conduct to the ventricle with a very
prolonged AV delay. This is followed by resumption of sequential AV
pacing with both antegrade and retrograde concealed conduction as
described above.

Application of magnet (Figure 3B) not only alters the pacing mode
(synchronous DDD to asynchronous DOO) but also suddenly accelerates the
rate of AV pacing (70 bpm to 85 bpm). This creates a long-short sequence
in both atrium and ventricle which is expected to be reflected as a
long-short sequence to the diseased conduction system as well. This
interrupts concealed conduction from the ventricle, but allows prolonged
AV conduction (without Wenckebach) to manifest itself. The captured P
wave is masked by the conducted QRS complexes. Upon close inspection,
the ventricular pacing spikes can be appreciated towards the end of each
conducted QRS (with functional non-capture) consistent with expected
magnet response.

The asynchronous magnet rate response before elective replacement index
(ERI) is reached varies between different models of Medtronic
pacemakers. The more familiar response consists of 3 paced beats at a
rate of 100 bpm for threshold margin testing (TMT) followed by pacing at
85 bpm.\textsuperscript{2} In the newer generation of pacemakers such as
ours, it is set at 85 bpm without any TMT
(https://www.medtronicacademy.com/features/magnet-mode-feature). The
interval between the last captured ventricular beat
(3\textsuperscript{rd} beat) and first conducted QRS complex
(4\textsuperscript{th} beat) is noted to be 600 ms (100 bpm) in figure
4B. This is not purely coincidental but an indicator to the possibility
of intrinsic AV conduction occurring.

\textbf{3 | CONCLUSION}

A diseased AV conduction system as encountered in our patient may harbor
concealed/decremental conduction and is quite susceptible to
`long-short' sequences. This can yield interesting interactions between
conducted and paced rhythms even at relatively slower pacing cycle
lengths. However as showcased here, i) a systematic analysis of device
tracings/ECGs, ii) familiarity with magnet rate responses, and iii)
appreciation of complex electrophysiological mechanisms at play is
quintessential to `trouble shoot' any unusual pacemaker related
responses.

\textbf{REFERENCES}

.

Jazayeri MR. Role of concealed conduction and allied phenomena in the
genesis, maintenance, and termination of cardiac
arrhythmias.\emph{Pacing Clin Electrophysiol} . 2019;42(7):779-804.

Jacob S, Panaich SS, Maheshwari R, et al. Clinical applications
of~\textbf{magnets}~on cardiac rhythm management devices.\emph{Europace}
. 2011;13:1222-1230.

iase PD, Lowe MD, Chow AWC. A randomized

double-blind crossover trial of triventricular versus biventricular pac-

ing in heart failure. Eur J Heart Fail. 2012;14:495-505

\textbf{FIGURE LEGEND}

\textbf{FIGURE 1}

A, Baseline 12 lead Electrocardiogram (ECG) with lead II magnified in
the lower panel showing wide complex rhythm at a rate of 70 bpm (lower
rate of pacemaker) with intermittent narrower QRS complexes (140 ms) of
a different morphology. B, Long rhythm strip recorded while application
of magnet showing transition from wide to narrower complex rhythm (85
bpm). The magnet response starts after the arrow. C, Upper left hand
panel depicts a previous ECG without pacing (intrinsic rhythm). The
upper right hand panel shows the 12 leas long rhythm strip ECG recorded
during magnet response. The lower panel shows a magnified view of leads
aVF, V1, and V2. Atrial pacing spikes (black arrows), onset of magnet
response (blue arrow), and atrial/ventricular rates are also illustrated
(please also refer to discussion).

\textbf{FIGURE 2}

A, Device electrogram showing onset and end of magnet response is shown.
Atrial pacing (AP), ventricular pacing (VP), and ventricular sensing
(VS) are annotated by the device. B, Device tracing demonstrating
responses to atrial pacing at different rates (40-110 bpm)

\textbf{FIGURE 3}

A, Baseline long rhythm ECG strip (upper panel) with concurrent ladder
diagram explanation (lower panel). Annotations for atrium (A),
atrio-ventricular conduction system (AV), ventricle (V), atrial pacing
(AP), ventricular pacing (VP), and ventricular sensing (VS) are noted in
the lower panel. B, Magnet Response: Long rhythm strip recorded during
application of magnet (upper and mid panel) with concurrent ladder
diagram explanation (lower panel). Long --short sequence created by
magnet application at the atrial and ventricular level is denoted in the
upper panel (please refer to discussion).\selectlanguage{english}
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