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 (4th 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. 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.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
(3rd beat) and first conducted QRS complex
(4th 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.