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.