Silent Sabotage: A Challenge in Cardiac Resynchronization

We present the case of a 15-year-old male with a history of congenital complete atrioventricular block diagnosed at 29 weeks of gestation, treated transplacentally with corticosteroids and salbutamol. His mother had systemic lupus erythematosus. He was born prematurely at 33 weeks with a low birth weight (2035 g), and a permanent epicardial pacemaker was implanted on day five of life. At 10 months, he was hospitalized for pacemaker adjustment, complicated by pacemaker-related intestinal ischemia and peritonitis, requiring bowel resection and temporary pacing. At 12 months, a dual chamber epicardial pacemaker was implanted, followed by two additional reinterventions. At 8 years old, he underwent generator replacement with a Boston Scientific PROPONENT DR MRI device. At that time, his cardiac status was stable, with mild left ventricular dilation and a mildly reduced LVEF (49%). At 14 years old, a transvenous cardiac resynchronization therapy (CRT) pacemaker (VALITUDE CRT-P) was implanted due to worsening heart failure. According to the limited documentation available, the previous epicardial pacemaker was intentionally left in place, considering the patient's complex surgical history (Figure S1). The last known device interrogation was recorded at that time (not accessible for review). In august 2023, at 15 years old, after moving to our country, he presented at our center with progressively worsening exertional fatigue and episodic precordial pain over the past month. He had no history of recent infections. Echocardiography revealed an LVEF of 34%. ECG findings are shown in Figure 1, and CRT-P interrogation parameters are listed in Table 1 and Figure 2. Viral infection as a decompensating factor. Atrial flutter with rapid ventricular response and loss of biventricular pacing. Increased right ventricular pacing threshold with loss of biventricular pacing. Automatic switch of the mode of pacing of the previously implanted pacemaker. None of the above. Summarizing the initial work-up: ECG showed unipolar pacemaker rhythm at 72 bpm with a broad QRS (∼190 ms) with left bundle branch pattern and QRS negative from V1 to V6. Device interrogation revealed normal right atrial, right ventricle and left ventricular leads impedance, capture and sensing thresholds, but there was a marked reduction in right and left ventricular (BiV) pacing starting in July 2023. Concomitantly, an increased frequency of R-wave sensing in both leads was noted. This correlates with the patient's worsening heart failure symptoms. Further interrogation of the epicardial pacemaker (PROPONENT DR MRI) revealed that the device had been switched to “Safety Mode,” and its function was permanently limited to the following settings: VVI (V, ventricular pacing; V, ventricular sensing; I, inhibitory response to sensing) mode, rate of 72 bpm and output of 5.0 V @ 1.0 ms on unipolar pacing (Figure S2). This automatic mode is typically activated when there is a major device failure or loss of integrity of the central processing unit, limiting its functionality to basic pacing to extend the device's remaining life for as long as possible. Atrioventricular sensing, pacing, and AV delay programming are also disabled by design [1]. Since reprogramming the epicardial pacemaker was not possible, the CRT-P was adjusted to a heart rate of 80 bpm achieving permanent BiV (Figure 3) and leading to both clinical and echocardiographic improvement (LVEF 38%). Atrioventricular delay optimization was not performed as dysfunction was primarily attributed to electrical interference. It was also decided to explant the epicardial generator for further analysis to determine the trigger for its activation in Safety Mode. The patient was transiently lost to follow-up and was later evaluated in other country. He has now entered complete battery depletion of the epicardial pacemaker, and the device was never explanted. In this patient, CRT output was inhibited due to electrical interference from a previously implanted epicardial pacemaker, which had unpredictably switched to Safety Mode. This resulted in a loss of biventricular pacing and subsequent heart failure decompensation. Although leaving the older generator in place can help mitigate surgical risks, failure to anticipate automatic mode changes can undermine the effectiveness of CRT and result in electrical interference. Notably, pacemaker manufacturers have different responses to battery depletion or device failure, and understanding these varied behaviors is crucial when retaining an old device (especially in pacemaker-dependent patients) [2]. In real world practice, management is further complicated by the lack of detailed prior device information, which is common in the context of international mobility, where patients with complex heart disease may move between healthcare systems with incomplete historical device data and fragmented follow-up. When reprogramming the interfering pacemaker is impossible, adjusting the CRT-P settings can help bypass the suppression of biventricular pacing, potentially restoring optimal function. Extraction of the interfering pacemaker may also be considered as a potential solution. While pacemaker extraction carries inherent risks, particularly in patients with complex surgical histories, it may represent the only definitive solution to restore optimal pacing function and may be particularly useful in patients with erratic follow-up, such as in this case, where managing device interactions over time could be more challenging. A thorough evaluation by an experienced multidisciplinary team is pivotal to carefully weigh the benefits of extraction against the risks. This case highlights the critical role of careful device planning, interrogation, and troubleshooting in CRT patients with retained pacemakers. Recognizing electrical interference due to unexpected mode switches is essential to avoid inappropriate pacing behavior. Ideally, explantation of a previous device generator should be warranted when implanting a new pacing system, in order to minimize unnecessary risks to patients. The authors declare no conflicts of interest. 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