Discussion
Our adolescent patients had an upgrade at their first pacemaker replacement after 9 to 11 years of RV pacing, exhibiting improvement of LV function and exercise tolerance following restoration of AV synchrony and of ventricular activation via the His-Purkinje network. CCAVB in patients with a normal heart disrupts 2 of the 3 electrophysiological determinants of cardiac performance: atrioventricular synchrony and chronotropic response. Though CCAVB patients can have a normal physical development and, occasionally, a normal life expectancy, the majority have signs of LV dysfunction, atrial arrhythmias, or symptomatic heart failure at long term follow-up. When a pacing indication ensues, chronotropic response is restored at the expense of the loss of inter-/intraventricular synchrony, with or without restoration of AV synchrony depending on pacing mode (DDD vs VVIR).
In this perspective, key questions in CCAVB are yet unanswered owing to the impossibility to run methodologically correct studies in this pediatric population. The timing to consider pacing is individually based. The choice of the pacing site and mode has been mostly debated in literature (2, 6, 7): despite convincing evidence of superior cardiac performance and exercise tolerance with AV-synchronous pacing, VVIR mode seems a reasonable choice as the initial pacing strategy in small children to minimize intravascular hardware (risk of vein thrombosis) and lead malfunction, given the modest difference in clinical endpoints and quality of life in pre-adolescence (8, 9).
A first pacemaker implant or replacement in adolescence is a different setting, because key decisions are taken for long-term cardiac pacing in a view to restore - possibly - all the 3 electrophysiologic determinants of cardiac function. A mild LV systolic dysfunction, especially when associated to functional mitral or tricuspid regurgitation and initial left atrial enlargement, is a clinical hint of the RV pacing detrimental effect, and may promote upgrading the system to restore cardiac synchronicity. CRT is well known to improve cardiac function in RV pacing-induced heart failure/LV dysfunction (10); recent evidence, however, points toward a similar effect of HBP and CRT in this setting, both in terms of LV function and clinical status improvement (11). These results are not surprising, given the comparable efficacy reported for HBP and CRT in heart failure patients, with a possible beneficial effect of HBP also in non-responders to conventional CRT (12-14).
A single case of cardiac resynchronization via direct HBP in a CCAVB patient with RV pacing-associated cardiomyopathy has been reported to date (15), highlighting the concept of reversible LV dysfunction that can be corrected by restoration of the normal activation pathway via the His-Purkinje network. Our experience with milder degrees of LV dysfunction suggests a role for the preservation of LV mechanics at long term in CCAVB with persistent His-to-ventricle conduction, when the first replacement in adolescence offers the opportunity to upgrade the pacing system.
The ongoing development of dedicated tools and the improved skillfulness in HBP implantation have now set the ground for the expansion of this pacing modality also to CCAVB patients (14).
Limitations . The observations about these two patients need to be confirmed in a large series of CCAVB patients. Moreover, some caveats are to be considered, such as: persistence of conduction in the His-Purkinje network at long term; minimization of intravascular leads that dwell lifelong in young patients; device longevity when a moderate-to-high HBP threshold dictates a high current drain, as the number of replacements is a main predisposing factor to infection (16). Technical aspects like the amount of lead slack needed to accommodate for patients’ growth are better addressed in late rather than in early childhood.
Conclusions . Taken the abovementioned points as a cautious habit at innovation, we believe that HBP should be explored as the first-choice strategy in CCAVB undergoing pacemaker implantation in late childhood, and in CCAVB candidates to upgrading because of RV pacing-associated LV dysfunction.
Funding : no funding supported this article
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Table . Main characteristics of the two patients.