Disentangling rate effects from pacing-site effects on electrical and structural remodeling—and where the residual concerns actually live in a contemporary conduction system pacing system.
The question of whether left bundle branch area pacing (LBBAP) programmed at a lower rate limit of 60 bpm in a heart chronically conditioned to an intrinsic rate near 50 bpm can drive maladaptive electrical or structural remodeling is sharp, and the answer depends on disentangling two variables that pull in different directions: the rate change and the pacing site.
The concern about transitioning from a chronic 50 bpm baseline to a programmed lower rate of 60 bpm is biologically reasonable but probably overstated in magnitude. Several mechanisms deserve consideration.
Chronic bradycardia produces action potential duration (APD) prolongation, downregulation of IKr and IKs repolarizing currents, and increased transmural dispersion of repolarization—the well-established substrate for the classic pause-dependent or bradycardia-induced torsades de pointes seen in complete heart block. Raising the rate from 50 to 60 bpm moves the myocardium away from that substrate rather than toward it.
Reverse rate-dependence of IKr means APD shortens at faster rates, and the dispersion of repolarization that chronic bradycardia creates tends to regress with modest rate elevation. A 10 bpm increment is also a small perturbation relative to the diurnal heart rate range any individual experiences—particularly in a trained subject whose resting tone may sit in the 40s while threshold exercise drives rates well above 160. The myocardium is not "set" to 50 in any rigid sense; it is set to a distribution of rates.
Chronic rate elevation can cause tachycardia-induced cardiomyopathy (TIC), but the threshold for TIC is typically sustained rates above 100 to 110 bpm over weeks to months. 60 bpm is well below any plausible TIC threshold and is, if anything, closer to the rate band that population data associate with cardiovascular benefit in HFrEF cohorts.
What a chronically bradycardic, well-conditioned subject may actually notice with a 60 bpm LRL is a subjective shift—loss of the resting vagal tone signature. This is real but not pathologic. If it is symptomatically relevant, the programming lever is the LRL itself, or rate hysteresis on the atrial channel if the device supports it in DDD mode, rather than any structural concern.
This is where the LBBAP literature is genuinely informative, and where the situation differs meaningfully from right ventricular apical pacing at 60 bpm.
Selective LBB capture preserves the physiologic transseptal and apex-to-base activation sequence, which means the repolarization gradient is preserved. Non-selective LBB capture introduces a small parallel LV septal myocardial activation component, slightly widening the QRS and producing a modest, mostly inconsequential alteration of the repolarization vector. Neither has been associated with increased arrhythmic risk in the available follow-up data.
This stands in sharp contrast to RV apical pacing, which inverts the transmural repolarization gradient and is associated with increased VT/VF risk over years of follow-up.
The mechanistic case for LBBAP avoiding pacing-induced cardiomyopathy rests on preserved LV synchrony: no regional strain heterogeneity, no segmental hypoperfusion, and no chronic stretch-mediated fibrosis in the lateral wall. Cohort data out to 3 to 5 years from the major conduction system pacing groups (Vijayaraman, Sharma, Huang) show preserved or improved ejection fraction, no signal for increased ventricular arrhythmia burden, and in PICM populations, demonstrable reverse remodeling.
For patients with prior eccentric LV remodeling and LA dilation from high-burden RV pacing, the expected trajectory after upgrade to LBBAP is regression of those structural changes, not progression.
Prior PICM substrate generally develops under near-100% RV pacing. With DDD-LBBAP at LRL 60, ventricular pacing burden depends on intrinsic AV conduction. In a system with intact AV nodal conduction and appropriately programmed AV delays (or AV search hysteresis enabled), the ventricular pacing percentage may sit far below 100%, which further reduces any theoretical remodeling exposure. The actual Vp% at next interrogation is therefore a more informative variable than the LRL alone.
Not on the 50 to 60 bpm rate change. The more legitimate long-term questions for LBBAP specifically are:
The rate move from a chronic 50 bpm baseline to a programmed LRL of 60 bpm is unlikely to drive maladaptive remodeling or arrhythmogenesis on its own; if anything, it reduces the bradycardia-associated repolarization dispersion that constitutes a known pro-arrhythmic substrate.
The pacing site is the dominant variable, and LBBAP is currently the lowest-risk physiologic pacing option available for exactly the remodeling endpoints in question. The data are not yet infinite-horizon, but the mechanistic case and the 3 to 5 year follow-up both point away from the PICM and ventricular arrhythmia trajectory that characterizes long-term RV apical pacing.