Left ventricular paced activation in cardiac resynchronization therapy patients with left bundle branch block and relationship to its electrical substrate

Abstract

BACKGROUND Cardiac resynchronization therapy (CRT) uses left ventricular (LV) pacing to restore rapid synchronized LV activation when it is delayed in patients with myocardial disease. OBJECTIVE Although intrinsic LV activation delays are understood, little is known about reactions to LV stimulation and whether they are affected by QRS duration (QRSd), morphology, LV substrate, or choice of electrode pair. The purpose of this study was to test these interactions. METHODS In 120 heart failure patients with left bundle branch block (LBBB) and QRS .120 ms receiving CRT with quadripolar LV leads, device-based measurements of intrinsic activation delay (qLV) and paced inter-(and intra-) LV conduction times were evaluated at the proximal and distal LV bipoles. RESULTS During intrinsic conduction, qLV varied little between the proximal and distal pairs in patients with LBBB (n 5 120; age 68 6 11 years; 63% male; ejection fraction 25% 6 7%; 33% ischemic cardiomyopathy; QRSd 162 6 19 ms). A minority (30%) had conduction barriers (ie, gradients) (DqLV 29 6 8 ms vs 9 6 5 ms in patients without gradients; P ,.01), which occurred equally in ischemic and nonischemic patients. A majority were functional (and not scar-mediated), as they resolved with pacing in most patients (75%). Importantly, LV-paced conduction times were unrelated to baseline QRS morphology (LBBB 166 6 30 ms vs RBBB control 172 6 30 ms; P 5 NS), longer than intrinsic conduction (166 6 30 ms vs 129 6 28 ms; P ,.01), and varied significantly by electrode pair (ie, small distances) and etiology. Correlation between intrinsic activation delay (qLV) and LV-paced conduction time was poor (R 2 5 0.278; P ,.05). CONCLUSION LV-paced effect, which is core to CRT, is unpredictable based on conventionally used measures and should be considered during CRT optimization.

Reduction in the QRS area after cardiac resynchronization therapy is associated with survival and echocardiographic response

“…This could be interpreted as electrical substrate modification by CRT being an independent factor from electrical substrate presence. The finding of electrical substrate modification being an independent factor aids the previous findings that LV‐paced conduction times by CRT are unrelated to baseline QRS morphology and that a poor correlation exists between intrinsic activation delay (qLV) and LV‐paced conduction time 9 . Also, another study reported that ΔQRS area was associated with event‐free survival independently from QRS morphology 10 .…”

Optimizing lead placement for pacing in dyssynchronous heart failure: The patient in the lead

“…This may be explained by the substantial but unpredictable differences in LV propagation seen on ECG imaging, despite pacing adjacent LV electrodes. 32 Therefore, we may conclude that the influence of a variety of factors on optimal LVLP is reflected by the large individual variation in paced effects and hemodynamics, and LVLP is highly patient-specific. Because measures of both LV dP/dt max and stroke work were consistent across all studies, these findings emphasize the need for individualized targeting of the optimal pacing site.…”

“…39 Because of small differences in intrinsic activation delay when LVLP already is optimal, electrical indices such as LV-paced to RV-sensed wavefront propagation (ie, LVp-RVs) correlate poorly to QLV as well. 32 Therefore, LV-paced activation effects are highly unpredictable when based on stimulation site alone, despite widely variable acute electrical responses. 32,38 Where QLV is a measure of intrinsic electrical dyssynchrony, the difference between left and right paced-to-sensed interlead delays (LVp-RVs .…”

“…32 Therefore, LV-paced activation effects are highly unpredictable when based on stimulation site alone, despite widely variable acute electrical responses. 32,38 Where QLV is a measure of intrinsic electrical dyssynchrony, the difference between left and right paced-to-sensed interlead delays (LVp-RVs . RVp-LVs) can be used as a measure of paced LV-dyssynchrony.…”

Therapy for cardiac resynchronization: When left ventricular–only “fusion” pacing is not enough

“… 1 , 2 Moreover, these cannot be predicted from baseline QRS morphology or qLV. 3 The interaction of pacing and conduction barriers is important. When LV pacing facilitates conduction block, it will retard LV activation compared to intrinsic conduction ( Figure , middle).…”