Activation sequence modification during cardiac resynchronization by manipulation of left ventricular epicardial pacing stimulus strength

Background: Success of cardiac resynchronization therapy (CRT) depends on altering electrical ventricular activation (VA) to achieve mechanical benefit. That increases in stimulus strength (SS) can affect VA has been demonstrated previously in cardiomyopathy patients undergoing ablation. Objective: To determine whether increasing SS can alter VA during CRT. Methods: In 71 patients with CRT devices, left ventricle (LV) pacing was performed at escalating SS. Timing from pacing stimulus to right ventricular (RV) electrogram, ECG morphology, and maximal QRS duration on 12 lead ECG were recorded. Results: Demographics: Baseline QRS duration 153 +/- 25 ms, ischemic cardiomyopathy 48%, ejection fraction 24% +/- 7%. With increased SS, conduction time from LV to right ventricle (RV) decreased from 125 +/- 56 ms to 111 +/- 59 ms (P = 0.006). QRS duration decreased from 212 +/- 46 ms to 194 +/- 42 ms (P = 0.0002). A marked change in QRS morphology occurred in 11/71 patients (15%). The RV ring was the anode in 6, while the RV coil was the anode in 5. Sites with change in QRS morphology showed decrease in conduction time from LV to RV from 110 +/- 60 ms to 64 +/- 68 ms (P = 0.04). Twelve patients (16%) had diaphragmatic stimulation with increased SS. Conclusions: Increasing LV SS reduces QRS duration and conduction time from LV to RV. Recognition of significant QRS morphology change is likely clinically important during LV threshold programming to avoid unintended VA change.