The relationship of bipolar left ventricular pacing stimulus intensity to cardiac depolarization and repolarization in humans with cardiac resynchronization devices.

BACKGROUND: Myocardial depolarization can be altered by varying pacing stimulus output. This may have implications on response rates for cardiac resynchronization therapy (CRT). The purpose of our study was to determine the relationship of left ventricular (LV) pacing stimulus intensity to measures of depolarization and repolarization in humans with CRT devices.

METHODS: In 37 patients with a CRT device and bipolar LV leads, bipolar LV-only pacing from maximum output to threshold was performed. The presence of changes in depolarization was defined by predetermined changes in ECG morphology that accompanied a change in bipolar stimulus amplitude. ECG parameters and the EGMs were analyzed at various LV pacing stimulus intensities.

RESULTS: Changes in ECG morphology were apparent in 70% of patients. These occurred at a mean LV stimulus amplitude of greater than 2.7 +/- 0.8 V at 1 ms. Of the patients with changes in surface ECG, the transventricular conduction time decreased from 155 +/- 41 ms at low output to 141 +/- 39 ms at high output (P < 0.01). Despite a significant reduction in QRS duration with high output, mean QTc and JTc interval increased with increasing LV stimulus strength (539 +/- 45 vs 559 +/- 46 ms (P < 0.01) and 353 +/- 31 ms vs 377 +/- 32 (P < 0.01)).

CONCLUSION: Increased LV stimulus intensity, independent of RV anodal capture, is associated with faster transventricular conduction time, changes in myocardial depolarization, and longer QT intervals. These findings have important implications on the relationship of programmed LV pacing output to pacing-induced proarrhythmia and clinical CRT response rates.