Variation in Activation Time During Bipolar Vs Extended Bipolar Left Ventricular Pacing

Background: Cardiac resynchronization therapy (CRT) is typically delivered via quadripolar leads that allow stimulation using either true bipolar pacing, where stimulation occurs between two electrodes (BP) on the quadripolar lead, or extended bipole (EBP) left ventricular (LV) pacing, with the quadripolar electrodes and right ventricular coil acting as the cathode and anode, respectively. True bipolar pacing is associated with reductions in mortality and it has been postulated that these differences are the result of enhanced electrical activation. Materials and Methods: Patients undergoing a CRT underwent an electrocardiographic imaging study where electrical activation data were recorded while different LV pacing vectors were temporarily programmed. Results: There were no differences in the total electrical activation times or dispersion of electrical activation between biventricular pacing with bipolar or corresponding EBP LV vector configurations (left ventricular total activation time [LVtat] BP 74.70 +/- 18.07 vs EBP 72.4 +/- 22.64; P = 0.45). When dichotomized according to etiology, no difference was observed in the activation time with either BP or EBP pacing (LVtat BP ischemic cardiomyopathy 72.2 +/- 17.4 vs BP dilated cardiomyopathy 79.9 +/- 18.9; P = 0.38). Conclusions: Bipolar pacing alters the mechanical activation sequence of the LV and is associated with reductions in all - cause mortality. It has been postulated these benefits derive from improvements in electromechanical activation of the LV. Our study would suggest that true bipolar pacing does not necessarily result in more favorable activation of the LV or improved electrical resynchronization and other mechanisms should be explored.