Sieniewicz BJ, et al. - Investigators examined the link between factors known to impact left ventricular (LV) contractility while delivering biventricular endocardial (BIV_ENDO) pacing at a variety of LV endocardial (LV_ENDO) locations. The enrollment consisted of 104 patients incorporating 687 endocardial and 93 epicardial pacing locations. Identifying viable LV_ENDO tissue which exhibited late electrical activation for the detection of the optimal BiV_ENDO pacing site is key. Greater improvements in acute hemodynamic response were achieved by stimulating late activating tissue (LVLED >50%). Nevertheless, the optimal location was frequently not the site of latest activation.