ABSTRACT. The timing of local activation at left ventricular (LV) pacing leads is measured from the onset of the QRS complex to the peak of the LV electrogram (QLV). Pacing from the sites of late activation is associated with higher response rates to cardiac resynchronization therapy (CRTtechniques target the posterolateral wall of the left ventricle (LV) using a combination of radiographic appearance, pacing parameters, lead stability, and an absence of diaphragmatic stimulation to determine the final lead position. Unfortunately, CRT is associated with a nonresponder rate of 20% to 40%, depending on the definition used for response. One predictor of response to CRT is the timing of activation measured at the LV electrode relative to the onset of the QRS complex (QLV). Prolonged QLV (QLV > 95 ms) is associated with both an improved acute response to and marked long-term improvement with CRT. 3,4 Prospective measurement of QLV has been described