BACKGROUND A high percentage of biventricular (BiV) or left ventricular (LV) pacing in cardiac resynchronization therapy (CRT) devices has been associated with superior clinical outcomes. However, the percent ventricular (%V) pacing reported by CRT devices simply indicates the number of paces the device has delivered and not the proportion of pacing that has captured the LV effectively. OBJECTIVE The purpose of this study was to determine whether a beat-by-beat evaluation of effective pacing would provide a more accurate evaluation of CRT delivery. METHODS An automatic electrogram (EGM)-based algorithm that classifies each LV pace as effective or ineffective based on detection of QS/QS-r morphology on the unipolar LV EGM during pacing was developed and validated. LV EGMs that were recorded by 24-hour Hotter from 57 CRT patients were postprocessed. The percent effective CRT (%e-CRT) pacing was calculated by dividing the time spent in e-CRT pacing by the total time of the recording. RESULTS In this CRT cohort, the average %V pacing (94.8% +/- 8%) significantly overestimated the %e-CRT pacing (87.5% +/- 23%; P < .001). A significant minority of subjects (18%) had a discrepancy of at least 3 percentage points between %V pacing and %e-CRT pacing (mean 39% 41%). CONCLUSION Current device pacing diagnostics overestimate the amount of CRT pacing actually delivered. The new algorithm quantifies ineffective CRT pacing, which enables clinicians to identify patients with this issue and to address the reasons behind suboptimal CRT delivery.