Conduction times between the left atrium and right atrium (inter-atrial conduction time) will be measured at the time of implant, and left atrial contraction time will be measured by TTE at the follow-up appointment. For patients in whom the inter-atrial conduction time plus the left atrial contraction time is greater than the right-atrial to right-ventricular conduction time, it is not possible to optimize the AV delay. The proportion of patients who fall into this group will be the main endpoint of the study. Patients will be followed for this study until their follow-up appointment and TTE at 30 days post-implant. The total sample size of this study will be 50 patients. We expect to consent approximately 75 patients, expecting that 1/3 will not meet all entrance criteria or will withdraw early.