Loss of left ventricular capture from a chronic quadripolar coronary sinus lead was observed during development of an iatrogenic cardiac pericardial effusion. Subsequently, during pericardiocentesis left ventricular capture was reobtained. Reaccumulation of hemopericardium again resulted in repeated observable loss of LV capture. No previous documentation in literature review is noted. This phenomenon may have critical clinical value in predicting significant pericardial effusion or affect the clinical value of cardiac resynchronization therapy in patients with chronic pericardial effusions. To demonstrate a previously undocumented clinical phenomenon that significantly affect patient care and outcomes. N/A We present a case of an 83 YO male patient with iatrogenic pericardial effusion during transseptal puncture resulting in cardiac tamponade and observed loss of left ventricular capture. Our patient presented for Watchman implantation however after transseptal puncture prior to Watchman implantation hemopericardium developed with cardiac tamponade. Loss of LV capture resulted in RV only pacing until pericardiocentesis with placement of a pericardial drain was placed. After evacuation of 80cc volume and gross reduction in pericardial effusion size on transesophageal echocardiography with resolution of cardiac tamponade, LV capture was again noted. Device interrogation revealed stable lead thresholds. During clinical followup, reaccumulation of hemopericardium again resulted in loss of LV capture prior to clinical tamponade and again resolution of pericardial effusion resulted in regaining biventricular capture. Figure 1: Progressive loss of biventricular capture is noted during pericardial effusion accumulation leading to cardiac tamponade. Hemopericardium may result in loss of left ventricular capture from a chronic coronary sinus lead with stable thresholds. The observed loss of capture occurred prior to clinical cardiac tamponade and may offer an early qualitative measure of significant pericardial effusions.