Described herein are systems and methods for synchronizing electrical activity monitored by a plurality of external electrodes with supplemental cardiac data for assessing a patient's cardiac condition and configuring cardiac therapy. The supplemental cardiac data can include one or more markers indicative of a cardiac event occurrence and / or representative of at least one of a cardiac electrical signal, a heart sound, a cardiac pressure, a blood flow, and an estimated instantaneous flow waveform provided by a left ventricular assist device.