A leadless pacing device may include a housing, a distal extension extending distally of a distal end of the housing, one or more electrodes supported by the housing, a distal electrode supported by the distal extension, and a processing module located within an interior space of the housing and electrically coupled to the one or more electrodes supported by the housing and the distal electrode supported by the distal extension. The housing may be positioned within a coronary sinus and the distal extension may be positioned within a vessel extending from the coronary sinus. The processing module may determine whether cardiac events occurred based on near-field signals and/or far-field signals sensed using the one or more electrodes supported by the housing and the distal electrode. The processing module may generate cardiac stimulation pulses based on the determination of whether a cardiac event occurred and where it occurred.
