A medical system includes an implantable medical device that carries a first electrode on a distal end of the implantable medical device and a return electrode. The implantable medical device is configured to be positioned within a heart. The implantable medical device further includes a leadlet including a proximal end and a distal end. The proximal end is attached to the implantable medical device, and the distal end is configured to penetrate tissue of the heart. The leadlet carries a second electrode, and a body of the leadlet between the proximal end and the distal end is configured to flex when the distal end is implanted in the tissue of the heart. The implantable medical device further includes a fixation device attached to the implantable medical device. The fixation device is configured to affix the distal end of the implantable medical device to the heart.