Apparatus and methods are described, including apparatus for pacing a heart of a subject. The apparatus includes an implantable pulse generator (IPG) and a coiled lead connected to the IPG. The coiled lead includes a smaller-diameter coiled portion, a lumen of which having a first coil-lumen-diameter, and a larger-diameter coiled portion electrically in series with the smaller-diameter coiled portion, a lumen of the larger-diameter coiled portion having a second coil-lumen-diameter that is larger than the first coil-lumen-diameter. A perpendicular distance from a central longitudinal axis of the smaller-diameter coiled portion to the lumen of the larger-diameter coiled portion is greater than an outer radius of the smaller-diameter coiled portion, when the central longitudinal axis of the smaller-diameter coiled portion is parallel to a central longitudinal axis of the larger-diameter coiled portion. Other applications are also described.
