An improved pacemaker lead including a lead body supporting at least one flexible conductor element that provides an electrical signal path between a proximal connector element and a distal electrode. The lead body includes an insulating structure that protects the flexible conductor element(s) wherein the insulating structure is realized from a polymer blend of a thermoplastic polyurethane elastomer and an isobutylene block copolymer. The mole fraction of the isobutylene block copolymer of the polymer blend is in the range of 2-15% (most preferably on the order of 10%). The polymer blend of the insulating structure has a maximum tensile strength in the range of 20-40 MPa (most preferably in a range of 25-35 MPa). In the preferred embodiment, the hardness of the polymer blend can be characterized by a Shore hardness in a range of 70-80 A. The flexible conductor element(s) preferably include a coiled wire conductor defining a central axis with an outer surface facing radially outward away from the central axis and an inner surface facing radially inward toward the central axis, and the insulating structure surrounds at least the outer surface of the coiled wire conductor (and more preferably encapsulates the coiled wire conductor). The polymer blend of the insulating structure has reduced oxygen permeability, and thus provides improved resistance to environmental stress cracking and metal ion induced oxidation while maintaining the flexibility and desired tensile strength of the lead body.