An implantable lead, being either a fixed or retractable/extendable lead, having a distal tip electrode is adapted for implantation on or about the heart and for connection to a system for monitoring or stimulating cardiac activity. The electrode includes a mechanical fastener such as a fixation helix for securing the electrode to cardiac tissue, which may or may not be electrically active. The implantable electrode with a helical tip may comprise:
an electrode having a distal end and a proximal end; and
a helix disposed within the electrode, which helix is aligned along a radial axis of the electrode to the distal end; and
the implantable electrode having at least one feature selected from the group consisting of:
a) the helix having a coating of an insulating material on its surface,
b) the helix having its surface beyond the distal end of the electrode and the distal end of the electrode having a porous conductive surface at a base of the helix,
c) a porous conductive element, screen or mesh at a base of the helix, and
d) a porous conductive element at the end of the electrode having an insulating coating covering from 5-95% of the surface of the porous conductive element.
The electrode may further include an electrode tip having a porous electrical conductive element, such as a mesh screen, disposed on a surface at the distal end of the electrode tip, which can be used as a sensing or pacing interface with the cardiac tissue. The porous electrically conductive element will usually be positioned at the base of the helix (when extended if extendable/retractable). The mesh or screen may provide a guiding mechanism for the helix as it travels out of the electrode for securing the electrode to the heart or other muscle or organ. The guiding mechanism may include a groove within the mesh screen. Alternatively, the guiding mechanism includes a guiding bar on which the fixation helix rides on during extension or retraction. In combination with each of these high impedance tips, a drug delivery system is provided at least the majority of which drug delivery system is provided from a source within a volume which is within the structural walls of the lead and/or within a volume having its major dimensions and/or at least 50% of its volume generally within the projected area of the tip.