Bio-impedance may be used for navigation systems to chronically implant pacing and defibrillation leads in the heart using a non-fluoroscopic position sensing unit (PSU), such as a modified LocaLisa TM system from Medtronic Inc., which allows for variable frequency sampling of the position of electrode of a catheter. The PSU injects small AC signals via surface electrodes in three orthogonal axes, each on a slightly different frequency (e.g., near 30 KHz). Indwelling electrodes electrically connected to the PSU resolves the magnitude of induced voltage for each of the three frequencies, thus measuring voltage for each of the three axes. Voltages are divided by induced current to yield impedance in each axis for each electrode. Impedance is proportional to position within the body. Such a system requires that a conductive material, such as a retractable helical tip-electrode, be exposed during implantation. Since the tip is retracted during implantation, this disclosure provides a modified distal portion employing at least one aperture (or 'window') for fluid exposure of the helix-electrode and a deployable internal sleeve for covering the aperture(s) when the helix-electrode is extended.