text: The present invention relates generally to implantable medical leads, and more particularly to implantable leads placed by means of a guidewire.
  Most commercially available cardiac pacing and defibrillation leads are placed by means of a stylet which is inserted into a central lumen through the lead, and is used to assist in pushing the lead through the vascular system and guiding it to a desired location. More recently, leads placed by means of a guidewire extending entirely through the lead and out its distal end have been introduced. Although such leads are new to commercial distribution, the basic idea of the guidewire placed lead goes back quite some time. One early example of a guidewire placed pacing lead is disclosed in U.S. Pat. No. 3,769,984 issued to Muench. In this lead, a central lumen extending through the lead and out its distal end is provided which may be used for pressure measurement or for use of a guidewire for guiding the lead during its insertion. In this lead, the guidewire lumen extends along the entire length of the lead body. Similar leads are disclosed in U.S. Pat. No. 5,755,766 issued to Chastain et al, and U.S. Pat. No. 5,800,495 issued to Machek et al.
  In the context of coronary angioplasty catheters, the use of guidewires to place catheters within the vascular system has evolved to include the use of a "monorail" system, in which the guidewire lumen extends over only a distal portion of the catheter body. This basic approach has been adapted to cardiac pacing leads and cardioversion leads as well, as disclosed in U.S. Pat. No. 5,003,990 issued to Osypka, U.S. Pat. No. 5,755,765 issued to Hyde et al, U.S. Pat. No. 5,381,790 issued to Kenasaka and U.S. Pat. No. 5,304,218 issued to Alferness.
  In guidewire implanted leads, there is a danger of blood entering the lumen of the lead body through the opening at the distal end of the lead through which the guidewire exits. It has been suggested in the anonymous publication "Guidewire Placement of Electrical Lead" published as publication no. 35442 in Research Disclosure, October 1993, that a pierceable silicone rubber membrane may be located at the distal tip of the lead, to prevent fluid entry into the lead body during and after placement of the lead.