BIPOLAR LEADS FOR USE WITH PERMANENTLY IMPLANTABLE CARDIAC PACING SYSTEMS - A REVIEW OF LIMITATIONS OF TRADITIONAL AND COAXIAL CONFIGURATIONS AND THE DEVELOPMENT AND TESTING OF NEW CONDUCTOR, INSULATION, AND ELECTRODE DESIGNS

The unacceptable rate of mechanical failures, threshold problems, and recalls experienced with many coaxial bipolar cardiac pacing lead desi gns are reviewed in detail. To address these problems, redundant insul ation coradial atrial and ventricular tined leads (AL and VL, respecti vely) with iridium oxide electrodes were developed and subjected to ex tensive accelerated testing. There were no mechanical failures. The ne w lead body design proved to be much more durable than widely used tri filar MP35N configurations. The data reviewed and early and current te st results are strongly supportive of tightly coupled insulation being a major factor in improving lead durability as long as the insulating material is not stressed. In addition to improving flex life, insulat ion adherence to the conductor may reduce the potential for ionic degr adation. Pacing and sensing thresholds in animal studies of the new le ads were within the reported range for leads with steroid eluting elec trodes. A multicenter Canadian clinical trial was initiated with the f irst implant in early January 1994. By November 1995, 110 VL and 82 AL had been placed in 124 patients and followed for a mean of 11 +/- 6 m onths; maximum 21, total 1355. There were 60 males and 64 females with a mean age of 64 +/- 16 years, range 15-88. Primary indications for p acing were AV block in 61 patients, sick sinus syndrome in 53, vasovag al syncope in 4, and congestive heart failure in 7. Many patients had associated or primary tachyarrhythmias, including 111 with supraventri cular and 12 with ventricular. Forty-two percent of patients (52/124) had prior cardiac procedures, including 18 open heart surgeries and 20 AV nodal ablations. At implant, 8 lead characteristics were rated goo d or excellent in 90% (746/829) of evaluations. X-ray visibility was o f concern in 10% of patients (12/124). Three perioperative complicatio ns occurred, including displacement of one AL (1.2%) and one VL (0.9%) . There were no subsequent mechanical (connector, conductor, or insula tion) or functional (exit block, micro or macro displacement, or over- or undersensing) problems. Implant pacing thresholds (PT) at 0.45 ms were AL, 0.6 +/- 0.2 (74) and VL 0.4 +/- 0.2 V; impedance (Z) at 3.5 V output AL 373 +/- 77 (82) and VL 497 +/- 117 Ohm. sensing thresholds (ST) were AL 3.1 +/- 1.6 (74) and VL 10.3 +/- 4.9 mV. Ventricular lead data were obtained for all patients (N = 110). Atrial lead data are i ncomplete, because some patients were in atrial fibrillation during im plantation. After 12 months, AL PT at 1.5 V output was 0.18 +/- 0.10 m s (21) and at 2.5 V was 0.10 +/- 0.05 (22). Associated AL ST was 3.3 /- 0.9 mV(21) and AL Z 500 +/- 65 Ohm (25). After 18 months VL PT at 1 .5 V was 0.15 +/- 0.10 ms (9) and at 2.5 V output was 0.09 +/- 0.04 ms (9). Associated VL ST was >7.5 +/- 2.4 mV (9) and VL Z 497 +/- 105 Oh m (9). Follow-up time discrepancy is due to the VL being available 6 m onths earlier than the AL. There were no 30-day deaths and only one la te death at 10 months in a patient with chronic atrial fibrillation. D eath was unrelated to pacer or lead function. At 1 year, 68% AL (15/22 ) and 62% VL (24/39) captured at 0.5 V and less than or equal to 1 ms pulse width output. Innovative adherent insulation coradial bipolar le ad conductors of the design studied combined with coated iridium oxide electrodes provide for a negligible incidence of mechanical or functi onal failure with clinical follow-up now approaching 3 years. Excellen t acute and chronic sensing and pacing thresholds have been documented . Late thresholds have continued to improve gradually. Long-term clini cal pacing at less than or equal to 1. 5 V output with a large safety margin is feasible in essentially all patients. This coradial design p roduces very flexible <5 French bipolar redundantly insulated lead bod ies allowing both AL and VL to simultaneously pass through a single 10 French introducer sheath. The coradial design reduces potential insul ation, subclavian crush, and implant and late threshold problems versu s coaxial approaches. Because the leads are so slender with reduced me tallic content, some method of increasing radiopacity would help when using older fluoroscopes that are standard in many Canadian centers.