Search

P wave electrogram amplitudes and atrial stimulation thresholds were determined in eight Hanford miniature swine using a preshaped catheter with an “S” curve in the SVC, and a major lobe in the atrium to enhance electrode contact with the atrial wall. The catheter was designed for pacing and sensing in the DDD mode. P wave amplitudes were also ascertained with two commercially available VDD leads and compared to the data from the experimental catheters. The preshaped catheter used two 6-mm2 platinum iridium atrial electrodes with a 7-mm separation. Both atrial electrodes are on the same side of the catheter, facing outward on the major atrial lobe formed in the catheter. The P wave amplitudes were tested only in the differential bipolar configuration. For the eight preshaped catheters, the mean was 6.6 ± 3.8 mV while for the conventional leads it was 2.9 ± 1.6 mV. The mean atrial stimulation thresholds ranged from 1.1 t 0.2 V to 2.3 ± 1.2 V, with still lower thresholds of 0.9-1.3 V when using the parallel unipolar atrial electrode configuration, in which both parts of the bipolar atrial electrode are configured as a unipolar electrode. The data suggest that bipolar stimulation may be effective if sequential reverse polarity pulses are used to achieve cathodal stimulation from each electrode of the bipolar pair, on a beat-to-beat basis. (PACE 1997; 20[Pt. I]:1354-1358)

Adequate sensing of the cardiac electrogram is a necessary requisite for all demand cardiac pacemakers. In order to define better the effects of position of the sensing electrode on the ability of the pacer to detect the electrogram, a specially designed lead system with a movable, large surface area (50 mm2) proximal ring electrode and a small surface area distal tip electrode was placed transvenously in 12 mongrel dogs. As the sensing electrode was moved away from the right ventricular apex, there were slight but insignificant decreases in R wave amplitude. When the electrode was moved retrograde through the tricuspid valve into the atria there was a marked decrease in R wave sensing; however, this was accompanied by a significant increase in P wave amplitude. Therefore, with a large surface area electrode, direct contact with the myocardium is not needed for the detection of adequate P and R waves. In addition, a pacer lead system utilizing this sliding sensing electrode design could be used to optimize sensing position, either for atrial or ventricular sensing pacemakers.

A new segmented polyether polyurethane atrial pacemaker lead has been developed and tested acutely and chronically in dogs. This lead was constructed so that its tip could quickly and accurately be positioned in close proximity to the S-A node and provide long-term stability without the use of active fixation devices such as tines or screws. The acute intrinsic P-wave potentials seen at the S-A node area (9.7 +/- 4.7 mV) were superior to those detected in the appendage (4.7 +/- 2.8 mV) and coronary sinus (6.8 +/- 4.1 mV). There were no significant differences in pacing thresholds between sites. In long-term studies, transvenous (jugular) leads maintained their position for over three months without dislodgment in active unrestrained dogs with only a 25% decrease in intrinsic P-wave potentials. The chronic pacing thresholds for this lead were similar, or superior, to those reported for other pacemaker lead placement sites. The polyurethane coating material produces no adverse tissue reaction or thrombosis.

Urethane tubing has largely replaced silicone as the insulation of choice on cardiac pacemaker leads. Although urethane has been shown to be biocompatible, there have been reports of insulation defects. This study was on a segmented polyether polyurethane polymer which is applied and bonded directly to the wire. The SPU adds its strength and flexibility to the lead. SPU is heart settable, enabling special configurations (S-A node and J-Ieads) to be made without significantly affecting the O.D. The modulus of elongation (> 700%), and strength (> 6000 psi) of SPU are superior to silicone < 500% at 1200 psi) and of urethane < 400% at 6000 psi). Implants in rats, rabbits, and dogs have shown SPU to be essentially free of blood and tissue reactions. Long-term transvenous atrial and ventricular leads (up to 30 months) have been implanted in dogs without adverse affects and only a 25% decrease in chronic intrinsic P-wave and 20% in R-wave potentials. Threshold strength-duration curves showed changes similar to those reported for other pacemaker leads. Light and scanning photomicrographs show a freedom from cracks, crazing, and internal/external wear defects after over a cumulative 300 months implant experience. Polarized light has failed to show any stress related defects in the SPU coatings.