Screening of solid and porous materials for pacemaker electrodes

Histological Reactions of Porous Tip Endocardial Electrodes Implanted in Sheep

The tissue reaction caused by a sintered tip electrode is compared with that of a platinum-iridium tip electrode in a work which employs implantation in sheep and follow-up of the implant for up to 90 days. Clinical tests and anatomo-pathological examination of the histological sections were performed.

The results show a remarkably rapid fixation to the heart for the porous electrode with formation of limited inflammatory process, a thin fibrous capsule and modest alteration of the endocardial tissue. Conversely, the platinum-iridium electrode does not fix even in long periods and therefore continously rubs the endocardial surface.

Chronic inflammation and formation of remarkably thick connective capsule results, under which the heart seems mode deeply altered.

Biocompatibility considerations at stimulating electrode interfaces

The choice of biocompatible stimulating electrodes for various biomedical applications varies with the type of electrode-tissue interface, biomolecules present, electrolyte background, preparation of electrode, interfacial potential, current density, electrode material, porosity, geometry, and inflammatory response. Illustrative examples are given to demonstrate the importance of these parameters. Topics discussed are: A) DC electrodes applied to partially keratinized epithelial membranes; B) Variation of the electrical impedance and biocompatibility of stimulating electrodes with electrode potential and surrounding pH; C) Influence of electrode geometry, porosity and pore size on biocompatibility; D) Body defense mechanisms at the sites of implantable stimulating electrodes; E) Thrombus formation at stimulating electrode interfaces and F) Sterilization of electrodes to ensure biocompatibility.

Active fixation atrial leads: randomized comparison of two lead designs

Active fixation leads have reduced the incidence of lead dislodgement in patients with permanent pacemakers. However, theoretic concern that the tissue trauma associated with a myocardial screw-helix may increase the chronic pacing threshold of active compared to passive fixation leads has remained. Whether active fixation leads with a stimulating electrode that is independent of the fixation mechanism are associated with a lower chronic pacing threshold than leads utilizing a screw-helix for both fixation and stimulation is unknown. The present prospective, randomized study compared the acute and chronic atrial pacing and sensing characteristics of two unipolar active fixation leads, one utilizing a screw-helix for both fixation and electrical stimulation, the other with an active porous tip electrode and an electrically inactive helix. Patients were randomized to receive either a Medtronic 6957J lead with an electrically active myocardial screw-helix or a Cordis 329-101P lead with an inactive helix and a porous tip electrode. The baseline characteristics of the groups were comparable. At implantation, the 329-101P lead had a lower mean voltage threshold than the 6957J lead (0.61 +/- 0.16 V vs 1.05 +/- 0.34 V, P = 0.0004). There were no significant differences in atrial electrogram amplitude, slew rate, or lead impedance between the groups. At 6 weeks follow-up, there were no differences in the mean threshold voltage (1.85 +/- 0.36 vs 1.93 +/- 0.69 V), impedance (528 +/- 81 vs 530 +/- 118 ohms), or atrial electrogram amplitude (2.63 +/- 0.50 vs 2.42 +/- 0.95 mV) between the two leads. At long-term follow-up (mean 16.2 +/- 2.8 months, range 13.1-20.0 months) there were no significant differences in voltage threshold (1.65 +/- 0.61 vs 1.97 +/- 0.64 V), impedance (565.5 +/- 81.6 vs 617.7 +/- 146.7 ohms), or atrial electrogram amplitude (2.79 +/- 0.75 vs 3.10 +/- 1.53 mV). Thus, these results suggest that active fixation leads in the atrium with an electrode that is independent of the fixation mechanism do not provide chronic stimulation thresholds or electrogram amplitudes that are superior to those obtained with leads utilizing a myocardial screw-helix as both the active electrode and the fixation device.

Histological evaluation of porous titanium cardiac pacemaker electrode tips

An investigation of the tissue reaction to five porous titanium alloy pacemaker electrode tips inserted in sheep hearts was made. Attachment of the electrode tips to the endocardial wall was facilitated by fibrous tissue ingrowth into the porous material. Vascularized fibrocollagenous tissue was found within the pores of the tips. One tip was found to be infected. The presence of occasional multinucleated foreign-body giant cells within the porosity was related to the particulate features of the porous tip. The electrode tips had a reduced sensing impedance compared with conventional solid platinum tips. The present findings are consistent with previous observations relating to porous cobalt-chromium alloy and porous platinum-iridium alloy tips and suggest that porous titanium alloy may be a suitable pacemaker electrode tip material.

Characteristics of advanced porous and textured surface pacemaker electrodes

Reduced stimulation thresholds, improved sensing and better attachment have been claimed for totally porous and porous surfaced electrodes. In this study, the potential for clinical use of two new types of porous electrodes and a non-porous, textured high microsurface area electrode, has been evaluated by comparison with equivalent sized, smooth non-porous controls. Eighteen sintered and seven laser drilled porous electrodes, seven-non-porous textured electrodes, and sixteen controls, were implanted singly in the right ventricles of sheep. Measurements of threshold, pacing, sensing and bulk impedances were taken at regular intervals for up 180 days. At sacrifice, only three of the thirteen non-porous controls were attached. All laser porous electrodes, apart from two which were dislodged, were attached, as were eleven of fifteen sintered porous, and five of seven textured non-porous electrodes. Tissue ingrowth was found for both porous electrode types. Stimulation thresholds were not statistically different for all electrode types (p less than 0.05). Pacing and bulk impedances of the two porous and surface textured electrodes were significantly higher (p less than 0.10, p less than 0.05, respectively) than those of controls. The three new electrodes exhibited similar chronic sensing impedance values, 30% less than equivalent non-porous electrodes. The similar sensing performance of the porous and high microsurface area non-porous electrodes indicates that the nature of the external surface, rather than internal porosity, determines sensing impedance. All three new electrode types showed improved attachment and sensing compared with similar smooth electrodes. The laser porous electrode, which permits fixation by tissue ingrowth and maintains simplicity of construction, is promising for routine clinical use.

Comparison of pacing electrodes of different shape and material--recommendations

Recommendations for comparing different pacing electrodes to one another are presented. In most reported studies, such comparisons fall short of the intended goal because they are made with electrode parameters that are not constant. The suggestions presented include timetables for acute and chronic threshold measurements, conditions for measuring lead impedance, and the relationship between rheobase and chronaxy. In addition, the effects of polarization of the electrodes are discussed.

Porous electrodes: concept, technology and results

Porous electrodes were designed and implanted to optimize sensing and pacing performance of the cardiac pacemaker. These electrodes were manufactured to create an effective, atraumatic endocardial interface in order to (1) decrease chronic stimulation thresholds and pacemaker current drain; (2) facilitate fibrous protein tissue ingrowth for reduced electrode displacement; and (3) to create a large electrolytic surface area of contact for reduced polarization voltage, thus improving impulse and electrogram transmission. Thus far, clinical results demonstrate reduced long-term threshold and decreased current drain. Long-term advantages of porous electrodes remain to be determined.