Long-Term Performance of Endocardial Leads with Steroid-Eluting Electrodes

During a three-year period, 48 steroid-eluting leads (Medtronic* 4003 and 4503] have been implanted in 39 patients. 23 were implanted in the atrium and 25 in the ventricle. 36 patients with 45 leads have been observed for more than 12 months. Stimulation thresholds have been followed using pulse generators with variable output (vario): 12 atrial and 11 ventricular leads, 4 atrial leads and 11 ventricular leads have been followed using pulse generators with variable pulse-width (auto-threshold], P-waves have been followed using telemetry or sensitivity programming (23 atrial leads). R-waves have been followed using telemetry in 11 ventricular leads. Mean stimulation thresholds after 2 years are 0.7 V ± 0.2 in the atrium and 0.8 V ± 0.3 in the ventricle. P-waves after 2 years are of magnitudes allowing a sensitivity setting of 2.5 mV in 12 cases and of 1.5 mV in 2 cases; R-waves have in all cases been high enough to permit lowest programmable sensitivity setting. During the observation time, two patients have died from nonpacing-related causes. The results obtained from this investigation document low stimulation thresholds and good sensing levels in both ventricle and atrium using the steroid-eluting electrode with no significant changes after 6 weeks postimplant.     

Clinical Experience with Steroid-Eluting Unipolar Electrodes

In continuing search of low chronic threshold leads, a new concept of electrode design which is capable of delivering corticosteroids at the myocardial tissue interface has been made available by Medtronic. Twenty-three patients, 17 females and 6 males, were either implanted with 4003 (n = 21) or 5023 (n - 2) steroid-eluting electrodes in the ventricular chamber. Pacing modes utilized were WIM (n = 13) or DDD (n = 10). Pulse generators used were Medtronic (7005. 8317, 8329) Pacesetter (285) and Intermedics (283). Thresholds at the time of implantation at 0.50 msec pulse width were 0.40 ± 0.02 volts at 0.66 ± 0.05 milliamps. Resistance and R wave measured were 565.43 ± 22.07 ohms and 9.24 ± 1.06 mv, respectively. Chronic thresholds were checked on routine follow-up visits by either decreasing pulse width and for pulse amplitude. Data is being reported between 1 and 88 (23.22 ± 4.35) weeks. Pulse width threshold at 2.5 volts were 0.10 msec (n = n) and 0.05 msec or lower (n = 12). At 5.0 volts no loss of capture was seen at 0.05 msec (n = 22) except in one patient at 0.10 msec. Pulse width thresholds in the first 24 weeks were lower than 0.20 msec at 2.5 volts (n = 15) and less than 0.70 msec, at 0.8 volts (n = 6). No loss of sensing was seen by electrocardiographic analysis at the time of threshold checks with the pulse generator at standard setting of the R wave. Thus, in this initial report, the steroid-eluting electrodes have demonstrated very low thresholds both in the early and chronic follow-up phase. Demonstration of consistently low thresholds, avoiding initial peaking, will permit routine low output setting without compromising safety and thus prolong the life of the pulse generators.     

Acute and Long-Term Atrial and Ventricular Stimulation Thresholds with a Steroid-Eluting Electrode

Thirty leads with a steroid-eluting electrode (Medtronic 4003 and 4503) have been implanted in 24 patients, 11 in the atrium and 19 in the ventricle. Six patients received the steroid lead in both atrium and ventricle. The stimulation thresholds were followed using Elema pulse generators with Vario function in 15 patients (11 atrial leads and 10 ventricular leads) during 11 +/- 3 months (mean +/- SD). At 0.5 ms pulse duration the mean atrial and ventricular thresholds were 0.55 Volt +/- 0.22 and 0.39 Volt +/- 0.22 respectively at implant, 0.94 Volt +/- 0.13 and 0.82 Volt +/- 0.16 after 12 months. During the entire follow-up period both atrial and ventricular mean thresholds never exceeded 1 V.     

A Comparison of Platinized Grooved Electrode Performance with Ring-Tip Electrodes

One of the determinants of the capture threshold of an endocardial pacing lead is the configuration of the electrode tip. To evaluate whether micro- and macroporous electrodes have better initial and chronic thresholds than nonporous electrodes, acute and chronic capture thresholds, stimulation impedance and sensing thresholds were determined in 22 patients in whom a ventricular ring-tip electrode and a unipolar, dual chamber pacemaker with bidirectional telemetry had been implanted. These values were compared to those obtained from 25 patients receiving an electrode constructed with a platinized groove surface at the time of implant of an identical pulse generator. The ventricular capture threshold at implant was 0.7 V +/- 0.3 at 0.6 msec pulse width for both groups. The capture threshold was significantly greater in the ring tip electrode group at follow-up periods of 1 month (1.1 V +/- 0.5 vs 1.6 V +/- 0.6, P less than 0.008), 4 months (1.0 V +/- 0.2 vs 1.7 V +/- 0.8, P less than 0.002), and 10 months (1.2 V +/- 0.4 vs 1.7 V +/- 0.5, P less than 0.04) following implantation. The stimulation impedance at the time of implantation was lower in the ring-tip electrode group (530 ohms vs 603 ohms, P less than 0.03), but thereafter no significant difference was seen between the two groups. The acute and chronic sensing thresholds were similar in both groups. While the microporous electrode had significantly lower chronic capture thresholds, the magnitude of this difference is small, and probably clinically inconsequential.(ABSTRACT TRUNCATED AT 250 WORDS)     

Acute and Long-Term Ventricular Stimulation Thresholds with a New, Iridium Oxide-Coated Electrode

Efforts have been made to design electrodes that significantly reduce not only the acute and chronic stimulation thresholds, but also attenuate the early peaking phenomenon and polarization. At two voltage levels (2.7 V and 5.4 V, respectively), we evaluated the right ventricular stimulation thresholds obtained with a new, iridium oxide-coated electrode in ten patients who received a VVI pacemaker. Measurements were mode at implant and at multiple intervals for 1 year. Pulse width stimulation thresholds at implant were as follow: 0.04 ± 0.008 msec at 2.7 V, 0.03 ± 0.004 msec at 5.4 V; values at 2 weeks were 0.14 ± 0.06 msec at 2.7 V, 0.07 ± 0.025 msec at 5.4 V; values at 3 months were 0.09 ± 0.03 msec at 2.7 V, 0.05 ± 0.01 msec at 5.4 V; values at 1 year were 0.08 ± 0.02 msec at 2.7 V, 0.04 ± 0.01 msec at 5.4 V, The maximal increase of 0.11 ± 0.05 msec occurred at 2.7 V, 2 weeks after implant. Our results indicate that this new electrode provides low acute and long-term stimulation thresholds, as well as an attenuated early peaking phenomenon, being able to stimulate safely at 2.7 V even early after implant.     

A Comparison between Conventional and Basket Transvenous Electrodes

Three transvenous electrodes, two of a conventional type and one in the shape of a wire basket, were compared. In order to make a valid comparison, the electrodes were all attached to the same kind of lead. The conventional large and small surface electrodes showed no difference in early complication rate. The wire basket electrode, however, had significantly fewer early complications than the others. This was true whether the patients underwent a two- or a one-step procedure, and whether the surgery was done by experienced or inexperienced pacemaker surgeons. When inserted by experienced surgeons in a one-step procedure, the wire basket electrodes had an early complication rate of only 1.2%. Since this unrefined electrode that allows tissue ingrowth for improved anchoring has shown such good results, it is likely that other electrodes that have been perfected, of a hollow or porous design, will also be advantageous to the patients and show good results.     

Time Trends in the Intracardiac Potential Recorded by Pacemaker Telemetry: Comparison Between Steroid-Eluting Small Area Electrodes

We assessed the time course of electrograms sensed both in the atrium and ventricle by two different steroid-eluting electrodes: Medtronic Capsure SP (with an area of 5.5 mm2) and Z (with an area of 1.2 mm2). We considered 68 unipolar electrodes: 31 atrial (19 Capsure SP 4523 and 12 Capsure Z 4533) and 37 ventricular (24 Capsure SP 4023 and 13 Capsure Z 4033) implanted in 47 consecutive patients (30 men and 17 women, with an age of 72 +/- 9.4 years). The pacemaker model was Medtronic Elite 7077-7086 (DDD-DDDR) in 25 patients and Medtronic Legend 8419-8424 (VVIR-AAIR) in 22 patients. The endocavitary signal (all patients had spontaneous rhythm) was telemetrically obtained by a Medtronic 9790 device and acquired on a personal computer at implantation and 7, 30, and 180 days thereafter. The signal was studied both in the time domain and in the frequency domain by spectral analysis. The following parameters were calculated: amplitude (A): peak-to-peak value of the complex; slew rate (SR) peak negative first derivative; F0: frequency at which the power spectrum reaches its maximum value; and bandwidth (Bw): expressed as the distance between the -3 dB points and statistically analyzed by a two-way analysis of variance with factors "time" (four measurements) and "electrode" (Capsure SP and Z) and repeated measurements on the former. Ventricular sensing: no time or electrode effect (P > 0.1 in all comparisons) was found for F0, Bw, or SR, while a time effect (P < 0.04) not dependent on the type of electrode was found for the amplitude of the signal. In particular, a significant increase was found between the measurement at 6 months and that at implantation (P < 0.004). Atrial sensing: A, F0, and bandwidth were not affected by time or electrode (P > 0.09), while SR behaved differently over time (P < 0.05) in the two electrodes (the Capsure Z showed an increase at sixth month [P < 0.04] compared to implantation). In conclusion, the Medtronic Capsure SP and Z electrodes proved to be valid and substantially equivalent as far as concerns the measurement of the intracardiac potential despite the difference between their surface areas. Further studies should be devised to assess whether transitory decreases of atrial Bw in the first month of follow-up observed in a few patients for both electrodes could be responsible for clinical episodes of sensing deficit.     

Permanent Cardiac Pacing with Electrodes of a New Type of Fixation in the Endocardium

To improve electrode construction the following main problems have been considered: (1) reliable initial fixing in the endocardium, and (2) reducing the area of contact surface and improving threshold values. In this article we have described three original electrodes: 1) an endocardial electrode with a multi-edged tip (contact area 28 mm2), which gives high electric field strength and low thresholds; 2) a spreading tip electrode, which was created on the basis of morphological data. The tissues grow into the spread spaces of this tip and ensure better stability (contact area 17.8 mm2); and 3) a double-screw-in electrode which differs from the other corkscrew types. The contact end (surface area--10.4 mm2) consists of two sickle-shaped hooks. The sickle handle is 0.7 mm in length which prevents further hook penetration in the wall of the heart. It was found that a multi-edged electrode and electrodes supplied with a fixation device are, in terms of energy consumption, more effective compared to electrodes with spherical or cylindrical tips of the same area of contact surface. The double-screw-in endocardial electrode has useful features: reliable fixation and a small contact surface area and, therefore, a low threshold value. Thirty-five double-screw-in electrodes were inserted into an atrial position and 28 into a ventricular position. We have not observed any displacement of such electrodes during the past two years.     

Comparison of Electrical Characteristics Between a Steroid-Eluting Single-Pass VDD Lead and a Standard Steroid-Eluting Ventricular Lead

Compared to regular ventricular leads, single-pass VDD leads have two additional floating electrodes proximal to the ventricular tip, which enables them to detect atrial signals. Because of the latter, VDD leads are thicker than ventricular leads, which could affect ventricular pacing. The purpose of the present study was to compare ventricular pacing of a steroid-eluting single-pass VDD lead (CapSure VDD, Medtronic; n = 107) with the same steroid-eluting regular lead (CapSure SP, Medtronic; n = 39) implanted in the ventricle; both leads were connected to the same types of pacemakers. At implantation, pacing thresholds were measured at 0.5-ms pulse duration and impedance by means with the PSA. At discharge, as well as after 1 and 3 months, pulse duration thresholds were determined at 2.5 V pulse amplitude and impedance by telemetry. At implantation, pacing thresholds and impedance were not different in the VDD (0.38 ± 0.16 V; 691 ± 122 Ω) and ventricular lead group (0.44 ± 0.17 V; 648 ± 150 Ω). During follow-ups, no differences in pulse duration threshold were detected between the two groups neither at discharge (VDD = 0.05 ± 0.03 ms; ventricular 0.05 ± 0.02 ms), nor after 1 (VDD = 0.05 ± 0.02 ms; ventricular 0.08 ± 0.07 ms) and 3 months (VDD = 0.06 ± 0.03 ms; ventricular 0.09 ± 0.10 ms). There were also no significant differences for impedance at discharge (VDD = 675 ± 113 Ω; ventricular = 594 ± 113 Ω), after 1 (VDD = 678 ± 131 Ω,; ventricular = 627 ± 112 Ω) and 3 months (VDD = 652 ± 99 Ω; ventricular = 628 ± 105 Ω). Pacing thresholds and impedance were neither significantly different at implantation nor during follow-ups between patients with steroid-eluting VDD leads and patients with an equivalent ventricular lead indicating that the thicker VDD lead does not affect ventricular pacing.     

Long-Term Threshold Stability with Porous Tip Electrodes

Experience with 163 unipolar tined porous endocardial electrodes is reported. One patient required repositioning of the electrode because of exit block. There were no other complications in the entire series of patients. All of the patients had low chronic stimulation thresholds. The mean pulse width 24 hours after implantation was 0.0534 ± 0.0128 ms. Seventy-four patients were restudied six months after implantation. The mean pulse width threshold was then 0.07432 ± 0.0775 ms. Fifty-four patients were evaluated one year after lead implantation. The mean pulse width threshold was then 0.0611 ± 0.0230 ms. The pulse generator was reprogrammed to a lower pulse width in all of the patients. This permitted a substantial prolongation of the pulse generator life. The cost effectiveness of the pulse generator was also greatly improved by pacing with reduced pulse widths. In an additional 16 patients, the voltage amplitude was reduced from 5.0 volts to 2.5 volts. This permitted an even greater increase in the pulse generator longevity.     

A Randomized Comparison of a Bipolar Steroid-Eluting Electrode and a Bipolar Microporous Platinum Electrode: Implications for Long-Term Programming

Differences in acute and chronic pacing thresholds were compared in patients receiving either the Medtronic Model 4004 steroid-eluting lead or the Medtronic Model 4012 microporous platinum lead. Patients (n = 35) were randomized at the time of implant to receive either a steroid-eluting (n = 17) or a microporous (n = 18) lead. Pacing thresholds were determined within 24 hours and at 2, 4, 6, 12. 24, and 52 weeks postimplant. By 2 weeks postimplant. pacing thresholds measured at 0.8, 1.6, 2.5, 3.3, and 4.2 V were significantly lower in the steroid lead group compared to the microporous Jead group (P < 0.05). At 24 weeks, the voltage threshold at 0.3 msec was 0.8 V in 88% of patients with a steroid lead whereas this threshold was only observed in 33% of patients with the microporous lead (P < 0.01). At 52 weeks the pacing energy measured at 1.6 V, twice pulse duration threshold, was significantly lower in the steroid lead group (0.81 ± 0.59 μJ) compared to the microporous lead group (1.25 ± 0.60 μJ, P < 0.05). Thirteen patients in the steroid lead group and 9 patients in the microporous lead group have been programmed at a pulse amplitude of 1.6 V since the 24-week follow-up visit. These patients have been followed for a minimum of 6 months without documented failure to capture. This study shows that pacemaker/lead systems with stable chronic low thresholds can be safely programmed to low pulse amplitude settings. This practice will prolong the longevity of pulse generators.     

Comparison of a Porous-Surfaced with a Totally Porous Ventricular Endocardial Pacing Electrode

Porous endocardial ventricular electrodes, re-cent innovations in cardiac pacing technology, have been shown to perform in a su-perior manner compared to conventional solid electrodes with respect to such parameters as stimulation threshold, sensed R-wave amplilude, and source im-pedance, This experimental study compared the electrophysiological performance of a porous-surfaced and a totally porous electrode, two fundamental design vari-ations of the porous eJectrode concept which are in current clinical use. Six porous-surfaced (1 mm length, 2.3 mm diameter, 8.8 mm² outer surface area, pores < 25 μn) and six totally porous (1.3 mm length, 2.0 mm diameter, 7,5 mm- outer surface area, pores 100 to 150 μm) flanged ventricular endocardial eleclrodes were implanted into the right ventricular apex of 12 dogs. Stimulation threshoids under constant current and constanl voltage conditions at pulse durations of 0.1, 0.25, 0.5, 0.75, 1.0, 1.5, and 2.0 milliseconds, sensed peak-to-peak R-wave amplitudes, and source impedance ivere measured at implant and at l, 4, 8, 12,16, 24, and 30 weeks (explant) thereafter. Analyses of voriance on the data for the strength-durolion curves at expiant and the threshold-time curves at l ms pulse duration indicated highly significant differ-ences between the performance of the two types of electrodes, the porous-surfaced electrodes displaying average Stimulation threshoids approximateiy 30% lower than the totally porous electrodes. Student's t tests indicated the magnitude of the sensed R-wave to be maintained over the 30 week period for the porous-surfaced electrodes but to decrease by about 20% for the totally porous electrodes. In addition, the chronic source impedance of the porous-surfaced electrodes was significtmtly Jess (about 257%) than that of the totally porous electrodes. These differences indi-cate better overall electrophysiological performance for the porous-surfaced electrodes as compared with the totally porous electrodes. Both types of electrodes, however, operate well within the limits of highly acceptable function and therefore rep-resent attractive designs for clinical use in endocardial pacing.     

Assessment of Long-Term Stability of Chronic Ventricular Pacing Thresholds in Steroid-Eluting Electrodes

Sixteen patients with Medtronic 4003 steroid-eluting electrodes implanted in the ventricular position were followed over 5 years. In each patient a special type of Medtronic 2443 pacemaker was implanted to allow programming of output at 1.35 V. Chronic threshold values in these patients measured at an output of 1,35 V were stable over the first 18 months of follow-up. Mean values were: 0.06 ± 0.03 msec at 6 months and 0.08 ± 0.02 msec at 18 months; these did not differ from each other significantly. However, during the period from 18 to 36 months postimplantation, a significant increase in mean pacing threshold was observed: 0.08 ± 0.02 msec at 18 months postimplantation versus 0.14 ± 0.05 msec at 36 months (P < 0.01), After 36 months, the chronic pacing threshold remained stable until the end of the 5-year follow-up period. Further long-term study of chronic threshold behavior of steroid-eluting electrodes measured at low amplitudes is warranted.     

Vagal Stimulation Reduces the Severity of Maximal Electroshock Seizures in Intact Rats: Use of a Cuff Electrode for Stimulating and Recording

J. Zabara showed that repetitive vagal stimulation (VS) prevents or ameliorates convulsive seizures in dogs. We have studied the effects of VS on maximal electroshock seizures (MES) in intact rats: (1) A 5 wire cuff electrode was developed for stimulating and recording from the vagus. Compound action potentials (AP) were recorded and strength-duration curves obtained for A and C fibers. There is a monotonic relationship with a negative slope between heart rate (HR) and AP amplitude. C fibers remain excitable for 25 days after cuff implant. (2) The anticonvulsant efficacy of VS is directly related to the fraction of vagal C fibers stimulated and the frequency of stimulation. (3) The anticonvulsant efficacy of VS has been established using two rat models of human epilepsy. VS abolishes the extensor component of the tonic phase of a MES and shortens or prevents tonic seizures induced by pentylenetetrazol (PTZ). (4) VS appears to act via release of large quantities of the inhibitory mediators GABA and glycine throughout large volumes of the brain. (5) It is rational to test VS in man as a treatment for intractable seizures.     

Steroid-Eluting Epicardial Pacing Electrodes: Six Year Experience of Pacing Thresholds in a Growing Pediatric Population

Indications for pacemaker implantation in the pediatric population often include sinus or atrioventricular node dysfunction following surgery for congenital heart defects. However, patient size, cardiac defects, and vascular and valvular concerns may limit transvenous lead utilization. Since the epicardial surface of these patients often exhibits variable degrees of fibrosis from scar tissue formation or pericardial adhesions, chronic low output (2.5/1.6 V, 0.3 ms) epicardial pacing from implant is not currently recommended in children due to frequent threshold changes and electrode exit block. As a result, pacing in children is often viewed as a less efficient system than in adults. The addition of steroid combined with newer low threshold electrode designs however stabilizes the electrode-tissue interface and eliminates postimplant changes seen with standard smooth surface electrodes potentially permitting efficient chronic pacemaker application to all patient ages. The stability of chronic low output epicardial pacing with steroid-eluting electrodes was prospectively studied in 22 patients (ages 2 days-18.5 years, median 3.5 years) for up to 6 years. Chronic pulse width thresholds were compared according to implant site and association of prior cardiac surgery. A total of 26 pacing leads were implanted. The acute implant mean pulse width threshold (2.5 V) for all the electrodes studied was 0.10 ms ± 0.05 ms. Stable low thresholds were maintained for up to 6 years without significant variation from implant. Mean ventricular pulse width thresholds (0.12 ms ± 0.05 ms) were significantly higher (P < 0.001) than atrial thresholds (0.06 ms ± 0.03 ms) at implant and throughout the study period. The thresholds in the patients following cardiac surgery were comparable to those without previous cardiac surgery (P = NS). Stable low thresholds may be chronically maintatined for up to 6 years for epicardial steroid-eluting electrodes irrespective of pacing site or associated cardiac surgery.     

Clinical Performance of Steroid-Eluting Pacing Leads with 1.2-mm2 Electrodes

To raise pacing impedance and reduce battery current drain, new tined steroid-eluting leads were developed with 1.2-mm² hemispherical electrodes, instead of conventional 5–8 mm². Twenty-two unipolar J-shaped atrial leads and 25 unipolar ventricular leads (models 4533 and 4033, respectively) were implanted in 33 consecutive patients and followed for a mean of 25 months (range 18–29). Handling characteristics of atrial leads were found favorable. The leads slipped easily into the right atrial appendage and were easy to position. Handling characteristics of ventricular leads were satisfying, but more efforts had to be applied to cross the tricuspid valve. Special care was taken to avoid perforation of the myocardium due to the small lead tip. Following implantation, four ventricular and one atrial lead exhibited instability of pacing thresholds that resolved spontaneously within 1–3 days of implantation. Except for this, no lead malfunctioned. The reoperation rate was zero. The mean electrogram amplitudes of 15 mV (ventricle) and 4 mV (atrium), and the mean chronic pacing threshold of 0.085 ms at 1.6 V (app. 0.43 Vat 0.5 ms) were comparable with the best values seen in the literature on passive fixation leads. The rest of the electrophysiological parameters were enhanced: mean pacing impedances were 984 Ω (acute) and 900 Ω (chronic), mean slew rates 3.26 V/s (ventricle) and 1.75 V/s (atrium), mean acute voltage threshold at 0.5 ms was 0.25 V, mean current and energy thresholds calculated at 0.5 ms were 260 μA and 32 nJ (acute) and 478 μA and 103 nJ (chronic). The electrical characteristics of these leads provide for increased pacemaker longevity in combination with substantial safety margins for pacing and sensing.     

Corrosion of Pacemaker Electrodes

As improvements in pacemaker components and design increase pacemaker life, problems with other components may emerge such as corrosion of the electrodes. Explanted electrodes were examined under a scanning electron microscope and the degree of corrosion was graded and correlated with the reasons for expiantation, the status of the explanted pacemaker, and the duration of the implant. Fifty-six explanted electrodes were removed for loss of capture or sensing, pacemaker extrusion, broken wires, or after death. A numerical grading system was used to qualitate the degree of corrosion. The pacemaker itself was retrieved in 50 cases, permitting evaluation of the entire pacing system. The dc component of the pacemaker output was measured. Without She aid of magnification most electrodes appeared shiny and smooth. With one exception all corrosion was microscopic. Significant corrosion was seen on all pacemaker electrodes that had been connected to pacemakers with dc offset currents greater than 5 microamperes. All except the "youngest" explanted electrode showed some degree of corrosion. The data suggest that corrosion was directly related to the duration of implantation. With the possible exception of one case, no consistent adverse clinical effects were observed, even when electrodes were heavily corroded. Whether or not corrosion eventually will progress to the point that clinical problems become manifest cannot be predicted at this time.     

Paired Comparisons of Steroid-Eluting and Nonsteroid Endocardial Pacemaker Leads in Dogs: Electrical Performance and Morphologic Alterations

The effects of steroid elution from endocardial pacemaker electrodes on electrical performance and the thickness and cellularity of the reactive fibrous connective tissue formed around the stimulating electrode (peri-electrode tissue) were determined. Comparison was made with a nonsteroid electrode implanted in the same cardiac chamber (right ventricle) in each of six dogs for 6 weeks. Paired Students' t-tests showed that steroid-eluting leads had significantly (P less than .05): (1) lower voltage stimulation thresholds (as determined in sequential measurements made on the conscious animals during the experiment and on the anesthetized dogs at termination of the study); (2) less fibrous connective tissue formation around the electrode surfaces; and (3) fewer cells per unit area of peri-electrode fibrous connective tissue. There were also fewer (P less than .10) mast cells in the reactive connective tissue surrounding steroid-eluting leads. The thinner reactive connective tissue surrounding the steroid-eluting electrodes was correlated with lower voltage stimulation thresholds (r = 0.7, P less than .01). This is consistent with the hypothesis that the effect of the peri-electrode connective tissue is to increase the virtual surface area of the electrode, decreasing current density in adjacent stimulatable tissue. The relatively fewer total cells and mast cells in the peri-electrode connective tissue of the steroid-eluting electrodes suggest that the observed differences in fibrous connective tissue thickness, and therefore voltage stimulation threshold, may be related to a relatively decreased population of inflammatory cells due to the anti-inflammatory properties of the steroid.     

An Investigation into the Electrical Ablation Technique and a Method of Electrode Assessment

Ablative techniques, using standard defibrillators and commonly available cardiac catheters, have been applied to the His bundle and bypass tracts for the management of arrhythmias. We have done in vitro studies of the physical effects of these high energy electrical impulses delivered via different pacing electrodes. Unipolar impulses of 10 to 400 joules were delivered via three U.S.C.I. bipolar electrodes and three Vitatron Helifix electrodes immersed in Ringer's solution. The effects were recorded on 35 mm still film, video tape, and high speed cine film. Pressure, voltage, and current were measured. The U.S.C.I. bipolar electrodes and the Vitatron Helifix electrodes safely withstood repeated delivery of 400-joule impulses which produced similar flash shapes. Each took the form of an incandescent, spherical "fire-ball" centered around the exposed electrode surface. The mean diameters of the "fire-ball" for 10 to 400 J using the U.S.C.I. electrodes were 5-24 mm and 3-20 mm for the Helifix catheter electrodes. Peak pressure excursions of over an atmosphere were observed 3 cm from the electrode tips. Higher pressures, lower voltages, and larger currents occurred using the U.S.C.I. pacing lead. The simple, 35 mm time exposure technique showed that at low energies the flashes appeared to emerge in a retrograde manner from the U.S.C.I. catheters and more distally from the Helifix electrode. This suggested that the latter might be more effective with lower energy impulses. It is concluded that lower energies should be used to take full advantage of the active fixation electrode.