Search

The unacceptable rate of mechanical failures, threshold problems, and recalls experienced with many coaxial bipolar cardiac pacing lead designs are reviewed in detail. To address these problems, redundant insulation coradial atrial and ventricular tined leads (AL and VL, respectively) with iridium oxide electrodes were developed and subjected to extensive accelerated testing. There were no mechanical failures. The new lead body design proved to be much more durable than widely used trifilar MP35N configurations. The data reviewed and early and current test 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, insulation adherence to the conductor may reduce the potential for ionic degradation. Pacing and sensing thresholds in animal studies of the new leads were within the reported range for leads with steroid eluting electrodes. A multicenter Canadian clinical trial was initiated with the first 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 months; 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 pacing were AV block in 61 patients, sick sinus syndrome in 53, vasovagal syncope in 4, and congestive heart failure in 7. Many patients had associated or primary tachyarrhythmias, including 111 with supraventricular 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 good or excellent in 90% (746/829) of evaluations. X-ray visibility was of concern in 10% of patients (12/124). Three perioperative complications occurred, including displacement of one AL (1.2%) and one VL (0.9%). There were no subsequent mechanical (connector, conductor, or insulation) 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 omega. 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 incomplete, because some patients were in atrial fibrillation during implantation. After 12 months, AL PT at 1.5 V output was 0.18 +/- 0.10 ms (21) and at 2.5 V was 0.10 +/- 0.053 (22). Associated AL ST was 3.3 +/- 0.9 mV (21) AL Z 500 +/- 65 omega (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 omega (9). Follow-up time discrepancy is due to the VL being available 6 months earlier than the AL. There were no 30-day deaths and only one late death at 10 months in a patient with chronic atrial fibrillation. Death was unrelated to pacer or lead function. At 1 year, 68% AL (15/22) and 62% (24/39) captured at 0.5 V and < or = 1 ms pulse width output. Innovative adherent insulation coradial bipolar lead conductors of the design studied combined with coated iridium oxide electrodes provide for a negligible incidence of mechanical or functional failure with clinical follow-up now approaching 3 years. Excellent acute and chronic sensing and pacing thresholds have been documented. Late thresholds have continued to improve gradually. Long-term clinical pacing at < or = 1.5 V output with a large safety margin is feasible in essentially all patients. This coradial design produces very flexible < 5 French bipolar redundantly insulated lead bodies allowing both AL and VL to simultaneously pass through a single 10 French introducer sheath. (ABSTRACT TRUNCATED)

The clinical utility of single lead, atrial synchronous, ventricular pacing (VDD), for patients with normal sinus function and heart block is well established. Atrial stimulation, unavailable in VDD systems presents a significant disadvantage. DDD pacing systems however, require the introduction and positioning of two separate leads. The acute human study discussed evaluated a modified version of a commercially available VDD lead with a preshaped lobe, capable of both sensing and pacing the right atrium. “P” waves and atrial stimulation thresholds were determined in five patients. The mean P-wave was 2.5 ± 1 millivolts. Atrial stimulation in the unipolar configuration was 1.6 ± 0.5 volts and 1.7 ± 0.9 volts in the bipolar configuration. These acute stimulation thresholds and sensing amplitudes were comparable to conventional DDD pacing systems. Therefore, single-lead DDD pacing may be feasible and warrants further study.

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)

Some concepts derived from modern literature on the physics of cardiac muscle conduction regarding optimization of the design of electrode systems for detection of atrial depolarization and a preliminary clinical corroboration of the concepts are presented. The concepts of most import are 1) that the mechanical dimensions of sensing electrodes in all planes have to be as small as possible relative to the dimensions of the extracellular waveform dimensions associated with depolarization action potentials, and 2) that the distance between bipolar electrodes should be tuned to be greater than the dimensions of the extracellular waveform to avoid subtractive interference patterns associated with saltatory propagation in aging cardiac muscle. Additionally, in the practical application of chronic atrial sensing from a single lead for use in an implanted VDD pacing system, it is also important to consider electrode size and its effect on impedance level.

A 73-year-old male required a permanent pacemaker for complete heart block. After placement of the dual-chamber pacemaker, the patient developed random, repetitive ventricularly paced coupling at the upper rate limit. The diagnosis made from the surface electrocardiography was one of ventricular tracking of occult (T wave obscured) atrial ectopic activity. Examination of the telemetered atrial electrogram (EGM) revealed a diagnosis of intermittent far-field atrial sensing of the terminal ventricular depolarization. The value of obtaining an atrial electrogram during dual-chamber pacing is discussed.

Electrode studies have been performed with dead animal tissue and a variety of other materials immersed in saline solution and compared with studies in the canine heart (live and arrested) in an attempt to delineate both normal and anomalous signals sensed by pacemaker electrodes or obtained during diagnostic electrogram recording of cardiac activity. The data from these studies could be useful for defining the origin of artifacts and a variety of other phenomenon such as “fractured” QRS complexes, acute ST segment elevations, His bundle oscillatory signatures, and unexplained potentials synchronously associated with cardiac events. The studies verify that artifacts can be generated in an electrolyte medium by rubbing electrodes against insulators or biologic materials and by inducing motion between common pole materials of an active electrode system. The studies suggest that some of the grasping electrodes in current clinical use may be subject to self-generating artifacts associated with cardiac-induced frictional motion between the constituent materials employed in the electrode design.

A ventricular inhibited demand cardiac pacemaker was inhibited in the presence of intimate contact between the active temporary bipolar electrode ring and a retained inactive permanent bipolar electrode tip. Electromagnetic interference effects, lack of insulation, lead breaks, and loose connections were all ruled out as the cause of pacemaker suppression.

To define the effects of endocardial positioning on the sensing electrode's ability to detect the cardiac electrogram, a specially designed coaxial lead system with a movable, large surface area (50 mm 2 ) proximal ring electrode and a small surface area distal tip electrode was placed transvenously in 17 mongrel dogs. When the sensing ring electrode was used in a unipolar mode, there were slight but insignificant decreases in R-wave amplitude while the ring remained in the ventricle. When the electrode was moved out through the 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. When the ring electrode was used in conjunction with the tip as a conventional bipolar system, the sensed R wave remained at uniformly high levels except when the two electrodes were very close together at the tip. P-wave amplitudes were not detected in the bipolar mode until the ring entered the atria.

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.

After years during which pacers of very similar design and capabilities were provided by a small number of manufacturers, many different lithium, halogen, rechargeable, and nuclear power sources are now available. The variety of chemistries, methods of construction, and sealing techniques used in the batteries of the different manufacturers is almost unlimited. This has made it necessary for physicians who implant and follow pacers to acquire a general knowledge of the field if they are to make an informed choice of pacemaker power source for implantation and if they are to manage recalls with a minimum of patient and physician trauma. More experience is required before it can be definitely determined which of the new pacer power sources will prove superior, but when coupled with well-designed, hermetically sealed pulse generators, all are capable of providing continuous pacing for at least 5 years and the 10-year pacemaker is now a probability.

Tests were conducted on rechargeable mercury-zinc pacemaker batteries under simulated and actual biologic conditions, using a variety of discharge rates and charging schedules. In tests on 96 cells at a 6.4 milliampere (ma) discharge, recharging once every 15 months of simulated pacing at a 25 microampere ( μ a) drain, the earliest cell failure occurred after an equivalent of 50 years of pacing. The mean pacing equivalent for all 96 cells was more than 140 years. In 6.4 ma discharge tests on 24 cells, recharging once every 8 days of simulated pacing, only 1 cell in 24 failed after an equivalent of more than 500 years of pacing (actual time 2 years). In tests on 13 cells pacing at a 200 μ a drain without recharging, the simulated mean duration of pacing before total discharge was 4.8 years. Seven other cells at a 200 μ a drain with periodic recharging continue to function normally after more than 7 years of actual time, simulating 56 years of pacing at a 25 μ a drain. Cardiac pacemakers using the rechargeable mercury-zinc cell have been implanted in animals for more than 2 1/2 years and in patients for more than 1 year with all units continuing to function satisfactorily. It has been demonstrated unequivocally that a rechargeable mercury-zinc pacemaker will function continuously for more than 4 years without recharging and that periodic recharging will extend pacing life far beyond that predicted for lithium and nuclear primary power sources.

With exposed metal at the electrode tissue interface (8 mm2, 28 mm2, 57 mm2), myocardial threshold stimulation impedance increased as pulse duration was lengthened, with left ventricular intramyocardial stimulation, and with the smaller surface area electrode. An 0.5 mm2 differential-current-density electrode, which eliminated direct metal-to-tissue contact at the electrode-myocardial interface, was associated with notably higher impedances than each of the three metal tip electrodes and did not show increasing impedance levels with changes in pulse duration, confirming the minimization of polarization energy losses with this device. The majority of electrode, electrode tissue interface, and myocardial variables that are characterized by high threshold stimulation impedance are associated with low threshold energy requirements for pacing and reduced pacemaker power source drain. No accurate information about sensing impedances can be derived from current knowledge of pacing impedance.

Demand cardiac pacemaker functions are under study that provide new methods to distinguish between cardiac activity and pulsatile electromagnetic interference (PEMI). All known forms of currently marketed ventricular inhibited demand pacemaker (VVI) functions can be inhibited by high level pulsatile electromagnetic interference. The recent introduction of shielded circuitry to protect against disruptive (inhibiting) EMI has reduced pacemaker sensitivity to interference. However, EMI received via the cardiac lead/electrode can still mimic cardiac activity. This mimicing occurs as a consequence of detection by defribrillator protection structures or by amplifier saturation from RF artifacts insufficiently suppressed by input QRS bandpass filters. The new functions under development employ a separate EMI detection receiver for controlling the pacemaker mode to minimize inhibition by PEMI.

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.

While many rechargeable pacers, including a commercial unit have been developed using nickel-cadmium cells, they are in general limited by low cell capacity, short life without recharging, the need for frequent recharging, indirect monitoring of battery voltage by stimulation rate changes and gradually falling cell capacity with each cycle at body temperature. We have developed a low-drain rechargeable pacemaker, hermetically sealed in a seamless container, that will stimulate continuously for 4 yr without recharging. Our battery is a silver modified 1000 mA hr rechargeable version of the mercury-zinc cell proven by years of use in over 95% of pacers used clinically to date. It has functioned for over 6 yr in real time tests and in 120 accelerated tests has always simulated over 50 yr of pacing. Our pacer is small and thin, with less than one-half the displacement of current nonrechargeable units. Electromagnetic recharging energy is painlessly transmitted through the intact skin by a tuned coil, using a unique system unaffected by variations in implant depth. A telemetry oscillator permits noninvasive monitoring and telephone transmission of cell voltage, stimulation rate, implant depth, and optimal recharging coil location. Six animal and 3 clinical implants continue perfect function after up to 2.5 and 1 yr, respectively. A 6 yr life is proven and a greater than 20 yr life is anticipated.

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.

Some telemetrically monitored pacemakers are capable of detecting and transmitting either the P- or R-wave, however, none are able to transmit the complete electrogram (EGM) during normal and paced rhythms. A new lead system has been developed that, when used in conjunction with a telemetrypacemaker system, permits the transmissionofthe complete EGM and is not adversely affected by pacemaker outputs or afterpotentials. Detecting electrodes are placed in both right atrium and ventricle in order to optimize EGM detection. Electrically separate from the pacing-sensing electrodes, the A-V Data probes are structurally part of a transvenously placed ventricular lead no larger than a conventional bipolar lead (2.3 mm). The EGM telemetry system is also compatible with telephone monitoring systems. The A-V data lead has consistently demonstrated its ability to accurately detect the EGM during normal sinus rhythms as well as during atrial, ventricular, and A-V sequential pacing. In addition, this system can detect and transmit EGM abnormalities such as ectopic beats and retrograde conduction. Eight A-V data lead, telemetry monitored VVI pacemakers have been implanted in dogs for over two years. This system has provided the means to noninvasively detect and telemetrically record the entire normal and abpormal EGM, even in pacer dependent patients.

No telemetrically monitorable pacemakers are capable of detecting and transmitting the complete electrogram (EGM) during normal and paced rhythms, although some are capable of detecting and transmitting either the P- or R-wave. A new lead and telemetry-pacemaker system has been developed that permits the transmission of the complete EGM and is not adversely affected by pacemaker outputs or after-potentials. Electrodes are placed in the right ventricle and atrium to optimize EGM detection. These A-V data probe electrodes are electrically separate from the pacing-sensing electrodes but are structurally part of a coaxial ventricular lead that is no larger than a conventional bipolar lead (2.3 mm). The telemetry system is also compatible with telephone monitoring systems. The system has consistently demonstrated its ability to detect accurately the EGM during normal sinus rhythms and does not affect atrial, ventricular, or A-V sequential pacing. In addition, this system can detect and transmit EGM abnormalities, including ectopic beats, retrograde conduction, and ischemic changes. A-V data leads with telemetry monitored VVI pacemakers have been implanted in dogs for over two years. This system will provide the means to detect and telemetrically record noninvasively the entire normal and abnormal EGM, even in pacer dependent patients. (PACE, Vol. 7, November-December, 1984).

Although the Secretary of Health, Education and Welfare recently stated that hermetic sealing of all implanted cardiac pacemakers, as recommended by the Navy in 1969, is inadvisable, several small manufacturers are already marketing hermetically sealed devices (CPI, Pacesetter), all of the over 600 nuclear pacers implanted to date have been hermetically sealed, and belatedly the major manufacturers are developing hermetically sealed units. Total hermetic sealing of all implantable electronic devices has been technically feasible for years, and it is essential that device legislation include mandatory encapsulation standards.

Since 1967, three series of rechargeable single-cell silver-mercuric oxide-zinc pacemakers have been implanted in dogs with complete heart block. The five nonhermetic units in series 1 failed after less than or equal to 18 months, primarily due to prototype cell deficiencies, although one cell functioned for eight years. The six units in series II contained improved cells, but failed due to gradual transepoxy fluid absorption after less than or equal to 31 months. All rechargeable cells were salvaged and dried, and, seven years after their manufacture, they continue to power pacing circuits. Series III now totals 20 doubly hermetically sealed units, tested for up to three years (total more than 300 months or 26 years), with no pacemaker failures. Accelerated tests indicate a minimum life of more than 50 years. A clinical trial is in progress.