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)

Cardiac Surgical Procedures↗

PubMedDate not supplied

Advances in ventricular synchronous demand cardiac pacemakers.

New ventricular synchronous demand (VVT) pacemaker functions have been developed that provide improved performance in electromagnetic interference (EMI) environments and improved sensing of ventricular premature contractions. All previous ventricular synchronous pacemakers had an inherent design conflict between choosing the optimal (relatively long) input refractory interval needed for limiting the maximum pulse delivery rate and choosing the optimal (relatively short) input refractory interval (sensing dead time) following either a sensed or paced cardiac contraction. The necessary compromise resulted in a device that in the presence of EMI, stimulated at a rate (approximately 150 ppm) which was dangerously fast for certain patients, yet was insensitive to early poststimulation (400 to 500 msec) premature ventricular contractions, resulting in a risk of T-wave stimulation. Partly because of these deficiencies, the VVT function has not been widely employed by the medical profession: instead, the ventricular inhibited (VVI) pacemaker has evolved as the treatment of choice for the cardiac patient with intermittent heart block, even though total inhibition by certain EMI radiators is a potentially serious problem. New VVT structures have been developed that allow control separation of the maximum EMI discharge rate and the sensing refractory interval by employing independent input and output refractory intervals. New low-power digital devices provide building blocks applicable to the developed architectures.

Arrhythmias, Cardiac↗

PubMedDate not supplied

Improved waveform generator and cardiac simulator for sensitivity testing of ventricular programmed pulse generator.

The need for an easily generated, simulated QRS complex to facilitate preimplantation sensitivity testing of ventricular programmed pacemakers has led to the adoption of the sine squared waveform as the standard test pulse, by the Pacemaker Standards Subcommitte of the Association for the Advancement of Medical Instrumentation (AAMI). A simple circuit is presented that provides an accurate sine squared pulse with easily adjustable pulse width (base frequency). A suggested circuit for incorporating this sine squared pulse generator into a complete pacemaker test waveform generator/cardiac simulator is also presented.

Electronics↗

PubMedDate not supplied

The induction of myocardial damage by open-chest low-energy countershock.

Anesthetized rabbits were used to study the effects of internal electrical defibrillation on the heart. Single electrical shocks of 0.5, 1.0, and 3.0 Wsec/kg or five consecutive shocks of 0.5 Wsec/kg spaced at either 1 shock per min, per 2 min, or per 3 min, were used. Half of the rabbits given a single shock of 1 Wsec/kg or multiple shocks at a spacing of 1 shock per min were given intravenous injection of dexamethasone sodium phosphate (6 mg/kg) 5 min before receiving the defibrillatory shocks. Damage to the hearts was assessed by electrocardiography and histopathology. Myocardial damage was evident at the lowest energy levels used. All indices showed that the degree of damage was increased by increasing the energy level of single shocks or by decreasing the time interval between multiple shocks. The steroid pretreatment did not appear to protect the hearts from damage caused by the shocks.

Animals↗