"In-line" bipolar, steroid-eluting, high impedance, epimyocardial pacing lead.

Recent advances in electrode surface designs have eliminated traditional threshold differences between endo- and epicardial pacing leads. Since the epicardial approach offers the potential of direct left ventricular pacing and the transvenous approach may not be feasible or warranted in all instances, more advanced leads are being designed to optimize epicardial pacing capabilities. This study was conducted to evaluate a bipolar epimyocardial lead. Six immature canines (age 3 months) were instrumented. The lead (Medtronic model 10389) is a single-pass, "in-line" bipolar electrode with low current drain and high impedance, with an intramyocardial steroid-eluting cathode and nonsteroid epicardial anode. Twelve ventricular leads were implanted (two per animal) and the animals followed for 6 months with weekly analysis of pacing and sensing capabilities. Results at explant were compared with implant values and showed no significant differences between sensed R waves or in R wave slew rates in unipolar or bipolar modes. Explant lead impedances remained high in both modes: bipolar, 1658 +/- 331; and unipolar, 1327 +/- 308 omega (P < 0.05). Chronic voltage (V) threshold at 0.5 ms showed no significant change from implant values during the study: unipolar, 0.3 +/- 0.06 versus 1.0 +/- 0.8; and bipolar, 0.4 +/- 0.06 versus 1.6 +/- 1.2. Histologic review showed negligible fibrous reaction at the electrode-tissue interface. This study introduces a high impedance, low threshold, "in-line" bipolar pacing lead design capable of stable chronic pacing with implant facilitated by a single suture technique.

Thyroid dysfunction effect on cardiac stimulation and sensing parameters

We report our findings in a chronic canine study performed to evaluate the effects of thyroid funciton alteration on atrial and ventricular pacing parameters. Each of five dogs with normal, healthy myocardia, received four unipolar Medtronic contempory pacing leads - model 4011 Target Tip and model 4023 Capsure SP ventricular leads, model 6957 atrial screw-in and an additional lead, either model 4523 Capsure SP or model 4511 Target Tip passive fixation lead. Baseline electrical data and blood levelof T3, T4, electrogram amplitude and slew rate in both the atrium and the ventricle for each electrode type studied during hypothyroid vs hyperthyroid states. Minor decreases in ventricular thresholds were identified. Hyperthyroid induction did not chage either sensing or stimulation parameters from the euthyroid vaseline values.

Polyurethane elastomer biostability.

Polyurethanes have unique mechanical and biologic properties that make them ideal for many implantable devices. They are subject to some in vivo degradation mechanisms, however. Polyester polyurethanes are subject to hydrolytic degradation and are no longer used in long-term implanted devices. Polyether polyurethanes, while hydrolytically stable, are subject to oxidative degradation in several forms, including environmental stress cracking and metal ion oxidation. Mineralization is also known to occur. A new polycarbonate polyurethane has superior biostability in early in vivo qualification tests compared to the polyether polyurethanes, including no evidence of hydrolysis, ESC or MIO.

The encapsulation of polyurethane-insulated transvenous cardiac pacemaker leads.

When cardiac pacemakers are implanted, the tranvenous route is typically preferred. For dual chamber pacemakers, an atrial and a ventricular lead are required. Based on postmortem examination of 101 canines with polyurethane insulated leads implanted from 10 days through 13 years, encapsulation of these leads is initiated by thrombus secondary to endothelial damage and/or blood flow perturbations. Organization of thrombus results in a vascularized collagenous capsule. With continued blood flow perturbation, more thrombi can form and reorganize to cause the collagenous capsules to grow with implant time. Under certain conditions, the encapsulating sheath can differentiate into cartilage, mineralized tissue, and even bone. The least commonly encapsulated area is in or just superior to the annulus of the valve. The most common locations for capsule formation are within the upper right atrium (where two leads are often bound together by the tissue) and the right ventricle. The ventricular sheaths are often adhered firmly to tricuspid valve structures. The presence of relatively large, friable, partially organized thrombi on chronic leads is not unusual, even after more than 10 years' implantation. It is recommended that chronic leads be imaged prior to attempts to remove them to detect the presence and location of embolizable structures.

Improved epimyocardial pacing: initial experience with a new bipolar, steroid-eluting, high impedance lead design.

Epicardial pacing typically is associated with decreased pacing and sensing capabilities compared with the endocardial approach. Since endocardial pacing is neither appropriate nor possible in all instances, this study was conducted to evaluate a new concept in a chronic epimyocardial lead design in six 3-month-old growing dogs. The new bifurcated lead (Medtronic model 10401) is a low current drain, high impedance, steroid-eluting, bipolar design. The implant is facilitated by a suture attached with an atraumatic needle. Twelve ventricular leads were implanted (2 per animal) and followed for 6 months with weekly analysis of pacing and sensing capabilities. Results at explant were compared with implant values. There were no significant differences between implant and explant in sensed R waves, or in the slew rate of the R wave in unipolar or bipolar modes. Lead impedances at explant remained high in both modes: bipolar, 1550 +/- 223; unipolar, 1234 +/- 262 omega (P < 0.05). Chronic voltage (v) threshold at 0.5 msec showed no significant change from implant values during the study: unipolar, 0.4 +/- 0.2 vs 0.7 +/- 0.3; bipolar, 0.5 +/- 0.4 vs 1 +/- 0.5. Histologic evaluations of the electrode tissue interface demonstrated negligible fibrotic capsule formation. This study introduces a new, easily implanted, high impedance, low threshold, bipolar epimyocardial pacing lead design with excellent chronic pacing and sensing characteristics.

Anatomical and morphological evaluation of pacemaker lead compression.

In recent years, pacemaker lead failure due to compressive damage has been reported with increasing frequency. To document the mechanism of this failure, we evaluated explanted mechanically damaged leads with electrical testing, optical microscopy, and in some cases, scanning electron microscopy (SEM). In addition, we performed an autopsy study to measure the compressive loads on catheters placed percutaneously through the costoclavicular angle, as well as by cephalic cutdown. Of the 49 explanted compression damaged leads with enough clinical data for analysis, all had been placed by percutaneous subclavian puncture. Our autopsy data confirmed the significant increase in pressures generated in the costoclavicular angle for medial percutaneous subclavian catheterization (126 +/- 26 mmHg) compared to a more lateral percutaneous subclavian puncture (63 +/- 15 mmHg) or a cephalic cutdown (38 +/- 13 mmHg) (P < 0.01). In vivo coil compression testing documented loads up to 100 pounds per linear inch of coil and a compressive morphology by SEM identical to that seen in the clinical explants. Pacemaker leads appear to be susceptible to compression damage when placed by subclavian venipuncture. When possible, leads should be placed such that they avoid the tight costoclavicular angle.

Programming eccentricities of an activity sensing pacemaker.

The Activitrax rate responsive pacemaker system has enjoyed wide popularity but minor engineering eccentricities have occurred and have been reported. We report another unusual feature seen in Activitrax models 8400, 8402, and 8403. This feature consists of continuing in a temporary mode while a different permanent mode was programmed. These eccentricities of the programming features are not very commonly seen, however, they can be somewhat perplexing to the physician following the patient. These unusual features are no longer present in the newer models being manufactured.