M. Schaldach

Improvement of stenting therapy with a silicon carbide coated tantalum stent

State of the art cardiovascular stent materials are a compromise between bulk properties and surface related properties. As a consequence, deficiencies in both characteristics lead to serious limitations of stenting therapy. Beside a dissatisfying X-ray visibility of current stent materials, which hinders precise angiographic control of the stent during implantation, insufficient hemocompatibility causes subacute vessel occlusions despite stringent anticoagulant medication. Additionally, bleeding complications result which further limit the therapeutical success. Therefore it is essential to develop a new coronary stent with improved material properties for the bulk of the stent and its surface. This is realized by a hybrid concept. The stent is manufactured from tantalum, having a high inherent radio-opacity. The stent is coated with amorphous silicon carbide, optimized for hemocompatibility. An appropriate deposition technology to maximize coating adhesion was developed. Amorphous silicon carbide was investigated in vitro and in vivo to assess its suitability for coronary stents.

Sputter-deposited TiN electrode coatings for superior sensing and pacing performance.

SCHALDACH, M., ET AL.: Sputter‐Deposited TiN Electrode Coatings for Superior Sensing and Pacing Performance. The sensing and pacing performance of pacemaker electrodes is characterized by the electrochemical properties of the electrode/tissue interface affecting tissue reactions and the kinetics of the ionic exchange. The usually smooth metallic electrode surface results in a high pass filter characteristic. To better match the electrodes filter characteristic to the spectral content of the depolarization signal, various combinations of electrode shape, material and surface structure have been researched. The electrode with sputter‐deposited TiN coating presented in this report has been designed to meet the demand for low acute as well as chronic thresholds and superior sensing performance not only with respect to spontaneous activity but also regarding the detection of the evoked response. The clinical results obtained with this electrode prove the excellent pacing and sensing properties resulting from minimized polarization losses and optimized filtering of the signal to be detected, respectively. The acute and chronic clinical advantages over previous concepts are attributed mainly to the biocompatibility of the material used and the microcrystalline surface structure achieved by the coating process. The design concept of the new electrode is presented together with the clinical results obtained. While the advancements in microelectronics and battery technology have certainly formed the basis for the development of pulse generators featuring an ever increasing versatility of functions at the same or even smaller pacemaker dimensions, from a point of view of pacing system performance the development of improved electrode concepts as the one presented must be regarded as equally indispensable.

Autonomic nervous system controlled closed loop cardiac pacing.

A multicenter clinical study is presented, which focuses on the reestablishment of closed loop cardiac control in patients with chronotropic insufficiency. Using the information about sympathetic tone contained in the myocardial contractility, it is possible to reconnect the heart rate to the physiological control mechanisms. Intracardiac impedance is measured with the ventricular electrode and the ventricular inotropic parameter (VIP) is derived from that. The VIP serves directly as input to the control of heart rate by the pacemaker. Over 200 patients have received autonomic nervous system (ANS) controlled pacemakers. The patientpacemaker system was investigated in different ways. This included standard exercise tests, long‐term studies of every day activities over 24 hours, psychological, and pharmacological challenges. To prove the validity of the approach we specifically looked at (1) the appropriateness of changes in paced heart rate with sympathetic tone during exercise, (2) the correlation between heart rate and sinus rate, if detectable, and (3) the correlation between the echocardiographically determined preejection period (PEP) and the VIP controlled heart rate.

Automatic adjustment of pacing parameters based on intracardiac impedance measurements.

SCHALDACH, M.: Automatic Adjustment of Pacing Parameters Based on Intracardiac Impedance Measurements. The selection of the parameters used for rate control is determined not only by technical feasibility, but also by patient considerations. Technical feasibility considerations include long‐term stability and reliability of the sensor, as well as susceptibility to interference. The patient considerations relate to the patients physical state, including the various physiological and pathologic conditions. The rate response must be in proportion to circulatory demand. The specificity of the rate response is of particular importance for the patient with low cardiac reserve. The development of future pacemakers aims at providing additional patient benefits, with a reduction of the effort associated with initial parameter selection and patient follow‐up. This will be achieved by utilizing a better understanding of the integration of the control mechanisms for the entire cardiovascular system under physiological and pathological conditions. To support the development of the future pacemakers, we must utilize realistic multiparametric models of the cardiovascular system. These models will assist the evaluation of potential algorithms for integrating multisensor signals into a single pacing rate. The parameterization and validation of these models are important issues to be addressed. Intracardiac impedance measurements in conjunction with microprocessor controlled signal processing and improved lead technology provide a great variety of practical applications for physiological control of the pacing rate and the automatic adjustment of pacemaker parameters. The concepts of an “intelligent” pacemaker capable of automatic control in response to changes in the pacing requirements under a variety of physiological and clinical conditions are presented.

Improvement of stimulation and sensing performance of bipolar pacemaker leads

The quality of stimulation electrodes is determined by their pacing and sensing performance. During the past three years experimental efforts demonstrated, that both properties are significantly improved by coating the tips of unipolar leads with titanium nitride due to its high porosity resulting in a significant increase in Helmholtz capacity. However, the stimulation performance of bipolar leads is still poor due to the smooth surface of the indifferent ring electrodes. Therefore this paper deals with the question of suitable ring coating materials. It is shown that sputtered iridium coatings are superior to titanium nitride coatings under anodical polarization, whereas titanium nitride should be used for the cathodically polarized tip only.

Design of digital filters

Schlüsselwörter: Digitale Filter, Phonokardiogramm Die Anwendung digitaler Filter in der biomedizinischen Technik bringt gegenüber Analogfiltern wesentliche Vorteile. Vor allem erleichtert die große Flexibilität digitaler Systeme die Anpassung an das jeweilige Meßproblem, wobei in der vorliegenden Arbeit die Optimierung des Signal-Rauschverhältnisses bei biologischen Summensignalen wie Elektrokardiogramm (EKG) und Phonokardiogramm (PKG) im Vordergrund steht. Während bei der Realisierung von digitalen Filtern im allgemeinen Filtercharakteristiken durch digitale Computeralgorithmen approximiert werden, die einen sehr hohen Rechenaufwand erfordern, werden in der vorliegenden Arbeit digitale Filterprinzipien vorgestellt, die ein Minimum an Rechenzeit, Zeitaufwand bei der Verwirklichung und geringe Anforderungen an Spezialkenntnisse digitaler Filteralgorithmen erfordern. Die Wirksamkeit des Verfahrens wird an der phonographischen Analyse der »beat to beat«Bestimmung der fetalen Herzfrequenz demonstriert.

Electroactive coating of stimulating electrodes

Electrophysiologic examinations as well as the electrotherapy of heart diseases require electrodes which distinguish themselves by outstanding sensing and pacing performance. Both, the sensing and the stimulation behaviour are determined by the interface between electrode and tissue. To avoid energy losses and distortions of heart signals having components down to 1 Hz, the interfaces impedance has to be very small in the corresponding frequency range. Towards this goal a new coating has been developed: electrolytically deposited electroactive iridium oxide. On the electrodes surface iridium oxide forms a long-term stable reversible redox system which changes its oxidation state according to the applied potential. The decrease of impendance is demonstrated by impedance spectroscopy: the low-frequency limit is lowered by more than three orders of magnitude to a value of 0.4 Hz. The reason is the electroactivity of this material which is caused by reversible proton incorporation into the coating, as is proven by cyclic voltammetry. Due to the low interface impedance well below 1 Hz, the coated electrodes fulfil the requirements for medical applications. Outstanding sensing behaviour was shown by measuring the monophasic action potential in dogs, which is possible without any distortions.

A FRACTALLY COATED, 1.3 MM2 HIGH IMPEDANCE PACING ELECTRODE

Minimizing the geometric surface area of pacing electrodes increases impedance and reduces the current drain during stimulation, provided that voltage (pulse‐width) thresholds remain unchanged. This may be feasible by coating the electrode surface to increase the capacity of the electrode tissue interface and to diminish polarization. Ten unipolar, tined leads with a surface area of 1.3 mm2 and a “fractal” coating of Iridium (Biotronik SD‐V137) were implanted in the ventricle, and electrogram amplitude (unfiltered), slew‐rate, pacing threshold (0.5 ms), and impedance (2.5 V; 0.5 ms) were measured by the 5311 PSA (Medtronic). On days 0, 2, 5, 10, 28, 90, 180, 360 postimplant, sensing threshold (up to 7.0 mV, measuring range 1–14 mV on day 360 only) and the strength duration curve (0.5–4.0 V; 0.03–1.5 ms; steps: 0.5 V; 0.01 ms, respectively) were determined, the minimum charge delivered per pulse (charge threshold), and the impedance were taken from pacemaker telemetry (Intermedics 294–03). Data were compared with those of an earlier series of 20 unipolar, tined TIR‐leads (Biotronik) with a surface area of 10 mm2 and a “fractal” coating of titanium nitride. With the model SD‐V137 versus TIR, intraoperative electrogram amplitudes were 15.1 ± 6.1 versus 14.4 ± 3.9 mV(NS), slew rates 3.45 ± 1.57 versus 1.94 ± 1.06 V/s (P < 0. 05), pacing thresholds 0.16 ± 0.05 versus 0.52 ± 0.15 V (P < 0.01) and impedance measurements 1,136 ±175 versus 441 ± 73 Ω (P < 0.0001), respectively. During follow‐up, sensing thresholds were the same with both leads. Differences in pulse width thresholds lost its significance on day 28 but resumed on day 360 (SD‐V137: 0.08 ± 0.04 ms; TIR: 0.16 ± 0.06 ms at 2.5 V; P < 0.01). With an electrode surface of 1.3 mm2, charge per pulse and impedance consistently differed from control, beingO.15 ± 0.15 versus 0. 66 ± 0. 20 μC (P < 0.001) and 1,344 ± 376 versus 538 ± 79 Ω, respectively, one year after implantation (P < 0.0001). In summary, “fractally” coated small surface electrodes do not compromise sensing; by more than doubling impedance against controls they offer pacing thresholds (mainly in terms of charge) that are significantly lower than with the reference electrode.

Increased monophasic action potential dispersion in endothelin-1-induced ventricular arrhythmias.

The aim of this study was to investigate the changes in monophasic action potentials (MAP) from different sites in the heart and to determine MAP dispersion during endothelin-1 (ET-1) infusion. Standard ECG, left ventricular anterior, right ventricular lateral, right ventricular septal, and right ventricular apical MAPs and intra-arterial blood pressure were monitored in seven anesthetized open-chest mongrel dogs. After radiofrequency atrioventricular node ablation, ventricular pacing (70/min) was performed and intracoronary ET-1 (60 pmol/min) was administered into the left anterior descending coronary artery. Both MAPd90 and MAPd90 dispersion increased significant during ET-1 infusion. The onset of spontaneous monomorphic and polymorphic sustained ventricular tachycardias (sVT) was observed in five dogs (around 40 min), and nonsustained VTs (nsVT) developed in another two dogs. The increases in MAP and MAP dispersion lasted until the appearance of polymorphic nsVTs and sVTs, but at the time of these VTs this difference decreased. At the termination of the experiments, ventricular fibrillation occurred in six cases. In four cases third-phase early afterdepolarizations were recorded. Our results suggest that increased MAP dispersion and development of EAD contribute to the arrhythmogenic action of ET-1, and these phenomena might explain the pathogenesis of a wide variety of ventricular arrhythmias with different morphology observed in this study.

Titannitrid-Herzschrittmacher-Elektroden - Pacemaker Electrodes Made of Titanium Nitride

The sensing and pacing performance of pacemaker electrodes is characterized by the electrochemical properties of the electrodes/tissue layer; the usually smooth metallic electrode surface results in a high pass filter characteristic. Consequently, the detected intracardiac signals, which control the implantable systems, are not optimally matched to the spectral contents of the depolarisation signal. To avoid interference caused by noise (EMI, muscle potentials, etc.) a shift of the frequency of the band pass towards the lower frequency spectrum is required. As previously reported, the electrochemical properties of sintered and surface-treated electrodes prove the predicted improvement of sensing performance if titanium-nitride coated electrodes are used. Our results demonstrate their superiority above all the other electrodes presently known. The advantages can be referred to the micro-crystalline surface structure achieved by sputter-deposited electrode coatings and the kinetics of the ionic exchange. Furthermore, the acute thresholds achieved with the TiN-systems were significantly better than those of the smooth metallic surface. These results were also confirmed for chronic implants and are attributable to the known biocompatibility of titanium and its alloys.