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 @actal" 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 omega (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, being 0.15 +/- versus 0.66 +/- 0.20 microC (P < 0.001) and 1,344 +/- 376 versus 538 +/- 79 omega 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.

Pacing threshold variation was studied over 24 hours in 20 patients with different passive fixation low threshold leads. Measurements were taken every 30 minutes by an external unit which combined Holter monitoring and temporary output programming of the pacemakers implanted. Thresholds were determined at 0.25; 0.50; 0.75 ms as the minimum voltage providing for capture with a resolution of 0.1 V. From these data, rheobase and chronaxie values were approximated. Thresholds were found to vary by no more than 0.3 V. Changes relative to the lowest threshold recorded reached 300% in terms of voltage and 400% with the rheobase-chronaxie-product which calls into question the validity of relative safety margins in the setting of very low threshold leads. Data indicate that low voltage settings and narrow safety margins in absolute terms are feasible with modern pacing leads.

A estimulação de dupla câmara (DDD) com eletrodo único usando eletrodo atrial flutuante está limitada em função dos altos limiares encontrados para captura atrial. Avaliamos um sistema novo de cabo eletrodo para estimulação atrial que utiliza dois anéis atriais com pulsos de onda quadrada simultâneos unipolares lançados com polaridade oposta. O primeiro pulso é aplicado ao pólo distal do eletrodo e é positivo, o segundo pulso é aplicado ao pólo proximal do eletrodo e é negativo, ambos em relação à carcaça do gerador. O atraso entre os dois pulsos é programável entre 0,0 ms e 1,0 ms. A distância entre os anéis a nível atrial é de 10 mm e a distância entre o pólo distal atrial e o eletrodo ventricular unipolar pode ser selecionado entre 11, 13 e 15 cm. O posicionamento dos anéis a nível atrial é selecionado de acordo com a medida do limiar de estimulação do pulso bifásico simultâneo, incluindo manobras respiratórias para confirmar a captura/sense contínuos. O gerador de pulso tem uma única conexão para o eletrodo e a capacidade de aplicar os pulsos programáveis de onda quadrada com polaridades opostas, com atrasos programáveis de 0,0 a 1,0 ms. O gerador pode ser programado para estimulação VDD com eletrodo único. Este sistema foi implantado em 4 pacientes com bloqueio AV total e função sinusal normal. Os limiares de estimulação atrial e diafragmático foram medidos com várias configurações do pulso, larguras e atrasos, intra e pós-operatórios. A porção média do átrio direito foi selecionada como a melhor posição para os anéis atriais com captura contínua durante inspiração profunda. As medidas intra-operatórias e pós-operatórias (48 horas) foram: Limiar Intra-operatório Pós-operatório atrial unipolar 3,2 + 0,47 V não realizada atrial (bifásico simultâneo) 1,6 + 0,37 V 3,37 + 0,84 V diafragmático acima de 7 V 5,21 + 0,3 V sense de onda P 2,35 + 1,3 mV 1,27 + 0,8 mV Os pacientes tiveram alta com o gerador programado no modo VDD. A evolução de 30 dias após o implante mostrou perda de captura atrial transitória durante inspiração em 1 paciente, apesar da ausência de outras variações no limiar, comparado com a evolução de 48 horas pós implante. Em conclusão, a estimulação bifásica simultânea reduz o limiar de estimulação atrial, ao mesmo tempo em que mantém o sense atrial adequado com uma margem de segurança em relação à estimulação diafragmática.Single lead, dual chamber (DDD) pacing with an atrial floating electrode is limited by high atrial capture thresholds. We evaluated a new atrial stimulation lead with two atrial ring electrodes on a single lead and overlapping unipolar square wave pulses of an opposite polarity. The first pulse is applied to the distal electrode and is positive, the second pulse is to the proximal electrode and is negative, both with respect to the pacemaker housing. The delay between the two pulses is programmable between 0.0 ms to 1.0 ms. The distance between the atrial electrode rings is 10 mm and the distance from the distal atrial ring to the unipolar ventricular electrode can be selected to be 11, 13 or 15 cm. The positioning of the atrial ring is selected according to the measured overlapping biphasic pulse thresholds, including respiratory maneuvers to confirm continuous capture/sensing. The pulse generator has a single lead connection and the capability of delivering the two atrial square wave opposite programmable pulses, with delays from 0.0 to 1.0 ms. The generator may be programmed to the single lead unipolar VDD mode. This system was implanted in 4 patients with complete AV block with normal sinus mode fuction. Atrial and diaphragmatic thresholds for various pulse configurations, widths and delays were measured intra and postoperatively. The midportion of RA was selected as the best location for the atrial rings with continuous capture during deep inspiration. The intra and postoperative (48hrs) measurements were: Threshold Intra-op POstop. (48hrs) Atrial unipolar 3.2 + 0.47 V not measurable Atrial (overlapping biphasic) 1.6 + 0.37 V 3.37 + 0.84 V Diaphragmatic above 7 V 5.21 + 0.3 V wave sensing 2.35 + 1.3 mV 1.27 + 0.8 mV The patients were discharged in the VDD mode. Evaluation thirty days after showed transient loss of atrial capture during inspiration in one patient in spite of absence of other threshold changes compared to the 48 hours evaluation. In conclusion, overlapping biphasic stimulation reduces the atrial pacing thresholds while maintaining adequate atrial sensing with a safe margin regarding diaphragmatic stimulation.

Article Signalanalyse intrakardialer Signale vor supraventrikulären und ventrikulären Tachyarrhythmien was published on January 1, 1996 in the journal Biomedical Engineering / Biomedizinische Technik (volume 41, issue s1).

EINLEITUNG: Vielfältige elektrophysiologische Untersuchungen des monophasischen Aktionspotentials (MAP) zeigen, daß es den zeitlichen Verlauf des Transmembranpotentials mit großer Genauigkeit widerspiegelt [1]. Der zeitliche Verlauf des Transmembranpotentials der Myokardzelle ergibt sich aus der Summe der verschiedenen lonenströme, die durch die intraund extrazellulären lonenkonzentrationen bestimmt werden [3]. Da die Intensität der einzelnen lonenströme wesentlich durch das autonome Nervensystem (ANS) beeinflußt wird, reflektiert das myokardiale Aktionspotential die jeweilige ANS-Aktivität. Im Gegensatz zur Messung des Transmembranpotentials, welche nur an isolierten Gewebeproben möglich ist, läßt sich das MAP mittels am Myokard anliegender Elektroden messen. Im Hinblick auf eine breite klinische Anwendung eröffnet sich damit der Zugang zu den in diesem Signal enthaltenen Informationen auf zellulärer Skala [2]. bei Überstimulation nach lokaler Aktivierungszeit ausgewertet.

Article Vergleich Ag/AgCl und fraktal beschichteter Katheter zur Messung intrakardialer Signale was published on January 1, 1996 in the journal Biomedical Engineering / Biomedizinische Technik (volume 41, issue s1).

Article Effekte von antiarrhythmischen Medikamente auf die ventrikulär evozierte Reizantwort und das monophasische Aktionspotential was published on January 1, 1996 in the journal Biomedical Engineering / Biomedizinische Technik (volume 41, issue s1).

Possible band diagrams and electrical transport properties of aniso-type superconducting junctions - contacts in which the charge carriers are electrons in one electrode and holes in the other - are discussed. Special attention is given to p-n junctions between superconductors with a low density of mobile charge carriers, such as the high-Tc cuprates. It is predicted that most of such contacts are damped Josephon tunnel junctions with low critical current-normal-state resistance products due to a depletion layer at the interface. With increasing current bias the characteristics of the junctions exhibit a gradual transition to Esaki-diode behavior. Similar depletion layers may result in low critical currents at high-Tc-low-Tc junctions.

A brief summary of electric field effects on the normal and the superconducting state of high-T c superconductors is given. Novel experiments are presented in which electric fields are used to modulate the transport properties of weak links. Analogies between the effects of electric and magnetic fields on superconductivity are emphasized.

The influence of applied electric fields on the transport properties of weak links in YBa2Cu3O7−δ films has been investigated. It is found that the critical temperature Tc0 of epitaxial films containing weak links can be reduced by 10 K with applied electric fields of 6 MV/cm. In one case, Tc0 shifts of up to 25–30 K were observed. These are by far the largest Tc shifts reported for any superconducting system.

We report Hall effect measurements on epitaxial YBa2Cu3Oτ and Bi2Sr2CaCu2Oy films in the normal state and in the phase transition. The Hall coefficient RH (B∥c-axis, j∥ab-plane) of both compounds in the normal state is positive and strongly temperature dependent. The unusual temperature dependence of RH will be compared to the prediction of our calculation based on a renormalized two-dimensional tight binding model, which accounts to some extent for strong correlation effects. Below Tc the Hall coefficient, which is strongly dependent on both the temperature and the magnetic field, exhibits a peculiar sign reversal.

Photoemission data of n-type superconducting RE2−xCexCuO4−δ (RE = Pr, Nd and Sm) and Nd2CuO4−yFy are presented. An XPJ and an XPS resonant photoemission study reveals that the RE elements are trivalent with decreasing 4f-O 2p-hybridization from RE = Pr to Sm, whereas Ce has a valence of 3.6. The partial Cu 3d emission shows valence band satellites indicating the existence of strong electron-electron correlation. These results can be well reproduced by cluster calculations. An decrease of the on-site Coulomb repulsion energy U from RE = Pr to Sm is found in RE2−xCexCuO4−δ.

The lattice constants and the atomic positional and thermal parameters of tetragonal Nd 2− x Ce x CuO 4 were refined from X-ray (295 K) and neutron (1.5 K) powder data. A rare earth oxide phase was detected for x 0.20. Whereas the a -axis remains constant over the whole range of composition, a pronounced decrease of the c -axis with increasing amount of cerium is observed at both temperatures. The cell constants at 1.5 K are slightly shorter ( a = 94224(3), c = 12.0603(1)) than at 295 K ( a = 3.9457(2), c = 12.0764(9)) for x = 0.15. No deformation to lower symmetry was detected at 1.5 K.

We present photoemission data of n-type superconducting Re2–xCexCuO4–δ (Re = Pr, Nd, and Sm) and Nd2CuO4–y Fy. An XPS study of Cu core levels reveals that by doping with F, charge carriers of predominantly Cu 3d-character are induced. Further, the O 2p-band, hybridzed with the Cu 3d-band, is narrower and shifted towards higher binding energies compared with p-type high-Tc superconductors. The Re elements are trivalent with decreasing hybridization between the 4f-electrons with an O 2p-band at EB = 4 eV.

The electric field gradient (EFG) at111Cd probe atoms in YBa2Cu3O6+x has been studied by the perturbed angular correlation (PAC) spectroscopy. After annealing at temperatures below 1000 K the probe atoms are located at the Cu(1) site; above 1100 K they preferentially occupy the Y site, whereas in the intermediate temperature range they also reside on the Ba site. A reduction of the oxygen content from x=1.0 to 0.2 resulted in a decrease of the asymmetry parameter measured at the Ba site, but in an increase of it at the Y site, indicating there a local distortion of the YBa2Cu3O6+x lattice by the Cd impurity. Two different surroundings for the Cd atoms at the Cu(1) site were observed.

Photoelectron spectroscopy at the electron-doped superconducting cuprate Nd2-xCexCuO4-δ gives evidence to strong electronic correlations by valence band data as well as by Cu core level spectra which reveal that doping by electrons gives rise to an admixture of Cu 3d10 to the 3d9-state which is pure in Nd2CuO4-δ. An enhanced on-site Coulomb repulsion energy Udd and augmented Cu 3d-ligand hybridization is found for Nd2-xCexCuO4-δ in comparison with the p-type cuprate superconductors. A XPS study of the valence of the constituents reveals that Nd is trivalent with strong Nd 4f-O 2p hybridization, which is confirmed by resonant photoemission, whereas Ce has a 4f occupation number nf ≈ 0.45 as in CeO2.