Does reducing capacitance have potential for further miniaturisation of implantable defibrillators?

OBJECTIVE: To determine whether considerably smaller capacitors could replace 125 microF capacitors as the standard for use in implantable defibrillators. METHODS: Measured energy, impedance, voltage, and current delivered were compared at defibrillation threshold in 10 mongrel dogs for defibrillation using 75 microF and 125 microF capacitors alternated randomly. Defibrillation was attempted with biphasic shocks of comparable tilt between an endocardial lead in the right ventricular apex and a "dummy" active can of an experimental implantable device placed in the subpectoral position. RESULTS: A reduction of capacitor size of 40% was associated with an increase in voltage of 21% and in current of 22%. With a 65% tilt, no significant differences were found between the two capacitances with respect to the impedance or energy required for defibrillation. CONCLUSIONS: Multiple advances in electrode material, electrode configuration, shock morphology, and shock polarity have reduced defibrillation energy requirements. Smaller capacitors could be used in implantable cardioverter/defibrillators without a major decrease in effectiveness.

Bipolar pacemaker leads: new materials, new technology.

This commentary is in response to a review published earlier in this journal. It is intended to provide additional information and supplement the original paper. A short review of the failure mechanisms of polyurethane pacing lead materials is provided. Two specific degradation mechanisms, environmental stress cracking and metal ion oxidation, are discussed. Environmental stress cracking has been extensively studied and is a well understood failure mechanism. Methods for reducing the problem have been developed and tested in vivo. As a result, stress cracking failures can be virtually eliminated. Metal ion oxidation failures now dominate pacing lead recalls. Two new materials, polycarbonate urethanes and ethylenetetrafluoroethylene, have been introduced as insulators for pacing leads. These materials do not fail by stress cracking and preliminary test results are very positive.

Automatic target detection and recognition in multiband imagery: a unified ML detection and estimation approach.

Multispectral or hyperspectral sensors can facilitate automatic target detection and recognition in clutter since natural clutter from vegetation is characterized by a grey body, and man-made objects, compared with blackbody radiators, emit radiation more strongly at some wavelengths. Various types of data fusion of the spectral-spatial features contained in multiband imagery developed for detecting and recognizing low-contrast targets in clutter appear to have a common framework. A generalized hypothesis test on the observed data is formulated by partitioning the received bands into two groups. In one group, targets exhibit substantial coloring in their signatures but behave either like grey bodies or emit negligible radiant energy in the other group. This general observation about the data generalizes the data models used previously. A unified framework for these problems, which utilizes a maximum likelihood ratio approach to detection, is presented. Within this framework, a performance evaluation and a comparison of the various types of multiband detectors are conducted by finding the gain of the SNR needed for detection as well as the gain required for separability between the target classes used for recognition. Certain multiband detectors become special cases in this framework. The incremental gains in SNR and separability obtained by using what are called target-feature bands plus clutter-reference bands are studied. Certain essential parameters are defined that effect the gains in SNR and target separability.

Digital beat-to-beat cardiotachometer.

A beat-to-beat cardiotachometer counting from 27 to 199 beats per minute has been constructed using digital integrated circuits. Power consumption is reduced, and accuracy enhanced, by incorporating a programmed read-only memory. This circuit technique can be employed for other devices, such as digital thermometers, in order to circumvent a nonlinear relationship between voltage and the physiological variable.

Sky-radiance gradient measurements at narrow bands in the visible.

Accurate calibrated measurements of the radiance of the daytime sky were made in narrow bands in the visible portion of the spectrum. These measurements were made over several months and were tabulated in a sun-referenced coordinate system. The radiance as a function of wavelength at angles ranging from 5 to 90 deg was plotted. A best-fit inverse power-law fit shows inversely linear behavior of the radiance versus wavelength near the Sun (5 deg) and a slope approaching inverse fourth power far from the Sun (60 deg). This behavior fits a Mie-scattering interpretation near the Sun and a Rayleigh-scattering interpretation away from the Sun. The results are also compared with LOWTRAN models.

Over-the-horizon optical propagation in a maritime environment.

An experimental-theoretical program aimed at characterizing over-the-horizon (OTH) propagation via marine atmospheric aerosols is described. Pathloss measurements as a function of transmitter-receiver azimuth-elevation angles for 1.06 microm and 0.53 microm over two OTH links are reported. The first link involves an overwater range of 63 km with 19-km and 40-km horizons; the second involves an overwater range of 128 km with a 19-km horizon. Two theoretical models, one based on particulate single scatter and the other based on particulate multiple scatter, are presented and compared with the pathloss measurements obtained over the 63-km link. Good agreement between both models and experiments is shown. No quantitative comparison between the models and the 128-km experiment is made because of the lack of accurate meteorological data over this extended path.

Blue-green pulsed propagation through fog.

Measurements and analysis of a blue-green pulsed propagation through fog have identified three distinct regions for energy transport. Region I small number of attenuation lengths tau in the path (0 32): the direct beam and the forwardscattered beam have decayed to the point where the diffusion type multiple-scattered radiation is the dominant energy received. This component does not decay exponentially but results in large spatial, angular, and temporal spreading. This paper presents quantitative data on Region II.

The patient alert system--pilot study of built-in warning systems for pacemakers.

Malfunction of a pacing system can be life-threatening for a pacemaker-dependent patient. It would be desirable for implantable pulse generators to have a built-in automatic warning system capable of alerting the patient as soon as a potentially dangerous disorder is detected. In this study, seven patients (mean age, 72.6 +/- 10.7 years) with slow, chronic atrial fibrillation underwent implantation of a dual-chamber pulse generator with a custom-made "alert electrode" connected to the atrial port of the pulse generator to stimulate the underlying pectoral muscle. The muscle was temporarily stimulated while the pacemaker was in VVIR mode. The lowest amplitude sufficient to alert the patient (perception threshold) was 1.6 +/- 0.58 V at 0.45 ms during implantation and 1.2 +/- 0.5 V at 0.45 ms chronically. In a second phase, alerts outside of the office were issued using a special software routine capable of delivering stimuli at programmable date and time.

New ICD-technologies: first clinical experience with dual-chamber sensing for differentiation of supraventricular tachyarrhythmias.

Inappropriate ICD therapy for supraventricular arrhythmias remains an unsolved problem and may lead to serious clinical situations. Current algorithms for differentiation of supraventricular and ventricular arrhythmias are based on ventricular sensing solely and, therefore, lack sensitivity and specificity. This preliminary analysis from a multicenter trial comprises data from the first 26 patients who received a Res-Q Micron active-can ICD (Sulzer Intermedics) with a ventricular defibrillation lead and an additional bipolar lead for atrial sensing. Digitized atrial and ventricular waveform storage as well as interval charts from 102 induced and 30 spontaneous arrhythmia episodes were prospectively collected and analyzed with regard to appropriateness of ICD therapy. From all 132 arrhythmia episodes, high-quality stored dual-chamber intracardiac electrograms (IEGM) could be retrieved for further analysis: in 40 (30%) episodes, atrial fibrillation (AF with rapid ventricular response 22, AF with VT 9, AF with VF 9) was identified as the underlying intrinsic rhythm, and inappropriate ICD therapy was delivered in 4/22 (18%) episodes of AF with rapid ventricular response. In the remaining 92 (70%) episodes, sinus rhythm was the underlying atrial rhythm (SR with VT 13, SR with VF 79), and no inappropriate therapy was observed. Three of 22 (15%) high-energy shocks delivered for ventricular arrhythmias (VT 9, VF 9, rapid AF 4) terminated AF at the same time. In total, there were 3 complications (2 atrial lead dislodgments, 1 revision for bleeding). Both atrial lead dislodgments occurred in the 2 patients with passive-fixation leads compared to none in the 24 patients with active-fixation leads (p = 0.003). In conclusion, dual-chamber sensing and waveform storage of the new Res-Q Micron offer very helpful diagnostic tools for the detection of inappropriate ICD-therapy. Placement of an additional atrial lead is safe and does not interfere with proper ICD function. However, for avoidance of atrial lead dislodgment, active fixation leads are recommended. With the tested active-can lead configuration, the efficacy of successful atrial cardioversion by high-energy shocks delivered for ventricular arrhythmias seems to be low.

Quantitative, low cycle, crack initiation fatigue testing of fine wires and CENELEC standard pacing coil.

Uniaxial fatigue testing was performed on different diameters of fine wires made from MP35N. The fatigue limits of the wires differed from each other based on the diameter of the wire. Multiaxial (shear) fatigue testing was also performed on a benchmark coil used to evaluate the fatigue life of all modern pacemaker leads (the CENELEC standard coil). A computer algorithm was used to quantify the maximum shear stress and strain on the coil. The bend radius, coil diameter, wire diameter, and pitch of the coil all affect the shear stress and strain and therefore the fatigue properties of conductor coils. Based on the analysis presented, it was determined that the portion of the CENELEC standard dealing with fatigue, when used in its present format, is not a valid fatigue test for pacemaker leads.

Comparison of defibrillation efficacy using biphasic waveforms delivered from various capacitances/pulse widths.

The efficacy of the biphasic waveform shock for the defibrillation of the ventricular myocardium has been reported by researchers and physicians. Although many authors have suggested that biphasic waveforms delivered from lower capacitances and shorter pulse widths could result in the reduction of the energy required for successful defibrillation, no report has described the smallest capacitance and pulse width yielding the lowest DFT. In this study, we compared efficacies of the biphasic waveform shocks and DFT safety margins among five different capacitances (175 mu f, 125 mu f. 100 mu f. 75 mu f, and 50 mu f) combined with 1-3 pulse widths. These experiments performed in six dogs used an endocardial lead/subcutaneous patch defibrillation electrode system. The average DFTs at E50 for 175 mu f (6.5/3.5 ms), 125 mu f (6.5/3.5 ms), 100 mu f (6.0/3.0 ms), 75 mu f (4.0/2.0) ms, and 50 mu f (3.0/2.0 ms) were 8.5, 10.0, 11.0, 14.0, and 16.5), respectively. These results indicate that a biphasic waveform delivered from a larger capacitance with a proper pulse width could achieve a higher defibrillation efficacy. All DFTs at E50 for all waveforms were compared to their deliverable energies and maximum stored energies. This comparison indicated a narrow DFT safety margin with capacitances below 100 mu f. Therefore, it is concluded that higher energy and higher leading edge voltage are required for a biphasic waveform delivered from a smaller capacitance with a shorter pulse width. Since the current capacitor technology provides a maximum voltage of 750 V using two capacitors in series, with the electrode impedance system used in this study, smaller capacitors appear to have a decreased probability of defibrillation success at a given energy.