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Biomedical subjects

W Irnich

Publications and source records attributed to W Irnich.

At least 37 records · Page 2Linked to original sources
PubMed1996

Electromagnetic interference of pacemakers by mobile phones.

The topic of interference of pacemakers by mobile phones has evoked a surprisingly strong interest, not only in pacemaker patients, but also in the public opinion. The latter is the more surprising, as in the past, the problem of interference has scarcely found the attention that it deserves in the interest of the patient. It was the intention of our investigation to test as many pacemaker models as possible to determine whether incompatibility with mobile phones of different modes may exist, using an in vitro measuring setup. We had access to 231 different models of 20 manufacturers. During the measurements, a pulse generator together with a suitable lead was situated in a 0.9 g/L saline solution, and the antenna of a mobile phone was positioned as close as possible. If the pulse generator was disturbed, the antenna was elevated until interference ceased. The gap in which interference occurred was defined as "maximum interference distance." All three nets existing in Germany, the C-net (450 MHz, analogue), the D-net (900 MHz, digital pulsed), and the E-net (1,800 MHz, digital pulsed) were tested in succession. Out of 231 pulse generator models, 103 pieces corresponding to 44.6% were influenced either by C- or D-net, if both results were totaled. However, this view is misleading as no patient will use C- and D-net phones simultaneously. Separated into C- or D-net interference, the result is 30.7% for C or 34.2% for D, respectively, of all models tested. The susceptible models represent 18.6% or 27% of today's living patients, respectively. All models were resistant to the E-net. With respect to D-net phones, all pacemakers of six manufacturers proved to be unaffected. Eleven other manufacturers possessed affected and unaffected models as well. A C-net phone only prolonged up to five pacemaker periods within 10 seconds during dialing without substantial impairment to the patient. Bipolar pacemakers are as susceptible as unipolar ones. The following advice for patients and physicians can be derived from our investigations: though 27% of all patients may have problems with D-net phones (not C- or E-net), the application should generally not be questioned. On the contrary, patients with susceptible devices should be advised that a distance of 20 cm is sufficient to guarantee integrity of the pacemaker with respect to hand held phones. Portables, on the other hand, should have a distance of about 0.5 m. Pacemaker patients really suffering from mobile phones are very rare unless the phone is just positioned in the pocket over the pulse generator. The contralateral pocket or the belt position guarantees, in 99% of all patients, undisturbed operation of the pacemaker. A risk analysis reveals that the portion of patients really suffering from mobile phones is about 1 out of 100,000. Nevertheless, it would be desirable in the future if implanting physicians would use only pacemakers with immunity against mobile phones as guaranteed by the manufacturers.

Electromagnetic Fields↗
PubMed1995

Magnetostimulation in MRI.

In national and international bodies, there is active discussion of appropriate safety regulations of levels of magnetic field strength in MRI. Present limits are usually expressed in terms of the switching rate dB/dt, but the validity of this is open to debate. Application of the fundamental law of electrostimulation is well-established, both on theoretical and experimental grounds. Application of this law, in combination with Maxwell's law, yields a very simple equation that we call the fundamental law of magnetostimulation. This law has the hyperbolic form of a strength-duration curve and allows an estimation of the lowest possible value of the magnetic flux density capable of stimulating nerves and muscles. Calculations prove that the threshold for heart excitation is much higher than those for nerve and muscle stimulations. Experimental results from us and other authors confirm the correctness of the derived laws for magnetostimulation. In light of these findings, proposed safety limits should be reconsidered.

Electric Stimulation↗
PubMed1995

Optimal truncation of defibrillation pulses.

The statement that the optimal pulse for defibrillation has not yet been discovered implies that an ideal pulse exists, but that it is different in shape, duration, and energy as compared to pulses of today. The optimum pulse is that which can defibrillate with lowest energy. Reduction of energy can be reached twofold: by looking for a pulse duration with lowest energy threshold, and by finding the optimal truncation with lowest refibrillating effect. Assuming that there is also a rheobase in defibrillation below which no defibrillating but probably a refibrillating effect exists, the exponential pulse should be truncated if it intersects with the rheobase. Combining the fundamental law of electrostimulation with this boundary condition allows for the mathematical solution of the above problem of optimal energy. Defibrillation can be optimized with respect to pulse duration or tilt and to energy efficiency. The most important parameter in determining other optimized parameters such as output capacitor is the chronaxie. The calculations reveal that the "concept of constant energy" does not accurately describe defibrillation, that today's implantable cardioverter defibrillator devices possess refibrillating tilts, that pulse durations should be programmed to values between 4 and 10 msec, and that smaller output capacitors around 30 microF would minimize the energy requirements. Whether optimized monophasic pulses are inferior or equal to biphasic pulses needs further experimental studies.

Defibrillators, Implantable↗
PubMed1995

Optimal truncation of defibrillation pulses.

The statement that the optimal pulse for defibrillation has not yet been discovered implies that an ideal pulse exists, but that it is different in shape, duration, and energy as compared to pulses of today. The optimum pulse is that which can defibrillate with lowest energy. Reduction of energy can be reached twofold: by looking for a pulse duration with lowest energy threshold, and by finding the optimal truncation with lowest refibrillating effect. Assuming that there is also a rheobase in defibrillation below which no defibrillating but probably a refibrillating effect exists, the exponential pulse should be truncated if it intersects with the rheobase. Combining the fundamental law of electrostimulation with this boundary condition allows for the mathematical solution of the above problem of optimal energy. Defibrillation can be optimized with respect to pulse duration or tilt and to energy efficiency. The most important parameter in determining other optimized parameters such as output capacitor is the chronaxie. The calculations reveal that the "concept of constant energy" does not accurately describe defibrillation, that today's implantable cardioverter defibrillator devices possess refibrillating tilts, that pulse durations should be programmed to values between 4 and 10 msec, and that smaller output capacitors around 30 microF would minimize the energy requirements. Whether optimized monophasic pulses are inferior or equal to biphasic pulses needs further experimental studies.

Animals↗
PubMed1991

[The programming of cardiac pacemakers--wish and reality].

To test the extent and quality of pacemaker programming, 340 programmable pacemakers were selected out of 1,204 impulse generators removed post-mortem. Their actual program was compared with the nominal parameters when first delivered. 177 (52.1%) of the 340 pacemakers still had the same parameters as on delivery. The multiply programmable impulse generators with telemetry were more frequently programmed (61.2%) than the simple programmable ones (43.0%) or the multiply programmable ones without telemetry (43.7%). Rate was the most frequently programmed parameter, followed by impulse amplitude, sensitivity and impulse duration. Refractory time and hysteresis were only rarely changed from the initial state. Apparently, programmability was not thought to be important enough. Programming without telemetry was seemingly too complicated. The reasons for this attitude should be investigated so that the discrepancy between wishes and reality can be overcome.

Equipment Design↗
PubMed1990

The fundamental law of electrostimulation and its application to defibrillation.

Around the turn of the last century, there was an intensive discussion among physiologists as to whether there is a law describing the phenomena of electrostimulation and which formula may best approximate it mathematically. J.L. Hoorweg found in 1892 that the voltage at which a capacitor must be charged to elicit an excitation, was a function of the capacitance in an inverse correlation. G. Weiss reported in 1901 that according to his investigations a linear relationship existed between the duration of a pulse and the corresponding quantity of electricity applied and called it "formule fondamentale." We are now able to give the "fundamental formula" a physical interpretation that yields, as result, the electric field produced by the electrode acting on the excitable membrane. The electric field in the extracellular space is transformed by the cell geometry ratio: cell length to membrane thickness yielding a high transmembrane field capable of reducing the inherent electric field to its threshold level. The consequences drawn from this hypothesis are remarkable and (should) have an influence on all applications of electrostimulation including the discussions on defibrillation. The application of the stimulation theory to defibrillation yields as results: (1) The basic engineering principle of defibrillation is to produce an electric field within the ventricles of 400 V/m or more. An orthogonal pulse application may reduce the energy requirements, as more fibers are longitudinally reached by the electric field; (2) The shape of the defibrillation pulse and its polarity plays no role. Consequently it follows that biphasic pulses must be less efficient than monophasic pulses, if they are close to the chronaxie; and (3) The most serious disadvantage in today's defibrillation practice is its dose characterization in "energy"; but this physical quantity cannot be justified in the light of the fundamental law of electrostimulation.

Animals↗
PubMed1989

Structuring of service centres for economic and equipment efficiency.

Increasing costs in healthcare have stimulated discussions on whether installation of competent in-house service groups for equipment management could eventually aid in reducing expenditure. In the Federal Republic of Germany, a four-year programme was initiated in 1979 with the purpose of investigating the feasibility, efficiency and profitability of a clinical engineering service within hospitals. The results of this programme formed the foundation on which we developed guidelines for structuring such service centres, to enable other hospitals to profit from the experience gained and possibly to avoid repeating mistakes. Service groups must have sufficient personnel to be successful. As a rough estimate, approximately one service staff member per 100 beds is needed. A more sophisticated structuring demands analysis of the repair accounts to elucidate additional parameters. An optimised service centre can reduce the maintenance costs to about 60 per cent of their original value without in-house service. Structuring of service centres for cost-effectiveness requires a simultaneous increase in service quality, which should be the highest motivation in clinical engineering.

Cost-Benefit Analysis↗