Search PubMed⌕ Search

Biomedical subjects

Werner Irnich

Publications and source records attributed to Werner Irnich.

13 recordsLinked to original sources
PubMed2006

Do induction cooktops interfere with cardiac pacemakers?

AIMS: In induction cooktops, coils produce time-varying magnetic fields that induce eddy currents in the ferromagnetic bottom of a pot or pan, thereby heating it, while the cooktop itself remains cool. Interference with pacemaker sensing could conceivably be produced by voltages induced directly by induction or indirectly by leakage currents. METHODS AND RESULTS: A worst-case pacemaker-patient (PP) model representing left-sided implantation of a unipolar pacemaker was used for measurement of induced voltages, to judge whether induction cooktops could interfere with pacemaker sensing. Eleven induction cooktops of European manufacture were tested using the PP model. The pacemaker sensitivity with respect to 24 kHz voltages, amplitude-modulated at 100 Hz, was investigated in 244 devices. The current passing through the body of a grounded patient touching a metal pot was determined by measuring the voltage from hand to hand and between electrodes placed on the thorax to simulate an implanted unipolar pacing system underneath. The results obtained were complex. If the pot is positioned concentrically with the induction coil, the smallest pot produced the largest stray field, but the induced voltage always remained below the critical value of 100 mV. With eccentrically positioned large pots, voltages of up to 800 mV could be induced. The induced voltage could always be reduced to </=60 mV by maintaining a distance of 35 cm. The most sensitive pacemaker reacted at 90.5 mV. Because of leakage current, approximately 2% of the voltage between pot and ground appears across the pacemaker's sensing input. CONCLUSION: Patients are at risk if the implant is unipolar and left-sided, if they stand as close as possible to the induction cooktop, and if the pot is not concentric with the induction coil. Unipolar pacing systems can sense interference generated by leakage currents if the patient touches the pot for a long period of time. The most likely response to interference is switching to an asynchronous interference mode. Patients with unipolar pacemakers are at risk only if they are not pacemaker-dependent.

Adult↗
PubMed2006

Bioelectricity.

Explore the source record for details and available documents.

Cardiography, Impedance↗
PubMed2005

Do we need pacemakers resistant to magnetic resonance imaging?

AIMS: Manufacturers of pacemakers (PM) and of magnetic resonance imaging (MRI) devices state that MRI scanning of PM wearers is contraindicated. This paper tries to summarise which effects can interfere with PM, what can be hazardous, and how treatment of PM in MRI can be modified to guarantee compatibility. MATERIAL AND METHODS: All PM tested were from deceased patients. Reed contact thresholds and reactions were investigated in low magnetostatic fields and compared with those in strong magnetostatic fields. Influence of gradient fields on PM and heating due to radiofrequency (RF) pulses were estimated. Thirty Legal Medicine Departments were questioned whether deaths of PM patients during MRI are known. RESULTS: Reed contacts are influenced above 0.7 mT. In MRI fields only 28% of the PM in magnet mode remained so in all orientations. Of synchronous PM, 76% remained synchronous in all orientations. Gradient fields can influence sensing but cannot stimulate. Power density and temperature rise produced by RF fall rapidly with distance. Our question revealed six deaths. All suffered from sick-sinus-syndrome and all were not PM dependent. In three cases ventricular fibrillation was proven as the cause of death. DISCUSSION: Asynchronous pacing due to magnetostatic and gradient fields may be problematic in patients with spontaneous rhythm. To avoid them, PM triggered MRI scan restricted to refractory period is proposed. Neither inhibition of PM nor heating of the electrode poses real risks. So far, we have examined eight patients 12 times in MRI triggered mode without problems.

Equipment Design↗
PubMed2003

Postmortem in situ diagnosis of pacemakers and electrodes to detect dysfunction.

Cardiac pacemakers usually are very reliable, but sometimes malfunctions of the system occur. We conceived and developed a method to judge the functionality of pacemaker systems in deceased patients. The idea was to verify the hypothesis that more dysfunctions of implanted pacemaker systems go undetected than are detected and corrected. With the aid of a pre-amplifier and a digital storage oscilloscope, pacemaker pulse signals are derived from the surface of the thorax. The derived pulse shape offers information on the functionality of pacemakers and electrodes. Additionally the lead impedance is measured with a test pacemaker and its corresponding hand-held programmer. Synchronization properties can also be assessed with an external test pacemaker. So far 262 pacemakers have been investigated yielding an anomaly rate of 15%, comprising life threatening to annoying malfunctions. These results emphasize the forensic relevance and give reason for a discussion about the natural cause of death in these cases.

Electrodes, Implanted↗
PubMed2003

How to program pulse duration or tilt in implantable cardioverter defibrillators.

Implantable cardioverter defibrillators (ICDs) are available with independently programmable duration and tilt of the shock pulse waveform. Manufacturers do not, however, commonly advise how these parameters can be programmed for optimal clinical benefit. From theoretical considerations, the author recommends programming both parameters based on the measured lead system resistance R into which the shock is delivered. Assuming that the defibrillation pulse decline below the defibrillation threshold rheobase is undesirable because of the possibility of refibrillation. Mathematical relationships expressing optimal pulse duration and tilt as functions of the output time constant can be derived that are valid for monophasic pulses and the first phase of biphasic pulses. Two ICD manufacturers provide for programmable tilt (Medtronic GEM III, atrial channel) or both tilt T and pulse duration PD. (St. Jude Medical newest devices). Considering its output capacitance, it is recommended that the Medtronic Gem III should be programmed for T = 50% when R < 75 omega and 65% when R < 38 omega. The author considers programming tilt to 30% or 40% useless in clinical conditions. By the same reasoning, he recommends that the newer St. Jude Medical ICDs should be programmed to T = 50% if R < 75 omega and 60% if R < 41 omega, and PD = 5.5, 5.0, 4.5, 4.0, 3.5, and 3.0 ms for R < 75, 73, 62, 51, 40, and 32 omega, respectively. PD = 6 ms was considered clinically unrealistic. Programmable shock pulse duration and tilt are useful in optimizing defibrillation, but it is suggested that this can best be accomplished by programming these parameters with the guidance of theory as described in this discussion.

Defibrillators, Implantable↗
PubMed2003

Threshold measurements: ten rules for good measuring practice.

The following rules for professionally measuring thresholds are derived and discussed: RULE 1: Thresholds should be expressed as voltage averaged over pulse duration to get reproducible and comparable results! RULE 2: Pacing threshold measurements with exponentially decaying pulses should not be extended beyond 1.4 ms as that portion of the pulse below rheobase does not contribute to the stimulation effect! RULE 3: Threshold measurements are best carried out with fixed pulse duration and variable voltage! RULE 4: If threshold measurements are carried out in discrete steps, the steps should be chosen such that the relative step size is as equal as possible! RULE 5: Accuracy of threshold measurements is highly increased if the arithmetically averaged value of the last effective and the first ineffective pulse is defined as threshold! RULE 6: To determine strength-duration-curves, a linear regression of the quantity versus pulse duration should be calculated which yields simply the numerical values of the chronaxie and rheobase! RULE 7: To reach representative strength-duration-curves, measurements with at least four pairs of values must be carried out! RULE 8: Measuring defibrillation thresholds, the relative voltage step size should be chosen equally to have equal accuracy for all steps. RULE 9: If the result of a defibrillation threshold investigation does not reach significance, a too large voltage step size could be an explanation! RULE 10: Comparing intraindividually the threshold of two different defibrillation systems or parameter settings, the threshold ratios should be formed and averaged! Obeying these rules guaranties professional threshold measurements expressed as "rheobase" and "chronaxie" even with devices with discrete steps in parameter programming.

Cardiac Pacing, Artificial↗
PubMed2002

Georges Weiss' fundamental law of electrostimulation is 100 years old.

In 1901, Georges Weiss published a voluminous paper that was the result of the charge by the Commission International du Parc-aux-Prince to investigate whether there are measures to make mutually comparable and to classify the different devices that physiologists used for nerve and muscle stimulation. Georges Weiss was born August 26, 1859 in Bischweiler (Alsace, France). He trained as an engineer in Paris and afterward began his medical training and received his medical doctorate in 1889. In the same year he was appointed "Professeur Agrègé, " and "Préparateur" at the Department of Medical Physics of the Medical Faculty at Paris. He made many contributions to physiology, but his main field of interest was electrophysiology. At the end of the nineteenth century the measuring capabilities for electrical stimulation pulses were limited and stimulation theories were based more on speculation than on measurements. Weiss found a fascinating method to produce short-lasting pulses of defined amplitude and duration. He constructed bridges by conducting threads within a circuit that were then destroyed by an air rifle bullet driven by liquid carbonic acid to produce short-lasting pulses. To investigate double pulses, the measuring system was expanded in the same manner, now with four bridging threads. The experiments were carried out with remarkable accuracy. The results included: (1) the threshold quantity that is the voltage-time-product, is a linear function of the pulse duration; (2) there is always a minimum of the delivered energy dependent on pulse duration; (3) pulse shape plays no role in electrostimulation. The physiologists were not so impressed by the Weiss report as it did not really meet the requirements as expressed by the title. There was no measuring technology available at that time to measure the quantity of devices with short-lasting pulses in the millisecond range. No wonder that the importance of the findings was not really perceived by the scientific community. It can be concluded that if there are statements in the literature contradicting one of the above three Weiss theorems, one can infer that the investigation is questionable.

Electrophysiology↗
PubMed2002

Electronic security systems and active implantable medical devices.

How do active implantable medical devices react in the presence of strong magnetic fields in the frequency range between extremely low frequency (ELF) to radiofrequency (RF) as they are emitted by electronic security systems (ESS)? There are three different sorts of ESSs: electronic article surveillance (EAS) devices, metal detector (MDS) devices, and radiofrequency identification (RFID) systems. Common to all is the production of magnetic fields. There is an abundance of literature concerning interference by ESS gates with respect to if there is an influence possible and if such an influence can bear a risk for the AIMD wearers. However, there has been no attempt to study the physical mechanism nor to develop a model of how and under which conditions magnetic fields can influence pacemakers and defibrillators and how they could be disarmed by technological means. It is too often assumed that interference of AIMD with ESS is inevitable. Exogenous signals of similar intensity and rhythm to heart signals can be misinterpreted and, thus, confuse the implant. Important for the interference coupling mechanism is the differentiation between a "unipolar" and a "bipolar" system. With respect to magnetic fields, the left side implanted pacemaker is the most unfavorable case as the lead forms approximately a semicircular area of maximum 225 cm2 into which a voltage can be induced. This assumption yields an interference coupling model that can be expressed by simple mathematics. The worst-case conditions for induced interference voltages are a coupling area of 225 cm2 that is representative for a large human, a homogeneous magnetic field perpendicular to the area formed by the lead, and a unipolar ventricular pacemaker system that is implanted on the left side of the thorax and has the highest interference sensitivity. In bipolar systems the fields must be 17 times larger when compared to a unipolar system to have the same effect. The magnetic field for interfering with ICDs must be 1.7 stronger than that of the most sensitive unipolar pacemaker. The lowest interference thresholds measured over the last 10 years in the low frequency range (16 2/3 Hz-24 kHz) together with thresholds > 24 kHz that were supplied by the CETECOM study are listed. Both sets of data together with the coupling model, allow for judging which fields of ESSs could influence AIMDs. From measurements at gate antennas, it is possible to derive a "maximum allowed field" curve over the whole frequency range, below which no interference will occur. Comparison of data from literature with these maximum allowed fields confirm the correctness of the calculations. Thus, it is possible to predict interference situations in gates if the magnetic field is known. If all future pacemakers were to have the immunity against interference of the better 50% of today's pacemakers, the magnetic field ceiling values could be at least four times higher. The same is true if the ventricular sensitivity is routinely set at 7 mV. Pacemaker manufacturers should consider filter improvement with modern technology, but gate manufacturers should not claim the privilege of being out of bounds.

Defibrillators, Implantable↗