Biomedical subjects
Publications and source records attributed to W Irnich.
This paper outlines the moves being made by the International Federation for Medical and Biological Engineering to establish an International Clinical Engineering Board. The purpose of this board is to stimulate, through the establishment of National Examining Authorities, international agreement on the minimum requirements needed for the registration or certification of professional clinical engineers.
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Describing mathematically, the intensity duration curves of electrostimulation by hyperbolas, Lapicque introduced two terms which characterized the functional relationship: "Rheobase" was the lowest intensity with indefinite pulse duration which just stimulated muscles or nerves. "Chronaxie" was that pulse duration at which the threshold intensity was twice that of the rheobase. Up to now, both terms have never played an important role in cardiac pacing. However, it can be shown that the chronaxie, especially, is an important parameter--influenced by several factors such as electrode size, material, and stimulation mode--which may help match the generator to its electrode. Practical consequences may be derived from the concept of chronaxie: 1) A pulse duration longer than the chronaxie is not desirable because current consumption is increased without decreasing the threshold significantly. 2) Pacing with constant current needs twice that pulse duration of constant voltage stimulation. 3) Smaller electrodes are more favorable because the pulse duration they need may be reduced without losing safety. 4) Estimation of the safety margin with decreasing output of the generator or with programmable pulse duration is possible if the chronaxie of a specific electrode is known.
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Follow-up investigation over 12--84 months (mean 30 months) of 86 patients with the primary diagnosis of permanent total atrioventricular block revealed that spontaneous rhythm could be demonstrated in all but 17 patients. A-V conduction was re-established in 13, but in most of them it was on the basis of ventricular ectopic beats. These findings prove that in the described type of patient spontaneous rhythm after pacemaker implantation is common. Therefore, fixed-rate pacemakers are not indicated in these patients.
The inhibited pacemaker (VVI or AAI) has become the most popular in recent years because of its ability to combine a physiological advantage with economical current consumption in cases with spontaneous activity. One of its disadvantages is its sensitivity to external electromagnetic interference. Though today's pacemakers possess effective protection against most interference signals there may be instances in which patients are subjected to uncomfortable or even life-threatening situations. This is the case of "amplitude modulated" or "pulsed" fields with modulation frequencies in the physiological range. Fields of that sort have been found in the vicinity of a welder, an electric steel plant, and in medical practice where therapeutic currents were applied. Even touch-actuated switches may influence a demand pacemaker. However, these situations may be overcome by a device within the pacemaker for simple time analysis which can be carried out with few components. If electromagnetic fields of diathermy equipment are applied, today's pacemakers may react with intolerably high or low rates. They should, therefore, be avoided.
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Within the first 14 days after implantation, thresholds were measured at three transvenous pacemaker electrodes with different surface areas. It should be verified, 1. to what extent a correlation existed between maximal threshold increase and electrode surface, 2. how electrode impedance and 3. how the amplitudes of the R-wave voltages developed. The results were: At initial implantation, thresholds were all the lower the smaller the electrode-surface area was. On overage, 9-10 days after implantation the highest thresholds were reached. Thereby, in small surface area electrodes the threshold-increase factor was greater than in larger ones. In all cases the current threshold increase exceeded that one for voltage thresholds. 14 days later, thresholds had dropped again compared to the maximum. And this decrease was depending on the electrode-surface area also. The electrode impedance decreased after implantation to rise later on once more to 80-88% of the original value. The magnitude of the detected R-wave was independent on electrode-surface area. Indeed, using small surface electrodes, signal reductions up to 50% could be found. But 14 days later they reached again 80-95% of the initial amplitude. The key finding there is that when applying small surface-area electrodes and presuming suitable initial thresholds it seems to be possible to connect low-output pacemakers (output 5,4 V and impluse duration 0.25 ms or 4 V at 0.5 ms) to those electrodes. By this reduction of the safety margin, a considerable increase in pacemaker lifetime could be achieved.