A Model of Electrostimulation Based on the Membrane Capacitance as Electromechanical Transducer for Pore Gating.

BACKGROUND: Electrostimulation has gained enormous importance in modern medicine, for example, in implantable pacemakers and defibrillators, pain stimulators, and cochlear implants. Most electrostimulation macromodels use the electrical current as the primary parameter to describe the conventional strength-duration relationship of the output of a generator. These models normally assume that the stimulation pulse charges up the passive cell membrane capacitance, and then the increased (less-negative) transmembrane potential activates voltage-gated sodium channels. However, this model has mechanistic and accuracy limitations. NOVEL CONCEPT: Our model assumes that the membrane capacitance is an electromechanical transducer and that the membrane is compressed by the endogenous electric field. The pressure is quadratically correlated with the transmembrane voltage. If the pressure is reduced by an exogenous field, the compression is released and, thus, opening the pores for Na(+) influx initiates excitation. RESULTS: The exogenous electric field must always be equal to or greater than the rheobase field strength (rheobase condition). This concept yields a final result that the voltage-pulse-content produced by the exogenous field between the two ends of a cell is a linear function of the pulse duration at threshold level. Thus, the model yields mathematical formulations that can describe and explain the characteristic features of electrostimulation. CONCLUSIONS: Our model of electrostimulation can describe and explain electrostimulation at cellular level. The model's predictions are consistent with published experimental studies. Practical applications in cardiology are discussed in the light of this model of electrostimulation.

Investigation of pacemaker position, lead configuration, and sensitivity setting in pacemakers of 579 deceased patients.

AIMS: The position of the pacemaker (PM) system (right or left-sided, pectoral, or abdominal), the ventricular lead configuration (unipolar or bipolar), the programmed ventricular sensitivity setting (SS), and the ventricular sensitivity of the pulse generator (PG) against electromagnetic interference (EMI) are decisive parameters with respect to EMI behaviour of PGs. Three of these ventricular parameters were investigated in PM of 579 deceased patients. MATERIAL AND METHODS: We investigated PM function provided a regular stimulation pattern in 878 deceased PM patients before cremation. The PG was explanted and then measured in a bench test in the laboratory with respect to the programmed parameters. Further investigations were restricted to 579 patients with PGs implanted between 1998 and 2004. The following parameters were evaluated: (i) position of the PG, (ii) lead configuration, and (iii) programmed SS of the ventricular channel. SS was measured according to the European Pacemaker Standards. Out of 579 patients, 556 PMs were implanted pectorally, with 172 on the left side (30.9%) and 384 on the right side (69.1%). In 23 cases, the implantation site was unknown. Out of 579 PMs, 282 ventricular leads were unipolar (48.7%). Of the 297 bipolar leads (51.3%), 61 (20.5%) had a unipolar sensing function so that a total majority of 343 PM (59.2%) had unipolar sensing. The mean value of SS was 3.24 mV (range: 1.2-8.1 mV) for unipolar leads and 3.55 mV (range: 1.05-10.9 mV) for bipolar ones. The PGs with unipolarized bipolar leads were even more sensitive at 3.0 mV. Of the 579 PM systems, 0.67% possessed a combination of parameters: left side, unipolar and with SS < 2 mV. CONCLUSIONS: The results seemed to be paradoxical in that unipolar sensitivity was more sensitive than bipolar sensitivity. Less than 0.67% of patients possessed a worst case PM system with respect to EMI: a unipolar, left-sided PG with a ventricular SS < 2 mV. This implies that ∼2345 PM patients in Germany could be at risk. Out of the 61 PGs with unipolarized bipolar leads, 14 had never been programmed as they still possessed the shipping programming. Unipolar leads can be used with left-side implantation if the SS is 3 mV (median value of all leads in our study) or higher. This would largely improve the immunity of PGs to EMI in the future. This study also demonstrates that there is a need for educational measures.

Pacemaker sensitivity to 50 Hz noise voltages.

AIMS: European standards specify that pacemakers (PM) should be resistant to electromagnetic interference (EMI) up to an upper borderline voltage as a function of frequency. Electromagnetic interference fields should either remain below this upper borderline voltage level or be identified and isolated from the general population. Physicians caring for PM patients need to be aware of potential problems relating to EMI. For example, sensitivity should be programmed to avoid sensing EMI below the recommended borderline voltage level. The susceptibility of a pacemaker (PM) to 50 Hz noise is an important parameter of EMI and depends on the programmed sensitivity [sensitivity setting (SS)]. We studied SS and 50 Hz noise thresholds in a large population and determined the borderline SS, defined as the SS below which 50 Hz noise was sensed. Our results should be taken into consideration in programming the SS to protect patients from the adverse effects of EMI. METHODS AND RESULTS: Measurements were performed on 189 PMs explanted after death. All PMs studied were implanted in 1998 or later. Sensitivity setting and sensing configuration (unipolar or bipolar) were left as programmed during lifetime. The ventricular SS and 50 Hz noise thresholds were measured according to the European pacemaker standard. The signal-to-noise ratios (S/N) were derived from the heart test and noise test signal thresholds. The S/N for pulsed 50 Hz noise of five manufacturers tested ranged from 0.435 to 0.59. The S/N for 50 Hz continuous noise for four manufactures other than Medtronic was higher, ranging from 0.458 to 0.623. No PM showed a ratio of 1 or better. Medtronic PMs reacted differently to 50 Hz continuous noise than the other brands. In 24 Medtronic PMs, the continuous noise threshold was evaluated with two heart test signal amplitudes: either 10 mV or threshold level. In tests at threshold amplitudes, voltages between 0.1 and 0.85 mV elicited interference proving that Medtronic PM reacted extremely sensitively to noise. At 10 mV heart test amplitude, the noise threshold was inversely proportional to the SS, i.e. higher SS resulted in lower noise thresholds. Noise immunity increases with increasing heart test signal amplitude. CONCLUSION: All tested PMs reacted to pulsed 50 Hz waves as if they were heart signals and were inhibited. Continuous noise above noise threshold evoked asynchronous pacing at noise rate. All PMs had an S/N ratio <1, indicating that the heart signals were amplified less than noise. The European Standard requires that unipolarly sensing PMs tolerate noise up to 2 mV. However, an SS of 2 mV does not guarantee a noise tolerance of 2 mV. In order to fulfil this requirement, SS in the majority of PMs must be programmed >2 mV. In Medtronic PMs, the continuous noise threshold is paradoxical as it is higher with decreasing SS. As a good compromise in Medtronic PMs, SS should be ∼3 mV to guarantee sufficient protection from pulsed and continuous noise, assuming ventricular heart signals of 10 mV or more.

From defibrillation theory to clinical implications.

BACKGROUND: Our defibrillation theory claims that the mean voltage threshold is a hyperbolic function of pulse duration and that voltages below rheobase should be avoided as being counterproductive. Truncation of the pulse just at rheobase level yields minimal stored energy thresholds. To verify or falsify this theory, animal experiments were carried out. MATERIAL AND METHODS: In two animal experiments, 212 defibrillation thresholds in 22 swine were determined with different biphasic pulses of which 92 were optimally truncated in phase 1. Step-up test procedure was used with the first successful shock defined as "threshold." RESULTS: Experimental proof is gained that truncation according to "rheobase condition" shows lowest stored energy. A ranking order of stored energy thresholds demonstrates that (1) lower output capacitances reduce needed energy, and (2) pulse durations shorter or longer than optimal increase needed energy. The voltage-pulse-content threshold is linearly correlated with pulse duration. CONCLUSIONS: Truncation above or below rheobase increases the stored energy threshold. Voltage averaged during pulse duration is a hyperbolic function of pulse duration. The stored energy is reduced with decreasing output capacitance. The experimental results do not only fully verify our theory, they also suggest clinical implications: (1) the current usage of the "constant tilt concept" in implantable cardioverter defibrillator (ICD) should be abandoned in favor of "optimal truncation concept," (2) an algorithm developed for calculating optimal truncation proved to be useful so that incorporation into ICD for automatic adjustment is recommended, and (3) the output capacitance should be reduced from about 100 microF to 60 to 70 microF.

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.