Inhibition/filter characteristics and input impedances of QRS-inhibited demand pacemakers determined by in vitro studies

Sintered Platinum for Cardiac Pacing

Platinum electrodes for cardiac stimulation having a surface porosity in the range of 40 μm have been prepared with a sintering process.

These electrodes showed excellent in vitro characteristics (low polarization and sensing impedance). Animal testing proved good tissue acceptance and clinical trials in 31 patients gave the following results: threshold at implant 0,375 μJ (SD: 0.174 μJ), chronic threshold 1.15 μJ (SD: 1.05 μJ). The longest observation period was 7 months and the mean observation period was 17.1 weeks.

These results indicate that this type of electrode allows safe stimulation of the heart with low energy pulses; in particular a reliable safety margin could be obtained with pulses having the amplitude of the existing lithium batteries, avoiding the need for a voltage doubler. This would improve the battery utilization and the pulse generator longevity.

Simultaneous bipolar pacing and sensing during EPs

Bipolar pacing and sensing-recording on the same electrode is a problem during pacemaker implantation as well as during electrophysiological studies. A method is described that makes this possible. By using bipolar simultaneous sensing-pacing on the same electrode, the system can record local repolarization after a depolarization controlled by stimulation. The bipolar-paced evoked potential is useful as an indicator to drive a bipolar QT rate responsive pacemaker. Even more, during EPS it can be used to detect capture during tachycardia stimulation and to measure the exact refractory period.

Optimal pacemaker sensing with respect to amplitude and slew rate of intracardiac electrograms: theoretical analysis by computer simulation

The optimal amplitude and slew rate of intracardiac electrograms for pacemaker sensing were examined on a theoretical basis by computer simulation. The simulation was based on the concept that it is the voltage at the position of the pacing electrode in an electrical field of a moving electrical dipole. By changing the distance between the electrode and the myocardium and the moving velocity of the electrical dipole, simulated ECGs with arbitrary amplitudes and slew rates were generated by the computer and fed to a bandpass filter. This filter was equivalent to those assembled in some models of permanent pacemakers and had a center pass-band frequency of 50 Hz and a Q of 1.0. The outputs of the filter were measured. The results showed that, for pacemakers sensing, simulated intracardiac electrograms with high amplitude should have high slew rates and those with low amplitudes should have low slew rates, although the absolute values depend on the characteristics of the bandpass filter and the sensing threshold of the pacemaker.

Pulse width dependent increase in the pacemaker automatic interval

From a total of 51 patients equipped with rate and pulse width adjustable pulse generators (Microlith-P 0505, Microthin-PI 0522) implanted over the last 3 years, 10 (19.6%) showed an unexpected drop in pacemaker pulse rate during pulse width programming. For one of the pulse generators used (Microthin-PI 0522), unexpected rate decrease occurred in 7/13 cases (53.8%). For all except one patient, decrease in pacemaker pulse rate corresponded with the total refractory period of the pulse generator ( 320 ms), at a certain pulse width when rate drop first occurred. In seven of the patients the pulse generator automatic interval was extended from 13 ms to 171 ms beyond the refractory period. In two patients it was necessary to replace the pulse generators. Our study strongly proves that this abnormal pacemaker functioning is a result of sensing of the polarization voltage at the pacemaker electrode/tissue interface and/or the T-wave. The polarization voltage is highly dependent on the total pacemaker electrode/tissue interface impedance. Using typical values for pulse generator output and input resistance and output capacitance, Faraday resistance, Helmholtz capacitance and tissue resistance at the electrode/tissue interface it was shown mathematically that in some cases the polarization voltage alone would be of sufficient amplitude and slew rate for pacemaker inhibition. The study demonstrates an urgent need for change in the filter characteristics by making the pulse generators less sensitive in the low frequency region and reducing the polarization voltage by reducing the output circuit capacitance.