The Interdependence Between Electrogram, Total Electrode Impedance and Pacemaker Input Impedance Necessary to Obtain Adequate Functioning of Demand Pacemakers

Electrogram maximum derivatives (DMAX, SMAX) and electrogram amplitudes (AMAX, UMAX) (Figure 2), were studied in 71 cases during permanent pacemaker treatment. During the acute phase, (at first implantation), 29 patients were studied, and during the chronic phase, (at pulse generator replacement), 42 patients were studied. Of these patients, 27 (acute phase) and 36 (chronic phase) were studied for tissue impedance (RT) and interface impedance (Faraday resistance RF and Helmholtz capacity CH). DMAX and SMAX changed from 3.47 +/- 0.33 V/s (mean +/- SEM) to 2.46 +/- 0.23 V/s and 1.93 +/- 0.20 V/s to 1.32 +/- 0.12 V/s; p 0.8, p > 0.5. Patients with coronary heart disease were found to have significantly higher AMAX than patients classified as having rhythm disturbances of primary cause; p 0.5). RT was statistically significantly higher on the smaller compared with the larger surface electrodes (p > 0.005). There was a slight but not statistically significant fall in RT from acute to chronic phase (p > 0.2). RF ranged from 2.0-94.6 kohms. There was no statistically significant differences between the 8 mm2 compared with the 12 mm2 electrodes (p > 0.2). CH varied between 0.7 and 37.0 microfarads, with significantly lower values for the smallest electrodes (p < 0.05). In patients with electrograms of borderline amplitudes and maximum derivatives for being sensed, the low CH found with the small tip electrodes, will gave a higher risk of demand failure if the input impedance in the sensing circuit of a demand pacemaker is too low.