Temporary Pacemaker Treatment in Open Heart Surgery: Variation in Myocardial Threshold, Tissue and Interface Impedances in Man*

Abstract

Myocardial threshold and impedance of adequately insulated multicore metal electrodes (lengths l1 and l2) were investigated in 28 patients undergoing open heart surgery. Increase in current threshold from the pre-to postoperative period was: 607 +/- 102% (mean +/- SEM) with a constant-current pulse generator and 885 +/- 129% with a constant-voltage pulse generator. Tissue impedance (RT - initial impendance) calculated as voltage/current ratio 90 mus into the pulse changed from 564 +/- 34 omega before surgery to a minimum of 134 +/- 7 omega. Thereafter, there was a gradual increase in RT to 162 +/- 9 omega the day of electrode removal. In 25 of 28 patients the minimum values were reached the third to eighth postoperative day. Electrode/tissue interface impedances--Faraday resistance (RF) and Helmholtz capacity (CH)--were calculated from regression analysis of loaded and unloaded electrograms using the method of least squares. The RF showed a fall from 14.7 +/- 1.4 K omega to 5.2 +/- 0.3 K omega, and the CH (20-40 Hz) rose from 6.0 +/- 0.9 mu F to 15.5 +/- 0.8 muF preoperatively to the day of minimum tissue impedance. There were no further changes until the day of electrode removal. A significant positive correlation was found between CH (p < 0.002), current threshold (p < 0.005) and equivalent electrode length [lequ = l1 X l2/(l1 + l2)]. The electrode signal source impedance calculated from RT, RF and CH was of a magnitude not likely to contribute to demand failures. The low postoperative electrode impendance resulted in excessive load on the constant-voltage generator (condenser discharge type), rendering stimulation of the heart with reasonable current values impossible.

Ventricular Tachyarrhythmia Initiation in a Canine Model of Recent Myocardial Infarction. Comparison of Unipolar Cathodal, Anodal and Bipolar Stimulation

“…Late diastolic anodal threshold is approximately 3 times cathodal threshold in normal tissue [21,22], Thus, to avoid inconsistencies and differences in re fractory period measurements that might result from the use of drive pacing "at twice threshold' in each of the three pacing modes, it was decided to use the cathodal pacing mode for the eight ventricular drive beats throughout, and to vary the pacing mode for the last extrastimulus only, using either unipolar catho dal, anodal or the bipolar pacing mode for the extra stimulus ( fig. 1).…”