TEMPORARY PACEMAKER TREATMENT IN OPEN-HEART SURGERY - VARIATION IN MYOCARDIAL THRESHOLD, TISSUE AND INTERFACE IMPEDANCES IN MAN

Mymcardial threshold and impedance of adequately insulated multicore metal electrodes (lengths 11 and 12) 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 impedance) calculated as voltage/current ratio 90 μs into the pulse changed from 564 ± 34Ω before surgery to a minimum of 134 ± 7Ω. Thereafter, there was a gradual increase in RT to 162 ± 9Ω 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 Helm‐holtz 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 Ω to 5.2 ± 0.3 KΩ, and the CH (20–40 Hz) rose from 6.0 ± 0.9 μF to 15.5 ± 0.8 μF 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= 11± 12/ (11+12)]. The electrode signal source impedance calculated from RT. HF and CH was of a magnitude not likely to contribute to demand failures. The low postoperative electrode impedance resulted in excessive load on the constant‐voltage generator (condenser discharge type), rendering stimulation of the heart with reasonable current values impossible. Copyright © 1979, Wiley Blackwell. All rights reserved