External cardiac pacing using low impedance electrodes suitable for defibrillation: a comparative blinded study

Department of Medicine, Boston City Hospital, Massachusetts 02118.

OBJECTIVES. The objective of this study was to determine whether the threshold for successful cardiac pacing is affected by electrode impedance and whether this procedure can be successfully carried out through low impedance electrodes that are also suitable for defibrillation. BACKGROUND. Reintroduced in the early 1980s, external cardiac pacing utilizes large externally placed electrodes with a high impedance, in conjunction with a stimulator capable of producing an impulse of 20 to 40 ms in duration. On the basis of empiric observation, high impedance electrodes (> 500 omega) are believed to be optimal for external cardiac pacing. Such electrodes are unsuitable for defibrillation, a technique that is most successful when impedance is low. In view of the absence of controlled data to support this recommendation, as well as the desirability of using one set of electrodes for both pacing and defibrillation, we undertook the following study. METHODS. Thirty-two normal subjects underwent a total of 110 attempts at external cardiac pacing with either (or both) high or low impedance electrodes in combination with one or two commercially available external cardiac pacemakers. Each subject underwent pacing at least twice in a randomized double-blind fashion to determine the pacing threshold and level of discomfort. RESULTS. Individual subjects had a wide range of pacing thresholds but did not experience any greater discomfort with one pacemaker-electrode combination than with any other. Similarly, no pacemaker-electrode combination was superior to another in terms of pacing thresholds. The mean pacing threshold was 72.5 +/- 6 mA for the 40-ms impulse/high impedance electrode combination, 78.7 +/- 6 mA for the 40-ms impulse/low impedance electrode, 73.8 +/- 7 mA for the 20-ms impulse/high impedance electrode and 77.5 +/- 7 mA for the 20-ms impulse/low impedance electrode (p = NS for all comparisons). CONCLUSIONS. Contrary to previous belief, a high impedance electrode offers no advantage for external pacing in terms of either pacing threshold or discomfort level during pacing. This study demonstrates that it is feasible to perform external pacing through an electrode that is also suitable for defibrillation and suggests that a single external pacing-defibrillation electrode is all that is needed to perform these two procedures.