TRANSTHORACIC DEFIBRILLATION - IMPORTANCE OF AVOIDING ELECTRODE PLACEMENT DIRECTLY ON THE FEMALE BREAST

Objectives. This study sought to determine the effect on transthoracic impedance of placement of defibrillation electrodes on the female bre ast versus adjacent to or under the breast. Background. Transthoracic impedance is a major determinant of transthoracic current flow in defi brillation. For a given energy setting, a high transthoracic impedance reduces current flow and may adversely affect the ability of electric shocks to accomplish defibrillation. We hypothesized that the increas ed interelectrode tissue associated with placement of the apex defibri llation electrode on the female breast would result in increased trans thoracic impedance compared with electrode placement lateral to or und er the breast. Method. Transthoracic impedance was measured noninvasiv ely by passing a 5-V, 31.25-kHz square wave current through the chest and comparing the low level current flow to known references. We measu red transthoracic impedance associated with three different apex defib rillation electrode positions-on the breast, under the breast and late ral to the breast-in 25 women (brassiere size 34A to 48C, 25 to 75 yea rs old, body weight 128 to 328 lb [58 to 148 kg] and 2 men. The measur ements were taken with a modified defibrillator that accurately predic ts transthoracic impedance without delivering an actual shock The meas urement sequence was random. Results. The average measured transthorac ic impedance,vith placement of the apex defibrillation electrode on th e breast was 95 +/- 25 ohms (mean +/- SD), under the breast 84 +/- 17 ohms and lateral to the breast 83 +/- 20 ohms (asterisk indicates p < 0.01 vs. on the breast by analysis of variance). The study cohort wa s also classified into two groups: large breasted (brassiere size grea ter than or equal to 40) and small breasted (brassiere size less than or equal to 39). The measured transthoracic impedances for the large-b reasted group were 112 +/- 20 ohms for on the breast, 94 +/- 13 ohms for under the breast and 98 +/- 19 ohms for lateral to the breast. Fo r the small breasted group, the similar transthoracic impedance measur ements were 81 +/- 21, 77 +/- 16 and 71 +/- 13 ohms, respectively. Co nclusions. In women, placement of the apex defibrillation electrode on the breast results in higher transthoracic impedance, which will redu ce current flow. We recommend placing the apex electrode lateral to or underneath the breast.