OBJECTIVE This study sought to determine the effect on transthoracic impedance of placement of defibrillation electrodes on the female breast versus adjacent to or under the breast. BACKGROUND Transthoracic impedance is a major determinant of transthoracic current flow in defibrillation. 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 increased interelectrode tissue associated with placement of the apex defibrillation electrode on the female breast would result in increased transthoracic impedance compared with electrode placement lateral to or under the breast. METHODS Transthoracic impedance was measured noninvasively 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 measured transthoracic impedance associated with three different apex defibrillation electrode positions--on the breast, under the breast and lateral to the breast--in 25 women (brassiere size 34A to 48C, 25 to 75 years old, body weight 128 to 328 lb [58 to 148 kg] and 2 men. The measurements were taken with a modified defibrillator that accurately predicts transthoracic impedance without delivering an actual shock. The measurement sequence was random. RESULTS The average measured transthoracic impedance with placement of the apex defibrillation electrode on the 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 was also classified into two groups: large breasted (brassiere size > or = 40) and small breasted (brassiere size < or = 39). The measured transthoracic impedances for the large-breasted 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. For the small breasted group, the similar transthoracic impedance measurements were 81 +/- 21, 77 +/- 16 and 71 +/- 13* ohms, respectively. CONCLUSIONS In women, placement of the apex defibrillation electrode on the breast results in higher transthoracic impedance, which will reduce current flow. We recommend placing the apex electrode lateral to or underneath the breast.