Transthoracic impedance: differences between men and women with implications for impedance cardiography

Impedance cardiography (IC) is a reliable noninvasive technique for monitoring stroke volume (SV) and cardiac output. Transthoracic impedance (Zo) is one variable in the equation used for the calculation of SV. Thoracic impedance reflects the resistivity offered by tissues and air and the length and cross-sectional area of the thoracic volume. The purpose of this study was to evaluate possible differences in Zo between men and women. Measurements (Mean +/- S.D.) of Zo in 29 men (age 25.6 +/- 4.6 yr) and 35 women (24.2 +/- 6.0 yr) in the seated posture revealed Zo values were significantly (p less than 0.05) greater for the women (31.5 +/- 3.3 ohm) than the men (23.5 +/- 1.84 ohm). The observed differences in Zo cannot be attributed to thoracic length; distances between the monitoring electrodes were similar for the men (25.1 cm) and the women (25.7 cm). We hypothesize that the greater Zo observed for women is due to a smaller thoracic cross-sectional area and greater resistivity resulting primarily from relatively more fat tissue, smaller heart size, and lesser central blood volume as compared with men.