Sex differences in transthoracic impedance: evaluation of effects on calculated stroke volume index.

A recent publication noted that women and men differ in transthoracic impedance (Zo), with women having higher levels, and this might cause underestimation of stroke volume index (SVI) in women when impedance cardiograph (IC) techniques are employed, as Zo is a squared factor in the denominator of the calculation equation. This paper confirmed the observed sex differences in Zo, with women (n = 19) having significantly (p less than 0.0001) higher Zo than men (n = 19) in both seated and supine positions. However, women also had higher dZ/dt (first derivative of impedance signal over time) levels when seated (women: 2.48 +/- 0.58; men: 1.38 +/- 0.35; p less than 0.0001) and when supine (women: 2.62 +/- 0.56; men: 1.58 +/- 0.34; p less than 0.0001). dZ/dt is a factor in the numerator, and, therefore, calculated SVI did not differ across sexes in either position. It was speculated that the Zo difference is due to differences in fat levels. Controlling for percent body fat and subscapula skin-fold differences did not entirely eliminate the sex differences in Zo. However, a group of well-trained females tested later (n = 10) showed lower Zo levels (24.8 vs. 28.2) when compared to the original group of 19 women who were not well-trained. Differences may be due to fat/muscle ratio of the thorax.