The effect of haematocrit on transthoracic electrical impedance and on the calculation of cardiac output by an impedance cardiograph.

The Transthoracic electrical impedance (TEI) was measured using the Bomed NCCOM3 non-invasive cardiac output monitor in 27 patients with polycythemia rubra vera (PCRV) and in a control group of 27 patients with normal haematocrits. The mean haematocrit was 49.4% (SD 2.61) in the patients with PCRV and 42.1% (SD 2.95) in the control group, a difference that was significant (p less than 0.001). The mean TEI was also significantly higher in patients with PCRV than in the control group (p less than 0.05), the respective values being 31.25 (5.48) Ohms and 27.5 (3.31) Ohms. The mean values for cardiac output (CO) and cardiac index (CI) were similar in both groups.