Effects of changing haematocrit, ventricular rate and myocardial inotropy on the accuracy of impedance cardiography.

1. The accuracy and limitations of the non-invasive impedance cardiograph technique were examined in dogs with electromagnetic flow-transducers mounted on the aortic root over a wide range of physiological conditions of anaemia, heart rate, stroke volume and myocardial inotropy. 2. The in vivo blood resistivity (rho-haematocrit relationship is linear and slightly inverse, and is thus opposite to the curvilinear, direct relationship of the bench-derived rho-haematocrit relationship. At haematocrit 41%, in vivo rho is 135 ohm.cm (s.e.m. = 1.0, n = 134) and rises only to 143 ohm.cm (s.e.m. = 1.6, n = 134) as haematocrit falls to 26%. 3. When in vivo rho is used in the Kubicek formula for stroke volume (SV) calculation, the instrument is linear and accurate for heart rates over 38-156 min-1. Thus 82% of all points fall within +/- 20% of the line of equal value over stroke volumes ranging from 8-46 ml (N = 3, n = 105). The standard error of the estimate for pooled data is +/- 2% of the mean impedance stroke volume value of 22.2 ml. The instrument tends to overread at heart rates lower than 60 min-1.