Improvement in intracardiac impedance volumes by field extrapolation

The measurement of volume by electrical impedance is complicated by non-homogeneous current distribution resulting from small current sources, by the irregular shape of the ventricle, and by loss of current to surrounding anatomical structures. A mathematical technique, field extrapolation, was developed to partially correct the current distribution. The technique mathematically transforms measured potentials into the potential distribution which would result from infinitely distant current sources. The linear correlation coefficient between impedance stroke volume or cardiac output using field extrapolation and thermodilution stroke volume or cardiac output was 0.83 (n = 86) in 11 dogs and 0.76 (n = 92) in 12 patients. The average linear correlation coefficient between impedance stroke volume and integrated aortic flow in four dogs was 0.83 +/- 0.09 (n = 49) using field extrapolation.