THE INACCURACY OF KUBICEKS ONE-CYLINDER MODEL IN THORACIC IMPEDANCE CARDIOGRAPHY

The validity of a one- and a two-cylinder model, underlying thoracic i mpedance cardiography (TIC), was investigated by studying the length d ependence of the impedance parameters Z(0), (dZ/dt)(min), and stroke v olume (SV). It can be shown that, within a one-cylinder model, all par ameters are directly proportional to the length, whereas, if the volum e conduction of the thorax and the neck are modeled separately, Z(0) a nd (dZ/dt)(min) are expected to be linear dependent and SV will be non linear dependent upon the length. The expectations were compared to re sults from in vivo measurements. Two electrode arrays were studied, in which the caudal recording electrode position was varied; SV was calc ulated using Kubicek's equation. Except for small distances, the resul ts showed a nearly linear relation between the parameters and the leng th. Regression analysis of the linear part revealed statistically sign ificant intercepts (p < 0.05). Neither the intercept nor the nonlinear part can be explained by a one-cylinder model, whereas a model consis ting of two cylinders serially connected describes the experimental re sults accurately. Thus SV estimation based on a one-cylinder model is biased due to the invalid one cylinder model. Corrections for the Kubi cek-equation need to be developed in future research using this two-cy linder model.