| Introduction:Impedance cardiography is an impedance change curve that is measured on the chest surface. It carries abundant information for the motions of heart and vessels. It can indirectly reflect the volume changes of human heart and vessels. It is also closely related to physiological and pathological changes of cardiovascular. Therefore, impedance cardiography can be used to noninvasively measure cardiac function parameters and monitoring the hemodynamic changes, which makes it a promising technique. Theoretically, however, it is unreasonable to speculate that the impedance cardiography is caused by only aortic volume changes. A large number of studies have already demonstrated that impedance cardiography detected with current techniques represents mixed impedance changes that are induced by the volume changes of aortic, left and right pulmonary vascular, and left and right ventricular. Therefore, the cardiac function, which is judged basing on impedance cardiogrphy, reflects the function status of both left and right ventricle. In the present study, we aimed to separate the impedance changes of the left heart, right heart and great vessel from the mixed signals of the chest. Materials and Methods:1、A plastic barrel filled with dilute aqueous salt solution was used to simulate chest. Different sizes of copper rods placed in the barrel were used to simulate thoracic vessels in different states. The impedance values were measured under different conditions. The associations between the impedance variations on the chest surface and impedance changes of intrathoracic vessels were analyzed.2、We designed 6-lead electrodes model to detect chest surface impedance, and established a 6-simultaneous equations group. Through referencing algebra heavy frame method solution equation group, we separated impedance components of aortic(AO), blood vessel in left lung(PL), blood vessel in right lung(PR), left ventricular(LV) and right ventricular(RV) from the mixed impedance changes on the chest surface.3、We delivered cardiac catheter into the left hearts of 6 adult dogs using conventional human left heart catheterization method. By stimulating the ventricular wall, we induced ventricular extrasystole and artificially changed the heart stroke volume in dogs. The c-wave amplitude changes of AO, PL, PR were then measured on and after ventricular extrasystole.4、we measured the wave amplitude changes in 180 normal adults who were categorized by age into young, middle-aged and older groups. The amplitudes of c-wave, x-wave, b-wave, o-wave, and the intervals of Q-C, Q-B in these adults were statistically analyzed.5、Through comparative analysis of the differential diagrams of reconstructed cardiac impedance plethysmography in patients with cardiac dysfunction and controls with normal cardiac functions, we examined the mechanisms for the presence of normal and abnormal Q waves in the mixed impedance on chest surface. Results: 1、The impedance changes of a single vessel in chest was positively associated with the length of the vessel(Lb), the ratio of the vessel impedance to the basic impedance(ΔZb/Zb0), and basic impedance between two electrodes at thoracic surface(Zs0), while it was inversely associated with the distance between the vessel and the connection line of two electrodes(d). The impedance caused by multiple blood vessels was the sum of the impedance changes of each blood vessel on the chest of surface).2、the waves were anti-phase between the component of left and right ventricular and the component vasculars in reconstructed impedance diagram. The waveform of impedance component of the ventricle was similar to the physiological diagram of ventricular volume changes as well as the diagram of ventricular volume changes measured by radionuclide method. The lowest point of waveform of impedance component of the ventricle was generally in the same phase of the ventricular volume on the smallest state.3、At ventricular premature beat provocation, the C-wave amplitude of AO, PL PR components increased 47.1± 19.4%, 21.4 ± 11.8%, and 14.3± 9.2%, respectively. The C-wave amplitude changes of PL and PR components were lower than that of AO component(p < 0.001).4、In the five impedance components of normal group, the C-wave amplitude change of the aorta is the biggest, followed by the left and right pulmonary vasculars and the LV and RV. The amplitude of C and X-wave of impedance component, and the amplitude of b and O-wave of ventricular impedance component decreased with the age of the subjects. The amplitude of O-wave of ventricular impedance component was significantly higher than that of vascular impedance component(AO, PL, PR, p < 0.001).5、In thoracic impedance differential, the amplitude of C-wave of the normal group was significantly larger than that of the abnormal group(P<0.001), whereas the amplitude of O-wave of the normal group was significantly smaller than that of the abnormal group(P<0.05). In the reconstructed impedance differential cardiogram, the amplitude of O-wave of vascular impedance component was smaller than that of ventricular impedance component of the normal group(P<0.001). The amplitude of O-wave and the O/C ratio of the abnormal group were significantly higher than those of the normal group(P<0.001). Conclusion:We successfully separated the impedance change components for the AO, PL, PR, LV and RV from the mixed impedance changes that were measured on the chest surface. The reconstructed impedance could be used to represent the physiological parameters of thoracic cardiovascular, as indicated by waveform analysis and data on dog model. Our data therefore indicate that it is feasible to evaluate the cardiac function of the left ventricle(LV) by using impedance cardiography technique. Our study also demonstrated that the O wave in the thoracic impedance graph is derived from the superposition of the O waves of the impedance change components for the aorta, blood vessels in lung and ventricles. In addition, the O/C ratio of pulmonary vascular component can be used as indicators of the left ventricular diastolic function. |
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