Influence of physiological sources on the impedance cardiogram analyzed using 4D FEM simulations

Abstract

Impedance cardiography is a simple and inexpensive method to acquire data on hemodynamic parameters. This study analyzes the influence of four dynamic physiological sources (aortic expansion, heart contraction, lung perfusion and erythrocyte orientation) on the impedance signal using a model of the human thorax with a high temporal resolution (125 Hz) based on human MRI data. Simulations of electromagnetic fields were conducted using the finite element method. The ICG signal caused by these sources shows very good agreement with the measured signals (r = 0.89). Standard algorithms can be used to extract characteristic points to calculate left ventricular ejection time and stroke volume (SV). In the presented model, the calculated SV equals the implemented left ventricular volume change of the heart. It is shown that impedance changes due to lung perfusion and heart contraction compensate themselves, and that erythrocyte orientation together with the aortic impedance basically form the ICG signal while taking its characteristic morphology from the aortic signal. The model is robust to conductivity changes of tissues and organ displacements. In addition, it reflects the multi-frequency behavior of the thoracic impedance.

Effect of patent ductus arteriosus and patent foramen ovale on left ventricular stroke volume measurement by electrical velocimetry in comparison to transthoracic echocardiography in neonates

“…An effect of pathological shunts (ventricular septal defects) on EV was previously reported [36]. Thus, the present findings regarding the influence of left-to-right shunts on EV recordings are consistent with theoretical considerations and prior literature [8,15,36,46,47]. Further, our data suggest that shunts considered hemodynamically irrelevant according to echocardiography criteria via a PDA and/or PFO can affect accuracy of measurements of LVSV EV in comparison to LVSV TTE , thus identifying these shunts as a 'source of error' in EV recordings.…”

“…The higher LVSV values obtained by EV compared with TTE in the presence of PFO may be explained by the fact that EV measurements reflect changes in flow in any vessels parallel to the distance between the measuring electrodes, including systolic flow in the systemic and pulmonary arterial system [15]. Therefore, EV theoretically detects an increase in the combined LVSV plus right ventricular stroke volume output caused by any shunt.…”

“…The presence of both shunts should have a variable effect on LVSV and right ventricular stroke volume. The EV method depends on a variation of transthoracic electrical impedance during the cardiac cycle, with a decrease in trans-aortic impedance with laminar alignment of erythrocytes during systole [13,15,28,29,33]. Additional blood flow via an open PDA may theoretically result in an increase in LVSV as measured by EV, as the EV method cannot distinguish blood flow in the aorta from blood flow in a PDA (or any other vessel) flowing partially parallel to the aorta.…”

“…Likewise reproducibility is an important factor for trending ability, yet a method might be reproducible but unable to track changes correctly. 'Sources of error', i.e., influencing factors related to individual patients impairing measurement by the tested CO monitoring method [8,[13][14][15], should be identified. And last, in order to justify usage of a monitoring method, a potential 'clinical benefit' of the measured parameters for the examined patient group should be assessed [16][17][18][19][20].…”

Systematic variability in ICG recordings results in ICG complex subtypes – steps towards the enhancement of ICG characterization

“…This variability of the X point has led to different definitions of X points, which consequently have produced different detection algorithms, which are tailored to each definition [22]. Additionally, this alteration of the X point has been observed, reported [21] and studied previously [19,20]. In previous studies [19], [20], the physiological sources that might contribute to building ICG signals (ΔZ and dZ/dt) were investigated.…”