Electrode Positioning to Investigate the Changes of the Thoracic Bioimpedance Caused by Aortic Dissection - A Simulation Study

Abstract

Impedance cardiography (ICG) is a non-invasive method to evaluate several cardiodynamic parameters by measuring the cardiac-synchronous changes in the dynamic transthoracic electrical impedance. ICG allows us to identify and quantify conductivity changes inside the thorax by measuring the impedance on the thorax during a cardiac cycle. Pathologic changes in the aorta, like aortic dissection, will alter the aortic shape as well as the blood flow and consequently, the impedance cardiogram. This fact distorts the evaluated cardiodynamic parameters, but it could lead to the possibility to identify aortic pathology. A 3D numerical simulation model is used to compute the impedance changes on the thorax surface in case of the type B aortic dissection. A sensitivity analysis is applied using this simulation model to investigate the suitability of different electrode configurations considering several patient-specific cases. Results show that the remarkable pathological changes in the aorta caused by aortic dissection alters the impedance cardiogram significantly.

Keywords: Aortic dissection; impedance cardiography; numerical simulation; sensitivity analysis.

Fig. 1

a) Intimal tear in the aorta [2]. b) Aortic dissection types (Stanford system) [3].

Fig. 2

The spatial average time-dependent cross-sectional radius of the aortic arch and the descending aorta during one cardiac cycle.

Fig. 3

The spatial average time-dependent blood velocity in the aortic arch and the descending aorta.

Fig. 4

Orientation and deformation of RBCs in a blood vessel during the systole and diastole.

Fig. 5

The blood conductivity changes as a function of reduced average velocity 〈v/R〉 for different haematocrit (H) levels.

Fig. 6

Simulation model setup. a) 3D view – b) 2D bottom view.

Fig. 7

Flow disturbances around the dissection in case of an aortic dissection.

Fig. 8

Damage factor DF as a function of the radius of the false lumen.

Fig. 9

Source electrode pairs and measurement electrode pairs positions.

Fig. 10

Values of ${\widehat{Y}}_{n,m}^{\mathit{PCE}}(t)$ reflecting the discrepancy between the healthy and dissected conditions for 20-time steps and all proposed electrode combinations.

Fig. 11

Maximal discrepancy ${\widehat{Y}}_{\mathit{max}}^{\mathit{PCE}}$ for the fourth time step and each electrode configuration. Colours show source electrodes; blue: injection from A, red: injection from B, yellow: injection from C; numbers show the measurement electrodes.

Fig. 12

Sensitivity analysis on a. ${\widehat{\mathit{HC}}}_{C,4}^{\mathit{PCE}}(t)$ , b. ${\widehat{\mathit{DC}}}_{C,4}^{\mathit{PCE}}(t)$ , c. ${\widehat{Y}}_{C,4}^{\mathit{PCE}}(t)$ .

Fig. 13

Changing of ${\widehat{Y}}_{\mathit{max}}^{\mathit{PCE}}$ by the damage factor for injection from source electrodes C (inj C) and measurement from five electrode pairs (m1 to m5).

Fig. 14

a. ${\widehat{\mathit{HC}}}_{C,4}^{\mathit{PCE}}(t)$ and ${\widehat{\mathit{DC}}}_{C,4}^{\mathit{PCE}}(t)$ for different damage factors, b. ${\widehat{Y}}_{C,4}^{\mathit{PCE}}(t)$ for different damage factors.