Background: Transbrachial electrical impedance measurement is a medical technique dedicated to therapeutic use in cardiology. Based on dielectric properties of human body tissues,
transbrachial impedance measurement aims at
evaluating hemodynamic parameter changes within the arm. For this purpose, a high frequency, constant magnitude, low amperage and alternating current is injected in the left arm by a pair of spot electrodes. Impedance will be then deduced from Ohm’s law by measuring output voltage with another pair of electrodes placed between current injecting ones.
Impedance Measurement Limitations: It comprises a user friendly and non-invasive measurement technique. However, it is known to
yield imprecise measurements as diverse factors could alter the measured impedance signal. In the arm, pulsatile impedance changes are assumed to be generated by blood resistivity. Some other factors such as overall arm impedance changes, brachial artery diameter changes, the number of electrodes used, electrode locations, body movements or damp skin can also increase the risk of error. All these facts explain why this technique is not enough reliable for medical purpose.
Aims of Numerical Simulations: The ultimate purposes of finite element simulations include:
- Reproducing physiological impedance changes for …
- … Enhancing understanding of physiological phenomena which are responsible for impedance changes during a transbrachial impedance measurement and …
- … Identifying factors which cause impedance signal errors
The main difficulty encountered with these numerical simulations is to
dynamically reproduce physiological transbrachial impedance changes. But simulation results are all the more necessary since they can unveil new explanations concerning wrong estimations of impedance signal measured. They can also allow to approve electrical impedance use for hemodynamic phenomena characterizations pursuant to literature data.
Personal contribution to the subject: Numerical simulations have already been done by research groups: factors influencing impedance changes from transthoracic impedance measurement were investigated with 3D CST Studio Suite (Dassault System). Here, the work should bring an additional angle to impedance signal study by focusing on
transbrachial impedance measurement technique. In contrary to previous model which have two ring electrodes,
the model developed in this work is designed with
four spot electrodes.
Work done: A simplified
CAD model representing transbrachial impedance measurement was generated thanks to CST modelling tools. It is composed of a blood-filled cylinder (brachial artery) surrounding by adynamic tissue cylinders with high impedance. Four spot electrodes are inserted into outside tissue cylinder. A literature review has allowed to define the dimensions for getting an anatomical model with consideration of
inter-subject morphological variabilities. Each tissue component of the model is characterized by its specific dielectric properties for the frequency of study. The model is discretized by
tetrahedrons.
Simulations with the model should be ran with
Low Frequency Solver (
electroquasistatic solver type). Unfortunately, the results were not satisfying as admittance matrix (admittance is the inverse of impedance) were calculated between current input electrodes. Hence, results were corresponding to a transbrachial impedance measurement with two electrodes. As the interest is to have numerical results for
four spot electrodes impedance, high frequency simulations with
Frequency Domain Solver were performed. It is now possible to get impedance from current input and voltage output.
Other CST functionalities are also of my interest. From CST Voxel Family and CST Female Visible Human, new models have been created alongside the development of simplified CAD model. The current work on four-electrode impedance measurement models from CST data is yielded to obtain a reliable and optimal
tetrahedral mesh.
Results: Numerical brachial impedance results for current simplified CAD model ranges from 413 to 449 Ohms when brachial artery diameter ranges from 1,5 to 4,8 mm. A literature review showed that physiological brachial impedance must range from 23,0 to 128,3 Ohms.
Work to be done: Comparing brachial impedance results with brachial impedance literature data emphasizes an overestimation of brachial impedance with the simplified CAD model. Next step should include a complete study of the model for identifying impedance sources of error (material properties, way to stimulate electrodes, electric field in the model, …).
Perspectives: For next few weeks, comparison between results from simplified CAD model and models generated from CST data will be the opportunity to evaluate and improve performances of the simplified CAD model. Thanks to appropriate impedance change simulations during a full cardiac cycle, it will be at least possible to assess diverse hemodynamic parameters.
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