Analysis of the Frequency-Dependent Sensitivity Distribution for Lower Forearm Impedance Plethysmography

OBJECTIVE: The assessment of hemodynamic parameters plays an important role for the diagnosis and detection of cardiovascular diseases. One of the promising techniques for the non-invasive and continuous monitoring of arterial hemodynamics is the impedance
plethysmography (IPG), which aims to measure the pulsatile volume change of superficial arteries. Crucial for the success of an IPG measurement is the selection of a setup that effectively targets the artery. APPROACH: This work analyzes the influence of the injection
frequency alteration on the sensitivity distribution of an IPG measurement and the sensitivity towards the radial artery. The analysis is performed both in simulation and using study data obtained from 41 participants. The simulation consists of a finite element model representing the lower forearm and a fixed IPG setup, in which the sensitivity distribution is analyzed for different excitation frequencies between 3 kHz and 100 kHz. To allow for comparison, the IPG study utilizes the same five excitation frequencies between 3 kHz and 100 kHz. MAIN RESULTS: The results of the finite element simulation show that increasing excitation frequency shifts the region of large sensitivity away from the artery and especially towards deeper muscle tissue layers. The analysis of the study data supports
this hypothesis by demonstrating that lower-frequency excitation yields larger relative pulsatile changes than larger excitation frequencies, thus indicating a larger sensitivity towards the pulsatile artery. SIGNIFICANCE: The results of this paper show, that injection frequency directly affects the sensitivity regions. Analysis of the FEM and study results indicate that the largest sensitivity towards the radial artery occurs at the smallest injection frequency. Thus, comparison of IPG measurements performed at different excitation frequencies should be interpreted carefully, comparability between individual measurements may be limited.