Introduction: Impedance Cardiography (ICG) is a non-invasive technique allowing for the continuous monitoring of patients haemodynamic parameters, such as Cardiac Output and Stroke Volume, which can be analysed in a clinical setting. Traditionally, ICG's are recorded on the thoracic region. However, long-term ambulatory ICG from the brachial artery along the left upper-arm offers highly-attractive clinical applications with user-compliant arm-ICG wearable device, providing reliable trend-based indicators of cardiac contractility in novel methods for cardiovascular disease diagnosis.
Methods: Seven healthy cases were considered in this study, with arm-ICG and conventional thorax-ICG data wirelessly recorded simultaneously with the Lead-I ECG. ICG real-time denoising performance for an optimised band-pass (0.5-8Hz) Butterworth IIR-filter, for an appropriate Savitzky-Golay FIR-filter design and for the combination of both denoising processes were assessed. For each subject-case and mode, a 700ms "noiseless” ICG-vector was extracted by signal-averaged of 600 ICG beats, against which Pearson correlation (p) residual-noise level RMS and S/N-ratio (SNR; signal being the noiseless ICG-vector) were computed as denoising performance metrics on a beat-by-beat basis. Further denoising performance assessment was by their impact (error-rate) on the accuracy of ICG waveform parameters: the ICG peak-amplitude B (Ω/s) and Ventricular Ejection Time VET (ms), used for estimating Stroke-Volume.
Results: Denoising performance mean metric p for Butter-IIR-filtering, SG-FIR-filter and their combination on the arm-ICG were: p=0.87, p=0.86 and p=0.88 respectively, and on the thorax-ICG: p=0.97, p=0.97 and p=0.98 respectively. The mean residual-noise RMS performance metric on the arm-ICG were: 0.087mV, 0.087mV and 0.077mV respectively, and mean SNR: 9.75, 7.85 and 12.46 respectively.
Conclusions: Comparative assessment results by the 3 denoising performance metrics (p, RMS and SNR) consistently indicated highest performance for the combined (Butter-IIR)+(SG-FIR) denoising filter process, which also enabled the highest accuracy performance on arm-ICG waveform Stroke-Volume related parameters determination (B and VET), with error-rate statistics below 10% on average.