A Cost-Effective, Noninvasive Dual Impedance-Based Wearable for Continuous Monitoring of Cardiovascular Health: Development of the Analog Front-End

Abstract

Vulnerable populations are considered high-risk for CVD development by experiencing a combination of low socioeconomic status, reduced access to healthcare, and/or genetic traits that increase susceptibility to CVD risk factors. Due to elevated mortality rates from CVDs in these areas, there is a need for low-cost, wearable devices that are capable of monitoring cardiovascular health for early detection and prevention. One approach for predicting the onset of CVDs includes the tracking of blood pressure (BP) and heart rate. This work focuses on the development of the analog front-end for a bioimpedance-based device that is cost-effective, noninvasive, and cuffless for the monitoring of cardiovascular health by continuous detection of heart rate (HR) and pulse transit time (PTT), which is another method that has the potential to be used for indirect BP measurement without the need for a cuff.

Bioimpedance (BIO-Z) is a favorable modality for CVD health monitoring by allowing for deep tissue penetration and conformability to a wearable format. The continuous measurement of BIO-Z corresponds to the impedance plethysmography waveform (IPG) where HR can be detected and monitored. A dual-IPG format permits continuous PTT acquisition and continuous, cuffless BP measurement. Therefore, two subsystems for dual-IPG measurement were designed and tested: the current driving and voltage sensing networks. The circuits were made into a prototype and its performance for continuous HR/PTT measurement was assessed through direct comparison with a benchtop, impedance cardiography system.

A second iteration of the dual-IPG device was fabricated into a compact, printed circuit board. An automatic feedback loop that continuously detects the magnitude of the injected current and shuts the system down if the applied current exceeds 1 mA and/or electrical safety standards was integrated and evaluated. The characteristics and accuracy of the current monitor was investigated by comparing the system’s output with a multimeter. Calibration curves were then obtained and utilized for converting the system’s output to the standard impedance unit: ohms. Lastly, after validating the calibration approach, an in vivo study was conducted to obtain the HR, PTT, basal impedance and pulsatile impedance of six test subjects.