A Novel Measurement System to Simultaneously Obtain Three Impedance Plethysmograms and One Electrocardiogram Using Only Four Dry Electrodes

Pulse transit time (PTT) is a cardiovascular (CV) parameter of interest that is obtained by measuring the difference of two arterial pulse waves (APWs) and is useful for determining arterial stiffness or cuffless blood pressure. However, at present, obtaining it requires the use of a large number of sensors and/or complex devices, which limits its use by nonspecialists. To overcome these drawbacks, this work develops the design guidelines for a novel measurement system capable of simultaneously measuring one electrocardiogram (ECG) lead and three APWs using only four dry electrodes in contact with the hands or wrists. To this end, the system uses impedance plethysmography (IPG) with a single current source without multiplexing or isolating strategies. Its design involves addressing the technical limitations of combining a four-electrode and two three-electrode impedance measurements in a single system by designing the appropriate conditioning circuits based on synchronous demodulation. Following the proposed guidelines, an example of system implementation is also presented, and the feasibility of measuring the desired signals is tested in several practical cases. These include some of the most common electrode types, shapes, and placements, on fingers or wrists, that may be found in possible future applications of the proposed measurement system, such as smart watches, smartphones, or body composition analyzers, among others. The measured signal-to-noise ratio (SNR) (41 and 22 dB) and CMRR (55 and 25 dB) for the impedance plethysmograms measurements with four and three electrodes, respectively, were in agreement with the expected ranges for the correct operation of the system.