Cardiovascular Monitoring System. Electrocardiograms (ECGs) represent a non- invasive approach for measuring and recording the fluctuations of cardiac potential. This is the most widely used and effective diagnostic tool that physicians have used for decades to identify heart- related problems such as different forms of arrhythmias.

Although many arrhythmias are not life- threatening, some results from weak or damaged heart such as myocardial infarction (MI) that may lead to cardiac arrest, if not managed immediately . After a heart attack, patients are required to receive immediate medical attention, which, otherwise, may turn fatal. These complications can be avoided if any inconsistency in cardiac activity is detected and treated in an early stage that calls for outpatient ambulatory monitoring of ECG.

Some rare, serious arrhythmias (e. Brugada Syndrome, Arrhythmogenic Right ventricular Cardiomyopathy, Long QT syndrome, hypertrophic Cardiomyopathy) are infrequent and only detected on prolonged monitoring. Figure 2a shows one cycle of a typical ECG signal. In a conventional 1. ECG system, electrical activities of the heart along 1. Ag- Ag. Cl electrodes (hydrogel method/wet ECG), which are affixed to some specific parts of the body. Figure 2b shows the placement of the electrodes in a standard of 1.

ECG system. The electrodes contain conducting gel in the middle of the pad that functions as a conduction medium between the skin and the electrode. This conducting gel has potential toxic and irritant effects on the skin and is thus not best suitable to use for long–term ambulatory monitoring system though currently, it is the only system available . However, only a few numbers of electrodes are used in ambulatory ECG monitoring system at the cost of limited information (Figure 2c). A continuous ambulatory monitoring device requires a wearable and portable system that could be used comfortably without affecting an individual’s daily activities. Andreoni et al. The electrodes were made from silver based conductive yarns.

Instead of using any conductive gel, the electrodes relied on body sweat, an electrolyte medium, to improve the conductivity of the skin- electrode interface and signal quality. The device also included a Sp. O2 sensor and a three- axis accelerometer for fall detection, and it could transmit the data over low- power Bluetooth 4. An elastic fabric- made ECG vest was presented in . The electrodes were fabricated from Ni/Cu coated compressed urethane polymer foam that was enclosed by an Au- coated conductive taffeta fabric.

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The ECG measured by the proposed system exhibited high correlation with the simulated signal, although measurements on real subjects were not shown. Due to the conductive and flexible nature of the substrate, the skin- electrode impedance was low and remained stable over a longer period of time, thus reducing the electrode motion noise. Jeong et al. In addition to that, they used a very high bias resistor at the input of the pre- amplifier, which assisted further in reducing electrode motion artifacts. A noise cancellation and peak detection algorithm was performed on the raw ECG data to find out the QRS complex, and HR, although a detailed description of the algorithm was not provided.

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The authors reported achieving high sensitivity and high accuracy in detecting QRS complexes. The electrodes used in the ECG systems presented in . Nemati et al. The cotton functioned as the dielectric material between the electrode and the skin.

The signal processing and communication modules were mounted on a small two- layer PCB board. Power consumption was minimized by selecting low power electronic components for the system, ANT protocol for wireless communication as well as by adopting idle mode signal sampling technique. However, the rigid electrodes can be inconvenient to the users and may induce motion artifacts in the signal. In order to minimize the common mode interference, an additional driven right- leg (DRL) electrode was used in . This DRL electrode is usually placed at a distant site, far from the measurement electrodes and thus requires a long wired connection, which may not be convenient for wearable and long- term monitoring systems. Komensky et al. In order to increase and stabilize the input impedance, two anti- parallel connected diodes were used for biasing that has advantages over resistors .

The ECG measurements on stable subjects were reasonably well, although the P waves were indistinguishable, which might be attributed to the electrodes’ position on the body or the absence of the common electrode. On the other hand, the measurements during walking were greatly affected by the motion artifacts, but the QRS complexes were still recognizable.

Many researchers have developed and made use of piezoelectric pressure sensors for measuring the HR by sensing the arterial pulse wave generated by the periodic contraction and relaxation of the heart. A wireless HR monitoring device was presented in . A piezoelectric film pressure was used to sense the in- ear pulse waves (EPW) and convert it to an electric current.

A knowledge- based algorithm was implemented in a microcontroller that could detect the pulse peak in real time from the signal after performing a morphological conversion. However, the pressure variance, and thereby the peak height of the pressure waves can be affected by body movements that introduce error in HR estimation. In addition, an ear- mounted device is inconvenient for long- term use. A similar system was proposed in . They used carbon black/silicone rubber nanocomposite as the flexible piezoresistive material.

High sensitivity and linearity of the pressure sensor was achieved by forming microstructures at the contact surface of two piezoresistive layers. They also proposed a low- cost analog signal processing (ASP) system that could perform denoising, data processing, and HR measurement. Yoon et al. The pressure sensor was fabricated on a polyimide substrate with a small window. A thermally evaporated silver electrode was spin- coated with a polyvinylidene fluoride- trifluoroethylene (P(VDF- Tr. FE)) piezoelectric layer. The pressure variation in the radial artery causes mechanical stress on the piezoelectric layer, resulting in potential variation across the electrodes.

Tajitsu et al. The piezoresistive material was made from nonwoven acrylate- modified polytetrafluoroethylene (PTFE) fabric that was fabricated using electro- spinning. The PTFE was deposited on an aluminum electrode on a polyethyleneterephthalate (PET) film. The pulse wave measured from the wrist by this sensor had similar pattern as the ECG signal and showed high accuracy as well as less vulnerability to motion- induced noise. Some researchers have exploited system- on- chip (SOC) technologies to integrate both analog and digital signal processing units for on- chip ECG signal processing. Izumi et al. The chip was designed to perform data acquisition, process ECG and accelerometer signals, and communicate with the smartphone. The R- peak detection and HR estimation was performed by utilizing short- term autocorrelation (STAC) between a template signal and the measured signal.

The chip was fabricated using a standard 1. CMOS technology. The system was reported to consume ~1. An ultra- low- power ASIC was designed for cardiovascular monitoring in . The PGA offered wide dynamic range, self- biasing capability, and low supply voltage requirement.

A DC voltage was added to the baseline drift and R–peaks were detected by comparing the QRS complex signal with the shifted baseline. The system required only 5. W of power and can operate continuously for one year with a 0. Ah thin- film battery, thereby making it suitable for long- term monitoring applications.

Helleputte et al. Each channel measures ECG as well as electrode- tissue impedance (ETI), which is found to be strongly correlated with the motion artifacts.

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