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Recently, an investigation conducted around the world by the WHO demonstrated that cardiovascular-related disease has become the number one killer worldwide (World Health Organisation (WHO), 2017). This is very serious consternation for everyone living in this world. In short, Cardio Vascular Disease (CVD) is any disease or condition that affects the structure or the function of the heart. The big problem here is that unlike in the case of other diseases where the cause can be linked to a specific pathology, CVDs causes are various. This is because any element failure in the Cardio Vascular System (CVS) will automatically expose the individual to CVDs. If a preventive measure is not taken to tackle the problem timely, the affected individual may become a victim of sudden death. Including the heart which acts like a pump of the system, many CVDs cases are related to the blood vessel circulatory canals. Just to name, a few of them include the coronary artery diseases, heart attack, arrhythmias, heart failure, heart valve disease, congenital heart disease, heart muscle disease, aorta disease, blood vessel disease (WebMD, 2016). In view to counteract this phenomenon, some general prescriptive and preventive actions include physical activities, healthy nutrition and regular checkups (Association, 2015). The other dilemma is that in most cases, CVD evolves silently and before one could notice that something isn't working well in the body, it is already too late. Since CVDs are directly related to the circulatory blood fluid canal, and the canal being a closed circuit, logically, any burst or obstruction in this canal will affect the rate of blood fluid. Since the blood fluid rate is somehow equivalent to the rate at which the heart is pumping the blood into the vessel, the Heart Rate (HR) monitoring becomes a major concern. This is the main motivation driving the development of a multitude of devices available in the market today for HR monitoring. Besides the variety of existing HR monitoring devices, affordability is still a big problem especially in countries of lower-income revenues. Unfortunately, it was also proved that most affected people leaving with CVDs reside in the third world (World Health Organisation (WHO), 2017). Early detection of CVDs is an advantage because some of them can be treated. However, left untreated, they can lead to serious adverse health problems including hypertension, atrial fibrillation, and stroke. This is one further reason which motivated the invention of the Photoplethysmography (PPG) based HR monitoring. Due to its embedded advantages, which include simplicity, lower cost, reliability, non-invasive, mobility and so on, this technology has become the most prominent amongst its peers.
PPG technology is a non-invasive optical technique that measures blood volume changes (Allen, 2007). In this concept, light is shone at the tissue and the backscattered light is then measured by a photodetector. When light travels through the tissue, it is reflected, scattered, and absorbed. In PPG, the photodetector collects the light that is absorbed in tissue and the resulting signal represents the absorption changes due to the variations in blood volume (Allen, 2007). The PPG signals oscillate with the heart cycle period, due to the rhythmical increase and decrease in the tissue blood volume, resulting in a periodically changing transmission of light (Nitzan. M, Babchenko.A, Khanokh.B, Landau.D, 1998). The signal produced by the photodetector is called photoplethysmogram and contains information on the blood light absorption. While the absorbance of some tissues (e.g., bone, muscle) is a constant and can be termed as the DC component of the PPG signal, the absorbance of arterial blood pulsations can be considered as an AC component (Woods. AM, Queen. JS, Lawson. D., 1991). This AC component contains each individual's unique information, although being a very small portion of the whole signal; it can be extracted and amplified to describe the HR activities.