# PPG and Bioimpedance-Based Wearable Applications in Heart Rate Monitoring--A Comprehensive Review
## 4 Biological Signals for Wearable-Based HR
### Photoplethysmography  
In most applications, photoplethysmography (PPG) is used in pulse oximetry to assess the oxygenation proportions of hemoglobin in order to derive oxygen saturation (SpO2) [34]. A PPG signal is non-stationary and quasi-periodic [9]. A typical PPG waveform is shown in Figure 6a. There are two components of a PPG signal [35]--the pulsatile AC component, synchronous with the blood flow during a heartbeat, and a slowly varying DC component which is associated with respiration and thermoregulation. It is often assumed that the major frequency component of PPG is based on average pulse rate around 1 Hz, and harmonic frequencies are added up to construct a PPG waveform. The low-frequency component (< 0.5 Hz) generally reflects the respiration (0.15-0.4 Hz) and motion artifacts (< 0.1 Hz) [36]. Here, as is also the case in EBI signals, each single cycle exhibits slight waveform differences due to the potential influence of external factors [37]. In PPG measurements, skin is illuminated with a light-emitting diode (LED) with either green, red, or infra-red (IR) light and a photodetector is used to detect changes in light intensity caused by a heartbeat. These changes are then converted to a voltage signal, called a PPG signal [35]. Since IR light is absorbed by the blood in vein vessels depending on levels of vasodilation, vasoconstriction, and oxygenation, both variations in blood volume pulse (BVP) due to the cardiac cycle [38], and oxygen saturation (SpO2) [39], can be measured  
Figure 5: Electrocardiography (ECG), electrical bioimpedance (IMP) and impedance cardiography (ICG) signals (the first derivative of the IMP signal) with the defined cardiac fiducial points (cardiac events) [33]. Figure 6: Measurement of PPG signals—(**a**) typical PPG waveform; (**b**) transmission PPG measurement mode; (**c**) reflection PPG measurement mode. by wearable devices exploiting the PPG measurement technology. Photodiodes can be positioned either in transmission or reflection PPG measurement modes (see Figure 6b,c). In transmission mode, the LED and photodetector are placed opposite to each other. The photodiode detects light passing through the respective body part. In reflection mode, both the photodetector and the LED are placed on the same side of the respective body part. The photodetector detects backscattered or reflected light from tissue and blood vessels. The transmission mode is limited to recording the PPG signal at body sites which are thin, such as fingers and earlobes [40; 41]. In contrast, in the reflection mode, the PPG signal can be recorded at any body part [42; 43; 41]. Hence, a wrist-worn PPG sensor is usually a reflective type [44], which transmits light on a wrist and identifies the arterial blood volume change by the amount of reflected light during the systolic and diastolic phases of the cardiac cycle. Apart from respiratory rate and monitoring of chronic respiratory conditions [45], analysis of the morphology of pulse waves measured with PPG sensors has been used to obtain parameters related to cardiovascular activity [46]; for example, HR in real time [47; 48], heart rate variability (HRV), and blood pressure [49]. The quantities derived from PPG signals include the number of calories burned during physical exercise and the metabolic equivalent of task (MET) [50]. PPG Waveform Features  
Features derived from PPG waveforms can effectively be used to provide a more complete cardiovascular profile of the patient than bioparameters alone. This approach has gained popularity in employing various machine learning algorithms to do the task. For example, patient classification based on various degrees of coronary artery obstruction in [37], and blood pressure estimation in [51]. **Characteristic PPG waveform points**  
A close-up view of a single cardiac cycle is presented in Figure 7a.