# PPG and Bioimpedance-Based Wearable Applications in Heart Rate Monitoring--A Comprehensive Review
## 4 Biological Signals for Wearable-Based HR  
### Electrical Bioimpedance (EBI)  
In relation to medical care, electrical bioimpedance (EBI) has been used to study small changes in the transthoracic impedance related to respiration or heartbeat non-invasively. EBI measurements can be conducted in two electrode configurations: bipolar and tetrapolar [12]. In the tetrapolar electrode configuration, the changes in transthoracic impedance are measured by injecting a fixed-amplitude sinusoidal alternate current \(l\cos(\omega t)\) inside the thorax via an electrode pair and calculating the electrical impedance by measuring the potential drop across the body or tissue segment by another pair [13,14]:  
\[U(t)=M_{Z}(t)\cdot\cos(\omega t+\varphi_{Z}(t)) \tag{1}\]  
where \(M_{Z}(t)\) and \(\varphi_{Z}(t)\)are the module and phase of the bioimpedance. In other words, the electrodes that measure voltage are separated from those sourcing the current. In a bipolar configuration, however, there are only two electrodes--simultaneously both for measuring and excitation current to the tissue. A tetrapolar electrode configuration reduces the effect of skin-electrode contact impedance [15]. This measurement principle is shown in Figure 3a, electrodes on both sides of the chest (green and blue) pass a constant current across the chest, \(I(t)\),with an impedance detection circuit (yellow) measuring the voltage drop \(U(t)\)caused by intrathoracic blood volume. The total impedance of the thorax at rest consists of electrical resistances of the components of the thorax--adipose tissue, heart and skeletal muscles, lungs, vessels, bone and air [16]. The principle of impedance cardiography is shown in Figure 3b. The colored circles show the position of the electrodes on the body.  
Bioimpedance signal is a quasi-periodic signal with four phases during a cardiac cycle (see Figure 4) [18]:  
1. Systolic phase--the left ventricle of the heart beats to push blood through the arteries and blood volume increases. Initial decrease in impedance.
2. Diastolic phase--the heart rests between beats and blood volume leaves the sensing area. Impedance increases.
3. The second, smaller impedance decrease due the arrival of the reflection wave.
4. The final impedance increase as the reflected blood volume leaves the sensing area.  
Hence, the change in impedance is inversely proportional to the change in blood volume [19]. EBI has been used in the following medical applications--electrical impedance tomography [20], body composition analysis [21], cardiac output monitoring [22], respiration activity monitoring [23], diagnosis of heart failure, myocardial infarction and ischemia [24], impedance plethysmography for continuous assessment of volume changes in the lungs [25; 26], heart [27; 28], veins and peripheral arteries [29; 30] and blood pressure by measuring the pulse from two separate locations on the wrist [31].  
EBI Waveform Features for Cardiovascular Monitoring  
Figure 4 also shows several features that can be extracted and potentially used in the monitoring of various hemodynamic parameters, such as heart rate and blood pressure [18]. These features are grouped in three categories:  
Figure 4: Bioimpedance signal during a single cardiac cycle and derived features for hemodynamic analysis. Amplitude features are in blue, timing features (STT: slope transit time, IBI: inter-beat intervals) are in yellow and area features are in green. \(A_{dia}\) is an area between the diastolic point and the maximum slope point, \(A_{slope}\) is the area between the maximum slope point and the systolic point, \(A_{sys}\) is the area between the diastolic and the systolic point [18].  
Figure 3: Bioimpedance measurement based on electrode placement. (**a**) Intrathoracic impedance measurement technique [6] and (**b**) impedance cardiography [17].