## 4 Measurement Results of HR and HRV with Optical Homodyne and Heterodyne Technique
### Feature Extraction in VCG and Its Mapping with ECG  
The oxygen-rich blood returns from the lungs and flows into the heart's upper left atrium. The oxygen-depleted blood returns from the rest of the body and flows into the right atrium. Both the left and the right atria become filled with blood. The sinus node (SA) in the right atrium produces a small electrical pulse. The systole and diastole are the two processes that are associated with the period of contraction and relaxation of the heart, respectively. The electrical pulse causes the heart contraction. Due to this, mechanical vibrations are originated and propagate through different tissue layers before being detected on the skin with an optical sensing technique [40]. The factors which influence the vibration signal are the time and amplitude characteristics of the electrical pulse and the mechanical response of the heart. These factors depend on specific characteristics, such as body mass index (BMI) and the medical history for the disease. It is important to map the electrical activity of the heart with its mechanical response, as it provides useful information about the activity of the heart. In the following section, the mapping of ECG and VCG is performed, and its significance is explained. 1. P wave: The P wave represents the depolarization of the left and right atrium. It corresponds to atrial contraction. The point of local maximum in the ECG for the P wave corresponds to the point of the first local minima \(U_{1}\) in the VCG, as shown in Figure 24. The opposite nature of the electrical and mechanical signal is due to atrial contractions [41];  
Figure 23: VCG signals obtained for different values of \(HR\): VCG1, 72 bpm; VCG2, 90 bpm; VCG3, 102 bpm. 2. PR interval: During the PR interval, the electrical pulse moves from atria to ventricles. The PR interval in the ECG has a duration of approximately 120-200 ms. The contractions of ventricles generate vibration patterns that are observable in the VCG plot. When the ventricle contracts, a dominant negative deflection is generated. Thus, it is possible to distinguish between a heartbeat and a beat drop. The atrial contractions result in positive deflections. The PR interval in the ECG is mapped to the interval between the first local minima \(U_{1}\) and the second zero crossing of the VCG signal;
3. QRS complex: The point Q in the ECG corresponds to the point of first local maximum \(V_{1}\), while the point of minimum S in the ECG corresponds to the point of second local minima \(U_{2}\) in the vibrocardiogram signal, as shown in Figure 25. The R wave in the ECG corresponds to the zero crossing of the VCG signal [42]. The zero crossing is a point which is located at the center of \(V_{1}\) and \(U_{2}\). It is located exactly at the center of the first maxima and the second minima of the VCG signal. The total time duration of the QRS signal represents the depolarization of ventricles. During this time interval, the chest wall vibration reduces significantly from maximum to minimum;  
Figure 24: Mapping of P wave of ECG and \(U_{1}\) wave of VCG. Figure 25: Mapping of QRS complex of ECG and \(V_{1}U_{2}\) complex of VCG. 4. ST segment: The T wave represents ventricular repolarization. It appears as a small wave after the QRS complex. The ST segment starts at the end of the S wave and ends at the beginning of the T wave. The ST segment is an isoelectric line that represents the time between depolarization and repolarization of the ventricles. The T wave in the ECG corresponds to the \(V_{2}\) wave of the VCG, as shown in Figure 26;
5. QT interval: The QT interval begins at the start of the QRS complex and finishes at the end of the T wave. The QT interval in the ECG is mapped to the \(V_{1}V_{2}\) interval of the VCG. It represents the time taken for the ventricles to depolarize and then repolarize [43]. The complete VCG cycle is mapped to the ECG cycle for the heart period (HP). Figure 27 shows the significance of features in the VCG with reference to the ECG.