# Unobtrusive Vital Sign Monitoring in Automotive Environments--A Review
## 2 Unobtrusive Sensing of Physiological Parameters and Features  
As mentioned above, the term _vital signs_ typically refers to five physiological parameters: HR, RR, BP, BT and SpO\({}_{2}\). Since no attempts have been conducted to equip cars with unobtrusive BP monitoring, this vital sign will not be further addressed in this review. In 2012, [PERSON] et al. [14] presented a comprehensive overview of contact-based (ECG and PPG) and non-contact (CW and PW (UWB) radar, laser vibocardiography [15], magnetic induction, capacitive ECG, ultrasound transmitter) heart monitoring systems. In 2014, [PERSON] et al. [16] presented a review on heart rate and heart rate variability measurements. Apart from conventional measurement approaches (ECG and PPG), experimental approaches were also presented, including the measurement by thermal imaging, imaging using visible light, optical vibocardiography, doppler radar, capacitive ECG and heart rate extraction from speech. Advantages and disadvantages of the experimental approaches were discussed. In the same year, [PERSON] et al. [17] presented a feasability study for several experimental heart rate measurement methods. The authors separated the methods into two categories, depicted as \"direct methods\" measuring the electrical heart activity (capacitive ECG) and \"indirect methods\" measuring mechanical displacement of the patients body (microwave and ultrasound distance measurement, signal from microphone). In 2015, [PERSON] et al. [3] from RWTH Aachen University provided a methodological review on nine different non-contact sensing modalities targeting the monitoring of cardiorespiratory activity. As described in this paper, there are three main categories of physical effects that can be sensed unobtrusively: bioelectrical, mechanical and thermal effects. Figure 1 provides an overview of the different effect categories, and how they relate to organ activity as well as to the measured vital signs. The activity of the heart can be observed by measuring body surface potentials which are generated by the electrical excitation of heart muscle cells. The measurement of these body surface potentials is called ECG or in the case of capacitively coupled measurement capacitive ECG (cECG), see Section 3, and provides extensive information about the cardiac cycle. Any intrathoracic organ motion will have mechanical effects. Such mechanical effects include the displacement of organ boundaries, deformation of surfaces and the displacement of fluids and may modulate the displacement of the body surface and the superficial perfusion, but also the intrathoracic impedance distribution. Body surface displacement can be obtained by radar, ballistocardiography (BCG), seismocardiography (SCG) (see Section 4) or video (see Section 5) and has been used for HR as well as RR monitoring. The superficial perfusion and its effect on the optical properties of the skin is the basis for PPG and PPG imaging (PPGi). These techniques provide information about the cardiac cycle and SpO\({}_{2}\) and are described in detail in Section 5. While variations in intrathoracic impedance distribution can also be observed with bioimpedance spectroscopy (BIS) and impedance cardiography (ICG), up to today only magnetic induction (MI) measurements allow unobtrusive (i.e., electrode-free) impedance analysis. In fact, MI has already shown its capability to measure HR and RR (see Section 6). The body surface temperature, which can be monitored in an unobtrusive way by radiation-based thermography as described in Section 5, is modulated by changes in the body's core temperature by direct conduction of heat between the organs and blood to the skin. Next to this, the body surface temperature is also modulated by heart and lung activity via heat convection by blood and air and, therefore, allows RR monitoring. In principle, body surface-based HR measurement could also be possible, but today's thermography devices do not provide the necessary sensitivity.