Remote Photoplethysmography (rPPG)
Remote photoplethysmography (rPPG) is a technology that provides a non-contact measurement of changes in tissue blood volume by analyzing the properties of light reflected from the tissue. [58]
Similar to conventional photoplethysmography (PPG), commonly known as pulse oximetry [59], rPPG technology measures variations in the optical absorption of light and the spectrum of its reflectance, which arise due to the circulation of blood and its corresponding levels of oxygenation. An important feature is that the absorption of light varies depending on the oxygenation state. The diffuse reflectance of different excitation wavelengths can therefore be used to assess the relative differences in absorption between oxygenated (HbO2) and deoxygenated (HbR) haemoglobin. With each heartbeat, pulsatile blood flow generates small oscillations in the reflected light, resulting in a minimal change in skin colour that cannot be seen with the naked human eye but can be easily detected using artificial intelligence. Advanced signal processing and/or deep learning models are then applied to extract colour variations of the skin from video frames using a camera. [60]
Advanced signal processing is then used to extract colour variations on the skin surface from video images taken with (see Figure 5), for example, a normal smartphone camera. [61] Although it was not clear for some time how deep the detection goes in terms of vascular lumen thickness, it has now been proven that deep pulsatile vessels can also be derived using the rPPG technique. [62] This technique can also be used to indirectly differentiate between venous or arterial thrombosis in free flaps during the postoperative phase. [63] In case of compromise of the vascular pedicle, rPPG can also lead to waiting without further compromising the flap, provided the vascular anatomy has been previously visualized by CT angiography. [64]