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]