4 Reflective Arterial Pulse Oximetry for New Measuring Sites …
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measurement technique available only allowed transmissive PPG measurement that
can be applied to body parts that are transparent for the measurement light, i.e., finger,
earlobe, or tooth, as well as arms, legs, and feet in infants. Although it is known that
reflective PPG (rPPG) is feasible because scattering predominates over absorption
in human tissue, the pulsating arterial component of the measurement signal is about
ten times smaller compared to the pulsating signal component measured in the transmissive measurement mode and, therefore, a more accurate measurement technique
was required.
Fortunately, technological progress substantially improved the analog and digital
signal processing thereby making rPPG possible. This measurement method is no
longer limited to a few body parts but can be applied on any skin surface that offers
sufficient dermal perfusion [12]. A new wearable sensor concept can be developed
with new possibilities for mobile monitoring and for routine clinical use with regard to
unobtrusiveness, measurement robustness, and suitability for everyday application.
4.2 Motivation for PPG Measurement in the Inner/Outer
Ear Channel
For body sensors intended for use in mobile vital sign monitoring, the parameters
‘diagnostic reliability’ and aspects of ‘usability’ must be considered. Regarding inear PPG sensors, both requirements seem to be in balance. The ear channel provides
both physiological and user-related advantages; both these aspects are discussed
below [13].
4.2.1 In-Ear PPG from a Physiological Viewpoint
From the physiological viewpoint, the ear channel is an ideal measurement side.
Between the heart and the ear-channel cardiovascular diseases are very rare. In
contrast, the peripheral system often suffers from insufficient perfusion. This can
be due to the influence of negative temperature, or to cardiovascular diseases (e.g.,
as a result of diabetes), blood stasis, or centralization of blood flow. For PPG sensors
that are attached to the periphery (e.g., the fingertip), these contraindications often
lead to a disrupted readout, or may even inhibit measurement. However, sensors
that are applied close to the body core (close to vital organs like heart, brain, etc.)
perform valid measurements even in critical situations, since endogenous control of
the human body aims to maintain a constant perfusion of vital organs, such as heart,
lungs, brain, etc. Therefore, modern rPPG sensors focus on the forehead or the ear
due to their proximity to the brain. In addition to constant perfusion, the ear channel
also has the advantage of constant temperature conditions.
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measurement technique available only allowed transmissive PPG measurement that
can be applied to body parts that are transparent for the measurement light, i.e., finger,
earlobe, or tooth, as well as arms, legs, and feet in infants. Although it is known that
reflective PPG (rPPG) is feasible because scattering predominates over absorption
in human tissue, the pulsating arterial component of the measurement signal is about
ten times smaller compared to the pulsating signal component measured in the transmissive measurement mode and, therefore, a more accurate measurement technique
was required.
Fortunately, technological progress substantially improved the analog and digital
signal processing thereby making rPPG possible. This measurement method is no
longer limited to a few body parts but can be applied on any skin surface that offers
sufficient dermal perfusion [12]. A new wearable sensor concept can be developed
with new possibilities for mobile monitoring and for routine clinical use with regard to
unobtrusiveness, measurement robustness, and suitability for everyday application.
4.2 Motivation for PPG Measurement in the Inner/Outer
Ear Channel
For body sensors intended for use in mobile vital sign monitoring, the parameters
‘diagnostic reliability’ and aspects of ‘usability’ must be considered. Regarding inear PPG sensors, both requirements seem to be in balance. The ear channel provides
both physiological and user-related advantages; both these aspects are discussed
below [13].
4.2.1 In-Ear PPG from a Physiological Viewpoint
From the physiological viewpoint, the ear channel is an ideal measurement side.
Between the heart and the ear-channel cardiovascular diseases are very rare. In
contrast, the peripheral system often suffers from insufficient perfusion. This can
be due to the influence of negative temperature, or to cardiovascular diseases (e.g.,
as a result of diabetes), blood stasis, or centralization of blood flow. For PPG sensors
that are attached to the periphery (e.g., the fingertip), these contraindications often
lead to a disrupted readout, or may even inhibit measurement. However, sensors
that are applied close to the body core (close to vital organs like heart, brain, etc.)
perform valid measurements even in critical situations, since endogenous control of
the human body aims to maintain a constant perfusion of vital organs, such as heart,
lungs, brain, etc. Therefore, modern rPPG sensors focus on the forehead or the ear
due to their proximity to the brain. In addition to constant perfusion, the ear channel
also has the advantage of constant temperature conditions.