5 Peripheral Venous Dynamics, Venous Oxygen Saturation …
95
that arterial oxygen saturation could be measured noninvasively by quantifying the
frequency spectra of the light emissions coming through the tissue [3], which made
heating the tissue (used in former measuring setups) [4] obsolete. This device was the
precursor for all the modern pulse oximeters [5–8]. In 2015, Aoyagi was recognized
with the IEEE Medal for innovations in healthcare technology for his “… pioneering
contributions to pulse oximetry that have had a profound impact on healthcare” [9].
In contrast to the advances in pulse oximetry, the monitoring of the venous oxygen
saturation (SvO 2 ) measurement continues to be conducted invasively. Of the available
methods, the invasive in vitro measurement of the extracted venous blood is the gold
standard in SvO 2 measurement. The in vivo monitoring of blood gas through an
intravascular catheter (e.g. in a. pulmonaris) is another technique that is clinically
accepted [10]. A photoplethysmographic based solution is described in the literature,
which uses a small finger cuff [11]. Using the cuff, external pressure is applied to
modulate the venous flow in the vascular segment downstream of the cuff. From
the pulsatile venous blood flow produced by this occlusion maneuver, SvO 2 can be
calculated. One of the major impediments in this method is that using a cuff, only
very small blood volume changes in the venous blood flow are possible and therefore,
the ability to detect venous oxygen saturation by this method is very limited.
5.3 Photoplethysmographic Measurement of the Peripheral
Venous Oxygen Saturation
This chapter describes a novel non-invasive method for the determination of the
peripheral, dermal venous oxygen saturation (SvO 2 ) in human extremities. In its
minimum configuration, this PPG based venous oxygen saturation measurement
method consists of
• a flat, flexible optoelectronic sensor that can operate in reflection or transmission
mode, depending on the selected tissue area, with at least one photodetector and
two light sources (preferably one in the red wavelength region and the other in IR
wavelength region) whose light illuminates the assessed skin area,
• a sensor control and evaluation unit, in which the output of the sensor signals
is digitized so that the PPGs at the two different (red and IR) wavelengths are
separated and made available for further processing,
• a digital filter unit which detects the arterial DC and AC signal components of the
PPG signals at the two (red and IR) wavelengths,
• a second digital filter unit that extracts the venous DC and AC signal components
from the PPG signals and
• a processor that evaluates the venous and arterial oxygen saturation using the AC
and DC signal components filtered by the two digital filters portrayed above.
The proposed venous oxygen saturation (SvO 2 ) and arterial oxygen saturation
(SpO 2 ) measurement method employ the well known venous muscle pump test
95
that arterial oxygen saturation could be measured noninvasively by quantifying the
frequency spectra of the light emissions coming through the tissue [3], which made
heating the tissue (used in former measuring setups) [4] obsolete. This device was the
precursor for all the modern pulse oximeters [5–8]. In 2015, Aoyagi was recognized
with the IEEE Medal for innovations in healthcare technology for his “… pioneering
contributions to pulse oximetry that have had a profound impact on healthcare” [9].
In contrast to the advances in pulse oximetry, the monitoring of the venous oxygen
saturation (SvO 2 ) measurement continues to be conducted invasively. Of the available
methods, the invasive in vitro measurement of the extracted venous blood is the gold
standard in SvO 2 measurement. The in vivo monitoring of blood gas through an
intravascular catheter (e.g. in a. pulmonaris) is another technique that is clinically
accepted [10]. A photoplethysmographic based solution is described in the literature,
which uses a small finger cuff [11]. Using the cuff, external pressure is applied to
modulate the venous flow in the vascular segment downstream of the cuff. From
the pulsatile venous blood flow produced by this occlusion maneuver, SvO 2 can be
calculated. One of the major impediments in this method is that using a cuff, only
very small blood volume changes in the venous blood flow are possible and therefore,
the ability to detect venous oxygen saturation by this method is very limited.
5.3 Photoplethysmographic Measurement of the Peripheral
Venous Oxygen Saturation
This chapter describes a novel non-invasive method for the determination of the
peripheral, dermal venous oxygen saturation (SvO 2 ) in human extremities. In its
minimum configuration, this PPG based venous oxygen saturation measurement
method consists of
• a flat, flexible optoelectronic sensor that can operate in reflection or transmission
mode, depending on the selected tissue area, with at least one photodetector and
two light sources (preferably one in the red wavelength region and the other in IR
wavelength region) whose light illuminates the assessed skin area,
• a sensor control and evaluation unit, in which the output of the sensor signals
is digitized so that the PPGs at the two different (red and IR) wavelengths are
separated and made available for further processing,
• a digital filter unit which detects the arterial DC and AC signal components of the
PPG signals at the two (red and IR) wavelengths,
• a second digital filter unit that extracts the venous DC and AC signal components
from the PPG signals and
• a processor that evaluates the venous and arterial oxygen saturation using the AC
and DC signal components filtered by the two digital filters portrayed above.
The proposed venous oxygen saturation (SvO 2 ) and arterial oxygen saturation
(SpO 2 ) measurement method employ the well known venous muscle pump test