4 Reflective Arterial Pulse Oximetry for New Measuring Sites …
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hypoxia study, the correlation between SpO 2 and SaO 2 is identified by applying the
oximeter under test to healthy persons while reducing their blood oxygen saturation
over a specific period of time.
This controlled hypoxemia is achieved by decreasing the oxygen partial pressure
in the breathing air and increasing the nitrogen partial pressure. Arterial blood gas
analysis provides comparative values.
A representative result requires a sufficient number of volunteers in the study. The
regression analysis of SpO 2 and SaO 2 is done retrospectively. Based on these results,
a general calibration function can usually be found that is often approximated by
SpO 2 = AR + B R
2
+ C
(4.1)
In the development process, this calibration specification is permanently
embedded in the pulse oximeter system [8]. Noise, external influences, or individual
physiological differences can lead to limitations in oxygen measurement. European
guidelines require a difference in effective value of ≤4.0% in the range of 70%–100%
SaO 2 . The difference in effective value is defined by
A rmse =
n
i=1 (SpO 2i − SaO 2i ) 2
n
(4.2)
where SpO 2i are the measured values and SaO 2i are the comparative values. The study
was performed at the University Medical Center of Schleswig Holstein (Lübeck,
Germany) [11]. Ten healthy volunteers (non-smokers, three women and seven men,
age range 22 years–37 years) were included in the study in accordance with the
recommendation of the FDA. The study was approved by the local Ethical Committee
and written informed consent was obtained. The subjects were motionless in a beachchair position. Sensors were applied to the ear and the sensor cable was temporarily
fixed (Fig. 4.5). SaO 2 was influenced by changing the composition of the breathing air
(nitrogen/oxygen relation). SaO 2 was reduced stepwise from 70%–100% (Fig. 4.6).
4.3.2 Heart Rate
The heartbeat is defined as the contraction of the heart muscle in combination with
the resulting cardiac output. By means of the electrocardiogram (ECG), the R-peak of
the QRS complex usually defines the time point of the heartbeat. The QRS complex
is associated with the depolarization of the heart muscles. Therefore, it can also be
seen as the initial time point of mechanical heart contraction [9]. After the heart
contracts, a pulse wave propagates through the arteries to the periphery. Hence, a
time delay exists between the heartbeat and the subdermal blood volume pulse at the
periphery. Nevertheless, the pulse transit time is usually constant and therefore does
not influence the heart rate measurement accuracy with PPG. In rPPG, heartbeat can
85
hypoxia study, the correlation between SpO 2 and SaO 2 is identified by applying the
oximeter under test to healthy persons while reducing their blood oxygen saturation
over a specific period of time.
This controlled hypoxemia is achieved by decreasing the oxygen partial pressure
in the breathing air and increasing the nitrogen partial pressure. Arterial blood gas
analysis provides comparative values.
A representative result requires a sufficient number of volunteers in the study. The
regression analysis of SpO 2 and SaO 2 is done retrospectively. Based on these results,
a general calibration function can usually be found that is often approximated by
SpO 2 = AR + B R
2
+ C
(4.1)
In the development process, this calibration specification is permanently
embedded in the pulse oximeter system [8]. Noise, external influences, or individual
physiological differences can lead to limitations in oxygen measurement. European
guidelines require a difference in effective value of ≤4.0% in the range of 70%–100%
SaO 2 . The difference in effective value is defined by
A rmse =
n
i=1 (SpO 2i − SaO 2i ) 2
n
(4.2)
where SpO 2i are the measured values and SaO 2i are the comparative values. The study
was performed at the University Medical Center of Schleswig Holstein (Lübeck,
Germany) [11]. Ten healthy volunteers (non-smokers, three women and seven men,
age range 22 years–37 years) were included in the study in accordance with the
recommendation of the FDA. The study was approved by the local Ethical Committee
and written informed consent was obtained. The subjects were motionless in a beachchair position. Sensors were applied to the ear and the sensor cable was temporarily
fixed (Fig. 4.5). SaO 2 was influenced by changing the composition of the breathing air
(nitrogen/oxygen relation). SaO 2 was reduced stepwise from 70%–100% (Fig. 4.6).
4.3.2 Heart Rate
The heartbeat is defined as the contraction of the heart muscle in combination with
the resulting cardiac output. By means of the electrocardiogram (ECG), the R-peak of
the QRS complex usually defines the time point of the heartbeat. The QRS complex
is associated with the depolarization of the heart muscles. Therefore, it can also be
seen as the initial time point of mechanical heart contraction [9]. After the heart
contracts, a pulse wave propagates through the arteries to the periphery. Hence, a
time delay exists between the heartbeat and the subdermal blood volume pulse at the
periphery. Nevertheless, the pulse transit time is usually constant and therefore does
not influence the heart rate measurement accuracy with PPG. In rPPG, heartbeat can