IEEE Solid-States Circuits Magazine - Fall 2020 - 36

IEEE SOLID-STATE CIRCUITS MAGAZINE	

HP OLV-5100 Ear Pulse Oximeter

FIGURE 1:  A timeline of advancements in respiration monitoring from 1956 to 2020 [14]-[22]. PtcO2: partial pressure of O2; PaO2: arterial partial pressure of O2; PPG: photoplethysmography; ASIC: application-specified IC; pH: acidity; OLED: organic light-emitting diode; OPD: organic photodetector.

2018: Lim et al. publish flexible
PtcO2 monitoring optode using
organic photodiodes and LEDs [19].

1993: Larsen, Linnet, and Vesterager propose
a solid-state glass pH electrode for PtcCO2.

2020: Worcester Polytechnic
Institute Integrated Circuits and
Systems Lab demonstrates
2019: SenTec introduces a
an ASIC for a wearable
combined PtcO2 and PO2
PtcO2 sensor [22].
sensor with pulse oximetery
(OxiVent) [41].
2007: Eberhard publishes a method
to optically measure PtcCO2.
1956: Clark invents a 1972: Aoyagi and Kishi develop
pulse oximetery. (Source: [16];
platinum-based O2
reprinted with permission from
electrode [14].
the American Physiological Society.)

2020: Berkley demonstrates
an ASIC for an implantable
PtcO2 sensor [21].
2019: Northwestern University
demonstrates a wearable pulse
oximeter on a flexible
substrate [20].
FA L L 2 0 2 0	

1976: First commercial
transcutaneous gas
sensors are available.

1995: Masimo introduces
algorithms to reduce errors
in pulse oximeters due to
motion artifacts and poor
perfusion.

2015: Fitbit releases
the charge HR,
a wrist-warn
fitness tracker
with a PPG heartrate
monitor.
1958: Severinghaus and Bradley
invent a CO2 and O2 monitoring device.
(Source: [15]; reprinted with
permission from The American
Physiological Society.)
1972: Lübbers group
demonstrates that PtcO2
values match PaO2 values
when the skin is heated
to 43-44°.

36	

loaded with O2, which is also called a
patient's SpO2.
Pulse oximeters are valuable tools,
as they enable medical practitioners
and nonprofessionals, who have only
minimal basic training, to easily monitor O2 saturation and heart rates. Pulse
oximeters measure peripheral O2
saturation. O2 saturation and O2 partial pressure (or tension) are related
through the dissociation curve, given
in Figure 2, which in many cases
enables O2 saturation to be used as
a surrogate measurement of O2 partial pressure.
Pulse oximeters are not perfect,
and they do have some drawbacks.
Many factors, such as acidity (pH),
temperature, hemoglobin count,
and the partial pressure of CO 2 ,
can shift the dissociation curve on
an individual basis. Additionally,
pulse oximeters have been found
to provide inaccurate results and
to have limited performance in certain settings. Different skin colors
can affect the absorption of the
sensing photons; this ca n cause
erroneous readings if not corrected
[29]-[31]. Low blood pressure and
vasoactive drugs can also cause
false readings [31], [32]. Another
issue related to poor perfusion is
that SpO2 readings from finger-tip
pulse oximetry change with limb
temperature, particularly if the fingers and hands are too cold [33].
Motion artifacts from movement, a
widely studied problem, can cause
false readings and interrupt continuous measurements [34]-[36].
One major issue involves the presence of carboxyhemoglobin and
methemoglobin, such as in carbon
monoxide poisoning and methemoglobinemia. These molecules have
a bs or b a nce patterns similar to
oxyhemoglobin and therefore cause
falsely elevated readings [31].
The relative ease of use and ability to measure SpO2 and heart rate
make the pulse oximeter a valuable
tool from the operating room to the
ambulance and the home environment. However, some physicians have
concerns about overreliance on this
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