3 Pulse Oximetry for the Measurement of Oxygen Saturation …
59
noted that the light transmitted through the earlobe exhibited pulsatile variations.
While attempting to eliminate these variations, he discovered that the ratio of pulsatile
signals measured at two different wavelengths could be related to the oxygen levels
of the arterial blood. Aoyagi’s method also eliminated the requirement to know the
intensity of light that was entering the tissue-under-test. In March 1974 Aoyagi and
his team announced the first pulse oximeter model OLV-5100. This was a significant
development as it reduced the number of wavelengths necessary down to two for
measurement of SpO 2 from the eight used in the Hewlett-Packard instrument [29].
The model OLV-5100 employed a tungsten light source and two narrow band filters to
obtain red and IR monochromatic lights. Unfortunately, these filters blocked majority
of the light intensity from the source, resulting in very low levels of light available
for the measurement of oxygen saturation. All these early instruments suffered from
one or more of the following drawbacks [30]:
(a) Lack of adequate calibration procedures.
(b) Difficulty in differentiating tissue, arterial blood and venous blood.
(c) Error introduced due to unknown optical path length.
In the late 1970s, several groups began developmental work in pulse oximetry
using Aoyagi’s idea and fingertip probes were introduced. With subsequent developments in semiconductor technology, leading to the invention of the solid-state
devices such as LEDs, photodiodes and microprocessors steered the current era of
modern pulse oximetry. LEDs generated required narrowband light with controlled
wavelengths, exactly the type of light required to vastly improve the signal quality of
oximeters. In 1981, Nellcor and Ohmeda (now GE) introduced commercial pulse
oximeters utilizing small LEDs and photodiode mounted directly on the sensor
probe applied to the patients. Today, there are many manufacturers producing pulse
oximeters with elevated levels of confidence in the readings of oxygen saturation
[31].
In course of time, pulse oximetry has revolutionized the concept of clinical monitoring of blood oxygen saturation since electrocardiography. The American Society
of Anaesthesiologists (ASA) made the pulse oximeter to be a standard for intraoperative monitoring in 1990 [32]. Since then, pulse oximetry has become the standard
technique for monitoring oxygenation during procedural sedation, anaesthesia, post
anaesthesia care unit, intensive care unit (including neonatal intensive care unit), and
recovery from anaesthesia.
3.7 Principle of Operation of a Pulse Oximeter
Pulse oximeters derive their name since they operate on the pulsatile portions of red
and IR PPG signals to estimate the oxygen saturation in arterial blood. It is seen from
Eq. (3.1), to compute SpO 2 , the concentrations of oxy and deoxy haemoglobin, N HbO
and N Hb , must be known. To extract these concentrations from PPG signals, all the
present-day pulse oximeters utilize Beer-Lambert’s law. Beer-Lambert’s law states
59
noted that the light transmitted through the earlobe exhibited pulsatile variations.
While attempting to eliminate these variations, he discovered that the ratio of pulsatile
signals measured at two different wavelengths could be related to the oxygen levels
of the arterial blood. Aoyagi’s method also eliminated the requirement to know the
intensity of light that was entering the tissue-under-test. In March 1974 Aoyagi and
his team announced the first pulse oximeter model OLV-5100. This was a significant
development as it reduced the number of wavelengths necessary down to two for
measurement of SpO 2 from the eight used in the Hewlett-Packard instrument [29].
The model OLV-5100 employed a tungsten light source and two narrow band filters to
obtain red and IR monochromatic lights. Unfortunately, these filters blocked majority
of the light intensity from the source, resulting in very low levels of light available
for the measurement of oxygen saturation. All these early instruments suffered from
one or more of the following drawbacks [30]:
(a) Lack of adequate calibration procedures.
(b) Difficulty in differentiating tissue, arterial blood and venous blood.
(c) Error introduced due to unknown optical path length.
In the late 1970s, several groups began developmental work in pulse oximetry
using Aoyagi’s idea and fingertip probes were introduced. With subsequent developments in semiconductor technology, leading to the invention of the solid-state
devices such as LEDs, photodiodes and microprocessors steered the current era of
modern pulse oximetry. LEDs generated required narrowband light with controlled
wavelengths, exactly the type of light required to vastly improve the signal quality of
oximeters. In 1981, Nellcor and Ohmeda (now GE) introduced commercial pulse
oximeters utilizing small LEDs and photodiode mounted directly on the sensor
probe applied to the patients. Today, there are many manufacturers producing pulse
oximeters with elevated levels of confidence in the readings of oxygen saturation
[31].
In course of time, pulse oximetry has revolutionized the concept of clinical monitoring of blood oxygen saturation since electrocardiography. The American Society
of Anaesthesiologists (ASA) made the pulse oximeter to be a standard for intraoperative monitoring in 1990 [32]. Since then, pulse oximetry has become the standard
technique for monitoring oxygenation during procedural sedation, anaesthesia, post
anaesthesia care unit, intensive care unit (including neonatal intensive care unit), and
recovery from anaesthesia.
3.7 Principle of Operation of a Pulse Oximeter
Pulse oximeters derive their name since they operate on the pulsatile portions of red
and IR PPG signals to estimate the oxygen saturation in arterial blood. It is seen from
Eq. (3.1), to compute SpO 2 , the concentrations of oxy and deoxy haemoglobin, N HbO
and N Hb , must be known. To extract these concentrations from PPG signals, all the
present-day pulse oximeters utilize Beer-Lambert’s law. Beer-Lambert’s law states