56
J. Kumar V. and K. A. Reddy
3.6.2 Arterial Blood Gas Analysis and Co-Oximetry
Prior to the widespread use of the present-day non-invasive pulse oximeter, the
arterial blood gas (ABG) analysis and Co-Oximetry were the main methods employed
for the measurement of arterial oxygen saturation. As its name implies, the ABG test
is conducted by taking a blood sample from an artery and performing complete gas
analysis. For this purpose, either the radial artery at the wrist or the brachial artery at
the elbow would have to be punctured. The common practice was to draw the samples
at regular intervals and analyze using in vitro blood gas analyzer. The sampling can
be several times a day or even several times an hour. In addition to the SaO 2 , the
ABG analyzer provides the pH value, P O 2 , P C O 2 and the bicarbonate concentration.
Co-Oximeter or haemoximeter calculates the actual concentrations of the Hb,
HbO, COHb and MetHb but again requires a sample of blood drawn from an artery.
It works on the spectrometric principle using four different wavelengths of light and
is also capable of measuring the fractional SaO 2 . The most accurate method available
for measuring the four clinically relevant haemoglobin species is the Co-Oximeter.
Due to this fact, it is accepted as the “gold standard” against which other methods of
oxygen measurement in blood are compared [14].
However, both these methods are invasive and risky as puncturing an artery may
result in spasm, excessive bleeding, vessel obstruction and infection [15]. Because
both these methods are time-consuming, invasive and provide SaO 2 readings only at
the times at which the samples are drawn, non-invasive methods that provide continuous measurements were needed. Pulse oximetry is such a non-invasive method that
can measure arterial oxygen saturation continuously without extracting blood. Pulse
oximetry has now become the most popular method employed for the determination of SaO 2 . When SaO 2 is determined using the principle of pulse oximetry, it is
customary to indicate it as SpO 2 . In the method of pulse oximetry SpO 2 is computed
using a couple of photoplethysmographs [9].
3.6.3 Photoplethysmography
Hertzman pioneered Photoplethysmography, a non-invasive electro optic method,
that provides information on the blood volume changes at a test site on the body
[16]. A Photoplethysmogram (PPG) is the signal extracted from either the reflected
or transmitted light, obtained by illuminating a part of the body of interest. Reflective
type PPG can be obtained close to the skin on any part of the body. Transmission type
PPG can be obtained only from an accessible extremity such as earlobe or finger.
Obtaining a PPG employing the reflected light is already dealt with in Chaps. 1
and 2. To obtain a PPG through the transmitted light, a light source of wavelength λ
having a constant intensity I INλ is placed on one side of an extremity, say fingertip or
earlobe, and the transmitted light I oλ through the finger or earlobe is detected by a
J. Kumar V. and K. A. Reddy
3.6.2 Arterial Blood Gas Analysis and Co-Oximetry
Prior to the widespread use of the present-day non-invasive pulse oximeter, the
arterial blood gas (ABG) analysis and Co-Oximetry were the main methods employed
for the measurement of arterial oxygen saturation. As its name implies, the ABG test
is conducted by taking a blood sample from an artery and performing complete gas
analysis. For this purpose, either the radial artery at the wrist or the brachial artery at
the elbow would have to be punctured. The common practice was to draw the samples
at regular intervals and analyze using in vitro blood gas analyzer. The sampling can
be several times a day or even several times an hour. In addition to the SaO 2 , the
ABG analyzer provides the pH value, P O 2 , P C O 2 and the bicarbonate concentration.
Co-Oximeter or haemoximeter calculates the actual concentrations of the Hb,
HbO, COHb and MetHb but again requires a sample of blood drawn from an artery.
It works on the spectrometric principle using four different wavelengths of light and
is also capable of measuring the fractional SaO 2 . The most accurate method available
for measuring the four clinically relevant haemoglobin species is the Co-Oximeter.
Due to this fact, it is accepted as the “gold standard” against which other methods of
oxygen measurement in blood are compared [14].
However, both these methods are invasive and risky as puncturing an artery may
result in spasm, excessive bleeding, vessel obstruction and infection [15]. Because
both these methods are time-consuming, invasive and provide SaO 2 readings only at
the times at which the samples are drawn, non-invasive methods that provide continuous measurements were needed. Pulse oximetry is such a non-invasive method that
can measure arterial oxygen saturation continuously without extracting blood. Pulse
oximetry has now become the most popular method employed for the determination of SaO 2 . When SaO 2 is determined using the principle of pulse oximetry, it is
customary to indicate it as SpO 2 . In the method of pulse oximetry SpO 2 is computed
using a couple of photoplethysmographs [9].
3.6.3 Photoplethysmography
Hertzman pioneered Photoplethysmography, a non-invasive electro optic method,
that provides information on the blood volume changes at a test site on the body
[16]. A Photoplethysmogram (PPG) is the signal extracted from either the reflected
or transmitted light, obtained by illuminating a part of the body of interest. Reflective
type PPG can be obtained close to the skin on any part of the body. Transmission type
PPG can be obtained only from an accessible extremity such as earlobe or finger.
Obtaining a PPG employing the reflected light is already dealt with in Chaps. 1
and 2. To obtain a PPG through the transmitted light, a light source of wavelength λ
having a constant intensity I INλ is placed on one side of an extremity, say fingertip or
earlobe, and the transmitted light I oλ through the finger or earlobe is detected by a