3 Pulse Oximetry for the Measurement of Oxygen Saturation …
55
The functional oxygen saturation in arterial blood is a measure of how much
oxygen the arterial blood is carrying as a percentage of the maximum it could carry.
If N Hb is the concentration (in molL
−1 ) of haemoglobin without oxygen (reduced
haemoglobin, Hb) and N Hb represents the concentration of oxygenated haemoglobin
(oxyhaemoglobin, HbO) in arterial blood, then
% SaO 2 =
N HbO
N Hb + N HbO
100 %
(3.1)
Functional oxygen saturation (% SaO 2 ) in Eq. (3.1) is derived assuming the
concentrations of dysfunctional haemoglobins, namely, carboxy-haemoglobin and
methaemoglobin in arterial blood are negligible [11]. If both functional and dysfunctional haemoglobin are present in the arterial blood, then oxygen saturation is
expressed as a fractional oxygen saturation and is given by.
% Fractional SaO 2 =
N HbO
N Hb + N HbO + N M Hb + N C O Hb
100 %
(3.2)
Here N C O Hb and N M Hb are the concentrations of carboxyhaemoglobin and
methaemoglobin. Rearranging Eq. (3.1) results in
% SaO 2 =
N HbO /N Hb
1 + N HbO /N Hb
100 =
Q
1 + Q
100
(3.3)
Where Q = N HbO /N Hb , the ratio of oxyhaemoglobin to reduced haemoglobin.
Under normal physiological conditions, a healthy, non-smoking person should have
arterial oxygen saturation (% SaO 2 ) between 94 and 100%. If % SaO 2 goes below
90% life-threatening complications may arise. Saturations lower than 90% may
be caused by chronic obstructive pulmonary disease (COPD), excessive bleeding,
smoking and malfunctioning blood vessels, especially capillaries.
Once oxygen is supplied to various parts of the body, the returning blood in the
veins will be depleted of oxygen. The functional oxygen saturation of venous blood
(SvO 2 ) is about 75% [12].
3.6.1 Measurement of Oxygen Saturation (Oximetry)
Three methods of oximetry are now in clinical use [13]:
(i) Invasive (in vitro) Co-Oximetry and arterial blood gas (ABG) analysis.
(ii) Invasive fibre optic based oximetry to determine oxygen saturation in arterial
flow, mixed arterial-venous flow or intra cardiac flow.
(iii) Non-invasive pulse oximetry to monitor arterial oxygen saturation at any part
of the body.
55
The functional oxygen saturation in arterial blood is a measure of how much
oxygen the arterial blood is carrying as a percentage of the maximum it could carry.
If N Hb is the concentration (in molL
−1 ) of haemoglobin without oxygen (reduced
haemoglobin, Hb) and N Hb represents the concentration of oxygenated haemoglobin
(oxyhaemoglobin, HbO) in arterial blood, then
% SaO 2 =
N HbO
N Hb + N HbO
100 %
(3.1)
Functional oxygen saturation (% SaO 2 ) in Eq. (3.1) is derived assuming the
concentrations of dysfunctional haemoglobins, namely, carboxy-haemoglobin and
methaemoglobin in arterial blood are negligible [11]. If both functional and dysfunctional haemoglobin are present in the arterial blood, then oxygen saturation is
expressed as a fractional oxygen saturation and is given by.
% Fractional SaO 2 =
N HbO
N Hb + N HbO + N M Hb + N C O Hb
100 %
(3.2)
Here N C O Hb and N M Hb are the concentrations of carboxyhaemoglobin and
methaemoglobin. Rearranging Eq. (3.1) results in
% SaO 2 =
N HbO /N Hb
1 + N HbO /N Hb
100 =
Q
1 + Q
100
(3.3)
Where Q = N HbO /N Hb , the ratio of oxyhaemoglobin to reduced haemoglobin.
Under normal physiological conditions, a healthy, non-smoking person should have
arterial oxygen saturation (% SaO 2 ) between 94 and 100%. If % SaO 2 goes below
90% life-threatening complications may arise. Saturations lower than 90% may
be caused by chronic obstructive pulmonary disease (COPD), excessive bleeding,
smoking and malfunctioning blood vessels, especially capillaries.
Once oxygen is supplied to various parts of the body, the returning blood in the
veins will be depleted of oxygen. The functional oxygen saturation of venous blood
(SvO 2 ) is about 75% [12].
3.6.1 Measurement of Oxygen Saturation (Oximetry)
Three methods of oximetry are now in clinical use [13]:
(i) Invasive (in vitro) Co-Oximetry and arterial blood gas (ABG) analysis.
(ii) Invasive fibre optic based oximetry to determine oxygen saturation in arterial
flow, mixed arterial-venous flow or intra cardiac flow.
(iii) Non-invasive pulse oximetry to monitor arterial oxygen saturation at any part
of the body.