User manual

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bound to hemoglobin, we identify it as oxyhemoglobin (oxy-Hb) or deoxyhemoglobin
(deoxy-Hb). The volume of oxygen transported in blood is thus directly dependent on the
amount of hemoglobin in the arterial blood. Besides oxy-Hb and deoxy-Hb, hemoglobin also
exists as carbaminohemoglobin (Hb-carbamate) with bound CO
2
and methemoglobin (met-
Hb), which is not capable of binding oxygen.
The individual states of hemoglobin have different physical qualities due to different
chemical bonds. That is why oxy-Hb has a different shape of the absorption spectrum curve
compared to deoxy-Hb (absorption of Hb-Carbamate and met-Hb may be neglected due to
their concentration in blood in physiological conditions). For deoxy-Hb it is true that it
absorbs red light more. On the contrary, oxy-Hb absorbs infrared radiation more. To find out
the oxy- and deoxy-hemoglobin concentrations, it is sufficient to use two radiation
wavelengths; the LED (light emitting diodes) with a wavelength of 660 nm and 940 nm are
usually used. Absorption curves of oxy-Hb and deoxy-Hb are depicted in Fig. 4.3.
Wavelength of 750 nm can be considered as a transition from red into infrared spectrum area.
The place in which the oxy-Hb and deoxy-Hb absorption curves intersect is called the
isobestic point.
600 800 900660 700 940750
λ (nm)
10
10
2
10
3
10
4
ε(λ)
(l·mol
-1
·m
-1
)
met-Hb
oxy-Hb
deoxy-Hb
Hb-karbamát
Fig. 4.3: Absorption spectra of the individual states of hemoglobin, loosely adapted from
[4.4].
To determine the O
2
saturation of arterial blood, we will thus measure the intensity of
transmitted radiation (light) for both the wavelengths. The transmitted radiation (light)
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