F-56 624074/07
F Pulse oximetry
Due to the difference in the absorption spectrum of the oxy-
and deoxyhaemoglobin by measuring the amount of light
passing through the measurement site at these two different
light wavelength and calculating the R/IR ratio it is possible to
asses an SpO
2
value using look-up tables (e.g. R/IR = 0.5 corre-
sponds to approximately SpO
2
= 100% and R/IR = 1.0 to
SpO
2
= 82%).
Light is absorbed by constant light absorbers such as skin,
tissue and venous blood, as well as by pulsating arterial blood.
For the calculation of SpO
2
, which is an approximation of arte-
rial oxygen saturation, the absorption caused by the constant
non-pulsating parts has to be removed. A plethysmogram is
the curve that visualizes the pulsating blood volume; it is deliv-
ered by the pulse oximeter (Figure F-24).
Figure F-24. Simultaneous pulse oximeter plethysmo-
gram and airway pressure recording with POPmax and
POPmin indicated on the plethysmogram
F.13 SpO
2
/FiO
2
For the diagnosis of ARDS and ALI, the PaO
2
/FiO
2
(P/F) ratio
index is used, where PaO
2
is the partial pressure of oxygen in
the arterial blood measured by arterial blood gas test, and FiO
2
is the fraction of inspired oxygen set on the ventilator. P/F is
used as a measure of blood hypoxia.
SpO
2
/FiO
2
(S/F) ratio is an approximation of the P/F, which in
contrast to P/F can be calculated noninvasively and continu-
ously. S/F ratio correlates well with the P/F ratio, in particular in
adults S/F ratios of 235 and 315, in children 201 and 263 corre-
spond to P/F ratios of 200 and 300. See the references in
Section F. 1 4.
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