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104 Algorithms and measuring principles
Conventional pulse oximetry assumes that all pulsations in the light
absorption are caused by the arterial pulse cycle. For this to work, the
venous blood in the sensor area must flow completely and therefore
constantly through the capillary bed. Conventional pulse oximetry
then calculates the ratio of the pulsatile to the mean absorption for
both wavelengths (660 nm and 940 nm).
The quotient of the two signals is then formed as follows:
R = S(660)/S(940)
The result R is used to read the corresponding SpO
2
value out of an
empirically calibrated table mapped in the oximeter software. These
tables were drawn up in trials with volunteers who underwent
temporary induced hypoxia. During these trials a conventional pulse
oximeter was used for the measurements; at the same time, arterial
blood was extracted and examined for its oxygen content.
Unlike conventional pulse oximeters, Masimo SET® pulse oximeters
assume that not only the arterial, also the venous blood flow varies
greatly. Changes to venous light absorption are regarded as a signifi-
cant source of interference to the pulse signal. The SpO
2
module
separates the signals for both wavelengths S(660) and S(940) into an
arterial signal S and a noise component N that are then used to
calculate the ratio R:
S(660) = S1 + N1
S(940) = S2 + N2
R = (S1 + N1) / (S2 + N2)
N1 and N2 are the noise components generated by venous blood.
The DST™ method (discrete saturation transform) isolates and there-
fore compensates for venous interference components.