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Pulse oximetry technique is a non invasive method useful to monitor the quantity of oxygen in hemoglobin, used in
medical diagnosis and clinical decision-making. It is based on the ratio between the red and infrared light absorbance
corresponding to the oxygenated (HbO2) and non-oxygenated (Hb) state of the hemoglobin. We develop the
mathematical model to obtain the oxygen saturation value observing that it depends on four known parameters: two
transillumination values assessed at the common pulse oximetry wavelengths (λ1 = 660 nm y λ2 = 940 nm), and the
extinction coefficients for the oxy- and deoxy-hemoglobin at these given wavelengths.
Analyzing the extinction curves for oxy- and deoxy-hemoglobin we note that at λ equal to 660 nm the HbO2 component
almost does not contribute to the attenuation of incidance when we transilluminate tissue (7.479x10-5 cm-1M-1). In this
case is the Hb component that gives the significant attenuation value (7.863x10-4 cm-1M-1). In 940 nm the extinction
coefficient of the Hb is 2.589x10-5 cm-1M-1 and we can ignore it when we count attenuation. At this λ we assume that the
pulsate component is only affected by the HbO2 (2.099x10-4 cm-1M-1). This analysis of hemoglobin extinction
coefficients in the absorption curves highlights the signal to noise ratio between these two oxygen dependent elements.
We are interested in accentuate the better contrast interval (λ pair), where this signal-to-noise ratio is higher, looking for
more transillumination information and more precise SO2 value.
We propose to use a transillumination waveform simulator to study the different effects (respiration, artifact body
movement, absorption, low perfusion, etc) presented in complex physiological signals and to know the optical path-integrated
behavior when we transilluminate tissue. This is practical for acquisition and processing transillumination
signals. The present work is the first part of a λ selection method to guaratee the optimum S/N for measurements in blood
using pulse oximetry and spectroscopic techniques at near infrared.
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