The fundamental physical property that allows the measurement of arterial oxygen saturation by pulse oximetry is that blood changes color with saturation. A pulse oximeter measures the 'color' of the arterial blood and correlates this 'color' with a given oxygen saturation which is then displayed. Hemoglobin, in its reduced or oxygenated state, strongly absorbs light at all wavelengths below approximately 630 nm which includes the entire visible spectrum apart from the red region. This is why the light from a flashlight placed against one side of the hand appears red when viewed from the other side. When blood is well oxygenated it does not absorb much red light, but as it desaturates it absorbs an increasing amount so that the blood takes on a darker appearance. The opposite behavior occurs in the near-infrared region (about 810-1000 nm) where hemoglobin absorbs more light when saturated with oxygen than when it is desaturated. For this reason, current pulse oximeters use two emitters, usually light-emitting diodes; one is designed to generate light in the red region, typically centered around 660 nm, and one which generates light in the near-infrared region, usually centered at 925 or 940 nm.
The saturation of a suspension of pure hemoglobin in a cuvette can be determined by measuring the ratio of light absorbed at a red wavelength (say 660 nm) to that absorbed at an infrared wavelength (say 940 nm); this ratio A660/A940 will have a one-to-one correlation with oxygen saturation. The absorption of light, at any given wavelength, is determined by directing a beam of light with a known intensity (the incident intensity) on the hemoglobin solution and measuring its intensity after passage through the solution (the transmitted intensity); absorption is defined as the ratio of the incident intensity to the transmitted intensity. Measurement in vivo is not quite so simple. In the body, there is both venous and arterial blood as well as bone and other tissue. Further, the hemoglobin is not homogeneously distributed throughout the blood but confined in red blood cells that are highly effective light scatterers, making it almost impossible to collect all the light that is not absorbed by the sample. These facts make direct measurement of the absorption of the arterial blood quite difficult.
How is it possible to measure the absorption of arterial blood only, so that arterial oxygen saturation can be measured accurately? Instead of measuring the ratio of absorptions, the solution is to measure the ratio of differential absorptions ( Fig 1), or at least an approximation thereof, which is defined as follows:
Fig. 1 The light generated by each emitter is modulated by the arterial pulsation as it passes through the tissue. The differential absorption is calculated by dividing the small change in intensity by the total intensity of the output light.
where dA is the differential change in absorption and D A is the physical measurement made to approximate the differential change in absorption.
Without going through the mathematics involved, suffice it to say that measuring a differential change in absorption rather than absorption itself simplifies the problem significantly. It is no longer necessary to know the incident intensity, so that instrument design is substantially simplified and the effect of scattering is minimized. The measurement now involves only the instantaneous change in absorption which in living tissue is mainly due to the change in pathlength of the arterial blood; absorption in venous blood, skin, and bone is almost negligible. The fact that differential rather than direct absorption is measured is what makes pulse oximetry possible.
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