Pulse oximetry

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Review pulse oximetry

Pulse oximetry is a noninvasive method by which arterial oxygenation can be approximated. It is based on the Beer-Lambert law and spectrophotometric analysis. When applied to pulse oximetry, the Beer-Lambert law essentially states that the intensity of transmitted light passing through a vascular bed decreases exponentially as a function of the concentration of the absorbing substances in that bed, and the distance from the source of the light to the detector.

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How does a pulse oximeter work?

A sensor is placed on either side of a pulsatile vascular bed, such as the fingertip or earlobe. The light-emitting diodes (LEDs) located on the opposite side of the sensor send out two wavelengths of light: one red (600–750 nm wavelength) and one infrared (850–1000 nm wavelength). These two wavelengths of light pass through the vascular bed to the sensor located on the other side where a photodetector measures the amount of red and infrared light received. Most pulse oximeters use wavelengths of 660 nm (red) and 940 nm (infrared).

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How is oxygen saturation determined?

A certain amount of red and infrared light is absorbed by the tissues (including blood) that are situated between the LEDs and photodetector. Therefore not all the light emitted makes it to the detector. Reduced (deoxygenated) hemoglobin absorbs much more of the red light (660 nm) than does oxygenated hemoglobin. Oxyhemoglobin absorbs more infrared light (940 nm) than does reduced hemoglobin. The photodetector measures the amount of light absorbed at each wavelength, which in turn allows the microprocessor to calculate a specific number (the SpO ₂ ) for the amount of deoxygenated and oxygenated hemoglobin present.

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How does the pulse oximeter determine the arterial hemoglobin saturation?

In the vascular bed being monitored, the amount of blood present constantly changes because of the pulsatile nature of blood flow. Thus the light beams pass not only through a relatively stable volume of bone, soft tissue, and venous blood, but also through arterial blood, which is made up of a nonpulsatile portion and a variable, pulsatile portion. By measuring transmitted light several hundred times per second, the pulse oximeter is able to distinguish the changing, pulsatile component (AC) of the arterial blood from the unchanging, static component of the signal (DC) comprised of the soft tissue, venous blood, and nonpulsatile arterial blood. The pulsatile component (AC), generally comprising 1% to 5% of the total signal, can then be isolated by canceling out the static components (DC) at each wavelength ( Fig. 20.1 ).

The photodetector relays this information to the microprocessor, which knows how much red and infrared light was emitted, how much has been detected, how much signal is static, and how much varies with pulsation. The microprocessor then sets up the red/infrared (R/IR) ratio for the pulsatile (AC) portion of the blood. The R and IR of this ratio is the total amount of absorbed light at each wavelength, respectively, for the pulsatile component of the arterial blood.

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What is the normalization procedure?

Normalization involves dividing the pulsatile (AC) component of the red and infrared plethysmogram by the corresponding nonpulsatile (DC) component. This scaling process results in a normalized R/IR ratio, which is virtually independent of the incident light intensity.

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How does the R/IR ratio relate to oxygen saturation?

The normalized R/IR ratio is compared with a preset algorithm that gives the percentage of oxygenated hemoglobin in the arterial blood (the oxygen saturation percentage). This algorithm is derived from volunteers, usually healthy individuals who have been desaturated to a level of 75% to 80%; their arterial blood gas is drawn, and saturation is measured in a standard laboratory format. Manufacturers keep their algorithms secret, but in general an R/IR ratio of 0.4 corresponds to a saturation of 100%, an R/IR ratio of 1.0 corresponds to a saturation of about 87%, and an R/IR ratio of 3.4 corresponds to a saturation of 0% ( Fig. 20.2 ).