23: Pulse Oximetry

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CHAPTER 23 Pulse Oximetry

Lyle E. Kirson, DDS, Renee Koltes-Edwards, MD

1 What is 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.

2 How important is pulse oximetry?

Physiologic monitors provide the anesthesiologist information that must be integrated into the overall clinical picture. Every anesthesiologist should know the limits of the various physiologic monitors, and every monitor that is used in the operating room will, under the right conditions, give false readings. Pulse oximeters are no exception, and the clinician should have a good idea as to when the readings may be spurious.

3 What are transmission pulse oximetry and reflection pulse oximetry?

As the names suggest, in transmission pulse oximetry light-emitting diodes (LEDs) transmit light through a vascular bed to a photodetector. In reflection pulse oximetry LEDs transmit light that is reflected back to a photodetector on the same side of the vascular bed as the LEDs. Transmission pulse oximetry is the most common form of pulse oximetry used in the operating room, although reflective pulse oximetry is becoming more popular because of the improved accuracy and ease of placement over various vascular beds. Transmission pulse oximetry is the subject of this chapter unless otherwise noted.

4 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 LEDs on one side of the sensor send out two wavelengths of light, one red (600 to 750 nm wavelength) and one infrared (850 to 1000 nm wavelength). Most pulse oximeters use wavelengths of 660 nm (red) and 940 nm (infrared). The two wavelengths of light pass through the vascular bed to the other side of the sensor where a photodetector measures the amount of red and infrared light received.

5 How is oxygen saturation determined from the amount of red and infrared light received and absorbed?

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

6 How does the pulse oximeter determine the degree of arterial hemoglobin saturation?

In the vascular bed being monitored, the amount of blood is constantly changing because of the pulsation caused by each heart beat. 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) made up 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 (Figure 23-1).

Figure 23-1 Transmitted light passes through pulsatile arterial blood (AC) and other tissues (DC). The pulse oximeter can distinguish the AC from the DC portion by measuring transmitted light several hundred times per second.

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

7 What is the normalization procedure?

Because the transmitted light intensities depend on the sensitivity of the detector and the individual intensities of the light sources and because tissue absorption can vary a great deal between individuals, a normalization procedure is commonly used. The normalization involves dividing the pulsatile (AC) component of the red and infrared photoplethysmogram by the corresponding nonpulsatile (DC) component of the photoplethysmogram. This scaling process results in a normalized R/IR ratio, which is virtually independent of the incident light intensity.

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

The normalized R/IR ratio is compared to a preset algorithm that gives the microprocessor the percentage of oxygenated hemoglobin in the arterial blood (the oxygen saturation percentage), and this percentage is displayed. 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 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% (Figure 23-2).

Figure 23-2 The ratio of absorbed red to infrared light corresponds to the appropriate percentage of oxygenated hemoglobin.

9 What is the oxyhemoglobin dissociation curve?

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