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).

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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.

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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).

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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?

It is a curve that describes the relationship between oxygen tension and binding (percentage oxygen saturation of hemoglobin) (Figure 23-3). Efficient oxygen transport relies on the ability of hemoglobin to reversibly load and unload oxygen. The sigmoid shape of the curve facilitates unloading of oxygen in the peripheral tissues, where the PaO2 is low. At the capillary level a large amount of oxygen is released from the hemoglobin, resulting in a relatively small drop in tension. This allows an adequate gradient for diffusion of oxygen into the cells and limits the degree of hemoglobin desaturation. The curve may be shifted to the left or right by many variables (Table 23-1).

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Figure 23-3 Oxyhemoglobin dissociation curve describes the nonlinear relationship between PaO2 and percentage saturation of hemoglobin with oxygen (SaO2). In the steep part of the curve (50% region), small changes in PaO2 result in large changes in SaO2.

TABLE 23-1 Left and Right Shifts of the Oxyhemoglobin Dissociation Curve

Right Shift Left Shift
Effects:

Decreased affinity of Hb for O2 (facilitates unloading of O2 to tissues)

Effects:

Increased affinity of Hb for O2 (decreases unloading of O2 from Hb)

Causes:

Increased PCO2
Hyperthermia
Acidosis
Increased altitude
Increased 2, 3-DPG
Sickle cell anemia
Causes:

Decreased PCO2
Hypothermia
Alkalosis
Fetal hemoglobin
Decreased 2, 3-DPG
Carboxyhemoglobin
Methemoglobin

Hb = hemoglobin, 2, 3-DPG = 2, 3-diphosphoglycerate.

10 Why might the pulse oximeter give a false reading? Part 1—not R/IR related

image The algorithm that the pulse oximeter uses to determine the saturation loses significant accuracy as saturation drops below 80% because, as mentioned, healthy volunteers have historically been used to establish these algorithms, and it would be unethical to desaturate someone below 80%. Therefore the percentage saturation below 75% to 80% is extrapolated from these data. As the saturation lowers, it becomes less accurate.
image Saturation is averaged over a time period of anywhere from 5 to 20 seconds. As a patient is desaturating, the reading on the monitor screen will be higher than the actual saturation. This becomes critical as the patient enters the steep part of the oxyhemoglobin desaturation curve because the degree of desaturation increases dramatically and may exceed the ability of the monitor to change rapidly enough to show the true level of oxygen saturation. Likewise, as a person’s saturation increases, the displayed reading on the screen will be lower than the actual saturation.
image Dark pigmentation of the skin may cause overestimates of oxygen saturation.
image Response time to changes in saturation is related to probe location. Response time is less with ear probes and greater with finger probes and may increase with hypoxia and vasoconstriction.
image If the monitor is not able to detect a good pulse during a period of desaturation, it may display an artificially high saturation reading. This is so because when the pulse oximeter cannot find a good pulsation, the prior saturation reading may freeze on the screen while the monitor continues to search for a pulse. Once that patient enters the steep part of the oxyhemoglobin dissociation curve, the difference in the displayed reading and the true saturation can be significant.
image Venous congestion, particularly when caused by tricuspid regurgitation, may produce venous pulsations that are interpreted as a signal. This will result in an artificially low reading as venous blood is interpreted to be arterial blood.
image Anemia, hypotension; poor perfusion at the site of measurement; and nail polish, especially blue or black polish, also lead to false readings.

11 Why might the pulse oximeter give a false reading? Part 2—R/IR related

As previously mentioned, the R/IR ratio determines the displayed saturation. Any circumstance that erroneously drives the R/IR number toward 1.0 will result in a saturation reading approaching 87%. The overwhelming majority of times, these circumstances develop in well-oxygenated patients and results in a falsely low displayed saturation. Of greater consequence, however, is when these circumstances occur in dangerous situations when the patient’s oxygenation is critically low and a falsely elevated saturation reading results. What can affect the R/IR number?

image Optical shunting takes place when some of the light from the LED does not pass through the vascular bed before reaching the detector and is a result of poor, incomplete probe placement. The R/IR number trends toward 1.0 (87%) and results in a falsely low measurement for a patient who is actually better saturated. However, if it occurs in a patient who is difficult to ventilate and is desaturating, falsely high measurement will be displayed regardless of how hypoxic the patient becomes.
image The penumbra effect, similar to optical shunting, occurs when the probe is not positioned appropriately and the path length between each of the LEDs and the photodetector are not equal, causing a relative distortion in the absorption reading. The R/IR ratio is driven toward 1.0.
image Motion artifact causes a low signal-to-noise ratio, alters the absorption detection of both the red and infrared light by the photodetector, drives the R/IR ratio toward 1.0, and results in false saturation readings.
image If the probe does not fit well on the finger or ear, fluorescent lighting and operating room lights, because of their phased light production (which is too fast for the human eye to detect) can cause false R/IR readings. When ambient light from either of the above sources is not eliminated, it is detected in both the red and infrared range and fools the microprocessor. As this occurs, the R/IR number is driven toward 1.0 (87% saturation). To minimize false readings from outside lights, pulse oximeters work in three phases, hundreds of times per second. During the first phase a wavelength of 660 nm is emitted, during the second phase a wavelength of 940 nm is emitted, and during the third phase no wavelength is emitted. This third phase allows the photodetector to measure ambient light contamination. The microprocessor then discards the ambient light contamination from its measurements. However, this solution to the ambient light problem is not totally effective; this is why manufacturers recommend placing opaque material over the oximeter probe.
image Dyshemoglobinemias carboxyhemoglobin (COHb) and methemoglobin (MetHb) pose a problem for accurate oxygen saturation measurement. At 660 nm, COHb absorbs light, much like oxygenated hemoglobin, causing an overestimation of true saturation. The influence of methemoglobinemia on SpO2 readings is more complicated. MetHb looks much like reduced Hb at 660 nm. However, more important, at 940 nm the absorbance of MetHb is markedly greater than that of either reduced or oxygenated Hb. Therefore the monitor reads it as absorption of both species, driving the R/IR number toward 1.0 and the saturation toward 87%. Therefore at a high SaO2 level the probe underestimates the true value; at a low SaO2 the value is falsely elevated.

12 The saturation plummets after injection of methylene blue. Is the patient desaturating?

Because of changes in light absorption (and alterations in the R/IR ratio), methylene blue fools the pulse oximeter into thinking that more reduced hemoglobin is present. The apparent, but false, SpO2 reading returns to normal within a few minutes.

13 Explain the difference between functional hemoglobin and fractional hemoglobin saturation

Functional hemoglobin saturation is the ratio of oxyhemoglobin to all the hemoglobin capable of carrying oxygen. Fractional hemoglobin saturation is the ratio of oxyhemoglobin to all hemoglobin, including dyshemoglobins (Hb + MetHb + COHb).

image

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14 Do pulse oximeters measure functional hemoglobin saturation or fractional hemoglobin saturation?

In the past pulse oximeters measured functional hemoglobin only because there was no method for detecting the dyshemoglobinemias with operating room oximeters. This is no longer true. There are now pulse oximeters that will provide readings for COHB, MetHb, and the normal hemoglobin species. Accomplishing this requires use of four LEDs and corresponding photodetectors.

15 Since the patient is oxygenated before anesthetic induction, if the pulse oximeter reaches 100%, does this indicate complete denitrogenation?

Replacing all alveolar nitrogen with oxygen provides a depot of oxygen that might be necessary if mask ventilation or intubation proves difficult. Hemoglobin may be completely saturated with oxygen before total pulmonary washout of nitrogen. Thus a reading of 100% in and of itself is not an accurate indication of denitrogenation.

16 Is the pulse oximeter a good indicator of ventilation?

A pulse oximeter gives no indication of ventilation, only of oxygenation. For instance, in the postanesthetic care unit a patient can have an oxygen mask delivering 50% or more oxygen and an SpO2 reading in the 90s, yet be hypoventilating and hypercapnic. In this situation the pulse oximeter gives a false sense of security. A better approach would be to administer less oxygen; and, as the pulse oximeter values decrease below 90, it would alert the nurse to tend to the patient. Arousing the patient from sleep, encouraging him or her to breathe deeply, and elevating the head of their bed are better strategies than just increasing the delivered oxygen concentration. Of course these interventions may not improve the situation considerably, but then the caregiver knows to pursue other reasons why the patient is not oxygenating. Always treat the cause, not the symptom; treat the patient, not the numbers.

17 Are there complications associated with the use of pulse oximetry probes?

Pressure necrosis of the skin has been reported in both neonates and adults when the probe has been left on the same digit for prolonged periods of time. Digital burns from the LEDs have been reported in patients undergoing photodynamic therapy.

KEY POINTS: Pulse Oximetry image

1. Use of pulse oximetry has allowed the anesthesiologist to rapidly detect and treat acute decreases in oxygen saturation.
2. As with all monitors, understanding its methods of operation and its limitations is critical to the delivery of safe care. Pulse oximeters can give falsely high and low numbers; understand the reasons why that is so.
3. Treat the patient, not the symptom! Oxygenation and ventilation are separate processes, and pulse oximetry does not assess adequacy of ventilation.

Website image

http://www.asa-abstracts.com

SUGGESTED READINGS

1. Barker S. Motion-resistant pulse oximetry: a comparison of new and old models. Anesth Analg. 2002;95:967-972.

2. Moyle J. Pulse oximetry, ed 2. London: BMJ Publishing Group, 2002.

3. Pedersen T., Moller A., Pedersen B. Pulse oximetry for perioperative monitoring: systematic review of randomized, controlled trials. Anesth Analg. 2003;96:426-431.

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