Special Feature
A Primer on Pulse Oximetry in the ICU
By Doreen M. Anardi, RN
Pulse oximetry has been widely accepted as an accurate, reliable, and inexpensive way to monitor oxygenation non-invasively. In fact, practitioners have become so comfortable with the information it provides that it is used in a variety of patient care settings, including operating rooms, ICUs, EDs, and outpatient clinics; indeed, some consider it to be "the fifth vital sign."1 This role in patient assessment has been "assigned" before scrupulous scientific experience has clearly determined appropriate indications and limitations of use. Outcome studies have been slow to emerge following the early influx of studies that described its accuracy and reliability. Even without these studies, the value of continuous monitoring of oxygenation in high-risk patients is common sense to most clinicians. This global acceptance of the technique must be balanced by a discerning eye as it is applied to a particular patient.
This article briefly reviews the basic principles of pulse oximetry, the clinical and technical factors that may affect accuracy, the clinical research that has been done, and lists clinical settings requiring special measures or awareness in order to avoid misleading or erroneous data.
How Pulse Oximeters Work
The ability of pulse oximeters to measure arterial oxygen saturation non-invasively relies on two principles of light transmission and reception, spectrophotometry and photoplethysmography.2 Spectrophotometry is based on Beer’s law, where the concentration of a substance can be determined by the spectra of light it absorbs, and Lambert’s law, in which the optical density of a homogenous medium is directly proportional to its thickness.3 Oxygen bound to hemoglobin can be measured because the color and optical density change with the amount of oxygen bound to it.2
Photoplethysmography uses light transmission through vascular tissue to differentiate arterial and venous blood flow by measuring pressure waveforms throughout the cardiac cycle.2 In the case of pulse oximeter probes, the percentage of oxygenated hemoglobin in the blood is determined when the light source passes through the finger. Most of the light is absorbed by structures that do not vary during the cardiac cycle, such as skin, bone, connective tissue, and venous blood. With each heartbeat, there is an increase in arterial blood and in the amount of light absorbed.3 Since normally most of the blood pulsating through the vascular bed is arterial blood, the combination of photoplethysmography (the measurement of arterial pulsations) and spectrophotometry (the measurement of oxygen bound to hemoglobin), along with the technologic advances of microprocessors and light emitting diodes (LEDs), has made selective measurement of arterial oxygen saturation possible.2,3
Design Features and Accuracy
The basic design of today’s pulse oximeter consists of a probe that is attached across an area with good vascular flow that allows transmission of the light source to a sensor or photo detector that measures the intensity of the different wavelengths of light. Fingers, toes, earlobes, and the bridge of the nose are most commonly used. The light source consists of two or three LEDs that emit light at known wavelengths: One or two that emit red light and one that emitts infrared light. The red wavelengths are commonly used because the light absorption characteristics of oxyhemoglobin and reduced (deoxygenated) hemoglobin are quite different.3 The infrared wavelength is similarly absorbed by both oxygenated and reduced hemoglobin, and so serves as a constant for comparison to the red light. The oxygen saturation is calculated by comparing the peak and trough absorption ratio of the red light to the peak and trough absorption ratio of the infrared light.
Pulse oximeter manufacturers formulated a calibration curve algorithm based on experimental data from healthy, non-smoking men breathing a variety of oxygen mixtures to obtain saturations from 70-100% that were verified by the gold standard, lab CO-oximetry.3 Pulse oximeters are accurate and precise in this range, but may not be reliable at lower oxygen saturations.3 Studies indicate that the 95% confidence limit of most pulse oximeters is ± 4%,2,4,5 (i.e., for a pulse oximeter reading of 94%, the true saturation could actually be 90% or 98%).
What Pulse Oximeters Actually Measure
The number of wavelengths of light emitted by a pulse oximeter determine the number of hemoglobin derivatives measured. Pulse oximeters measure "functional saturation" or the percentage of oxyhemoglobin compared to the sum of reduced hemoglobin plus oxyhemoglobin.3 The CO-oximeter in the lab can accurately measure oxyhemoglobin, reduced hemoglobin, carboxyhemoglobin, and methemoglobin, providing the clinician with the "fractional" oxyhemoglobin saturation.3 This is an important concept in patients who may have elevated levels of carboxyhemoglobin, fetal hemoglobin, or methemoglobin. To emphasize this concept, saturation derived from the pulse oximeter is more correctly abbreviated "SpO2" while arterial saturation determined from the CO-oximeter is abbreviated "SaO2."
Pulse Oximeter Algorithms and Resonse Times
Pulse oximeter algorithms use data smoothing schemes, averaging oxygen saturation readings over several heartbeats to avoid displaying the normal small pulse-to-pulse variation in measured oxygen saturation.2 These data smoothing schemes, the patient’s own circulation time, as well as the location of the pulse oximeter probe, affects response time and display of actual desaturation events. A number of investigators have measured average response time to desaturation events from simultaneously placed sensors: for ear sensors the time was 10-20 seconds, for finger sensors the time was 24-35 seconds, and for toe sensors the time was 57 seconds.2
The oximeter’s algorithm uses several artifact rejection schemes to avoid the display of false values of oxygen saturation from artifactual pulse waveforms, such as from motion or venous pulsations.2 Some older models of pulse oximeters search for a nonexistent pulse, increasing output from the LED and reception from the sensor. Actual pulsatile fluctuations can be produced by this activity.2
Technical Limitations of Pulse Oximetry: Practical Pointers
A number of patient and other factors can affect the clinical accuracy of pulse oximetry in the ICU, including:
Carboxyhemoglobin is primarily seen as oxyhemoglobin by the pulse oximeter, although it has absorption characteristics similar to both oxyhemoglobin and reduced hemoglobin at the wavelengths used. The displayed SpO2 will overestimate SaO2 approximately by the percentage of carboxyhemoglobin present.2,3 Pulse oximetry should be used with caution in patients with smoke inhalation or in other settings in which significant carboxyhemoglobinemia may be present.
Methemoglobin also interferes with accurate SpO2 readings. It has significant absorption at both wavelengths emitted by pulse oximeters, making the absorbance ratio almost 1. In the calibration curve used by most oximeters, an absorbance ratio of 1 corresponds to a saturation of 83-87%. The displayed pulse oximeter reading of 85% can be an overestimate or underestimate of the patient’s actual SaO2.2 This fixed SpO2 reading can be a diagnostic clue in the appropriate clinical setting. Methemoglobin is formed when iron in hemoglobin is oxidized from the ferrous to the ferric state. Increased levels can occur when normal intracellular mechanisms fail to prevent accumulation. Significant methemoglobin levels can occur congenitally or in some patients due to certain drugs. Methemoglobinemia has been induced by local anesthetics (lidocaine, benzocaine), nitrates (including topical silver nitrate), nitrites, metoclopramide, sulfa drugs, and edetic acid (EDTA). The lab CO-oximeter should be used to determine the true SaO2.3
Fetal hemoglobin does not affect the accuracy of pulse oximetry, although it may lead to erroneous CO-oximeter readings because of its similar absorption characteristics to carboxyhemoglobin. When fetal hemoglobin levels are high, CO-oximetry should not be used to verify pulse oximetry.
Intravascular dyes can interfere with accuracy of SpO2 readings. Methylene blue, the treatment for methemoglobinemia, absorbs light similar to the absorption of reduced hemoglobin, so it can artificially lower SpO2 readings. Other dyes such as indocyanine green and indigo carmine can affect readings to a lessor degree.
Hyperbilirubinemia can affect pulse oximeter readings when the serum bilirubin level is very high (e.g., more than 20 mg/dL). Available data on this effect are contradictory, and lab CO-oximeter measurement may be warranted in an icteric patient if there is any question about the accuracy of the indicated SpO2.2,3
Oxyhemoglobin Dissociation Curve
Effects of temperature, PCO2, and pH on the relationship between PO2 and saturation as indicated by pulse oximetry.
Fingernail polish, especially in blue or black tones, has been reported to interfere with SpO2 readings, either due to color interference or shunting of the light source away from the vascular bed.2 Nail polish should be removed from the finger or toe to which the sensor is applied.
Low perfusion pressure due to hypothermia, hypovolemia, hypotension, peripheral vascular disease, or vasopressor infusion may interfere with interpretation of the true signal from the background noise.2,3 Many oximeters will display an "inadequate pulse" message. Repositioning the probe may be helpful; the nasal septal anterior ethymoid artery is supplied by the internal carotid and may have a better pulsatile signal.3 Topical vasodilator ointment or warming the extremity may improve the signal.2,3 The earlobe is less affected by vasoconstriction than the fingers or toes.2
Increased venous pressure with venous pulsations due to right heart failure, tricuspid regurgitation, and blood pressure cuff above the probe can lead to erroneous SpO2 readings, because the pulsations of the venous blood will be interpreted as arterial and the saturation will be averaged with the arterial blood.3 Severe anemia with hemoglobin levels less than 5 g/dL may affect readings of SpO2; there is variability from patient to patient so lab CO-oximeter readings are warranted. The CO-oximeter reading will also report oxygen content so that the patients oxygen carrying capacity can be appropriately addressed.3
Excessive ambient light and external light sources can interfere with accurate SpO2 readings. Fluorescent lights, operating room lights, infrared heat lamps, and direct sunlight have been reported to alter SpO2 readings.2,3 The probe should be draped or shielded if light interference is suspected.2,3
Dark skin pigmentation has been reported by some investigators as causing difficulties with SpO2 measurements. Investigators from one study were unable to obtain a reading or had a poor signal strength message in 18% of darkly pigmented patients.2 Another study found poorer correlation of SpO2 and SaO2 in black patients compared to white patients.2,3 Some investigators suggest that optical shunting may be responsible for these occurrences. The transmitted light follows the path of least resistance and may reach the sensor without being exposed to the appropriate vascular bed.2 The calibration curves used to calculate SpO2 may have been derived from predominantly white volunteers and may not be accurate for more darkly pigmented patients.6
Relationship of SaO2 and PaO2
Because the purpose of SpO2 measurement is to infer SaO2 and PaO2, the relationship of these as demonstrated by the oxyhemoglobin dissociation curve must be taken into account when interpreting trends in SpO2. (See Figure.) Factors that shift the curve, such as changes in temperature, pH, and PCO2, will affect what the PaO2 is for that SaO2 and how readily oxygen is released to the tissues.2,3 Verification of correlation of SaO2 to SpO2 on some regular basis is necessary, as is the periodic determination of pH and PaCO2 for the clinician to assess tissue oxygenation.
The oxyhemoglobin dissociation curve also demonstrates why pulse oximetry is not suited to assessing hyperoxia or to prevent oxygen toxicity. At the upper flat portion of the curve and when hemoglobin is fully saturated, saturation does not change with increasing PaO2.
Clinical Applications
Pulse oximetry has become the standard of care in the operating room and post-anesthesia recovery room, where the occurrence of hypoxic events is not limited to high-risk patients or to certain surgical procedures, and the time it takes to detect hypoxemia is significantly shortened.3 A study done in the emergency department to identify changes in management based on triage pulse oximetry values, demonstrated that clinicians have more difficulty detecting mild-to-moderate degrees of hypoxemia in patients.7 Pulse oximetry has been used to document unsuspected hypoxemia in critically ill patients during transport and during invasive or diagnostic procedures.3
In a homogenous population of post-operative cardiac surgical patients, a study of pulse oximetry in post-operative care demonstrated a significant reduction of blood gas analyses in the patients weaned from mechanical ventilation using SpO2 data available at the bedside.5 In the other study group, where the SpO2 data were not displayed at the bedside, almost half of the patients had significant desaturations that were not clinically apparent. However, there was no difference in length of mechanical ventilation or ICU stay.
A before- and after- pulse oximetry study in a multidisciplinary ICU demonstrated a marginal overall decrease in blood gas determinations.8 The reduction was primarily in surgical patients and only on the night shift. The authors did not establish guidelines for pulse oximetry use, but suggested that standardized guidelines could greatly improve efficiency of care.8
The reliability of pulse oximetry in titrating oxygen in ventilator dependent patients was evaluated along with a survey of medical directors of how pulse oximetry was used in 25 hospitals.6 The major findings of this study were that the optimal SpO2 target value was dependent on the patient’s skin color, a SpO2 value of 92% was equivalent to a SpO2 value of 95% in black patients. Inaccurate oximetry readings were more common in black patients and overall accuracy in all patients decreased when SaO2 was less than 90%.6
Several ventilator management research protocols in patients with ARDS are in progress that use pulse oximetry to make patient care decisions. At least one of the protocols includes a decision algorithm for assessing the reliability of SpO2 in that patient.
Conclusion
Pulse oximetry is ubiquitous and utilitarian but it is not unconditional. Devices display a SpO2 value that is ± 4%; actual SaO2 may not correlate reliably for that patient and PaO2 has a large variability for a given saturation. Further, SpO2 doesn’t reflect tissue oxygenation or alterations in ventilation. A variety of technical and clinical factors affect the reliability of pulse oximetry in a particular patient. Most articles about pulse oximetry have concluded that more studies are needed to determine optimal use. That remains the case.
References
1. Neff TA. Chest 1988;94:227.
2. McCarthy K, et al. Pulse Oximetry. In: Kacmarek RM, et al. Eds. Monitoring in Respiratory Care. St. Louis: C V Mosby; 1993:309-346.
3. Schnapp LM, Cohen NH. Chest 1990;98:1244-1250.
4. Tobin MJ. Am Rev Respir Dis 1988;138:1625-1642.
5. Bierman MI, et al. Chest 1992;102:1367-1370.
6. Jubran A, Tobin MJ. Chest 1990;97:1420-425.
7. Mower WR, et al. Chest 1995;108:1297-1302.
8. Inman KJ, et al. Chest 1993;104:542-546.
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