Arterial blood gases (ABG) are the gold standard for assessing oxyhemoglobin desaturation and acid-base status. Arterial blood gas analysis calls for collection of a high-quality arterial specimen via an arterial puncture and collection of a contamination-free sample. The arterial blood is then analyzed using a calibrated blood gas analyzer, in duplicate, to quantitate the partial pressure for oxygen (PaO2), carbon dioxide (PacO2), and acidity (pH). Additionally, another small quantity of arterial blood is injected into a co-oximeter for determination of arterial oxygen saturation (SaO2), hemoglobin (Hb), and other hemoglobin species (COHb, MetHb).¹ From this analysis, the clinician is able to derive an overall picture of the patient's oxygen and acid-base status and render appropriate care.

During the past 30 years, noninvasive pulse oximetry has emerged as an alternative for assessing and continually monitoring arterial oxygen saturation (SpO2) in infants through adults.^2-4 The application of the noninvasive SpO2 measurement has expanded in clinical practice and is commonplace in hospitals, surgical suites, clinics, emergency areas, rehabilitation facilities, and other areas where there is a need to evaluate oxygen saturation.

Noninvasive oximetry is not perfect, and many studies have addressed some of the shortcomings. Factors impacting on oximetry performance and an accurate and reliable reading include motion artifact, poor tissue perfusion, ambient light, pigmentation differences, as well as electromagnetic interference.^5-8 While technologic and engineering advances have addressed these known shortcomings, no definitive solution has been derived to date to ensure an accurate SpO2 reading for any individual under any condition. This reflects the complexity of the physiology and broad application of the instrumentation across the age and ethnic diversity range. Perhaps the most important determinant for a high-quality measurement is the clinical understanding possessed by the individual using oximetry and careful consideration of its limitations and drawbacks.⁹

Notwithstanding, many technologic improvements have been made and have assisted clinical medicine in establishing pulse oximetry as an efficient instrument for evaluation of SpO2 for a diverse patient population.^10,12 Sensor technologies and advanced algorithms have not been able to completely remove the impact of artifact and potential spurious measurements, but they have greatly improved the accuracy and reliability of the measurement.^13-16

Oximetry uses the Lambert-Beer law, which describes the relationship between a colored substance, the length of the path on which light can pass through it, and the corresponding light absorbed by the substance.¹⁷ Clinical oximeters (transmission) use a light-emitting diode to emit two different wavelengths of light—red and infrared—and a photodiode light detector to measure the amount of light passing through the arterial bed. Arterial oxygen saturation, specifically SpO2, is estimated using various algorithms because the light-absorbing characteristics of hemoglobin differ between oxyhemoglobin and deoxyhemoglobin. Typical measurement sites for adults are the finger, toe, pinna or lobe of the ear, or forehead; for infants, they are the foot, palm of the hand, great toe, or thumb.

One troublesome area persists, and that is the signal-to-noise ratio induced as a result of low perfusion or motion artifact.^5,18 Low perfusion depresses the arterial pulse and pools blood while motion adds pulsatility to nonarterial blood, and for pulse oximeters, this pulsed blood (venous blood) is averaged with the red/infrared ratio generated by the pulsed arterial blood.¹⁹ Factors contributing to the measured SpO2 value include the venous blood saturation, the magnitude of motion artifact, and the strength of the arterial signal. When motion artifact is present, the instrument will return a lower arterial oxygen saturation reading. To obviate low perfusion inaccuracy and the impact of movement artifact on the measurement, two technologic approaches have been advanced: sophisticated signal processing and new sensor designs.²⁰ Each has improved the accuracy of pulse oximetry and has helped to eliminate dropped signals. Newer sensor technologies use reflectance rather than transmission to measure SpO2.²¹ With reflectance oximetry, the emitted light is backscattered rather than transmitted, thereby allowing the sensor to be placed on many anatomic locations throughout the body.²² This technology overcomes much of the artifact induced by low perfusion and motion or low pulse amplitude (eg, hypothermia, hypotension, or shock to mention a few) affecting the reading as commonly observed with measurement obtained from the extremities.^23-25 Studies suggest that the forehead is a suitable location for sensor placement because forehead blood flow is minimally affected by perfusion shifts and pulse amplitude changes in cold environments.^26,27 This technology was first introduced to the market in 1995 and used patented signal extraction pulse oximetry technologies and advanced signal processing algorithms to separate the arterial signal from nonarterial noise.²⁸

Oximetry Applications

When accurate knowledge regarding arterial oxygenation and acid-base status is required, arterial blood gases should be obtained. Preferably, the ABG should be obtained at the time of a SpO2 recording. The arterial blood gas values can then be compared to the SpO2 and a consensus regarding the accuracy of the SpO2 measurement ascertained. If the values agree, then oximetry can be used to check oxygen status and to monitor changes invoked through therapeutic interventions.

Oximetry is used to assess arterial blood oxygenation for many diverse patient groups with suspected cardiopulmonary disorders or in normals during heavy exercise or exposure to low partial pressures of oxygen. In general, the instruments perform well and reasonable readings for SpO2 are obtained. The clinical picture should always be correlated with the SpO2 obtained, however. If there is any doubt regarding the accuracy of the SpO2 measurement, an ABG should be obtained.

Oximetry has its broadest application as a method for continuously monitoring arterial blood oxygenation under a variety of conditions and circumstances. Infants and adults with cardiopulmonary disorders are routinely monitored for changes in arterial oxygen saturation using noninvasive pulse oximetry. Trending data is easily visualized, and adjustments to the prescribed therapeutic regime can be implemented quickly and efficiently. When supplemental oxygen is titrated, the therapist should remember that there is a lag time for the instrument to respond and for the body to equilibrate to the higher oxygen flow. Typically, the oximeter delay can range for half a minute to beyond 2 minutes. Thus, the therapist should review the manufacturer's specifications for the oximeter they are using and allow sufficient time for the response to equilibrate.

Low perfusion and motion artifact are particularly disturbing in the monitoring application. Pulse oximeters employed on clinical units typically have low alarm settings that will trigger when the SpO2 descends below the established trigger point. If these alarms are generated from physiologically valid data, then the oximeter has performed well and provided a service to all. If the alarms are triggered by dropped data due to motion artifact or other noise, however, then the false alarms are creating an environment whereby the attending clinical staff is desensitized to the alarm and calls the accuracy of the device into question. Thus, oximeter models that minimize the likelihood of dropped data resulting from motion artifact, low perfusion, or a combination of the two are desirable.²⁹ Again, redesigned sensors capable of being placed in areas where motion is not an issue and the vascular bed is optimal for measurement are being developed (eg, forehead sensors).

In Lung and Heart Disease

Patients with lung and heart disease experience episodes of arterial hypoxemia with a therapeutic goal to maintain arterial oxygenation above 90% SpO2, if clinically feasible. This is typically approached by titrating supplemental oxygen after all other underlying imbalances have been addressed (eg, low cardiac output, acid-base shifts, ventilation/perfusion defects, dead space to tidal volume relationships, etc).

For patients with chronic obstructive pulmonary disease (COPD), long-term oxygen therapy improves survival and hemodynamics in hypoxemic patients.^30-33 These established benefits are maximized if the arterial oxygen saturation is maintained above 90% for at least 18 hours per day.^34,35 Additionally, oxygen also appears to have a therapeutic effect on lung tissue and may assist in stabilizing the patient.³⁶ Thus, the effective delivery of supplemental oxygen cannot be undervalued in this cohort and efforts should be made to ensure 24-hour delivery and oxygen titrated to specific life events so as to maintain SaO2/SpO2 at its optimal therapeutic level.

In this cohort, the recommended time to monitor arterial oxygenation outside the admission is during sleep and/or exercise or when there is a complaint of excessive dyspnea or a change in the patient's medical condition. Sleep and exercise offer excellent opportunities to evaluate oxygen status, since each exerts variable and clinically relevant stress on the body. Further, each setting is routinely experienced by the patient and the importance of correcting even transient desaturation responses is emerging. It is important to realize that the oxygen prescription may be changed depending on the body's need during different levels of activity/inactivity. Pulse oximetry offers an excellent opportunity to evaluate patients over time and to customize the oxygen prescription to their unique needs.

During Exercise

Patients who complain of dyspnea on exertion and who do not have readily identifiable causes may undergo exercise testing with oximetry. If a desaturation response is noted and all other medical reasons have been addressed, titration of supplemental oxygen to maintain a target SpO2 during exertion may be warranted. Again, instrumentation minimizing artifact and eliminating spuriously low readings is highly recommended. An ABG may be obtained to correlate the SpO2 to the ABG. For diagnostic purposes, an arterial cannula is recommended as oximetry may miss small but significant changes in the PaO2 (eg, SaO2), and the ability to monitor changes in acid-base status will be essential in establishing an accurate diagnosis.

Another area for oximetry use is in ongoing monitoring of patients admitted to rehabilitation. Pulse oximetry can be used to assess baseline shifts and changes impacting exercise performance and the ongoing disease process. Based on patient symptoms and self-reported dyspnea, the therapist can address hypoxemia and its underlying cause in a timely and focused manner. If oxygen titration is prescribed, the patient can undergo an exercise trial while performing their usual rehabilitation exercise and a new oxygen prescription can be completed. Additionally, there is an emerging body of knowledge that maximizing supplemental oxygen administration during exercise will reduce dyspnea secondary to smaller tidal volumes and a reduced work of breathing, thereby allowing patients to exercise at higher workload levels and over a longer period of time. These adjustments will accelerate the training process and enhance function for the patient.

Recent Advances

By using signal extraction technologies and increasing the number of wavelengths emitted from the sensor (7+ wavelengths), the next generation oximetry technology called pulse co-oximetry was launched in 2005.²⁸ This instrumentation goes beyond the measurement of oxygen saturation to continuously and noninvasively measure carboxyhemoglobin, methemoglobin, pulse rate pleth variability index, and perfusion index.³⁷ This breakthrough technology extends the use of highly useful oximeters, allowing clinicians to detect and treat potentially life-threatening conditions earlier than before. In early clinical trials, co-oximetry yielded clinically acceptable results.^28,37

Oximetry Limitations

Limitations exist for pulse oximetry with SpO2 reflecting oxygenation only. Oximetry does not evaluate ventilation and is not a substitute for arterial blood gases, as it gives no indication regarding carbon dioxide, blood pH, or sodium bicarbonate levels—each highly useful in clinical decision processes. Further, saturation levels give no information as to the blood oxygen content, and this is essential in cases of severe anemia, where the patient's hemoglobin can be fully saturated. False low readings result from hypoperfusion, often resulting from cold or vasoconstriction secondary to the use of vasopressor agents, incorrect sensor application, highly calloused skin, and movement artifact (shivering), especially during hypoperfusion. False high and false low readings have been attributed to situations wherein hemoglobin is bound to something other than oxygen. In carbon monoxide poisoning, a false high reading may delay recognition of hypoxemia and delay effective treatment. Methemoglobinemia typically yields an oximeter reading in the mid 80s, while cyanide poisoning produces a high reading through a reduction in oxygen extraction. This reading is not false, yet it is misleading by not providing the correct clinical picture. Additionally, pulse oximetry measures only the percentage of bound hemoglobin, and in cases where carbon monoxide is present, the value will be high even though the patient is hypoxemic. This is where the new pulse co-oximeter can provide additional clinically relevant data in a timely and noninvasive manner.

Summary

Pulse oximetry is a highly valued, noninvasive technology for assessing hemoglobin oxygenation in patients, as well as healthy individuals placed in extreme environments or during exercise. Pulse oximetry should not, however, be substituted for arterial blood gases, as the ABG measurements will yield a high-quality clinical picture regarding not only oxygenation, but the ability of the patient to oxygenate, their acid-base status and different hemoglobin species, and other possibilities influencing the patient's medical course. With the introduction of advanced technologies such as reflectance, the reintroduction of an array of light-wave lengths, and greatly advanced signal processing, oximetry accuracy has improved, measurement artifact has been reduced, and the instrumentation has expanded its clinically relevant measurements. These refinements are allowing clinicians to establish a timely and more definitive treatment regime. Many variables can influence oximetry measurements, though, and it is essential that the therapist and physician understand the instrumentation's limitations and always consider the clinical presentation in light of inaccurate or misleading data.

Rick Carter, PhD, MBA, is professor and chair, Department of Health, Exercise and Sport Sciences; James S. Williams, PhD, is associate professor, Department of Health, Exercise and Sport Sciences, and adjunct associate professor of physiology, Texas Tech University, Lubbock. For more information, contact .

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