Pulse oximetry is a versatile tool used in a variety of settings, from outpatient oxygen monitoring to inpatient noninvasive oxygen saturation measurements. Plus, several exciting new applications are on the horizon.

The device decreases the need for frequent arterial blood gas analysis and prevents discomfort to the patient. In critically ill patients, however, when peripheral tissue is poorly perfused, the signal from the pulsatile blood flow is impaired leading to a less accurate estimate of SaO₂or less reliable signal.

In shock patients, pulse oximetry has been used successfully when mean arterial pressure is maintained above 60 mm Hg and SpO₂is higher than 90 percent, even when vasopressors were used.¹More recently, transcutaneous assessment of muscle oxygen saturation (StO₂ has become available and may be a reliable predictor of tissue oxygenation, particularly in trauma patients and with promising results in septic patients.²StO₂estimates tissue oxygenation by measuring the diffusion of extracellular oxygen into a heated sensor on the skin. It is the only noninvasive measure of tissue oxygenation currently available.³

SpO₂also is used to evaluate patients for long-term oxygen therapy and for the identification of people with possible obstructive sleep apnea. Pulse oximetry has been a vital tool in the assessment of sleep-disordered breathing. Its ability to precisely detect oxygen desaturations during sleep is a major factor in diagnosing sleep apnea.

Pulse oximetry has become a standard parameter to record clinically significant apnea and hypopnea events, according to the American Academy of Sleep Medicine task force.⁴Furthermore, the definition of apnea hypopnea events as required by the Centers for Medicare & Medicaid Services is to occur with a 4 percent desaturation.

Full polysomnography is the standard diagnostic test for sleep apnea, and it requires the patient to sleep in a well-equipped laboratory.

Nocturnal oximetry has been used for diagnosing obstructive sleep apnea-hypopnea syndrome with variable results. Although it offers faster assessment and potentially earlier treatment, it lacks a reliable accuracy or sensitivity to detect SDB and does not detect other sleep disorders.^5,6

More recently, manufacturers have developed ambulatory portable monitoring devices to screen and assist in the diagnosis of sleep apnea in high-risk patients.⁷These devices have been classified to different levels depending on their ability to record single-channel or multichannel recorders, including pulse oximetry.

Many studies are investigating the role of portable devices and their accuracy in diagnosing apnea and hypopnea events precisely.⁸Their failure rate is higher than that of in-laboratory PSG.Nonetheless, these devices may reduce the waiting time for diagnosis of sleep apnea and decrease costs associated with PSG. The validity of these portable home testing devices as a screening tool for SDB remains to be proven in large controlled studies.

Future advances

Several innovative applications of pulse oximetry recently have been reported:

• Continuous left-heart performance using a finger SpO₂in newborns and adults correlates strongly with the ejection fraction, which offers the possibility to detect early cases of congestive heart failure.^9,10

• Early first-day-of-life pulse oximetry screening in newborns is highly predictive in detecting congenital heart defects and other extracardiac disorders.¹¹The low SpO₂would then require other specific testing to confirm the diagnosis.

• Pulse oximetry may detect pulsus paradoxus (a decrease of more than 10 mm Hg in systolic blood pressure during inspiration) in pediatric patients after cardiac surgery.¹²Pulsus paradoxus can have significant diagnostic implications for many clinical conditions such as pericardial tamponade, heart failure, and shock.

• Preliminary data suggest the use of photoplethysmographic waveform of pulse oximetry may be useful in assessing the macrocirculation (systolic blood pressure) and the microcirculation (peripheral perfusion).

• Pulse oximetry may be used to provide a measurement of total hemoglobin concentration in a noninvasive and continuous way. This might enable clinicians to detect subtle anemia and monitor blood loss in critically ill patients at risk for bleeding or during blood transfusion. In addition, this application may be useful intraoperatively to monitor blood loss.

• A modified version of pulse oximetry may monitor cerebral oxygenation in patients with acute brain injury or during cardiac surgery.

Realizing the important applications of pulse oximetry and its role in improving patient care and patient survival, the World Federation of Societies of Anaesthesiologists recently launched the Global Oximetry initiative to increase oximetry utilization worldwide.

The long-term objectives of this initiative are to create appropriate oximetry solutions for low-income environments; to design systems, services, and infrastructures which will enable, promote, and sustain oximetry utilization; and to set new global standards for safer monitoring using oximetry.

Some limitations

Oximeters have several limitations that may lead to inaccurate readings and need to be considered during measurements and interpretations. One of the most important limitations is that SpO₂estimates

the arterial oxygen saturation (SaO₂, not the arterial oxygen tension (PaO₂. Therefore, the SpO₂measurements are not in linear relationship with actual oxygen content but rather in a sigmoid curve called oxygen dissociation curve. This means large changes in PaO₂at the extremes of this curve with minimal changes in the SaO₂ Other important limitations are that the presence of dyshemoglobinemia (i.e., methemoglobin and carboxyhemoglobin) or anemia may lead to falsely high or low values.

Carboxyhemoglobin has an absorbance similar to oxyhemoglobin at 660 nm, which will overestimate the oxygen saturation measurements.

There also is wide variation between oxygen saturation measured by pulse oximetry and blood oximetry in patients with sickle cell anemia (particularly hemoglobin SS-type).

In addition to the right shift of the oxygen dissociation curve, it is thought that the increased levels of nonfunctional methemoglobin and carboxyhemoglobin in untransfused patients with hemoglobin SS may explain these findings.¹³

In contrast, the effect on other types of anemia on SpO₂has been controversial. Most studies concluded hemoglobin concentration does not impact pulse oximetry accuracy until hematocrit value is less than 10 percent. (Normal range is between 34 percent and 44 percent.)^14,15Likewise, the presence of hyperbilirubinemia does not affect SpO₂measurements.¹⁶Skin color or pigmentation may affect the pulse oximetry accuracy. While one study in critically ill patients showed the difference between SpO₂and SaO₂was higher in black relative to white patients, a more recent study found no significant difference related to dark skin pigmentation at saturation levels higher than 80 percent in the three tested pulse oximeters.¹⁷Another important factor in the measured SpO₂is response delay between the change in oxygen saturation measurements and the display of these changes on the recorded signals.

This delay is dependent on several factors such as circulation time and the site of the oximeter probe.

In general, ear lobe probes are faster in detecting the SpO₂changes (10-20 seconds) than finger probes (24-35 seconds).¹⁸This variability in response (i.e., circulation time) could vary from one patient to another and is potentially a reflection of the cardiac function.¹⁹ For a list of references, visit www.advanceweb.com/respiratory.

Abdul Ghani Sankri-Tarbichi, MD, MSc, is assistant professor of medicine and Ayman O. Soubani, MD, is associate professor of medicine in the division of pulmonary, critical care, and sleep medicine at Wayne State University School of Medicine in Detroit.