Thorough understanding aids appropriate use.

We analyze peripheral oxygen saturation (SpO2), which represents the percentage of hemoglobin saturated with oxygen in the blood, with pulse oximetry. Hemoglobin delivers oxygen to tissues throughout the body via the circulatory system. Because tissue perfusion can’t be measured directly, SpO2 serves as a surrogate for tissue oxygenation and a basic indicator of a patient’s health. It’s crucial to patient care in various settings. The technology uses light transmission to measure blood oxygen saturation through a probe placed on the patient’s body, most commonly the finger or forehead. (See Oxygen transportation.)

Pulse oximetry is standard practice in most inpatient, outpatient, and home care settings. It’s a widely accessible, noninvasive, and inexpensive assessment tool compared to obtaining an arterial oxygen saturation (SaO2) level via arterial puncture, which is the gold standard for assessing oxygenation. An arterial blood gas measures multiple values, including but not limited to pH, PaCo2, PaO2, HCO3, and SaO2.The reliability of pulse oximetry to measure SpO2, typically within 3% to 4% of a patient’s SaO2, limits the need to perform the more invasive procedure.

Although nurses and other healthcare providers use SpO2 for clinical decision making, we should understand the limitations of pulse oximetry devices and how to use them appropriately.

History

The principles behind the use of oximetry technology have existed since the 1700s. Beer and Lambert’s study of light absorption and its relation to density established the Beer–Lambert law, which laid the groundwork for future technology.

In the late 1800s, researchers discovered that hemoglobin carries oxygen; however, not until the 1930s was the Beer–Lambert law used to confirm that light transmission could be used to measure oxygen saturation in blood. Scientists spent the mid-1900s trialing many methods to bring oximetry into patient care, with the first successful pulse oximeter created in 1981. Within 5 years, pulse oximetry became available worldwide with over a dozen brands on the market for use in healthcare settings.

Physiology and technology

Pulse oximetry measurement uses photoplethysmography (PPG) signals to measure and display SpO2. PPG signals use red light to measure light absorption via a direct current in body tissues and venous blood. It also uses infrared light to measure light absorption via an alternating current to gauge the changes in arterial blood flow. The ratio between the direct current and alternating current designates the oxygen saturation percentage expressed by a pulse oximetry sensor.

Frequently, SpO2 is characterized by a waveform called a plethysmograph (pleth). Analyzing the pleth can validate the accuracy of the value presented on the monitor. An adequate pleth exhibits one wave that correlates with each arterial pulse or heartbeat if an electrocardiogram is being monitored, with the exception of ectopic cardiac activity. Each wave should look similar in size and shape. (See Plethysmograph interpretation.)

Pulse oximetry limitations

Since pulse oximetry measurement relies on light transmission, any condition that alters light absorption can inhibit the oximeter’s ability to interpret SpO2. For example, the presence of nail polish, specifically darker colors, may block the light absorption needed to measure SpO2. Artificial nails are even more likely to impact light absorption. Excessive bright or fluorescent lights also may cause variability in light absorption, leading to false readings.

The effect of skin pigmentation on the accuracy of pulse oximetry has been long-studied. Originally, researchers found that patients with darker skin tones had lower reported SpO2 levels when using pulse oximeters. However, the research has since shifted to reflect that dark-skinned patients have overestimations in their reported SpO2 levels when compared to SaO2 levels obtained via blood draw. Occult hypoxemia occurs when an SaO2 registers less than 88%, despite a normally registering SpO2 of 92% or greater. These overestimations of SpO2 increase the risk of masking occult hypoxemia in patients with dark skin.

It is difficult to link and quantify how this healthcare bias impacts patients with dark skin, but occult hypoxemia leads to worsening patient outcomes, particularly when left untreated or when a delay in treatment occurs. According to Martin and colleagues, complications include prolonged hospitalizations, increased need for respiratory rehabilitation, and long-term oxygen requirements at home. Since this outcome disproportionality impacts patients with darker skin tones, as opposed to White patients, healthcare disparity occurs. The COVID-19 pandemic brought significant attention to this concern since SpO2 dictated most treatment.

Limitations associated with most available research include generalization of dark skin tones. Some evidence suggests that variability of pulse oximetry accuracy may exist among Asian, Latino or Hispanic, Native American or Indigenous, or mixed race/ethnicity patients compared to White patients. A tool such as the Fitzpatrick Scale can help classify skin by pigmentation rather than using the patient’s self-reported race. This scale separates skin pigmentation into six categories from lightest to darkest. Incorporating true pigmentation into future research may drive better understanding of variability sometimes represented by pulse oximetry measurement in patients of color. The Fitzpatrick Scale has some disadvantages. For example, it uses White skin as the baseline and it represents only six shades of pigmentation. Other scales, such as the Taylor Hyperpigmentation Scale, represents over 100.

Nursing considerations

Pulse oximetry sensors, typically placed on the fingers, toes, and earlobes because of the well-vascularized, pulsatile tissue, have a high level of accuracy. Patients find finger sensors relatively comfortable and convenient, and healthcare professionals find them simple to manage.

However, peripheral sensor placement can yield inaccuracy in the event of compromised perfusion. Chronic and acute conditions that limit blood flow can result in inaccurately transmitted light, making pulse oximetry measurement difficult. Patients with chronic arterial insufficiency, edema, and cardiac conditions that cause decreased cardiac output may experience inaccurate or unobtainable SpO2 measurements. Critically ill patients experiencing hypotension or hypothermia and those receiving vasopressors may have impaired peripheral circulation.

In addition, motion in the tissue being monitored may create artifacts and potentially false readings. Short, intermittent movements shouldn’t present a problem, but you may find it difficult to obtain an accurate reading in patients with constant motion in the fingers or toes. Earlobes serve as a good alternative site when peripheral perfusion may be compromised and digits aren’t capturing accurate measurements.

Pulse oximetry can be measured in some non-pulsatile areas. The forehead probe uses a reflective sensor to measure PPG signals. Forehead probes display similar accuracy to peripheral sensors in most patients. However, you might obtain increased accuracy using forehead probes in patients with impaired peripheral circulation. Forehead probes also may be well-suited for long-term use, especially in critically ill patients.

Whichever location you choose, assess the patient’s skin integrity and rotate sensor placement. In rare instances, patients can develop burns from pulse oximetry sensors. Thermal, chemical, and electrical burns have been documented in patients receiving SpO2 monitoring. Impaired blood flow to an appendage can decrease sensation and increase the risk of thermal injury. The risk of burns also can increase if equipment is used inappropriately or when using sensors for more than 8 hours without rotating the monitored site.

In addition to equipment troubleshooting, such as moving or replacing the sensor in the setting of poor pleth, consider physical assessment findings when interpreting SpO2 validity. Your nursing physical exam should include assessing for signs of hypoxemia, regardless of the SpO2 value. Some symptoms of hypoxia include decreased level of consciousness, restlessness, confusion, and tachycardia. Also watch for cyanosis, increased respiratory effort, reports of dyspnea, and pale or mottled extremities. These findings may indicate insufficient oxygenation and warrant further assessment, regardless of the SpO2 value on the monitor. (See Pulse oximeter application.)

One piece of the puzzle

Consider pulse oximetry as only one piece of the patient’s oxygenation status. The convenient, quick, and noninvasive reading shouldn’t overshadow the fact that inaccuracy can occur in some clinical situations. In addition to properly using and troubleshooting equipment, waveform interpretation and nursing assessment can help to substantiate all noninvasive SpO2 readings. In the event of uncertainty, measure oxygen saturation via arterial blood draw.

The authors are clinical practice leads at the Hospital of the University of Pennsylvania in Philadelphia

References

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