Pulse Oximetry – More Than Meets the Eye

• A pulse oximeter is a device used almost daily in healthcare. But do you actually know the technology behind it? 
• Pulse oximetry is a type of technology device worn on a patient’s finger that estimates oxygen saturation. 
• Pulse oximetry is so heavily relied on, but there is some considerations when measuring oxygen saturation in minority patients. 

Healthcare builds upon technological innovations to improve and impact patient care. One technology, the pulse oximeter, an essential tool amid the pandemic, is a non-invasive method to receive peripheral oxygen saturation in patients. But do we understand the basis of the technology and the potential physiological pitfalls and racial disparities?

Where Did the Pulse Oximeter Get Its Name

The components of the pulse oximeter respond to pulsations, such as those in pulsating capillaries, lending to the name pulse oximeter. Japanese bio-engineers Takuo Aoyagi and Michio Kishi developed the first pulse oximetry in 1972 by using new LED technology and measuring the ratio of pulsing red to infrared light absorption at the site of measurement.  

The first fingertip pulse oximeter provided a swift estimation of the peripheral oxygen saturation and valuable clinical data in an efficient, non-invasive, and convenient method.  

In healthcare, pulse oximeter readings, referred to as Sp02 readings, contain the following components: the (S) infers saturation; (P) shows pulse, or serum pressure, and (O2) measures oxygen.  

The abbreviation, in medical terminology, is a measure of oxygen attributed to hemoglobin cells within the circulatory system. Oxygen saturation is determined by calculating the ratio of oxygen–hemoglobin and deoxyhemoglobin.

Pitfalls of Pulse Oximetry

A correct assessment of blood oxygen saturation is a measurement on which diagnostic and treatment decisions are based. Technologies, regardless of their complexities, have their limitations.  

For the pulse oximeter, patients with atypical hemoglobin structure, anomalous hemoglobin levels, or hemoglobin bound carbon dioxide CO2, pulse oximetry readings are not an exact representation of oxygenation. For nurses, understanding the values, subtle nuances, and limitations of pulse oximetry allows for a more accurate picture of their patient’s tissue oxygenation and potential treatment decisions.  

In patients wearing fingernail polish, nail bed occlusion with acrylic may skew pulse oximeter results, or dynamic changes, such as surrounding blood vessels, blood-filled tissue, bone, and skin, cause unpredictable interferences, complicating calculations of pulse oximetry measurements.  

For the clinician, separating the effects of these extraneous variables on arterial blood oxygenation can be difficult. A 100% saturation reading on the pulse oximeter does not guarantee satisfactory central tissue oxygenation.  

In cases of profound anemia and a reduction in total O2 content of the blood, the O2 saturation may remain within normal limits, rendering the oximetry an ineffective measurement in the anemic patient.  

For example, in a patient with heavy vaginal bleeding and a hemoglobin (Hgb) concentration of 6.8 g/dL, the O2 binding capacity is 9.1 mL O2/dL. Delivering less than half the oxygen, yet the oximeter may still read 100%.  

In patients with sickle cell anemia having an oxygen dissociation curve, pulse oximetry is a poor measurement of the patient’s hypoxemia. Sickle cells patients may need an arterial blood gas to evaluate PaO2 and SaO2, providing a more exact measurement.  

Another consideration is patients with elevated carboxyhemoglobin (COHb) or carbon monoxide poisoning secondary to smoking, outdoor grilling, or car exhaust, pulse oximeters may report an inaccurate and elevated saturation secondary to the lack of light absorption within carboxyhemoglobin, but underlying tissue is not receiving enough oxygen. A pulse oximeter under these circumstances may present false high readings secondary to the binding of circulating carboxyhemoglobin. 

Health Disparities and Pulse Oximetry

There are multiple definitions of health disparities across the spectrum of healthcare. For minorities in the United States, health disparities take on numerous forms, and within pulse oximeter technology, recognition of skin pigmentation requires consideration.  

 The government, in Healthy People 2020, defines health disparity as “a particular type of health difference that is linked with social, economic, and/or environmental disadvantages.” These biases, inherent to the overall system, alter the healthcare landscape for minorities and contribute to higher rates of morbidity, mortality, and overall well-being.  

When required to address the disparities, invasive measuring, such as arterial blood gas (ABG), puts minority populations at a higher risk of rare, but potential complications. 

Regarding the African American, Asian, and Hispanic populations, occult hypoxemia occurred more than in white patients. Occult hypoxemia, detected on blood gas but not noted pulse oximetry, is a saturation of arterial blood gas <88%, despite a normal pulse oximetry reading ≥92%.  

The Bottom Line of Pulse Oximetry

Considering known discrepancies, practitioners continue to use these devices notwithstanding their inaccuracies for minority patients. Second generation oximeters use additional wavelengths of light and work for a variety of skin tones.  

Bottom line, consider additional data points in the identified population to correlate with clinical findings and avoid bias when measuring oxygen saturations in minorities.  

Read More