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Pulse Oximetry: Factors Impacting Accuracy


Source: AyuSutras


Pulse Oximetry is applied for nearly every patient we encounter both on the ground and in the air. This blog will center on: (1) how it works, (2) why it's indicated, (3) factors impacting accuracy (limitations)


The WHAT

A pulse oximeter is a portable or fixed device that is used at the point of care to quickly and easily monitor a patient's blood oxygen saturation. The most correct definition is that is measured what is directly bound to the Fe (iron) in the hemoglobin protein, in some pathological conditions it may not be oxygen. The great news about this tool is that it is non-invasive. As an added bonus - the device goes ahead and gives us the heart rate at no additional cost. The COVID-19 pandemic brought more of these devices into the home as well. Their clinical utility has likely never been higher - especially outside of direct clinical environments..... but don't skip the fine print (to be discussed later).


The HOW

The device works on a simple, but brilliant principle. The oximeter shines a light through your finger, earlobe, or toe (optimal sites can vary in smaller populations). Blood, specifically hemoglobin absorbs light at predictable and known wavelengths. Some calculations happen behind the scenes based on finger size, ambient light etc... The oximeter has a reference curve saved to its memory and incoming patient readings are compared to this reference, normalized and finally the numerical results are shared with us clinicians. For best readings, minimize patient movement and remove nail polish if possible. The middle finger and the thumb are preferred targets if you wanted to be picky about getting well defined plethysmography tracings. The "pleth" simply represents pulsatile changes. Amplitude (high or low), irregularity or even weak signals could be an indication that the patient's volume status is changing....when you notice changes, another patient assessment or at least some repeat vital signs may be warranted. Below are some examples or normal vs. abnormal.



The WHY

We know the cardiovascular system works hand in hand with other body systems to perfuse vital end organs, tissues, and cells. This two-way street ensure that oxygen gets deposited to the things that matter, while also ridding the body of metabolic end products and gases like carbon dioxide (CO2). If there is a lack of oxygen in the bloodstream, this means that your bodies tissues with not be oxygenated properly and this is a known health hazard that EMS should be able to recognize and fix swiftly. The pulse oximeter gives us a quick glimpse of some of the patient's clinical condition and hemodynamic (cardiopulmonary) status.


The LIMITATIONS (our biggest focus)

Remember as with any other "Vital sign", that trended changes over time may be more important and meaningful than one single measurement. Be aware that multiple factors can affect the accuracy of a pulse oximeter reading, such as poor circulation, skin pigmentation, skin thickness, skin temperature, current tobacco use, and use of fingernail polish. Review the information in the sections below to better understand how accuracy is calculated and interpreted.


If the waveforms/signals are small or weak - maybe the patient is in a low volume state

If the pleth signal is large and bounding - the patient could have a TBI or an aortic issue

If the pleth signal is chaotic, it could be benign patient movement or they could be presenting with a deadly dysrhythmia

  • Refer to the device labeling or the manufacturer’s website to understand the accuracy of a particular brand of pulse oximeter and sensor. Different brands of pulse oximeters and even different sensors (finger clip versus adhesive) may have a different accuracy level. Pulse oximeters are least accurate when oxygen saturations are less than 80%.

  • Consider accuracy limitations when using the pulse oximeter to assist in diagnosis and treatment decisions.

    • Use pulse oximeter readings as an estimate of blood oxygen saturation. For example, a pulse oximeter saturation of 90% may represent an arterial blood saturation of 86-94%.

    • When possible, make diagnosis and treatment decisions based on trends in pulse oximeter readings over time, rather than absolute thresholds.

As with any other medical device the FDA has to give its official approval before they hit the market. All premarket submissions for prescription use oximeters are reviewed by the FDA to ensure that clinical study samples are demographically representative of the U.S. population, as recommended by FDA guidance. As described in the guidance, the FDA recommends that every clinical study have participants with a range of skin pigmentations, including at least 2 darkly pigmented participants or 15% of the participant pool, whichever is larger. Although these clinical studies are not statistically powered to detect differences in accuracy between demographic groups, the FDA has continued to review the effects of skin pigmentation on the accuracy of these devices, including data from controlled laboratory studies and data from real world settings.


TAKE HOME MESSAGE:

Pulse oximetry has proven itself to be an invaluable tool for the assessment and management of patients in an emergency setting or even primary care, but it is not a perfect tool. Clinicians should be aware of the limitations of pulse oximetry, the factors that cause sampling errors, and the potential for misinterpretation when basing a critical clinical decision on an SpO2 reading.


To help lessen or mitigate potential inaccuracies, bundle this knowledge with other tools in your box. In your most critical patients, also consider the use of continuous ETCO2 monitoring as well. Remember those waveforms (also available on your heart monitor) are more real-time and only ONE breath behind. They can tell you a lot about their overall cardiac status, while giving you more clues around their pulmonary system. This tool is NOT only reserved for those intubated patients.


July 10, 2023

Author: Joshua Ishmael, MBA, MLS(ASCP)CM, NRP

Pass with PASS, LLC.

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