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Medicine Pulse Oximeters

jasperkent

jasperkent

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May 19, 2015
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I remember the first time a pulse oximeter was used on me in a hospital. I asked the nurse what it was and how it worked. She explained it to me. My very first thought was "Wouldn't the readings be affected by how dark or light the patient's skin is?"

I'm not some kind of savant and I've never studied medicine (apart from field research in pharmacology😉)....so how in fuck did this thought not occur to anyone who designed, tested, and used this device?
🙄

www.cnn.com

Scientists are searching for solutions after studies show pulse oximeters don't work as well for people of color | CNN

Often when Dr. Thomas Valley sees a new patient in the intensive care unit at Michigan Medicine in Ann Arbor, he clamps a pulse oximeter on their finger -- one of the many devices he uses to gauge their health and what course of care they might require, whether they are a child having seizures...
www.cnn.com www.cnn.com
 
Interesting that the effect seems to have no biasing in the standard deviation of the error.


The study did kind of a cool thing where they calculated blood oxygen by controlling the CO2 and O2 levels being breathed in and measuring what came out to get a direct measurement, then they measured patients blood oxygen saturation levels with pulse oximeters of different brands.

The worst thing about this is that the effect is at the largest magnitude at the lowest oxygen levels. These are times where accuracy is of the most importance.

(actual results just copypastad)


Background
It is uncertain whether skin pigmentation affects pulse oximeter accuracy at low HbO2 saturation.
Methods
The accuracy of finger pulse oximeters during stable, plateau levels of arterial oxygen saturation (Sao2) between 60 and 100% were evaluated in 11 subjects with darkly pigmented skin and in 10 with light skin pigmentation. Oximeters tested were the Nellcor N-595 with the OxiMax-A probe (Nellcor Inc., Pleasanton, CA), the Novametrix 513 (Novametrix Inc., Wallingford, CT), and the Nonin Onyx (Nonin Inc., Plymouth, MN). Semisupine subjects breathed air-nitrogen-carbon dioxide mixtures through a mouthpiece. A computer used end-tidal oxygen and carbon dioxide concentrations determined by mass spectrometry to estimate breath-by-breath Sao2, from which an operator adjusted inspired gas to rapidly achieve 2- to 3-min stable plateaus of desaturation. Comparisons of oxygen saturation measured by pulse oximetry (Spo2) with Sao2 (by Radiometer OSM3) were used in a multivariate model to determine the interrelation between saturation, skin pigmentation, and oximeter bias (Spo2 - Sao2).
Results
At 60-70% Sao2, Spo2 (mean of three oximeters) overestimated Sao2 (bias +/- SD) by 3.56 +/- 2.45% (n = 29) in darkly pigmented subjects, compared with 0.37 +/- 3.20% (n = 58) in lightly pigmented subjects (P < 0.0001). The SD of bias was not greater with dark than light skin. The dark-light skin differences at 60-70% Sao2 were 2.35% (Nonin), 3.38% (Novametrix), and 4.30% (Nellcor). Skin pigment-related differences were significant with Nonin below 70% Sao2, with Novametrix below 90%, and with Nellcor at all ranges. Pigment-related bias increased approximately in proportion to desaturation.

Free study
Effects of Skin Pigmentation on Pulse Oximeter Accuracy at Low Saturation
 
Pulse oximeters are not meant to be anything more than a ballpark measure, but it is close enough even with differing skin tones to get a measure of peripheral 02 perfusion that indicates general health conditions. If measurements need to be very accurate (like during open heart surgery) then one needs to have blood drawn and processed in a lab. Every hospital I ever worked with had devices that do that so that results are available in minutes.
 
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