accuracy of standard pulse oximetry at low saturations

[coffeebythelake]

at what range would you consider standard pulse oximetry to be accurate? SpO2 80-100%? 70-100%?

assuming good signal quality, no other confounding factors.

understood that pulse oximetry algorithms are validated based on studying healthy patient populations, plus as the hemoglobin-oxygen saturation curve starts to fall precipitously at ~80%, below this is hypoxemic but actual numbers unreliable. comments?

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[facted]

at what range would you consider standard pulse oximetry to be accurate? SpO2 80-100%? 70-100%?

assuming good signal quality, no other confounding factors.

understood that pulse oximetry algorithms are validated based on studying healthy patient populations, plus as the hemoglobin-oxygen saturation curve starts to fall precipitously at ~80%, below this is hypoxemic but actual numbers unreliable. comments?

I've heard the same. I would say below 80, I don't believe the listed number. Plus, honestly, what's the difference? It's low. Get the sat up the best you can.

 

[deleted171991]

"The microprocessors of pulse oximeters are calibrated using reference tables of actual SaO2 measurements performed using co-oximetry and compiled using data from exposing healthy volunteers to decreasing fraction of inspired oxygen (FiO2) to yield SaO2 ranging from 100 to 75 percent. Because it would be unethical to intentionally generate lower saturations in volunteers, values for an SaO2 less than 75 percent are obtained by extrapolation from these volunteer data. Pulse oximeter manufacturers claim that reported values between 70 and 100 percent are accurate to within ± 2 percent of the true value, while those between 50 and 70 percent are within ± 3 percent. In practice, the cut-off for acceptable accuracy is felt by many clinicians to be 80 percent (which usually reflects an arterial oxygen tension [PaO2] of approximately 50 mmHg at a pH of 7.4), and varies depending on the model of pulse oximeter used [4,22,23]. The high accuracy of one widely used type of pulse oximeter at SaO2 values ranging from 82 to 94 percent was confirmed in a study of 100 patients, although the accuracy of the instrument deteriorated at values outside these parameters [24]."

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[dannyboy1]

If they are turning blue it is bad.

 

[Bones411]

I always wondered how does this apply to patients with single ventricle physiology? They live at a sat of 80%. I used it as a trend more than anything but never found any good data as to accuracy in these patients.

 

[AdmiralChz]

If they are turning blue it is bad.

Worked with some seasoned staff who trained before the era of pulse ox. Following truth difficult intubations the signs of hypoxemia they’d see in a dark-skinned person would be bradycardia (very late) and dark almost black blood on incision. The latter happened to him multiple times during cardiac cases in the 70s and for over 10 years he would hold his breath during incision for this reason.

 

[coffeebythelake]

I always wondered how does this apply to patients with single ventricle physiology? They live at a sat of 80%. I used it as a trend more than anything but never found any good data as to accuracy in these patients.

their oxygen hemoglobin saturation curve is probably shifted with higher 2,3 DPG plus compensatory polycythemia

 

[dannyboy1]

Worked with some seasoned staff who trained before the era of pulse ox. Following truth difficult intubations the signs of hypoxemia they’d see in a dark-skinned person would be bradycardia (very late) and dark almost black blood on incision. The latter happened to him multiple times during cardiac cases in the 70s and for over 10 years he would hold his breath during incision for this reason.

Look at the lips. They go from dark to somewhat.....pale.

 

[pgg]

If they are turning blue it is bad.

For the residents ... Don't forget that while blue=bad, it may also be bad if they're not blue. There's a threshold quantity of deoxyhemoglobin before patients look blue. This is a recurrent written board question.

You need at least 5 g/dL of deoxyhemoglobin to get central (lips, oral mucosa) cyanosis. In a guy with a Hb of 16 that may be a sat around 80%, but if his hemoglobin is 8 then he may not look blue until he passes through 50%.

That JW with a Hb of 5 may only turn blue once rigor mortis has set in.

 

[deleted171991]

For the residents ... Don't forget that while blue=bad, it may also be bad if they're not blue. There's a threshold quantity of deoxyhemoglobin before patients look blue. This is a recurrent written board question.

You need at least 5 g/dL of deoxyhemoglobin to get central (lips, oral mucosa) cyanosis. In a guy with a Hb of 16 that may be a sat around 80%, but if his hemoglobin is 8 then he may not look blue until he passes through 50%.

That JW with a Hb of 5 may only turn blue once rigor mortis has set in.

Don't bet the house on it. That was a 1923 paper.

There are studies where cyanosis was detected at even 1.5 - 2.3 g/dL of deoxyhemoglobin. E.g. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC461172/pdf/thorax00267-0052.pdf

I don't have access from home to this one: https://journal.chestnet.org/article/S0012-3692(15)40605-1/pdf