Accuracy of Oxymeters

[loopy]

G'Day all,

Just a quick one for the doctors/Eric - from my understading, oxymeters work by passing a combination of visible and IR light through the finger, giving the frequency change of the received light as proportional to the oxygen saturation of the blood. How accurate would a finger mounted oximeter be in a blood shift scenario? I'm not sure how complete the shift is (although I imagine the fingers/toes would bear the brunt of it), but would this affect the readings on the meter at all?

Cheers,


Brad

[Noel]

My experience with oximeters as a medic was that they either work or they don't. That is, "accuracy" isn't a variable; if the thing gives you a number, it's a good number.

I never worked with a drowning/diving injury, but it's not uncommon to be unable to get a reading with a shocky patient (poor blood flow for whatever reason), and I was also taught that it might not be possible to get a reading with an extremely cold patient as well, due to vasoconstriction. (I worked in Southern California so never got a chance to find this one out for myself )

So I don't know whether the thing would work during extreme bloodshift or not. All I can say is that if you get a number it will be an accurate one.

It occurs to me, though, that most pulse-ox monitors can be used with an earlobe sensor. Wouldn't the earlobe be less affected by bloodshift?

Noel

[loopy]

Good point - cheers for the info too, that was pretty much what I was thinking. From what I've read, it's the ratio between transmitted red and IR light that matters, so if you did get a reading, it should be accurate.

Cheers, I'll look into the ear lobe,


Brad

[efattah]

During the end of some statics, when the blood shift is very strong, and saturation is low, many oximeters tell me 'LOW QUALITY SIGNAL', and this is often accompanied by ridiculous readings, for example saturations in the 10-20% range, while I'm still conscious and obviously at 40% SaO2 or more.

Further, in the pool, keep one hand on the deck, results in readings that are delayed by up to 2 minutes from the actual breath hold.

[Fitz-Clarke]

Oxygen saturation by pulse oximeter (SpO2), as compared with the "true" value obtained by arterial puncture (SaO2), are generally within 2 or 3 percent down to about 80%. This is pretty good, but pulse oximeters become progressively unreliable below 80%. We will usually do an arterial puncture on a patient when the SpO2 is in the 80s or lower, so we can get the true SaO2, and PO2 which is actually more meaningful.

It is worth pointing out that SpO2 needs to be put into context. It only measures Hgb saturation, not the amount of hemoglobin or oxygen carrying capacity. You can be severely anemic (low Hgb), yet still be fully saturated. Oxygen delivery to tissues is DO2 = SaO2 x Hgb x blood flow.

Physiologically it is PO2 (not SaO2) that determines gas unloading at the tissues, symptoms of hypoxia, and loss of consciousness. Free divers tend to blackout at PO2 around 25-35 mm Hg, but it also depends on CO2 level, and other things we haven't figured out yet. There is a lot of variability here though.

As a rough rule of thumb, you can think "90=60 and 60=30", meaning SaO2 90% = PO2 60 mm Hg and SaO2 60% = PO2 30 mm Hg, but many factors can shift the curve. The relationship between PO2 and SaO2 also depends on the Bohr shift related to CO2 and pH, so you can't really predict PO2 based on an O2 sat. For example, SpO2 of 80% can mean PO2 is anywhere from 45 to 60 mm Hg, depending on CO2, pH, and temperature.

It is true that it can be hard to get a reading from a sat probe on patients in shock, due to peripheral vasoconstriction, but if you can get a decent signal, and it is over 80%, it is probably reasonably good. The same should be true of divers.

As for using pulse oximeters in BH diving or apnea, they are probably good for general comparison between dives, and studying rates of drop and recovery, but the numbers become more inaccurate as they drop into hypoxia. Anything less than 60% is probably meaningless, except to say that you are very hypoxic.

Pulse oximeters are really great instruments. I don't know how we could survive without them nowadays in clinical practice. We call O2 sat the fifth vital sign (along with pulse, resp, BP, and temp). I will sometimes make decisions on admitting vs discharging patients in the emergency department based on their SpO2. There is definitely a place for these devices in simple apnea research studies, but their limitations need to be appreciated.

John Fitz-Clarke, M.D., Ph.D.
Dalhousie University
Halifax, Canada

[loopy]

Eric/Dr Fitz-Clarke,

Many thanks - this is exactly the kind of info I was after.

Cheers,


Brad

[shoutatthesky]

What about using one of the instruments whilst submerged?

[azapa]

Eric: now theres an interesting "add on" for the new X1!!! That would make safety depth/time alarms completely obsolete! I'm sure you could do it....

[efattah]

Oximeters are useless in the water. The blood shift means no blood is flowing to the areas being measured. I have tried it before and it doesn't work.

[trux]

Newer generations of pulse oximeters using digital signal processing allow reflective (non-translucent) sensors that can be then attached on practically any body part, not only the usual fingertip or ear lobe, where results may be strongly influenced by vasoconstriction. They might be better suitable for freediving. Also the sensors may be smaller and simpler, making it easier to manufacture waterproof sensors.

From Wikipedia:

The latest generation pulse oximeters use digital signal processing to make accurate measurements in clinical conditions that were otherwise impossible. These include situations of patient motion, low perfusion, bright ambient light, and electrical interference. Because of their insensitivity to non-pulsate signals, it is also possible to build reflectance probes that place the photodiode beside the LEDs and can be placed on any flat tissue. These can be used on non-translucent body parts, to measure pulses in specific body parts (useful in plastic surgery), or when more convenient sites are unavailable (severe burn victims). They are commonly applied to the forehead of patients with poor peripheral perfusion.

More about digital signal processing in pulse oximetry can be found here:
ScienceDirect - Computers in Biology and Medicine : Signal processing methods for pulse oximetry*1



There is also a nice explication of oximetry and related topics at Oximetry.org

image credit Oximetry.org

[azapa]

pushing this technology one can see that in 5 years time advanced computers may make freediving so much safer, that combined with the freediver saftey vest, and we may even have very safe sport on our hands.. good luck

[shoutatthesky]

Thanks Eric, Just to clarify -oximeters can be a useful training tool for dry statics only? I understand the sensor can be attached to an earlobe. Would this avoid the bloodshift problems?
I have read about and seen photos of Seb Murat using a syringe to collect first exhale after wet static. Is this a good method of measuring remaining oxygen stores?