Pulseox measures...?

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So I was thinking of something today.

A PulseOx measures the amount of Oxygen Saturation in the blood. It does this by working as a UV-Vis (or maybe IR, i can't remember which) spectroscopy device and measures % absorbance of a certain wavelength of light.

I understand all of that, but then what's a reading of 100% indicate? That all hemoglobin is bound by 4 O2 molecules (in the peptide subunits)? Because we learned in our physiology class that while O2-Heme saturation is near 100% in the lungs, in the body, its closer to 40%

O2 gas (lungs) <==> O2 (dissolved in plasma) <==> O2-Heme (in RBC) (this is binding of oxygen in the lungs)

CO2 (body) <==> CO2 (dissolved in plasma) <==> CO2 (in RBC, bound to allosteric site) + H2O <==> H2CO3 (aq in RBC) <==(Carbonic Anhydrase)==> H(+) + HCO3 (-) (This is the equation for the capillaries near the body)

So that should illustrate my point. Since we have exchanged the O2 near the capillaries, 97% SpO2 makes no sense! We have used up some of our O2 already and our saturation should be closer to 40% !!!

So what does that 97% mean? Hopefully a paramedic who has had more physiology than I have had (Physiological control systems in college, the equivalent of an intro physiology course) can answer this question?

Thanks!
 
That was my rant in another thread when someone said a nurse should have known what a "sat" meant in the hospital setting.

We do measure many different oxygen saturations from various locations in the body as well as do direct blood measurements or have calculated values for oxygen saturations. SpO2 is taken from the "pulse" which should be arterial. If I measured the venous or capillary O2 saturation near that pulse, it might be 75% with a PaO2 of 40 mm Hg. If I measure the O2 saturation from the IJ or PA which would be SjvO2 or SmvO2, which we can do continuously as well as drawing a sample from the line for calibration, we also have our norms and is the O2 saturation we may use in the ICU to titrate meds, fluids and ventilator settings rather than the SpO2. However, we would draw an Arterial Blood Gas to see where the SaO2 (measured or calculated) stands in relationship to the SpO2 and the patient's clinical condition.

An SpO2 of 97% may not always mean the patient does not require oxygen and many times a patient may be said to have ARDS by CXR and the A-a gradient. If a patient was on close to an FiO2 of 1.0, I would expect a PaO2 of over 400 mmHg. If it is only 90 on an FiO2 of 1.0, even thorough the SpO2 might be close to 97%, that patient is in serious trouble. Thus, I look first at the patient's clinical status before removing oxygen by pulse oximetry.

You also should be familar with the Oxyhemoglobin Dissociation Curve. Many factors will affect the O2 Saturation.
Good information with articles if you click on the images. The journal source is listed under the image.
http://images.google.com/images?hl=...&sa=X&oi=image_result_group&ct=title&resnum=4

Good explanation of oygen carrying capacity.
http://images.google.com/imgres?img...curve&hl=en&sa=X&um=1&ei=IBhfSqnDGJC4Nc644eMB

Another good article from that page.
http://images.google.com/imgres?img...curve&hl=en&sa=X&um=1&ei=IBhfSqnDGJC4Nc644eMB


Here is a good explanation of the A-a gradient and how altitude or barometric pressure affects PaO2.
http://www.madsci.com/manu/gas_aa.htm
BP is the term used for barometric pressure in this article so again, if someone said get me the BP in a hospital setting, it could mean different things. On a med-surg floor it could be a blood pressure and in Respiratory Therapy or Lab, it could mean barometric pressure since we have different devices to monitor it to keep our technology calibrated. So again before you use abbreviations whether accepted or not, one must think of their audience and setting.

(I personally prefer Psubscript B when writing barometric pressure.)

A great website for an overview of oxygen and includes pulse oximetry.
http://www.ccmtutorials.com/rs/oxygen/index.htm

In the section "How Pulse Oximeter Work", one might look at the scenarios especially #3 which discusses the COPD patient. It also may reference to hypoxemia and FiO2 which is what some do not understand when making statements about hypoxic drive.
http://www.ccmtutorials.com/rs/oxygen/page17.htm

Apologies if this is more than what you wanted but there is so much to know about a "sat" that is is difficult to talk about one thing and not mention something else. In essence, a pulse oximetery check in not as simplistic as it appears.


Sidenote: Alveolar Gas Equation for my reference to the A-a gradient.
Abbreviated version:

PAO2 = FIO2(PB-47) - 1.2(PaCO2)
 
Last edited by a moderator:
VentMedic said:
That was my rant in another thread when someone said a nurse should have known what a "sat" meant in the hospital setting.

We do measure many different oxygen saturations from various locations in the body as well as do direct blood measurements or have calculated values for oxygen saturations. SpO2 is taken from the "pulse" which should be arterial. If I measured the venous or capillary O2 saturation near that pulse, it might be 75% with a PaO2 of 40 mm Hg. If I measure the O2 saturation from the IJ or PA which would be SjvO2 or SmvO2, which we can do continuously as well as drawing a sample from the line for calibration, we also have our norms and is the O2 saturation we may use in the ICU to titrate meds, fluids and ventilator settings rather than the SpO2. However, we would draw an Arterial Blood Gas to see where the SaO2 (measured or calculated) stands in relationship to the SpO2 and the patient's clinical condition.

An SpO2 of 97% may not always mean the patient does not require oxygen and many times a patient may be said to have ARDS by CXR and the A-a gradient. If a patient was on close to an FiO2 of 1.0, I would expect a PaO2 of over 400 mmHg. If it is only 90 on an FiO2 of 1.0, even thorough the SpO2 might be close to 97%, that patient is in serious trouble. Thus, I look first at the patient's clinical status before removing oxygen by pulse oximetry.

You also should be familar with the Oxyhemoglobin Dissociation Curve. Many factors will affect the O2 Saturation.
Click to expand...

I know that curve all too well. It showed up on many exams last semester! The main factors we focused on were CO, CO2, O2, Acidity/Basicity, and Pressure effects.

I'll have to take a look at some of those articles, but I wanted to point out this that you mentioned:

"SpO2 is taken from the "pulse" which should be arterial. If I measured the venous or capillary O2 saturation near that pulse, it might be 75% with a PaO2 of 40 mm Hg."

Which is the point I was trying to make with the finger capillaries. Since the finger doesn't have an artery in it, shouldn't we get a reading of 75% instead of 97%?
 
Perfect! I stand corrected! I didn't realize there were still arteris down there, I had assumed they have become arterioles and capillaries by then!
 
Your pH can be 7.1 and pO2 at 130mmHg and the spO2 could be at like 80-90%. Something that could be missed if just rely on pulse ox to actually tell you everything.
 
silver said:
Your pH can be 7.1 and pO2 at 130mmHg and the spO2 could be at like 80-90%. Something that could be missed if just rely on pulse ox to actually tell you everything.
Click to expand...

Depending on what the SaO2 was, I would say the SpO2 would not be correlating very well.
 
Your pH can be 7.1 and pO2 at 130mmHg and the spO2 could be at like 80-90%. Something that could be missed if just rely on pulse ox to actually tell you everything.
Click to expand...

You might want to look at the difference between PaO2 and SaO2. You seem to have swung and missed on this one.

Depending on what the SaO2 was, I would say the SpO2 would not be correlating very well.
Click to expand...

What she said.
 
VentMedic said:
Depending on what the SaO2 was, I would say the SpO2 would not be correlating very well.
Click to expand...
wouldn't a high partial pressure of O2 (say noted in an ABG) not counter balance a low pH to make it look quasi-normal?

please explain more, because I am just kinda not super sure...
 
Last edited by a moderator:
silver said:
wouldn't a high partial pressure of O2 (say noted in an ABG) not counter balance a low pH to make it look quasi-normal?

please explain more, because I am just kinda not super sure...
Click to expand...

You are getting the right idea. The oxyhemoglobin dissociation will shift to the right with acidosis which will the oxygen saturation.

Review the links I posted earlier with the addition of this one:
http://images.google.com/imgres?img...rve&hl=en&sa=X&um=1&ei=ifhfSoXuHY3hlAexx-HfCQ

and see if the concept become clearer.
 
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