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Old 05-16-2013
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Angry Confusing O2 Saturation problems

Ok, I've been trying to figure this out, but Goljan says something else, and other resources like Wiki(and I checked the references) says something else.

Basically, O2 saturation is the number of O2 attached to hemeglobin at any time. So if its 4, its around 97-98%. Ok, we all know that.

The problem is, if lets say, instead of O2, CN or CO binds to it, technically the saturation still should be high, because the machine used to measure can only detect "BOUND HB", it can't tell what is Hb bound with. This is as I understand from the physics behind measuring O2 sat.

Goljan says that the O2 sat depends on pO2. So if pO2 is decreased, saturation will be decreased. However, in CO poisoining, there is no problem with pO2 (pO2 is normal, since there is no problem with pO2 getting into the blood - it has a problem of binding to Hb). Therefore, when CO binds Hb, pulse sat still should be high.

Moreover, CN poisoining decreases O2 extraction(it blocks oxygen usage, by inhibiting cyt oxidase), so the pulse sat in CN is also very high.

Can anyone please shed some light on this. Did I get the concept right? If not, what and why are the values of pulse sat in CN/CO or methemglobniema?

Or the machine used for pulse sat is able to differentiate the various kinds of Hb, and can only measure HbO2? Then it will make sense. Otherwise, I am lost..
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Old 05-16-2013
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From wiki:
https://en.wikipedia.org/wiki/Pulse_oximetry

Quote:
Since pulse oximetry only measures the percentage of bound hemoglobin, a falsely high or falsely low reading will occur when hemoglobin binds to something other than oxygen:

Hemoglobin has a higher affinity to carbon monoxide than oxygen, and a high reading may occur despite the patient actually being hypoxemic. In cases of carbon monoxide poisoning, this inaccuracy may delay the recognition of hypoxia (low blood oxygen level).
Cyanide poisoning gives a high reading, because it reduces oxygen extraction from arterial blood. In this case, the reading is not false, as arterial blood oxygen is indeed high in early cyanide poisoning.
Methemoglobinemia characteristically causes pulse oximetry readings in the mid-80s.
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Old 05-16-2013
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Well, Im not sure what is the problem here, but lets see if I can help you. I see you have your concepts right. It is true that when Hb is saturated with whatever it has, O2 or CO, the pulseoximeter will read close to 100%.

The O2 saturation IS dependent of the PaO2. That is why you have that Hb saturation sigmoid curve you already know so well.
An example: A patient suffers from COPD, he cannot breathe right, so his PaO2 will be low, therefore his HbSat% will also be low.Easy

In the case of CO poisoning, there is no problem with the PaO2, its just that that the CO will compete against the O2 for the Hb, and will displace it, therefore O2 cannot be carried to tissues (the only way to carry it to the tissue is with Hb). But the patient is still breathing air! So his PaO2 will remain the same. The patient is in an anoxic state because he cannot deliver that O2 he is breathing to the tissue, because all his Hb are already saturated with CO.

In the case of CN- poisoning. O2 cannot be used in the tissue because the pathway is blocked. So... the patient is breathing, O2 is dissolving in the blood, Hb is receiving the O2 and saturating well, taking the O2 molecules to the tissues needed, BUT surprise surprise, the cell cannot use that O2.

A few concepts to help you out:
-PaO2 is the partial pressure of O2 in the arterial blood. Its just the O2 that is inhaled and dissolved in the blood. You need to get P50 to saturate enough Hb.
-Hb saturation is the amount of gas molecules the Hb is carrying (prefarably O2, since that will keep the patient alive). In order to carry O2, you need to reach the P50.
-In CO poisoning the PaO2 is normal, therefore the amount of O2 getting dissolving into the blood is normal, but since the CO is binding with the Hb, there is no O2 molecule being actually carried and delivered to the tissues.
-The O2 saturation in a clinical setting can only be used to see if the lungs or respiratory tract are working in the patient.
-The O2 sat will be appear normal in CO and CN poisoning because the problem is delivering the O2 to the tissues, not the respiratory apparatus.

Last edited by medpack87; 05-16-2013 at 06:34 PM.
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Old 05-16-2013
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Quote:
Originally Posted by medpack87 View Post
Well, Im not sure what is the problem here, but lets see if I can help you. I see you have your concepts right. It is true that when Hb is saturated with whatever it has, O2 or CO, the pulseoximeter will read close to 100%.

The O2 saturation IS dependent of the PaO2. That is why you have that Hb saturation sigmoid curve you already know so well.
An example: A patient suffers from COPD, so his PaO2 will be low, therefore his HbSat% will also be low. So... the patient cannot breathe enough PaO2, so he will not saturate enough his Hb. Easy

In the case of CO poisoning, there is no problem with the PaO2, its just that that the CO will compete against the O2 for the Hb, and will displace it, therefore O2 cannot be carried to tissues. But the patient is still breathing air! So his PaO2 will remain the same. The patient is in an anoxic state because he cannot deliver that O2 he is breathing to the tissue, because all his Hb are already saturated with CO.

In the case of CN- poisoning. O2 cannot be used in the tissue because the pathway is blocked. So... the patient is breathing, O2 is dissolving in the blood, Hb is receiving the O2 and saturating well, taking the O2 molecules to the tissues needed, BUT surprise surprise, the cell cannot use that O2.

A few concepts to help you out:
-PaO2 is the partial pressure of O2 in the arterial blood. Its just the O2 that is inhaled and dissolved in the blood. You need to get P50 to saturate enough Hb.
-Hb saturation is the amount of gas molecules the Hb is carrying (prefarably O2, since that will keep the patient alive). In order to carry O2, you need to reach the P50.
-In CO poisoning the PaO2 is normal, therefore the amount of O2 getting dissolving into the blood is normal, but since the CO is binding with the Hb, there is no O2 molecule being actually carried and delivered to the tissues.
Thanks for the reply. I don't have a problem with the concept of pO2 or Hb-Sat. My problem only is the measurement of Hb-Sat and its values in CO/CN poisoning.

As I mentioned earlier, the pulse sat(the Hb saturation) is only able to measure 'Bound Hb'. It can't differentiate the various types of Hb. This is what I understand from the concepts of measuring Hb via pulse sat (from physics, and wiki affirms this fact).

My main dilemma is that the pulse sat measured by the device in CO poisoning should be normal, since it won't be able to tell apart Hb-CO or Hb-O2. It just measures the percentage of Hb bound to something (and that can be anything). You can refer to the extract I posted from wiki above to better understand what I am trying to explain.

Correct me if I am wrong in this concept. Thanks once again for the input. Really appreciate it!
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Old 05-16-2013
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Perhaps, I've mixed pulse-sat with oxygen saturation?
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Quote:
Originally Posted by Smashingdude View Post
Thanks for the reply. I don't have a problem with the concept of pO2 or Hb-Sat. My problem only is the measurement of Hb-Sat and its values in CO/CN poisoning.

As I mentioned earlier, the pulse sat(the Hb saturation) is only able to measure 'Bound Hb'. It can't differentiate the various types of Hb. This is what I understand from the concepts of measuring Hb via pulse sat (from physics, and wiki affirms this fact).

My main dilemma is that the pulse sat measured by the device in CO poisoning should be normal, since it won't be able to tell apart Hb-CO or Hb-O2. It just measures the percentage of Hb bound to something (and that can be anything). You can refer to the extract I posted from wiki above to better understand what I am trying to explain.

Correct me if I am wrong in this concept. Thanks once again for the input. Really appreciate it!
You are not wrong. The Sat will be normal in CO poisoning. And Pulse Sat is = to O2 sat, you are not mixing them.
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Old 05-17-2013
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You're right to begin with, the Hb Sat will be normal in the situation of CO poisoning.
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Old 05-18-2013
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Quote:
Originally Posted by Smashingdude View Post
Perhaps, I've mixed pulse-sat with oxygen saturation?
U don't seem to have any problem at all. Ur concept is so good and intact so don't worry much move on in ur studies!
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In USMLE, “Any fool can know. The point is to understand.”
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Old 11-25-2015
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although the post is old but since I passed by I would like to make a simple correction
O2 saturation of Hb is reduced in CO poisoning.
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