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Old 05-25-2011
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Question Oxygen Dissociation Curve for Carbon Monoxide and MetHemoglobin

Hi,

Just wondering, for carbon monoxide poisoning and methemoglobin in goljan he wrote normal PaO2 and decreased SaO2. However, both cause a left shift in ODC which means for the same PaO2, % SaO2 increases. Isn't there some contradiction there? Also, would there be any changes in pulse oximetry reading?

just a query on a really basic concept
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Originally Posted by Jackstraw1589 View Post
Hi,

Just wondering, for carbon monoxide poisoning and methemoglobin in goljan he wrote normal PaO2 and decreased SaO2. However, both cause a left shift in ODC which means for the same PaO2, % SaO2 increases. Isn't there some contradiction there? Also, would there be any changes in pulse oximetry reading?

just a query on a really basic concept
As far as I know, CO has higher affinity for Hb than O2, so this would bind CO to Hb, thus decreasing SaO2 (like you stated), so the amount of arterial O2 will remain the same because it will not diffuse into the RBC Hb.

No, this wouldn't show changes on Pulse Oximetry (measures O2 saturation), because O2 which is normally bound to Hb is replaced with CO.

(Im pretty sure, you may want to confirm)
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Quote:
Originally Posted by patelMD View Post
As far as I know, CO has higher affinity for Hb than O2, so this would bind CO to Hb, thus decreasing SaO2 (like you stated), so the amount of arterial O2 will remain the same because it will not diffuse into the RBC Hb.

No, this wouldn't show changes on Pulse Oximetry (measures O2 saturation), because O2 which is normally bound to Hb is replaced with CO.

(Im pretty sure, you may want to confirm)
Key Point here: There are two ways to measure SaO2.

1) SaO2 can be immediately estimated by pulse oximetry, BUT this only indicates what percentage of Hb sites are occupied. It does not differentiate between O2 and CO2, therefore pulse oximetry in CO poisoning is INCREASED when actually SaO2 is DECREASED

2) Arterial blood gases give a direct measurement of SaO2 and is a very accurate measurement. In CO poisoning ABG will show DECREASED SaO2
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Old 05-25-2011
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Originally Posted by apx85 View Post
Key Point here: There are two ways to measure SaO2.

1) SaO2 can be immediately estimated by pulse oximetry, BUT this only indicates what percentage of Hb sites are occupied. It does not differentiate between O2 and CO2, therefore pulse oximetry in CO poisoning is INCREASED when actually SaO2 is DECREASED

2) Arterial blood gases give a direct measurement of SaO2 and is a very accurate measurement. In CO poisoning ABG will show DECREASED SaO2
I dont know what you're trying to state, but Pulse Oximetry CANNOT differentiate O2 from CO, so the "saturation" (amount of Hb bound) will still be 100%.

In order to differentiate CO/O2 etc, you would use a Pulse Co-Oximeter.
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Originally Posted by patelMD View Post
I dont know what you're trying to state, but Pulse Oximetry CANNOT differentiate O2 from CO, so the "saturation" (amount of Hb bound) will still be 100%.

In order to differentiate CO/O2 etc, you would use a Pulse Co-Oximeter.
I guess there are 3 ways to measure Hb saturation then

1) Pulse oximeter: Non-invasive, does not distinguish between O2 and CO
2) Pulse CO-oximeter: Non-invasive, can distinguish between O2 and CO
3) ABG: can distinguish between O2 and CO2

Also, SaO2 is normally less than 100%, around 97% so in CO poisoning, the pulse oximeter often reads 100% which is what I meant by increased SaO2 in the above post
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Old 05-26-2011
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Quote:
Originally Posted by apx85 View Post
I guess there are 3 ways to measure Hb saturation then

1) Pulse oximeter: Non-invasive, does not distinguish between O2 and CO
2) Pulse CO-oximeter: Non-invasive, can distinguish between O2 and CO
3) ABG: can distinguish between O2 and CO2

Also, SaO2 is normally less than 100%, around 97% so in CO poisoning, the pulse oximeter often reads 100% which is what I meant by increased SaO2 in the above post
Yep, I think that's exactly right.

You're right about the 97% rather than 100% but oh well, close enough
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