O2 saturation in Methemoglobinemia and CO Poisoning

[Transformers]

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In both CO poisnoning and methemoglobemia, the affinity of the remaining Fe2+ increases for O2 creating a left shift. Then why do you see decreased O2-sat when you would expect it to stay at 100%? I think in my mind I equate Hb-affinity to O2 as the basis for %-saturation...

Afterall you would see 100% O2-Sat in fetal hemoglobin and decreased O2-sat with right shifts (exercise, increased BPG, Acid/Altitude, and high temperatures), riight?

 

[MLT2MT2DO]

In both CO poisnoning and methemoglobemia, the affinity of the remaining Fe2+ increases for O2 creating a left shift. Then why do you see decreased O2-sat when you would expect it to stay at 100%? I think in my mind I equate Hb-affinity to O2 as the basis for %-saturation...

Afterall you would see 100% O2-Sat in fetal hemoglobin and decreased O2-sat with right shifts (exercise, increased BPG, Acid/Altitude, and high temperatures), riight?

Just because hemoglobin has a higher affinity for oxygen doesn't mean that it is fully saturated with oxygen. In fact you know carbon monoxide has bound some of these sites so you know for a fact that oxygen can't be 100% saturated, but the sites that do have the oxygen bound don't want to let it go.

 

[tiedyeddog]

Just because hemoglobin has a higher affinity for oxygen doesn't mean that it is fully saturated with oxygen. In fact you know carbon monoxide has bound some of these sites so you know for a fact that oxygen can't be 100% saturated, but the sites that do have the oxygen bound don't want to let it go.

exactly. More detailed reading here: http://medicine.ucsf.edu/education/resed/Chiefs_cover_sheets/methemoglobinemia.pdf

There are four oxygen binding sites per Hb. If one is changed to Fe3+ the remaining ones are still Fe2+. The remaining ones bind O2 even more tightly, causing a left shift. This left shift ensures almost all Fe2+ iron is bound of O2. Think about it as if the Fe3+ is causing a permanent, strong complimentary binding.

This is kind of like how H+ can bind Hb (but not at the receptor site) and cause a right sided shift.

In the old days the machines couldn't read methemoglobin levels, so you only got a PaO2 and an O2 saturation. today, the machines can tell you % methemoglobin which will give you the diagnosis without even thinking about it.

 

[Transformers]

Well CO poisoning affects TWO things to the Hb sat curve:

1.) Downward shift: CO decreases O2 binding capacity (a reflection of Hbconc) hence decreased total Hbs and hence the down shift on the Hb curve
2.) Leftward shift: CO increases the affinity of the LEFTOVER Fe2+ to O2

The reason why I equate Fe2+ affinity with O2 sats, is I mean think about it, if you leftward shifted the heck out of that Hb curve (say with methemeglobenmia or CO poisoning), wouldn't your O2sats be 100%??

Same goes for acidosis or exercise or increased BPG...if these were super high, wouldn't you right shift the heck out of the curve causing your O2 sats to drop?

 

[tiedyeddog]

The reason why I equate Fe2+ affinity with O2 sats, is I mean think about it, if you leftward shifted the heck out of that Hb curve (say with methemeglobenmia or CO poisoning), wouldn't your O2sats be 100%??

No. The Hb binding CO and the ones changed to Fe3+ bind no oxygen. This makes your O2 sat less than 100....

For the right shift stuff you mentioned, you have to look at the specific tissue. The pH and CO2 levels change drastically from lungs to tissue.

 

[Transformers]

Thanks...I mean you can see the confusion....I think I relate % sat to affinity...the better question for %sat should be, can each 4 O2 fully bind to 1 RBC molecule? In the case of exercise for example, I'm assuming your O2 sats are still 100% despite decreased affinity because you don't have a problem with binding O2.

 

[dbeast]

This may be what you were referring to about the newer vs older machines, but I remember somewhere over the past 2 years, a lecturer told us that the pulse ox machine only reads how much hemoglobin is saturated, but not what it is that is saturating it. Therefore, if binding sites are occupied by CO, you'll still see 100% saturation even though the patient could be terribly hypoxic. Is that what you were talking about?

 

[tiedyeddog]

This may be what you were referring to about the newer vs older machines, but I remember somewhere over the past 2 years, a lecturer told us that the pulse ox machine only reads how much hemoglobin is saturated, but not what it is that is saturating it. Therefore, if binding sites are occupied by CO, you'll still see 100% saturation even though the patient could be terribly hypoxic. Is that what you were talking about?

yes.
https://en.wikipedia.org/wiki/Pulse_oximetry#Limitations

The old ones would read 100% for CO poisoning and always 85% for methemoglobinemia. They only use one band of wavelength.

here are the new ones, they use multiple bands of wavelength at the same time to determine the different data
https://en.wikipedia.org/wiki/CO-oximeter

I guess since they are relatively new they probably aren't everywhere? Not sure.... my knowledge is only useful for step 1, not the real world. 😉

 

[dbeast]

yes.
https://en.wikipedia.org/wiki/Pulse_oximetry#Limitations

The old ones would read 100% for CO poisoning and always 85% for methemoglobinemia. They only use one band of wavelength.

here are the new ones, they use multiple bands of wavelength at the same time to determine the different data
https://en.wikipedia.org/wiki/CO-oximeter

I guess since they are relatively new they probably aren't everywhere? Not sure.... my knowledge is only useful for step 1, not the real world. 😉

Nah our lecturers are just old guys who only do research so they wouldn't know about anything recent.