Hb Saturation and CO
 

Does CO contribute to Hb saturation at higher partial pressures of O2 (via taking up sites normally occupied by O2) or does Hb saturation refer only to O2 saturation (which would be lower than normal at higher pO2)?

Hb saturation is referring to O2 saturation.

I thought so too, but I read this:

http://www.meddean.luc.edu/lumen/Med...hysio/pf10.htm

Which says "Hb becomes 100% saturated with CO at PCO = 0.6 mm Hg" and "Hb becomes 100% saturated with O2 at PO2 = 600 mm Hg" so I was wondering whether the term Hb saturation is independent of the gases bound to the Hb (i.e. several gases can together saturate Hb at the same time).

Saturation just means gas binding to O2. So yes, CO can saturate hemoglobin, but no one is referring to CO when they say "your patient is satting at 88%." When a patient presents with dyspnea and someone tells you the "sat," they are talking about the O2 sat. If you were doing a research experiment or something with Hb and CO, you might refer to CO saturation. Unless it says CO saturation, assume it's O2 saturation. Have you come across a practice question that just said "Hb saturation?" I don't think I have.

It should be worth noting that standard pulse oximetry only detects bound vs unbound hb. Thus a person with CO poisoning can have a falsely high or 100% sat reading via pulse ox. They do make specialized monitors that can detect the difference.

Also, most questions I've seen usually give you SaO2 specifically when wanting you to figure out O2 content.

Most pulse oximeters do not selectively measure what gas is bound to Hb. They just measure what % of Hb is bound to gas. You can have a normal Hb sat in CO poisoning with increased partial pressure oxygen since CO is bound with more affinity. It's the excess CoO that causes ppO2 to increase/ get displaced.

good point.

Leave it to the dolts at COMBANK for this gem:

A male teenager has been rescued from a house fire. Which of the following would you expect on blood gas?
[3.8 %] ( A ) normal blood gas
[16.2 %] ( B ) increased hemoglobin saturation; increased bicarbonate
[25.4 %] ( C ) decreased hemoglobin saturation; decreased bicarbonate
[42.3 %] ( D ) decreased hemoglobin saturation; increased bicarbonate
[12.3 %] ( E ) increased hemoglobin saturation; decreased bicarbonate

Can a blood gas differentiate between CO binding and O2 binding, unlike a finger (point of care) pulse oximeter? Just because one tool works a certain way and is prone to certain errors doesn't mean all of the tools that measure that perimeter is.

Blood gas doesn't directly tell you anything about saturation. If the pO2 is decreased, however, you can infer that the O2 sat is decreased.

A blood gas analzyer typically only measures the PO2 and pH and calculates oxygen saturation by assuming a normal hemoglobin oxygen dissociation curve (although new technologies are emerging).

If you want to measure methemoglobin or carboxyhemoglobin (CO) then you need to use a co-oximeter which uses four different spectrophotometric wavelengths to directly measure oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin and methemoglobin.