Old Board Question

[pgg]

This is from one of the old exams. Seems like it should be an easy one.

A 54-year-old man is scheduled for open reduction of a fracture sustained when he jumped from a burning building. The carboxyhemoglobin concentration is 25%. Which of the following is the most reliable indicator of adequate oxygenation during general anesthesia?

(A) PaO2 of 300 mmHg
(B) pH of 7.38 with a PaCO2 of 41 mmHg
(C) Mixed venous PO2 of 45 mmHg
(D) Oxyhemoglobin saturation of 100% measured by co-oximeter
(E) SPO2 Of 100% measured by pulse oximeter

The official key says the answer is C.


A is clearly wrong because PaO2 may be normal or high despite CO hogging a significant portion of the available hemoglobin.

B is clearly wrong because pH and PaCO2 are measures of ventilation, not oxygenation.

I think C is wrong. Sure, mixed venous O2 sat is a decent one-number benchmark for oxygen delivery & uptake, but (like A) you can't count on the usual correlation between PaO2 and Hb saturation if there's a bunch of carboxyhemoglobin around.

I think D is less incorrect than C - at least co-oximetry is the right tool to use. And in the absence of delivery or uptake problems (eg sepsis, cyanide), which are not implied in the question, 100% oxyhemoglobin is reassuring.

E is clearly wrong because standard pulse-oximetry will give falsely high readings even with high levels of carboxyhemoglobin.


Am I misunderstanding this, or is the ABA key wrong?

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[lobelsteve]

Choose the best answer, not necessarily the right answer.

 

[SleepIsGood]

Pgg I like your reasoning. To me, D should be the answer. If on co-oximetry you can get 100% oxyHb saturation then you're 'safe'. It's what you would want.....

 

[Noyac]

I like D

 

[Planktonmd]

This is from one of the old exams. Seems like it should be an easy one.

A 54-year-old man is scheduled for open reduction of a fracture sustained when he jumped from a burning building. The carboxyhemoglobin concentration is 25%. Which of the following is the most reliable indicator of adequate oxygenation during general anesthesia?

(A) PaO2 of 300 mmHg
(B) pH of 7.38 with a PaCO2 of 41 mmHg
(C) Mixed venous PO2 of 45 mmHg
(D) Oxyhemoglobin saturation of 100% measured by co-oximeter
(E) SPO2 Of 100% measured by pulse oximeter

The official key says the answer is C.


A is clearly wrong because PaO2 may be normal or high despite CO hogging a significant portion of the available hemoglobin.

B is clearly wrong because pH and PaCO2 are measures of ventilation, not oxygenation.

I think C is wrong. Sure, mixed venous O2 sat is a decent one-number benchmark for oxygen delivery & uptake, but (like A) you can't count on the usual correlation between PaO2 and Hb saturation if there's a bunch of carboxyhemoglobin around.

I think D is less incorrect than C - at least co-oximetry is the right tool to use. And in the absence of delivery or uptake problems (eg sepsis, cyanide), which are not implied in the question, 100% oxyhemoglobin is reassuring.

E is clearly wrong because standard pulse-oximetry will give falsely high readings even with high levels of carboxyhemoglobin.


Am I misunderstanding this, or is the ABA key wrong?

How can you have 100% oxyhemoglobin while you have 25 % Carboxyhemoblobin?
If SVO2 is 48% it means that tissue oxygenation is adequate enough that enough oxygen extraction is taking place.
So, yes, it should be C.

 

[pgg]

How can you have 100% oxyhemoglobin while you have 25 % Carboxyhemoblobin?

So you're saying we should assume that the co-oximeter is malfunctioning? I don't know, it seems like a lot of mental bending to second-guess the options and bring additional confounding factors into play.

Along that line, why not assume that the device used to measure the mixed venous PO2 is malfunctioning too?

If SVO2 is 48% it means that tissue oxygenation is adequate enough that enough oxygen extraction is taking place.
So, yes, it should be C.

SvO2 is NOT 48%. Mixed venous PO2 is measured to be 45 mmHg, which under normal circumstances an be expected to correlate to a SvO2 in the 70s, which would be reassuring. But there are two problems with this -

First, in a patient with a lot of carboxyhemoglobin, you probably can't make the assumption that PO2 of 45 = SvO2 of 75.

Second, I'd view a SvO2 of 75 in this patient as either bad or as a sign of a broken instrument, since it implies that there's ZERO O2 uptake by tissues. If his arterial blood has 25% carboxyhemoglobin, it has at most 75% oxyhemoglobin. If the mixed venous oxyhemoglobin is also 75% ... where's the brain getting its O2 from?

 

[Planktonmd]

So you're saying we should assume that the co-oximeter is malfunctioning? I don't know, it seems like a lot of mental bending to second-guess the options and bring additional confounding factors into play.

Along that line, why not assume that the device used to measure the mixed venous PO2 is malfunctioning too?



SvO2 is NOT 48%. Mixed venous PO2 is measured to be 45 mmHg, which under normal circumstances an be expected to correlate to a SvO2 in the 70s, which would be reassuring. But there are two problems with this -

First, in a patient with a lot of carboxyhemoglobin, you probably can't make the assumption that PO2 of 45 = SvO2 of 75.

Second, I'd view a SvO2 of 75 in this patient as either bad or as a sign of a broken instrument, since it implies that there's ZERO O2 uptake by tissues. If his arterial blood has 25% carboxyhemoglobin, it has at most 75% oxyhemoglobin. If the mixed venous oxyhemoglobin is also 75% ... where's the brain getting its O2 from?

Agree , I thought the 45 was SVO2.
Mixed venous PO2 of 45% is normal and usually correlates with a normal arterial PO2, it would indicate that you are at least providing enough oxygen in the absence of other measurements.
This is not a well constructed question.

 

[Noyac]

Holy Crap, I'm glad I'm done with that test.

 

[dhb]

(A) PaO2 of 300 mmHg
(B) pH of 7.38 with a PaCO2 of 41 mmHg
(C) Mixed venous PO2 of 45 mmHg
(D) Oxyhemoglobin saturation of 100% measured by co-oximeter
(E) SPO2 Of 100% measured by pulse oximeter

A: if you have a PaO2 of 300 you're going to saturated whatever free Hb you have (good answer maybe not the best)
B: irrelevant
C: Doesn't tell you about arterial delivery and tissue extraction
D: best answer: you can't get better transport than that
E: not bad but not the best answer

 

[Winged Scapula]

FYI: Unless this is a "retired" board question (ie, they have released it for general use and it will not appear on future exams), I would suggest not posting such questions here.

The American Board of Surgery monitors the Surgery forums here and has subpoenaed SDN for the names of the users who posted in-training exam questions and those who participated in the discussion.

I would not be suprised if other boards also monitored goings on here.

 

[FuzzyBK]

I think the reason that Answer D isn't preferred is because the wording says 100% saturation. It doesn't make any reference to what the concentration of oxyhemoglobin might be. If your sat was 100% but your HgB was only 4.0, well your O2 delivery wouldn't be all that great. In this case if 25% is carboxyhemoglobin and his starting HgB was 10 then only 7.5 of that would be available for O2 delivery. At least, that's my understanding.

 

[Planktonmd]

I think the reason that Answer D isn't preferred is because the wording says 100% saturation. It doesn't make any reference to what the concentration of oxyhemoglobin might be. If your sat was 100% but your HgB was only 4.0, well your O2 delivery wouldn't be all that great. In this case if 25% is carboxyhemoglobin and his starting HgB was 10 then only 7.5 of that would be available for O2 delivery. At least, that's my understanding.

"Oxyhemoglobin saturation" is meaningless because when hemoglobin becomes oxyhemoglobin it means it is carrying oxygen, and if it becomes desaturated it means it is not oxyhemoglobin anymore.
So oxyhemoglobin is always 100% saturated with oxygen.
This why D is meaningless.
And this is why this question is not well designed.

 

[dhb]

I think the reason that Answer D isn't preferred is because the wording says 100% saturation. It doesn't make any reference to what the concentration of oxyhemoglobin might be. If your sat was 100% but your HgB was only 4.0, well your O2 delivery wouldn't be all that great. In this case if 25% is carboxyhemoglobin and his starting HgB was 10 then only 7.5 of that would be available for O2 delivery. At least, that's my understanding.

yes but if sats are 100% you're at maximal oxygen carrying capacity regardless of the level of Hb: you can't get more oxygen on board except if you go hyperbaric...

 

[BADMD]

This is from one of the old exams. Seems like it should be an easy one.

A 54-year-old man is scheduled for open reduction of a fracture sustained when he jumped from a burning building. The carboxyhemoglobin concentration is 25%. Which of the following is the most reliable indicator of adequate oxygenation during general anesthesia?
...

Am I misunderstanding this, or is the ABA key wrong?

Carbon monoxide intoxication is more than just a functional anemia. CO poisons the cytochromes and prevents effective electron transport. The purpose of HBO isn't really to get the CO out of the blood. With 100% O2, that can usually be done fairly quickly and before most patients are able to get in the chamber (since that usually requires a transfer). HBO is trying to get the CO out of the tissue cytochromes and improve cellular respiration.

It is oxygen delivery and oxygen utilization that is important in someone with CO poisoning. Since answer C is the only one that actually tries to measure delivery/consumption, it is likely the most correct answer.

 

[pgg]

FYI: Unless this is a "retired" board question (ie, they have released it for general use and it will not appear on future exams), I would suggest not posting such questions here.

It's from one of the publicly released old exams. 1994 or 1995, don't remember which.