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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?
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?