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?