Factual errors: Morris thinks he does not need to perform an arterial blood gas test (ABG) on one of the girls with carbon monoxide poisoning because her pulse oximetry shows 100% oxygen saturation. Pratt then tells him (incorrectly) that a pulse oximeter measures oxygen saturation in the blood, not oxygen bound to hemoglobin, and that therefore (correctly) the ABG is necessary. A pulse oximeter actually measures how much oxygen is bound to hemoglobin and the ABG measures oxygen dissolved in blood. Here is why the ABG is necessary: Whenever hemoglobin has something bound to it, it turns bright red, and the pulse oximeter measures this by detecting the color of the blood. The problem is that the pulse oximeter cannot tell WHAT is bound to hemoglobin, only that SOMETHING is bound to it. So, when a patient has carbon monoxide poisoning, a substantial portion of the hemoglobin will have carbon monoxide bound to it instead of oxygen. The pulse oximeter won't be able to tell the difference, though, because the blood will still appear to be bright red. (In fact, it will appear even brighter, as carbon monoxide binds more strongly to hemoglobin than oxygen does.) So, the ABG is necessary to show how much oxygen is dissolved in the blood, and therefore accurately represent how well the patient is oxygenated.
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