USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

What effect does Methemoglobin formation have on the oxygen-carrying capacity of Hemoglobin in Carbon Monoxide Poisoning?

1. Increased oxygen-binding capacity

2. Decreased oxygen saturation

3. Enhanced oxygen delivery to tissues

4. Decreased oxygen dissociation curve

The correct answer is: Decreased oxygen saturation

Methemoglobin formation leads to a decreased oxygen saturation of hemoglobin. In carbon monoxide (CO) poisoning, CO binds to hemoglobin with a much higher affinity than oxygen, which can displace oxygen from its binding sites and subsequently push the hemoglobin into a state where it can become oxidized to methemoglobin. When methemoglobin is present, the hemoglobin is unable to effectively bind and transport oxygen, which results in a reduction in the overall oxygen saturation of hemoglobin in the bloodstream. This decrease in oxygen saturation is critical to understand, especially in the context of CO poisoning, as it exacerbates tissue hypoxia. Although carbon monoxide already reduces the delivery of oxygen by limiting its availability and promoting hypoxia, the formation of methemoglobin further complicates the situation by impairing the capacity of hemoglobin to carry oxygen, leading to lower oxygen saturation readings despite the total hemoglobin levels appearing normal. In summary, the formation of methemoglobin in the context of carbon monoxide poisoning is significant in causing a decrease in oxygen saturation, complicating the physiological effects of CO on oxygen transport in the body.