A 28-year-old man comes to the emergency department due to anxiety, weakness, dyspnea, and headaches. He is a graduate student in chemistry and was accidentally exposed to nitrites in his research laboratory. Physical examination is notable for cyanosis that is not improved by administration of supplemental oxygen via facemask. Analysis of a sample of arterial blood drawn from this patient prior to any oxygen supplementation is most likely to reveal normal values for which of the following?

Question

A 28-year-old man comes to the emergency department due to anxiety, weakness, dyspnea, and headaches.  He is a graduate student in chemistry and was accidentally exposed to nitrites in his research laboratory.  Physical examination is notable for cyanosis that is not improved by administration of supplemental oxygen via facemask.  Analysis of a sample of arterial blood drawn from this patient prior to any oxygen supplementation is most likely to reveal normal values for which of the following?

Quizplus · Accepted Answer

11ec160b_36e8_2e87_9249_079e7e345854_MD0001_00 Iron bound to heme is normally in the reduced ferrous (Fe²⁺) state. Nitrites cause poisoning by inducing the conversion of this heme iron to the oxidized ferric (Fe³⁺) state, leading to the formation of methemoglobin. With iron in the oxidized ferric state, methemoglobin is unable to bind oxygen. In addition, the affinity of any residual ferrous iron in the hemoglobin tetramer is increased, causing a leftward shift of the oxygen-dissociation curve. However, the partial pressure of oxygen in blood, which represents the amount of oxygen dissolved in the plasma, is unchanged. Methemoglobinemia causes dusky discoloration of the skin (similar to cyanosis), and because methemoglobin is unable to carry oxygen, a state of functional anemia is induced. 11ec160b_36e8_2e88_9249_dbc2323395f3_MD0001_00 (Choices A, B, C, and D) Methemoglobin cannot bind to oxygen and therefore the oxygen content and oxygen carrying capacity of the arterial blood will decrease. The bound fraction of oxygen and oxygen delivery to the peripheral tissues will also decrease due to methemoglobin's inability to carry and deliver oxygen. Educational objective: Methemoglobinemia causes dusky discoloration of the skin (similar to cyanosis), and because methemoglobin is unable to carry oxygen, a state of functional anemia is induced. However, the blood partial pressure of oxygen will be unchanged as it is a measure of oxygen dissolved in plasma and is unrelated to hemoglobin function.

A 28-Year-Old Man Comes to the Emergency Department Due to Anxiety