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A 50-year-old man presents to the emergency department after an episode of hematemesis, which occurred while he was at home eating dinner. Past medical history is noncontributory. After further evaluation, the patient undergoes an urgent upper gastrointestinal tract endoscopy. Benzocaine is used for topical anesthesia. Midazolam and fentanyl are administered for sedation. During the procedure, the patient’s oxygen saturation declines to 86% on ambient air, which fails to improve with the administration of supplemental oxygen. Physical examination shows bluish discoloration of lips and fingertips. The lungs are clear to auscultation bilaterally. The cardiac examination is unremarkable. The laboratory testing is shown below. Which of the following is the most likely cause of this patient’s symptoms?

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Benzocaine p. 571

methemoglobinemia p. 693

Cyanide poisoning

induced methemoglobinemia p. 689


methemoglobinemia p. 693

Methemoglobinemia p. 693

local anesthetics and p. 571

Vitamin C (ascorbic acid)

methemoglobinemia p. 689


Methemoglobinemia is a disorder characterized by elevated levels of methemoglobin in the blood, which leads to tissue hypoxia.

Normally, our red blood cells are loaded with millions of copies of a protein called hemoglobin.

Each hemoglobin protein is made of four globin subunits, each with an iron containing heme group.

Oxygen can bind to the iron molecule, so each hemoglobin molecule can bind four molecules of oxygen.

The iron molecules, called ferrum in latin, are usually in the ferrous state, which means that the iron atom has lost two electrons to form Fe2+.

When iron is in the ferrous state, it can bind oxygen easily when it reaches the lungs, and release oxygen easily when it reaches the other tissues in the body that need oxygen.

Now, methemoglobin is an oxidized form of hemoglobin, and is normally spontaneously formed in our blood in small amounts.

In methemoglobin, one of the iron molecules is in the ferric state, which means that the iron atom has lost three electrons, instead of two, to form Fe3+.

The heme with the Fe3+ is like the lazy co-worker with a decreased ability to bind oxygen.

The other three heme groups still have iron in the Fe2+ state, and they try to compensate for the slacker by binding to oxygen more tightly.

However, this ends up being more harmful than helpful, as it prevents them from releasing oxygen to the tissues.

Too much methemoglobin can eventually lead to tissue hypoxia, so in order to protect ourselves, we have a few enzyme systems that convert methemoglobin to normal hemoglobin.

The most important one is cytochrome b5 reductase, also known as methemoglobin reductase, because it uses a NADH as a reducing agent to donate electrons to an iron in the Fe3+ state and reduces it to the Fe2+ state. So it’s like the boss that comes by to make the lazy heme productive again.

This enzyme is found in red blood cells and other cells like neutrophils, and helps to keep the level of methemoglobin in our blood very low, approximately 1% of total hemoglobin.

OK, but if the function of these enzyme systems is disrupted, methemoglobin levels get higher than normal, and we call this condition methemoglobinemia.

Methemoglobinemia can be congenital or acquired.

In congenital methemoglobinemia, there’s a problem with the synthesis of cytochrome b5 reductase.

Two main types of congenital methemoglobinemia exist and both are inherited in an autosomal recessive pattern.


  1. "Harrison's Principles of Internal Medicine" McGraw Hill Education/ Medical (2018)
  2. "CURRENT Medical Diagnosis and Treatment 2020" McGraw-Hill Education / Medical (2019)
  3. "Physiology, pathophysiology, and clinical management" Elsevier (2019)
  4. "Bates' Guide to Physical Examination and History Taking" LWW (2016)
  5. "Robbins Basic Pathology" Elsevier (2017)
  6. "Acquired Methemoglobinemia" Medicine (2004)

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