A 34-year-old man is brought to the emergency department directly from his workplace at a … — USMLE Step 2 CK

A 34-year-old man is brought to the emergency department directly from his workplace at a synthetic chemical and dye manufacturing plant. His coworkers report that he was transferring a vat of aniline-based chemicals when a spill occurred, soaking his clothing. Shortly after, he developed acute-onset shortness of breath, a severe headache, and confusion. His medical history is notable for major depressive disorder, for which he takes fluoxetine. Vital signs are temperature $37.2^\circ\text{C}$ ( $99.0^\circ\text{F}$ ), blood pressure $110/70\text{ mmHg}$ , pulse $118/\text{min}$ , and respiratory rate $24/\text{min}$ . Pulse oximetry shows an oxygen saturation of $85\%$ on room air. The patient is placed on $100\%$ oxygen via a non-rebreather mask, but his oxygen saturation remains at $85\%$ . Physical examination reveals an agitated man with prominent bluish discoloration of the lips and fingertips. Lungs are clear to auscultation bilaterally. Cardiac examination reveals tachycardia with a regular rhythm. An arterial blood gas analysis obtained while the patient is on $100\%$ oxygen shows:

Parameter	Value
pH	$7.32$
PaCO2	$31\text{ mmHg}$
PaO2	$210\text{ mmHg}$
HCO3-	$16\text{ mEq/L}$

Which of the following is the most appropriate next step in management?

Intravenous methylene blueAnswer
B
Intravenous furosemide and nitroglycerin
C
Intravenous piperacillin-tazobactam
D
Intravenous sodium bicarbonate
E
Oral cyproheptadine

Answer

Intravenous methylene blue

The clinical presentation of cyanosis, an occupational exposure to aniline dyes, and an oxygen saturation gap is diagnostic for methemoglobinemia. Aniline oxidizes the ferrous iron (

Fe^{2+}

) in hemoglobin to the ferric state (

Fe^{3+}

), which cannot bind oxygen. This also shifts the oxygen dissociation curve of remaining normal hemoglobin to the left, resulting in severe tissue hypoxia. Standard pulse oximetry reads a steady-state value of approximately

85\%

in the presence of high methemoglobin levels, regardless of the true arterial oxygen tension (PaO2), which remains normal or elevated on supplemental oxygen. The administration of methylene blue is the treatment of choice, as it provides an artificial electron acceptor for the enzyme NADPH methemoglobin reductase, facilitating the rapid reduction of ferric iron back to functional ferrous iron.

Step-by-Step Solution

Identify the toxicological syndrome based on the occupational exposure and clinical signs.

Exposure to aniline dyes causes oxidation of hemoglobin iron, leading to methemoglobinemia, which presents with cyanosis and altered mental status.

Aniline is a classic oxidizing agent encountered in dye, plastic, and chemical manufacturing.

Analyze the objective laboratory data for confirmation.

The patient demonstrates an 'oxygen saturation gap'—pulse oximetry reads

85\%

(the absorption plateau of methemoglobin), while the ABG shows a highly elevated PaO2 (

210\text{ mmHg}

) reflecting normal dissolved oxygen.

Pulse oximetry cannot accurately differentiate oxyhemoglobin from methemoglobin, but the ABG machine measures dissolved oxygen gas directly.

Select the appropriate targeted antidote.

Methylene blue is selected.

It acts as an artificial electron acceptor for NADPH methemoglobin reductase, reducing the abnormal ferric iron (

Fe^{3+}

) back to ferrous iron (

Fe^{2+}

Key Concept

Occupational exposure to aniline dyes or nitrites can cause methemoglobinemia. It is characterized by cyanosis that does not improve with oxygen, a normal PaO2 with spuriously low pulse oximetry, and dark blood. The definitive treatment is intravenous methylene blue.

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