A 57-year-old man undergoes bronchoscopy for a persistent cough productive of blood-tinged sputum. He denies chest pain, shortness of breath, wheezing, and weight loss. He is a long-term cigarette smoker. He is allergic to peanuts and shell fish. A physical examination does not reveal any significant abnormalities. He is sedated with fentanyl and midazolam, and a local anesthetic is applied. The bronchoscope is subsequently introduced into the trachea without difficulty. About 10 minutes into the procedure, he is noted to be cyanotic by the medical assistant. The procedure is aborted and supplemental oxygen is administered.
His blood pressure is 112/78 mm Hg, pulse is 115/min, and respiratory rate is 20/min. Pulse oximetry has fallen from 94% pre-procedure to 85% at present. There is central cyanosis. Vesicular breath sounds are heard bilaterally. Myoclonic jerks involving all extremities are apparent. There is no skin rash.
Immediate arterial blood gas analysis shows the following:

Which of the following is the most likely diagnosis?

Question

A 57-year-old man undergoes bronchoscopy for a persistent cough productive of blood-tinged sputum.  He denies chest pain, shortness of breath, wheezing, and weight loss.  He is a long-term cigarette smoker.  He is allergic to peanuts and shell fish.  A physical examination does not reveal any significant abnormalities.  He is sedated with fentanyl and midazolam, and a local anesthetic is applied.  The bronchoscope is subsequently introduced into the trachea without difficulty.  About 10 minutes into the procedure, he is noted to be cyanotic by the medical assistant.  The procedure is aborted and supplemental oxygen is administered.
His blood pressure is 112/78 mm Hg, pulse is 115/min, and respiratory rate is 20/min.  Pulse oximetry has fallen from 94% pre-procedure to 85% at present.  There is central cyanosis.  Vesicular breath sounds are heard bilaterally.  Myoclonic jerks involving all extremities are apparent.  There is no skin rash.
Immediate arterial blood gas analysis shows the following:

Which of the following is the most likely diagnosis?

Quizplus · Accepted Answer

11ec193a_9715_f98d_853d_a1f0bbeb686a_MD0002_00 The combination of local anesthetic use, clinical cyanosis with normal PaO₂ on ABG, and lack of improvement after supplemental oxygen administration is suggestive of methemoglobinemia. The vast majority of methemoglobinemia cases result from exposure to exogenous oxidizing substances (e.g., dapsone, nitrates, and local/topical anesthetics). Symptoms (e.g., cyanosis, dyspnea, lethargy, seizures, and even death) typically appear when methemoglobin levels are > 20%. The arterial partial pressure of oxygen is determined by the amount of dissolved oxygen in the plasma and its measurement is not affected by hemoglobin function. As a result, the blood partial pressure of oxygen is unchanged in methemoglobinemia. Pulse oximetry is unreliable since it uses the difference in light absorbance between oxyhemoglobin and deoxyhemoglobin to calculate oxygen saturation. Methemoglobin absorbs this light differently than normal hemoglobin, producing an erroneous value of around 85% when methemoglobin levels are high. Confirmation of the diagnosis requires a co-oximeter, which is a device that can accurately measure the presence of methemoglobin. The offending agent(s) should be discontinued in all patients with suspected methemoglobinemia, while symptomatic patients and those with a methemoglobin level > 20% require prompt administration of methylene blue (a reducing agent). Those in shock may also require blood transfusion or an exchange transfusion. (Choice A) Decreased lung compliance, severe hypoxemia on ABG, and bilateral radiographic infiltrates are the characteristic features of acute lung injury/acute respiratory distress syndrome. (Choice B) Patients requiring positive pressure ventilation are at risk for air emboli as a consequence of pulmonary barotrauma. Severe air embolism usually present with sudden onset cardiopulmonary collapse, and the typical ABG evidence of severe hypoxemia and hypercarbia (increased dead space). (Choice C) Anaphylaxis is unlikely without skin/musocal manifestations (e.g., hives, tongue swelling), respiratory signs (e.g., wheezing, stridor), or cardiovascular compromise (e.g., hypotension). (Choice E) Opioid overdose causes a reduction of respiratory rate and tidal volume, leading to a respiratory acidosis and hypoxemia. Educational objective: Methemoglobinemia should be suspected when there is cyanosis with a normal arterial PaO₂, "dark chocolate" blood that does not change color on oxygenation, or the presence of a saturation gap. The most common offending agent(s) are local anesthetics. Symptomatic patients or those with a methemoglobin level >20% require prompt administration of methylene blue. Those in shock may also require blood transfusion or exchange transfusion. __________ References: Methemoglobinemia related to local anesthetics: a summary of 242 patients. (http://www.ncbi.nlm.nih.gov/pubmed/19224791) Acquired methemoglobinemia: a retrospective series of 138 cases at two teaching hospitals. (http://www.ncbi.nlm.nih.gov/pubmed/15342970) Methemoglobinemia: etiology, pharmacology, and clinical management. (http://www.ncbi.nlm.nih.gov/pubmed/10533013)

A 57-Year-Old Man Undergoes Bronchoscopy for a Persistent Cough Productive