A 52-year-old female undergoes spine fusion surgery with 2 L of intraoperative blood loss. On POD#3 she develops new onset atrial fibrillation with plans to undergo transesophageal echocardiography (TEE) before cardioversion. After topicalization with 20% benzocaine spray, sedation with fentanyl and versed, the TEE rules out clot and confirms otherwise normal heart function. Electrical cardioversion is successful, but shortly afterward the patient becomes dyspneic and cyanotic. Despite adequate spontaneous ventilation with a non-rebreather mask, the patient remains cyanotic with a pulse oximetry saturation of 85%.
Which study would be most helpful in confirming the diagnosis?
A. CBCCorrect Answer: D
Acquired methemoglobin is a form of hemoglobin where heme is oxidized to the ferric Fe 3+ state. Affected hemoglobin is unable to reversibly bind oxygen causing a functional anemia. Normally, hemoglobin displays increased affinity to oxygen causing a left shift in the hemoglobin dissociation curve. Classic features of methemoglobinemia include cyanosis with a normal PaO2 , decreased SaO2 , and “chocolate brown blood.” Symptomatic methemoglobinemia usually occurs when levels exceed 10% of total hemoglobin. Patients with anemia are typically more sensitive to the effects of methemoglobinemia. Common acquired causes of methemoglobinemia are drugs such as dapsone, lidocaine, prilocaine, benzocaine, metoclopramide, nitroglycerin, and sulfonamides. Other substances such as antifreeze, aniline dyes, hydrogen peroxide, nitrates, nitrites, paraquat, and resorcinol can also cause this.
Deoxyhemoglobin absorbs more red light and oxyhemoglobin absorbs more infrared light. Pulse oximetry works by emitting these two wavelengths and then calculates how much of each is absorbed, thus determining the percent saturation. Methemoglobin, however, absorbs both red and infrared light equally making pulse oximetry inaccurate. At increasing levels, pulse oximetry reading will approximate 85%. Cooximetry measures the absorbance of additional wavelengths specific to other dyshemoglobins such as methemoglobin and carboxyhemoglobin. Methylene blue is the preferred treatment. Methylene blue has a potent, reversible inhibitory effect on MAO, so consideration of an alternative treatment like Vitamin C should be given if patients are at risk for serotonin syndrome. Given this patient’s high suspicion for methemoglobinemia based on sequence of events and clinical signs, blood gas with co-oximetry is most helpful with diagnosis. CBC, ultrasound, chemistry panel, and EEG may help rule out other less likely contributors but will not confirm the diagnosis.
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