Consider Methemoglobinemia After Ruling Out the Common Causes for a Low Pulse Oximeter Reading



Consider Methemoglobinemia After Ruling Out the Common Causes for a Low Pulse Oximeter Reading


Leena Mathew MD

Philip Shin MD

Walter L. Chang MD



A morbidly obese 39-year-old man presented for open gastric bypass surgery. He had a history of obstructive sleep apnea, requiring continuous positive airway pressure (CPAP) at night. He had no known allergies to medications. The anesthetic plan was to proceed with an awake fiber-optic intubation for general endotracheal anesthesia. He was given a treatment of aerosolized 4% lidocaine for 20 minutes before entering the operating room (OR). In the OR, Cetacaine (benzocaine 14%, tetracaine 2%, and butyl amino benzoate 2%) spray was applied to the posterior oropharynx.

A transtracheal block was performed using 3 mL of 2% lidocaine. An awake nasal fiber-optic intubation was completed without difficulty. Within 15 minutes of intubation, the patient’s pulse oximetry registered a progressive decline to 90% on a FiO2 of 100%. As the surgeon made the incision he commented that the blood on the field looked “dark like chocolate.” When an arterial blood sample was taken, it was also noted to be dark chocolate-colored. A presumptive diagnosis of methemoglobinemia was entertained. The arterial blood sample was sent for blood gas analysis and methemoglobin (MHb) levels. Because of technical difficulties in the laboratory, there was a delay in processing the blood gas sample. At this point the decision was made to treat empirically with methylene blue; accordingly, 70 mg of methylene blue diluted in 50 mL of normal saline was given intravenously over 10 minutes. After an initial drop in the pulse oximetry reading to 88%, the reading rose progressively to the high 90s. Ten minutes after the entire dose, pulse oximetry read steadily at 99% on 100% oxygen. The patient was extubated without difficulty at the end of the procedure and discharged to home several days later. The result of the MHb level, available on postoperative day 1, was 24.6%.

Anesthesiologists commonly use local anesthetics to anesthetize the airway and pharynx in preparation for awake fiber-optic intubations or endoscopies. A rare though potentially serious toxicity associated with the use of local anesthetics is methemoglobinemia (MHbemia). Anesthesiologists should understand the pathophysiology, presentation, diagnosis, and treatment of MHbemia to prevent and/or manage potential problems.


In the case above, the patient experienced arterial desaturation to 90% on a FiO2 of 100% oxygen. The differential diagnosis for a low pulse oximetry measurement (SpO2) included: pulse oximetry artifact, hypoxemia, abnormal hemoglobin variants, sulfhemoglobinemia, and MHbemia. Pulse oximetry relies on measurement of absorbed light from two diodes, one emitting light at 660 nm (red) and another emitting light at 940 nm (near-infrared). It is well known that pulse oximetry is distorted by artifacts arising from motion, ambient light, poor perfusion, and injected dyes including indigo carmine and methylene blue.

Hypoxemia is the most critical cause for low SpO2 and must be investigated urgently. Hypoxemia may result from hypoxia, hypoventilation, shunting, ventilation/perfusion mismatching, decreased partial pressure of oxygen in mixed venous blood, and rarely, diffusion abnormalities. Quickly ruling out these causes of a low SpO2 is paramount to caring for any patient. In this case, the patient was ventilated manually with 100% FiO2. The tube placement was verified to be correct with a fiber-optic scope; auscultation, and presence of end-tidal CO2. The nasotracheal endotracheal tube was suctioned and the breathing circuit checked for kinks. New-onset diffusion abnormality was highly unlikely.


METHEMOGLOBINEMIA

Only after ruling out the most common and critical causes for a low SpO2 should one consider the possibility of MHbemia as the cause. Methemoglobin is the oxidized form of hemoglobin in which the iron moiety of MHb is in a trivalent (Fe3+) ferric state rather than a divalent (Fe2+) ferrous state. MHb is unable to bind oxygen and thus reduces the oxygen-carrying capacity of blood. In addition, MHb shifts the oxygen dissociation curve to the left, thus impairing release of oxygen from the heme molecule to tissue. Significant levels of MHbemia can place patients in a state of functional anemia.

Jul 1, 2016 | Posted by in ANESTHESIA | Comments Off on Consider Methemoglobinemia After Ruling Out the Common Causes for a Low Pulse Oximeter Reading

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