YOU'RE WORKING in the special-procedure unit when a colleague calls you for help. Angela Bevivino, 53, is cyanotic despite 100% supplemental oxygen via non-rebreather mask. Her vital signs are: pulse, 110; BP, 130/90; respirations, 24; and Spo2, 90%. She says she feels dizzy and tired and is getting a bad headache. She also seems disoriented to time and place, but doesn't appear to be in respiratory distress.
What's the situation?
Ms. Bevivino had an upper gastrointestinal endoscopy an hour ago. Before the procedure, she was alert and oriented, with an Spo2 of 98% on room air. She's overweight, but has no other medical problems. The procedure was uneventful, but afterward, the staff nurse noticed that Ms. Bevivino was cyanotic and that her oxygen saturation, as measured by pulse oximetry, had decreased. However, despite supplemental oxygen and administration of naloxone, Ms. Bevivino is becoming increasingly cyanotic and weak, and her Spo2 isn't improving. When you obtain a blood sample for arterial blood gas (ABG) analysis, you notice that the blood is muddy brown in appearance.
The ABG results show a normal Pao2 value, which reflects plasma oxygen content-yet Ms. Bevivino is still cyanotic with signs and symptoms of hypoxia. The attending physician orders an immediate consult with a pulmonary specialist, who orders co-oximetry to measure methemoglobin content.
What's your assessment?
Ms. Bevivino has developed methemoglobinemia, a rare adverse reaction to the benzocaine spray used to anesthetize her throat before the endoscopy. Signs and symptoms of methemoglobinemia include cyanosis that doesn't respond to 100% oxygen therapy, a subnormal oxygenation saturation level by pulse oximetry, and muddy brown arterial blood that doesn't turn red when exposed to air.
Co-oximetry reveals a 35% concentration of methemoglobin in Ms. Bevivino's blood, confirming the diagnosis of methemoglobinemia. Although methemoglobin occurs normally in the blood, the level should be less than 2%.
Benzocaine acts as an indirect oxidant, converting the iron in erythrocytes from the ferrous to the ferric form, which impedes the hemoglobin's oxygen-carrying capability and causes functional anemia. Symptoms usually occur within an hour of medication administration.
Because methemoglobin can't carry oxygen, hemodynamic instability, coma, and death can result if the condition isn't promptly recognized and treated.
What must you do immediately?
The pulmonologist orders methylene blue, administered at 1 mg/kg (as a 1% solution) by slow I.V. infusion. Ms. Bevivino's cyanosis and hypoxic symptoms resolve and her Spo2 returns to baseline.
What should be done later?
Ms. Bevivino will be transferred to the ICU for monitoring and frequent lab testing to ensure that methemoglobin levels return to physiologic levels. (Additional doses of methylene blue may be given if needed, but the total dosage shouldn't exceed 7 mg/kg because the drug itself can cause methemoglobinemia.) After an hour, Ms. Bevivino is awake and oriented. She'll continue receiving oxygen by nasal cannula at 4 liters/minute until the next day, when she's discharged home with instructions to seek immediate medical attention if she develops shortness of breath, increasing fatigue, or chest pain.