Methemoglobinemia

Nursing, Dec 1995 by Mathews, Micky L, Mathews, Paul J

CYANOTIC AND SHORT OF BREATH, FRANK ROYAL, 28. HAS just arrived in your emergency department. He's very anxious and says he's feeling weak. You immediately take him to a treatment room and place him on a noninvasive vital signs monitor. His vital signs are: blood pressure, 110/70; heart rate, 106; respirations, 38 and shallow.

WHAT'S THE SITUATION?

Mr. Royal was hospitalized 3 weeks ago with Pneumocystis carinii pneumonia related to his diagnosis of acquired immunodeficiency syndrome (AIDS). He was discharged 10 days ago.

During your assessment, you find that he's taking numerous medications, including zalcitabine (Hivid) for human immunodeficiency virus infection and dapsone, an antibacterial, for follow-up treatment of his pneumonia.

Pulse oximetry shows an SpO sub 2 of 97% (normal is 95% to 98%). The doctor orders a stat arterial blood gas analysis. While drawing the blood, you notice it's rusty-colored. You send the specimen to the lab and connect Mr. Royal to a cardiac monitor, which reveals sinus tachycardia.

WHAT'S YOU ASSESSMENT?

The rusty color of Mr. Royal's blood, along with his symptoms and drug history, leads you to suspect methemoglobinemia--a possible adverse effect of dapsone. Another telltale clue: Mr. Royal continues to be cyanotic and short of breath, yet his SpO sub 2 is normal.

Methemoglobin is formed when iron in the ferric state fails to deoxidize to the ferrus state necessary for oxygen transport. The increased levels of methemoglobin leave less room for oxygen to bind to the hemoglobin.

WHAT MUST YOU DO IMMEDIATELY?

Methemoglobinemia can be life-threatening, so you need to quickly restore a normal level of oxygen saturation by reducing the methemoglobin level. You start Mr. Royal on oxygen (6 liters/minute via nasal cannula) and draw venous blood for more tests, including a complete blood cell count and blood chemistry, as ordered. You'll also draw an arterial blood sample for co-oximetry. which is the most effective way to determine the fraction of dysfunctional hemoglobin.

Mr. Royal's still having trouble breathing, so you apply a face mask to deliver 50% oxygen, as ordered. His lab test results are: pH, 7.39 (normal is 7.35 to 7.45); PaO sub 2 , 97 mm Hg (norm is 80 to 100 mm Hg); PaCO sub 2 , 38 mm Hg (normal is 35 to 45 mm Hg); and SaO sub 2 ,:98% (normal is 95 to 98%). The co-oximetry reveals a methemoglobin saturation of 31.3% (normal is less than 1%) and an oxyhemoglobin saturation of 66.9 (normal is 95% or above).

Pulse oximetry values and SaO2 tell you that the hemoglobin is saturated, but not with what. In this case, co-oximetry reveals that hemoglobin is saturated with methemoglobin, not oxygen.

The high methemoglobin saturation confirms the diagnosis of methemoglobinemia. The doctor orders 1 to 2 mg/kg of methylene blue (Urolene Blue) 1% solution given by intravenous infusion over 20 minutes. Alert Mr. Royal to the drug's unusual dark appearance and the discomfort he'll feel at the injection site during the injection. Also warn him that the drug will turn his urine, tears, and sweat a bluish color. Monitor his vital signs and response to treatment during the infusion.

WHAT SHOULD BE DONE LATER?

Mr. Royal will be admitted and his blood gases and hemoglobin closely monitored. He'll continue on oxygen and receive another dose of methylene blue in an hour if his symptoms continue (maximum dosage is 4 mg/kg).

The doctor will probably resume Mr. Royal's other medications--except for dapsone, which caused the methemoglobinemia. She's likely to prescribe pentamidine aerosol as a replacement.