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The Purple Parturient: Obstetric Anesthesia for a Patient with Congenital Methemoglobinemia
Abstract Number: S5C-1
Abstract Type: Case Report/Case Series
Introduction: Congenital methemoglobinemia (CmH) is a rare disorder characterized by decreased ability to convert methemoglobin (metHgb) to hemoglobin (Hgb). Normal serum metHgb levels are <2%; individuals with CmH have higher levels and are sensitive to oxidizing agents. In non-anemic patients, cyanosis develops at metHgb levels >10%, symptoms of hypoxia manifest at 20%, and death occurs at 60-70%.(1) Patients with CmH present a unique challenge to obstetric anesthesiologists for several reasons: standard pulse oximetry is inaccurate, metHgb levels may increase during pregnancy and labor (2), and a number of medications are oxidizing agents which can cause the formation of metHgb including ester local anesthetics, lidocaine, nitroglycerin, and metoclopramide. We present the case of a parturient with CmH who experienced a postpartum metHgb crisis.
Case: A 46 year old G2P0 with CmH was referred to high risk obstetric anesthesia clinic at 30 weeks for delivery planning. She was visibly cyanotic. Her pre-pregnancy venous metHgb levels were 9%. This rose steadily to 15% at the end of the 3rd trimester. She received pRBC transfusions every 2-4 weeks after week 30 for mild anemia and to provide a source of normal Hgb.
The patient presented for labor induction at 39+3 weeks. Admission Hgb was 12.1 g/dL, and metHgb 15.7%. A Masimo Radical-7® co-oximeter was utilized continuously and correlated well with serial venous metHgb. Due to fetal intolerance of early labor, the patient underwent Cesarean delivery under combined spinal-epidural anesthesia with 12mg hyperbaric bupivacaine, 15mcg fentanyl, and 150mcg morphine. Her metHgb peaked at 17.7% at the time of delivery with corresponding mild dyspnea and headache. IV methylene blue 1g was available but not used as she remained stable. She was discharged on postpartum day 4 with metHgb level decreased to 13%.
The patient was readmitted 4 weeks postpartum with severe fatigue, dyspnea, confusion, and headache. She was anemic with Hgb 7.8 and venous metHgb of 23%. Serum lactate was elevated at 5.2 mmol/L. History revealed that she had been eating large amounts of processed meat (known to contain nitrates and precipitate metHgb). She was given 2 units pRBC and received 1g methylene blue IV, with immediate reduction in metHgb to 2% and symptomatic resolution.
Discussion: Congenital methemoglobinemia (CmH) is a rare disorder with many implications for the obstetric anesthesiologist. For this patient we effectively used noninvasive co-oximetry to monitor metHgb levels, which correlated well with venous blood levels. Our patient’s metHgb increased steadily from 9% to 17% during pregnancy and delivery despite the avoidance of known oxidizing agents. Our case also highlights the postpartum period as a vulnerable time in which these patients may be susceptible to metHgb crisis, especially in the setting of anemia.
1) Ann Emerg Med. 1999;34(5):646-56
2) Jugosl Ginekol Opstet. 1978;18(3):321-4