Late Diagnosis of Congenital Methemoglobinemia in an Elderly Patient During Cardiac Surgery

ETHEMOGLOBIN (metHb), resulting from the oxidation of iron in hemoglobin from the ferrous state to ferric state, cannot bind oxygen and thereby leads to a displacement of the hemoglobin dissociation curve to the left. It can, therefore, impair tissue oxygen delivery. Methemoglobinemia, which is characterized by the presence of a higher-than-normal level of metHb, either can be congenital (due to Hb variants or deficiency of enzymes that convert metHb back to Hb) or acquired (caused by drugs or toxins 1,2 ). During the perioperative period, anesthesiologists should suspect methemoglobinemia when discrepancy is seen between arterial oxygen saturation and arterial partial pressure, with low SpO2 and normal PaO2. Diagnosis of methemoglobinemia requires measurements of metHb using a co-oximeter analyzer. The authors describe a case of congenital methemoglobinemia in a 78-year-old man, whose diagnosis was made during cardiopulmonary bypass (CPB) for cardiac surgery. Methemoglobinemia was undiagnosed 1 week before, despite the occurrence of low oxygen saturation and generalized seizures during a cervical block for carotid endarterectomy. CASE REPORT A 78-year-old man was admitted for carotid endarterectomy prior to a planned aortic valve replacement. He had a history of type-2 diabetes and arterial hypertension. He had an inflammatory pleural effusion with full recovery around 30 years ago. His surgical history included a total hip replacement in 2008, a meniscus surgery in 1970, and a surgery of the right shoulder in 2007. On physical examination, the patient was found to be in good condition without any cyanosis. A systolic murmur was heard. Echocardiography found an estimated aortic valve area of 0.66 cm 2 , an aortic valve gradient of 48 mmHg, and preserved left ventricular systolic function. Hemoglobin level, hemostasis, and basic metabolic panel were normal. Chest x-ray showed left thickened pleura. Reports of the preoperative pulmonary function tests were normal. No preoperative arterial blood gases (ABGs) were sampled. Carotid endarterectomy was performed initially under combined deep and superficial cervical plexus block. This regional anesthesia was performed with a nerve simulator (240 mg of 2% lidocaine and 90 mg of 0.75% ropivacaine were used in total) by a trained anesthesiologist, without immediate complications. The patient complained of pain at incision (40 min after regional anesthesia), requiring local infiltrations with 300 mg of 1% lidocaine by the surgeon. No intravascular injection was observed. Rapid conversion to general anesthesia was necessary for acute cyanosis, low SpO2, and generalized seizures occurring before carotid clamping (20 min after surgical lidocaine infiltration). Tracheal intubation was performed without complication. Under general anesthesia and mechanical ventilation with 100% oxygen, the cyanosis and seizures were resolved, but the SpO2 remained low at 90%. The patient was transferred to the postoperative intensive care unit (ICU) for persistent low SpO2. ABG showed normal PaO2 and SaO2. Chest x-rays did not show any additional parenchyma abnormalities. Physical examination revealed no cyanosis, and no dyspnea was observed during T-tube trial of spontaneous breathing. The patient was extubated 8 hours after the end of surgery, and was discharged 24 hours after his admission in ICU. The last recorded SpO2 was 92% in room air. Local anesthetic-induced systemic toxicity after regional anesthesia was considered as the diagnosis. Cardiac surgery (aortic valve replacement with a biologic heart valve) was performed 7 days after the carotid endarterectomy. In the operating room, physical examination revealed cyanotic lips and the SpO2 was 92% in room air. General anesthesia was induced with a target-controlled infusion of propofol and remifentanil and a bolus of atracurium. Anesthesia was maintained with remifentanil infusion and target-controlled infusion of propofol, which was adjusted to keep the bispectral index value between 40 and 50. During the procedure, neither preoxygenation before tracheal intubation with pure O2 nor