Methemoglobinemia in Bronchoscopy

Abstract

Methemoglobinemia results from oxidation of ferrous iron to ferric iron within the hemoglobin molecule. This molecule cannot bind oxygen and increases the affinity of normal hemoglobin for oxygen, which results in decreased oxygen offloading in peripheral tissues. At elevated levels, methemoglobinemia can cause dyspnea, cyanosis, and even death. Common local anesthesia agents have been correlated with methemoglobinemia. Bronchoscopy is a commonly performed clinical procedure which uses topical application of these anesthetics to provide patient comfort. Methylene blue is an agent thought to help reverse the effects of methemoglobinemia by facilitating the methemoglobin reductase system. Using multiple search engines including PubMed and the Cochrane Database, available data on cases of methemoglobinemia after bronchoscopy were pooled. Adult and pediatric cases were considered. Eleven cases were identified. Cases occurred from 1977 until present. Data gathering was complicated by the fact that a consistent reporting system was not used across cases. Arterial blood gas data and CO-oximetry reported levels of methemoglobin were reported where available. No patients died from methemoglobinemia. The most common finding across all patients was decreased peripheral oxygen saturation. Cyanosis was also frequently reported. There was a disparity between the low peripheral oxygen saturation which was reported and the pO(2) on the arterial blood gas. Dose and type of anesthetic agent varied widely across studies. Using topical anesthetic during bronchoscopy appears relatively safe. No fatalities from methemoglobinemia after bronchoscopy have been reported. A high suspicion for methemoglobinemia is required in patients who develop hypoxia or cyanosis postprocedurally. Access to CO-oximetry can confirm the diagnosis but the clinical picture is often sufficient to proceed with methylene blue treatment or observation, based on how severely the patient is affected. Patients who return to baseline can be considered for discharge home.