Methemoglobinemia

Abstract

The diagnosis of methemoglobinemia should be considered in patients presenting with cyanosis and hypoxia. A variety of frequently used medications are capable of inducing methemoglobinemia, with dapsone and benzocaine being common culprits. Unique features, such as a saturation gap and chocolate-brown-colored blood, can raise suspicion for methemoglobinemia. Typically, symptoms correlate with the methemoglobin level, and treatment with methylene blue is reserved for patients with significantly elevated methemoglobin levels. In the presence of comorbid conditions that impair oxygen transport, however, low-grade methemoglobinemia can become symptomatic and may warrant treatment.

A 32-Year-Old Female with AIDS,Pneumocystis jiroveciPneumonia, and Methemoglobinemia

“…Thus, a patient with 98% saturated arterial hemoglobin who develops a met-Hgb level of 10% can be expected to have pulse oximetry of 93% [22]. With higher levels of met-Hgb (>30%), the pulse oximetry tends to plateau around 85%, irrespective of true oxygen content or met-Hgb levels, and the arterial blood is characteristically chocolate brown in color [22, 23]. Definitive measurement of met-Hgb requires co-oximetry, a test which is not performed routinely in most ICUs, and must be ordered specifically.…”

“…Of note, due to a severe hemolytic reaction, methylene blue therapy is contraindicated in G6PD deficient patients. Methylene blue is recommended in symptomatic patients with met-Hgb levels >20%, and in asymptomatic patients with levels >30% [18, 23]. …”

PharmGKB summary

“…Methemoglobinemia is an increase in the methemoglobin (MetHb) content of red blood cells (RBCs) [26,27]. MetHb is formed when heme iron atoms within hemoglobin are oxidized and can no longer bind oxygen or carbon dioxide [26-30].…”

“…MetHb is formed when heme iron atoms within hemoglobin are oxidized and can no longer bind oxygen or carbon dioxide [26-30]. Normal levels of MetHb in circulating RBCs are around 1%.…”

Severe decrease in SpO2 and methemoglobinemia following subareolar isosulfan blue administration and clinical relevance

“…Methemoglobinemia may occur as a side effect of local anesthetic toxicity or inhaled NO administration. Peripheral oxygen saturation (SpO 2 ) in methemoglobinemia patients is low and is independent from real partial oxygen pressure (PaO 2 ) in blood (6). However, the relationship between SpO 2 decrease and the clinical condition of the patient, as well as whether treatment is required, remain unclear.…”