Pathophysiology:

• O2 binds iron portiono of Hemoglobin in Ferrous (Fe2+) state
• Methemoglobinemia is when iron exists in Ferric (Fe3+) state; cannot bind O2

Etiology:

• Heriditary:
• Can happen naturally but Cytochrome B5 converts back to Fe 2+
• Deficiency in Cytochrome B5 reductase enzyme
• Hemoglobin M: Favors Fe3+ (ferric) state
• Acquired:
• Dapsone, Chloroquine 
• Topical Anesthetics (benzocaine)
• Ingested Nitrites & Nitrates (well water)
• Antifreeze
• Aniline Dyes
• Poppers (inhaled nitrites used to enhance sexual actvity that induces Methemoglobinemia when ingested)

Clinical: symptoms secondary to Hypoxia

• Cyanosis
• Non-specific symptoms: headache, malaise, lightheadedness, lethargy
• Dyspnea
• Seizure, Coma, Shock
• Hypoxia that does not improve with Supplemental Oxygen
• usually around 85% on Pulxe Oximetry
• Methemoglobinemia absorbs same wave length as both oxygenated and deoxygenated Hemoglobin 
• Chocolate brown blood

Diagnosis: can send serum level 

Management:

• Methylene Blue
• Mechanism: Converts iron from Fe3+ to Fe2+ allowing oxygen to bind hemoglobin again 
• Indications:
• >25% Methemoglobin level 
• Symptomatic 
• Contraindications: 
• G6PD Deficiency: can precipitate hemolysis
• Relative: use of serotonergic agents as Methylene Blue can inhibit MAO and can increase risk for Serotonin Syndrome