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Methemoglobinemia is a sentinel health event (occupational) associated with exposure to aromatic amines and nitro compounds (aniline, TNT, nitroglycerin, toluidine, and nitrobenzene) and working in the explosive, dye, and rubber industries; [Mullan] Methemoglobin Concentration and Adverse Effects: 1 % = normal; 10-20% = chocolate brown central cyanosis; 20-45% = CNS depression; 45-55% = coma; [ATSDR Case Studies: Nitrate/Nitrite Toxicity] In some cases, methemoglobin-forming compounds cause denaturation of hemoglobin (Heinz bodies) followed by hemolytic anemia. It is not known why some individuals have methemoglobinemia and others experience hemolytic anemia after exposure to the same oxidants. [Sullivan, p. 376] Methemoglobinemia severe enough to require treatment is relatively rare in smoke inhalation victims. [Bizovi KE. Smoke Inhalation Among Firefighters. Occup Med. 1995 Oct-Dec;10(4):721-33.] Pulse oximetry cannot distinguish oxyhemoglobin from methemoglobin, and SpO2 is falsely elevated in methemoglobinemia. [Merck Manual, p. 370] Cyanosis becomes apparent when methemoglobinemia reaches 7-10%. Nitrates and nitrites also cause vasodilation with headache, tachycardia, hypotension, and syncope. Venous blood at the time of venipuncture appears brown-colored. Methylene blue is the antidote for patients who are not G6PD deficient and who have cardiorespiratory distress with methemoglobinemia >30%. Like CO, CN, and HS2 poisoning, methemoglobinemia is a form of chemical or systemic asphyxia. [AHLS, p. 97-99, 225] Drugs that may induce methemoglobinemia include benzocaine, lidocaine, prilocaine, chloroquine, dapsone, primaquine, sulfonamides, trimethoprim, phenazopyridine, phenacetin, and metoclopramide. [Olson, p. 262] Subacute methemoglobinemia, oxidative hemolysis, and Heinz body anemia may develop in susceptible individuals exposed to chemicals that can induce methemoglobinemia. Polycythemia may also be seen as a response to the chronic anoxia caused by the methemoglobinemia. Individuals with G6PD deficiency are susceptible to this disorder. [LaDou, p. 211] In vitro testing for potency showed the following: p-Dinitrobenzene > o-Dinitrobenzene > copper = nitrite >chlorite > chlorate. [See Hyperlink] |
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