see also: [[carbon monoxide]] , [[Cyanide]] see: [Austin tox - methaemoglobinaemia](x-devonthink-item://5929BC47-19BB-4931-BD63-503758A9CA4D) Antidote:: [[Methylene blue]] - Methemoglobin occurs when the iron atom in hemoglobin loses one electron to an oxidant, and the ferrous (Fe2+) or reduced state of iron is transformed into the ferric (Fe3+) or **oxidized state**. - Methemoglobinemia is defined as an abnormal elevation of the ==methemoglobin level above 1%.== *Etymology:* The “Met” is from “meta” ; the prefix is not directly related to the “[methyl](https://thiebes.org/2023/04/09/the-etymology-and-meaning-of-methyl-ethyl-propyl-butyl/)” in methylne blue. # Causes * [[Cyanide#Amyl Nitrite]] * **prilocaine** and benzocaine ([[Local anaesthetic systemic toxicity]]), lignocaine sometimes * Nitropruside or sidium nitrite, GTN * Dapsone * sulfa abx * [[Malaria|Antimalarials]] eg chloroquine * [[Caustic Ingestions|paraquat]] * G6PD + fava beans can cause haemolysis + methemoglobinaemia # Physiology ![[Pasted image 20230410114539.png]] - ↓ O2 carrying capacity > note: agents that cause MetHb can also cause [[haemolytic anaemia|haemolysis]] # Diagnosis - **pulse ox O2 sat unreliable:** usually reads 85-90% despite supplemental O2 and normal PaO2 on ABG Clinical features depend on MetHb fracture and compensatory response to reduced O2 carrying capacity: | MetHb % | Clinical features | | ------- | --------------------------------------------------------- | | <1 | normal | | <10 | asymptomatic | | 10 - 20 | cyanosis | | 20 - 50 | tachycardia, anxiety, headache, confusion, tachypnoea | | 50 - 70 | CNS depression, seizures, arrhythmias, metabolic acidosis | | > 70 | potentially lethal | # Management ![[9C157B33-512A-4210-8C43-9AB703ED72F0.png]] ## Methylene blue **Indications**: - Symptomatic drug-induced methaemoglobinaemia (signs of hypoxaemia with chest pain, dyspnoea or confusion). - Consider in asymptomatic patients with methaemoglobin (MetHb) levels >20%. - Met Hb >10% and co-existing anaemia / chronic lung disease / cardiac failure - note, pts with G6PD more susceptible to MetHb **Dose:** - *1–2 mg/kg* (0.1–0.2 mL/kg of 1% solution) IV slowly over 5 minutes. Follow with a normal saline flush to minimise venous irritation. - MetHb levels should be measured hourly until a consistent fall is documented. - Methaemoglobinaemia usually responds to a single dose. However, a further dose of 1–2 mg/kg may be repeated after 30–60 minutes if the initial response is inadequate. - In rare instances, such as dapsone poisoning, when methaemoglobin formation may continue for days, repeat dosing every 6–8 hours may be necessary for several days. - A single dose of 1–2 mg/kg has been suggested as adjunctive therapy in toxic shock states. **Endpoint:** Return of spontaneous circulation with stabilisation of haemodynamic parameters. Improvement in hypoxia. Infusion may be restarted if hypotension recurs on cessation. Confirm with serial methemoglobin measurements *Contraindications* - ==G6PD deficiency==: lack of NADPH in this condition causes methylene blue to be ineffective, as it cannot be reduced to leucomethylene blue. Haemolysis may also occur. - Renal impairment: dose needs to be reduced. - Methaemoglobinaemia reductase deficiency. - Nitrite-induced methaemoglobinaemia following the treatment of cyanide poisoning. - Hypersensitivity. # Related Questions ## amyl nitrite - [x] 1Q: [Cyanosis at music festival](x-devonthink-item://D4C19F6F-0718-4AD7-BDC4-34B460451B98?page=12) -- [Answer](x-devonthink-item://736EC9CD-AC9C-4588-BA1E-F4AD190CBA47?page=19) ## methaemoglobinaemia - [x] DUPLICATE Q: [Cyanosis at music festival](x-devonthink-item://D4C19F6F-0718-4AD7-BDC4-34B460451B98?page=12) -- [Answer](x-devonthink-item://736EC9CD-AC9C-4588-BA1E-F4AD190CBA47?page=19) - [ ] 4Q: [Nitrite toxicity](x-devonthink-item://1A2C485F-D4AD-4821-AFF2-452BA753717F?page=8) -- [Answer](x-devonthink-item://FD716379-1A77-4B5B-B257-1154995ECA6E?page=3)