Wednesday, 27 January 2016

Methaemoglobin

Pathogenesis
- Oxidisation of the haem of haemoglobin by free radicals or things like hydrogen peroxide and nitric oxide. 
- Shifts O2 dissociation curve to the left. 

Causes

- Hereditary / Congenital: Hb and NADH-MetHb reductase deficiency
- Acquired: 
  *   Medications eg. Amyl nitrite, Benzocaine, Dapsone, Lidocaine, Nitroglycerin, Nitroprusside, Phenacetin, Phenazopyridine, Prilocaine, Quinones, Sulfonamides (eg. sulfamethoxazole). Chloroquine. 
  *   Chemical agents eg. Aniline dye derivatives (shoe dyes, inks) Butyl nitrite, Chlorobenzene, Nitrate-containing foods, Isobutyl nitrite, Naphthalene, Nitrophenol, Nitrous gases, Silver nitrate, and Trinitrotoluene. Sodium nitrite - used in food preservation. 

Signs & Symptoms

- chocolate brown discoloration of the blood. 
- SaO2 readings go crazy 

0-10% - Features unlikely


10-30%- Mild effects 

Blue-grey ‘apparent’ central cyanosis, fatigue, dizziness, headaches

30-50% - Moderate effects – weakness, tachypnoea, tachycardia


50-70% - Severe effects 

stupor, coma, convulsions, respiratory depression, cardiac arrhythmias, acidosis

> 70% - Potentially fatal


Treatment

<20% - nothing
20 - 30% - oxygen therapy 
>30% - methylene blue
            1-2 mg/kg IV over 5 minutes - 1% (10mg/ml solution) 
            repeat up to 7 mg/kg 
SpO2 normally dives as you give the methylene blue. 
Recheck levels after an hour 

Interesting Note

Hydrogen sulfide poisoning is similar to cyanide poisoning and can be treated by inducing metHb. 




References

http://stemlynsblog.org/feeling-blue-at-st-emlyns/
http://www.rcemlearning.co.uk/modules/papa-smurf-has-a-seizure/
http://emergencymedicineireland.com/2011/07/why-methaemoglobinaemia-is-a-good-thing/
http://emergencymedicineireland.com/2011/07/why-methaemoglobinaemia-is-a-bad-thing/

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