methb
−Table of Contents
methaemoglobinaemia
introduction
- methaemoglobin (metHb) is a dysfunctional form of Hb that is incapable of carrying oxygen.
- high levels of metHb result in a form of hypoxia and can be lethal, particularly at levels > 70%.
- metHb results from either:
- an increase in the formation of the oxidised Fe3+ haem moiety
- eg. oxidising agents such as nitrites (amyl nitrite, sodium nitrite), nitrates (GTN), prilocaine local anaesthetic, dapsone, rifampicin, sulphonamides
- eg. rare congenital metHb due to abnormal Hb variant (Hb M)
- decreased conversion back to the reduced form
- eg. rare recessive congenital metHb (RCM) which are of several types:
- RCM Type 1 - decreased functional soluble form NADH cytochromeb5 methemoglobin reductase - these patients normally have metHb levels of 10-50% and include those patients formerly believed to have a separate Type 3 form.
- RCM Type 2 - decreased functional membrane-bound form as well as soluble form NADH cytochrome b5 methemoglobin reductase
- RCM Type 4 - defect in cofactor cytochromeb5 (only 1 case has every been reported!)
clinical picture
- most patients have exposure to one of the oxidising agents whether intentional self-harm, accidental, or iatrogenic.
- once metHb levels reach >30%, pulse oximeter readings generally reads 85% and becomes unreliable
- pulse oximetry reads light absorbance at only two wavelengths - 660 and 940nm
- saturation is calculated based upon the ratio of the readings from these two wavelengths
- ratios of 0.43, 1.0 and 3.4 correspond to oxygen saturations of 100%, 85% and 0% respectively assuming no metHb
- metHb has high absorbance for light at both wavelengths and high levels of metHb will result in a ratio ~1 and thus a saturation reading of 85%
- raised oxygen saturation gap
- = calculated SaO2 from arterial blood gas - pulse oximeter SpO2 reading
- also elevated in sulfhaemoglobinaemia and carboxyhaemoglobinaemia
- levels above 30% increasingly cause:
- clinically evident cyanosis
- 'chocolate-brown' blood
- clinical features of hypoxia including lactic acidosis, and eventually cardiac arrest.
Mx of methemoglobinemia
- supplemental oxygen
- remove exposure if possible (eg. remove topical prilocaine from infants skin)
- consider iv methylene blue (see below)
- ascorbic acid is too slow to be useful in the acute setting
- if methylene blue is ineffective or contraindicated, consider exchange transfusion or hyperbaric oxygen
iv methylene blue therapy
- methylene blue is reduced to leukomethylene blue by NADPH which is formed from the hexose monphosphate shunt and catalysed by NADPH metHb reductase
- leukomethylene blue then converts metHb to Hb
- high doses of methylene blue (eg. > 5mg/kg) or rapid administration will directly oxidise Hb and cause MORE metHb!
- the recommended iv doses in a patient with an intact circulation is reported to successfully convert metHb to Hb within 30-60min
- efficacy is reduced in the presence of haemolysis
contraindications to methylene blue
- G6PD deficiency
- methylene blue will NOT help in patients with a deficiency of NADPH such as those with G6PD deficiency
- use in such patients may not only increase metHb levels but may cause haemolytic anaemia
- patients on SSRIs
- methylene blue is a potent MAO inhibitor and use in patients on SSRIs may result in serotonin syndrome
indications for methylene blue
- symptomatic AND metHb levels > 20%
- asymptomatic AND metHb levels > 25%
administration
- 1-2mg/kg (0.1-0.2 mL/kg of 1% solution) iv over 5 min followed by a 20 mL normal saline flush 1)
- repeat MetHb concentrations every 30 minutes to monitor recovery
- this dose can be repeated in 1hr if still indicated but these are rarely required
1)
EMA 2010 22, 466-469
methb.txt · Last modified: 2020/05/27 06:19 by gary1