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methb

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
    • 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

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