Methaemoglobinaemia

Q: Why are Smurf’s Blue? 

A: Methaemoglobin (MetHb) of course!

MetHb is produced by oxidisation of the Iron in Haemoglobin (Hb) from Fe2+ to Fe3+

Fe3+ prevents Hb carrying oxygen (thus produces symptoms of hypoxia)

Often due to chemical ingestion, but may also be genetic

Treated with Methyl Blue & supportive measures

Clues to Diagnosis

emcrit.org

If you see the following think Methaemagobinaemia:

  1. Cyanosis FAILS to respond to O2
  2. SaO2 Low BUT paO2 Normal/High on ABG
    • SaOoften in 80’s or less (O2 attached to Hb)
    • paO2 will increase depending on FiO2
    • Remember: paOrepresents Odissolved in plasma, NOT the Oattached to Hb
  3. Arterial blood is Dark/Brown in colour
  4. Urine may be Brown/Black

Diagnosis

  • Blood Gas: Methaemoglobin is recorded on our blood gas samples

Methaemoglobin levels & Symptoms

  • 0-10%:    Asymptomatic in general
  • 10-30%:  Mild effects –  skin blue-grey, central cyanosis, SOBOE, anxiety, fatigue, dizziness, headaches
  • 30-50%:  Moderate effects – weakness, confusion, tachypnoea, tachycardia
  • 50-70%:  Severe effects – coma, seizures, respiratory depression, cardiac arrhythmias, acidosis
  • >70%:      Potentially fatal

Causes

Treatment

  • Basics
    • High flow oxygen
    • ABC’s
    • May require ICU
  • Toxbase  – for full guidance
    • Sodium Nitrite – Phone (UK NPIS 0344 892 0111)
  • Methyl Blue
    • Any of the following criteria treat with  – methylthioninium chloride (methylene blue) 1-2 mg/kg in 100 mL of 5% dextrose over 5 min
      • All patients with life threatening toxicity
      • All symptomatic patients with a MetHb concentration ≥30%
      • Consider treatment in patients with MetHb <30% if pre-existing disease also cause hypoxia
    • Warning: methyl blue may not work in G6PD, other inherited causes Dapsone toxicity, Chlorate toxicity etc..
      • Discus with Toxbase  and call (UK NPIS 0344 892 0111)
      • Exchange transfusion may be needed

 

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