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

If you see the following think Methaemagobinaemia:

1. Cyanosis FAILS to respond to O₂
2. SaO₂ Low BUT paO₂ Normal/High on ABG
   • SaO₂ often in 80’s or less (O₂ attached to Hb)
   • paO₂ will increase depending on FiO₂
   • Remember: paO₂ represents O₂ dissolved in plasma, NOT the O₂ attached 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

• Nitrates/Nitrites
  • Iso(amyl/pentyl/propyl/butyl) Nitrite (Poppers)
  • Sodium Nitrite (a food preservative – recently sold in “Suicide Kits”)
  • Nitric Oxide
  • GTN
• Antibiotics
  • Dapsone
  • Rifampicin
  • Antimalarials
• Local Anaesthetic (rare with lido/prilocaine)
• Others: inc. metoclopramide, pesticides
• Genetic (Rare and likely known)
  • GD6P deficiency
  • Pyruvate Kinase deficiency
  • Cytokine b5 reductase deficiency
  • Blue Fugates of Kentucky

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