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Cyanosis

Blue from lung, heart, or the blood itself?

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Educational first-pass aid. Reassess the patient, confirm doses and use local protocols.

1 · First minutes

  1. Central (tongue/lips) vs peripheral (nail beds only).
  2. SpO₂ stuck ~85% with a normal PaO₂ is a saturation gap — send co-oximetry (methemoglobin, CO).
  3. Chocolate-brown blood that does not redden on oxygen: methylene blue unless G6PD deficiency.

2 · Escalate now if

  • Cyanosis of the tongue/lips that does not correct with oxygen
  • SpO₂ near 85% with a normal PaO₂ (saturation gap)
  • Infant with mixing-lesion physiology
  • Recent topical anesthetic, nitrites, or dapsone
  • Shock with cyanosis

3 · Immediate workup

  • SpO₂, ABG with co-oximetry (MetHb, COHb) — a standard ABG PaO₂ will miss methemoglobin
  • Hyperoxia test in infants with suspected CHD

4 · Don’t miss

  • Methemoglobinemia
    SpO₂ ~85% despite O₂, chocolate blood, normal PaO₂, saturation gap. Benzocaine, dapsone, nitrites, aniline. Methylene blue 1–2 mg/kg IV unless G6PD deficiency (then ascorbic acid / exchange).
  • Hypoxemic respiratory failure (PE, pneumonia, shunt, high altitude)
    Central cyanosis that improves (or does not) with oxygen tells you shunt vs V/Q. Treat the lung/PE.
  • Cyanotic congenital heart disease / Eisenmenger
    The crying infant or the adult with repaired CHD — do not over-oxygenate some mixing lesions; call cardiology. Hyperoxia test at the bedside.

5 · Disposition lane

Admit

New unexplained central cyanosis, treated methemoglobinemia under observation, pneumonia/PE on therapy.

ICU

Refractory hypoxemia, methemoglobin with shock or level typically >30% (or symptomatic at lower levels), mixing lesions, massive PE.