Onset with drugs/anesthetics (benzocaine spray), well-water nitrites, dapsone
Lung or cardiac disease, altitude, diving
Known congenital heart disease
Cold exposure vs mucus-membrane involvement
/
Your objective: explain the approach, recognize important warning signs, then test your recall.
Abbreviations explained
- ACS
- Acute coronary syndrome
- OMI
- Occlusion myocardial infarction
- ECG
- Electrocardiogram
- PE
- Pulmonary embolism
- POCUS
- Point-of-care ultrasound
- QTc
- QT interval corrected for heart rate
- LBBB
- Left bundle branch block
- RBBB
- Right bundle branch block
- hs-cTn
- High-sensitivity cardiac troponin
- CTA
- Computed tomography angiography
- ICU
- Intensive care unit
Filter
The jobBlue from lung, heart, or the blood itself?
- Central (tongue/lips) vs peripheral (nail beds only).
- SpO₂ stuck ~85% with a normal PaO₂ is a saturation gap — send co-oximetry (methemoglobin, CO).
- Chocolate-brown blood that does not redden on oxygen: methylene blue unless G6PD deficiency.
- Oxygen will not fix a right-to-left shunt — find it.
Cyanosis is ≥4–5 g/dL of deoxyhemoglobin (central) or local stasis (peripheral).
The trap is that methemoglobinemia and sulfhemoglobinemia look blue with a “normal” PaO₂ and an SpO₂ stuck near 85%. Chocolate-brown blood that does not redden on oxygen is the bedside clue. Treat the airway and the cause — methylene blue for methemoglobin (not in G6PD).
Methemoglobinemia Critical
Key: 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) Critical
Key: Central cyanosis that improves (or does not) with oxygen tells you shunt vs V/Q. Treat the lung/PE.
Cyanotic congenital heart disease / Eisenmenger Critical
Key: The crying infant or the adult with repaired CHD — do not over-oxygenate some mixing lesions; call cardiology. Hyperoxia test at the bedside.
Sulfhemoglobinemia Emergent
Key: Similar picture to methemoglobin but does not correct with methylene blue — phenazopyridine, sulfonamides; supportive care.
Carbon monoxide is NOT cyanosis Emergent
Key: CO skin color is unreliable; cherry-red appearance is uncommon; SpO₂ is falsely normal — CO-oximetry. Listed here because it is the classic mis-association.
Peripheral cyanosis / Raynaud / cold Common
Key: Warm the patient; central mucous membranes should be pink. If they are not, it is not “just cold.”
Tick what your patient has — the banner updates as you go.
Central (lips, tongue) vs peripheral (hands, feet only)
Work of breathing, lung findings, cardiac murmurs, clubbing (chronic)
Blood colour on the gauze
chocolate-brown vs red
Bedside
- SpO₂, ABG with co-oximetry (MetHb, COHb) — a standard ABG PaO₂ will miss methemoglobin
- Hyperoxia test in infants with suspected CHD
Labs
- CBC (polycythemia, anemia changes the cyanosis threshold), co-oximetry panel
- Check G6PD history and serotonergic medications before methylene blue when feasible; consult toxicology urgently for treatment selection and alternatives
Imaging
- CXR, CTPA as the lung/PE picture dictates
- Echo for shunt and pulmonary hypertension
Medication safety reminder
Use this as a first-pass prompt; verify all medications, doses, concentrations, contraindications, weight, pregnancy status, and local protocols before administration.
Check indication, allergy, route, renal/hepatic risk, interactions, monitoring, and local formulary.
Discharge
- Clear peripheral cyanosis from cold that reverses with warming; known stable chronic cyanotic heart disease at baseline with clinic follow-up.
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.
Use objective reassessment and the local pathway.
Before the next decision · reassess and hand over
Use these learning prompts with the presentation’s pathway. They are not discharge criteria.
- Reassess: compare symptoms, observations and examination with the initial assessment and response to treatment.
- Warning signs for Cyanosis:
- 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
- Reconsider: check unresolved findings and alternative explanations, including the pitfalls below.
- Escalate: communicate deterioration, uncertainty or needs beyond the current setting.
- Plan the transition: identify outstanding results, responsibility for follow-up, patient understanding and specific return advice.
Clinical Pearls
- Saturation gap (pulse ox vs calculated SaO₂ from PaO₂) = dyshemoglobin until co-oximetry.
- Chocolate blood + SpO₂ 85% on oxygen = methemoglobin.
- Central vs peripheral is mucous membranes vs nail beds.
- CO is not a cyanosis diagnosis — do not skip co-oximetry because the patient is not blue.
Pitfalls
- Trusting SpO₂ and PaO₂ without co-oximetry misses methemoglobin and CO.
- Methylene blue is contraindicated in G6PD deficiency and can worsen hemolysis.
- Anemic patients may not look blue until they are profoundly hypoxemic (not enough hemoglobin to make 5 g/dL deoxy).
- Do not assume “peripheral” until you have seen the tongue.
- Oxygen will not fix a right-to-left shunt — find it.
PRACTICE REFRESHER
Rapid recall
Test your first action and the dangerous diagnoses before revealing the answer.
01 · First move
Before you scroll, what needs to happen first?
Central (tongue/lips) vs peripheral (nail beds only).
02 · Immediate threats
Name at least two diagnoses that cannot wait.
Methemoglobinemia · Hypoxemic respiratory failure (PE, pneumonia, shunt, high altitude) · Cyanotic congenital heart disease / Eisenmenger
PRIVATE TO THIS DEVICE
Your learning note
Capture a weak point, a teaching pearl, or a question to take to your next shift.
Review schedule starts when marked reviewed
Selected guidance · source check 2026-09-07. These 1 source links support selected teaching points, not a complete review of this topic. Check population, setting and local protocol before applying a recommendation.
- Rosen’s Emergency Medicine, 10th ed. (2023) — cyanosis
- Goldfrank’s / ACMT methemoglobinemia guidance
- AHA pediatric CHD / hyperoxia test teaching