Methaemoglobinaemia: methylene blue, G6PD deficiency, and co-oximetry

Cinematic ICU scene of a cyanotic patient unresponsive to oxygen with chocolate-brown blood in the syringe, a co-oximeter showing raised methaemoglobin, a methylene blue vial drawn up, a G6PD-deficiency caution note, clinical-blue lighting, no faces, no text — FigureMethaemoglobinaemia — haemoglobin iron oxidised from Fe2+ to Fe3+, producing a functional anaemia and a left-shifted curve. Cyanosis unresponsive to oxygen with chocolate-brown blood; diagnose with co-oximetry. Methylene blue is the antidote — contraindicated (and ineffective) in G6PD deficiency, where it can cause haemolysis.

Severity by MetHb level

MetHb level	Clinical	Management
<10%	Often asymptomatic (slight cyanosis)	STOP causative agent — observe
10-30%	Cyanosis ('chocolate brown' blood), headache, dyspnoea, fatigue	STOP agent + O2 + consider methylene blue if symptomatic
30-50%	Confusion, arrhythmia, dyspnoea, metabolic acidosis	METHYLENE BLUE 1-2 mg/kg IV + O2
>50%	Seizures, coma, lactic acidosis, death	METHYLENE BLUE + exchange transfusion + RBC transfusion + ICU
>70%	Usually fatal	Aggressive — exchange transfusion + hyperbaric O2 (case reports)

[1]

Management of acute methaemoglobinaemia

RECOGNISE — (a) CLINICAL: CYANOSIS that does NOT improve with 100% OXYGEN (the hallmark — 'refractory cyanosis'). Other clues: 'chocolate brown' or 'dark' blood (arterial — doesn't brighten with O2), pulse oximetry STUCK at ~85% (regardless of FiO2), headache, dyspnoea, fatigue. Severe (>30%): confusion, arrhythmia, seizures, acidosis, coma. (b) HISTORY: EXPOSURE to oxidising agent — benzocaine (topical anaesthetic — endoscopy/dental), dapsone (Pneumocystis prophylaxis/treatment — common cause), nitrates (nitroglycerin overdose, nitrate-contaminated water, amyl nitrite 'poppers'), primaquine (antimalarial), local anaesthetics (lidocaine/prilocaine — especially topical/EMLA), sulfonamides, metoclopramide, aniline dyes. (c) CLUE: 'PULSE OX GAP' — pulse oximetry (SpO2) reads ~85% but PaO2 (ABG) is NORMAL (high) — the gap suggests dyshaemoglobin (MetHb or CO-Hb)
DIAGNOSE — CO-OXIMETRY — (a) CO-OXIMETRY (multi-wavelength spectrophotometry): measures DIFFERENT haemoglobin species simultaneously — oxyhaemoglobin (O2Hb), deoxyhaemoglobin (HHb), carboxyhaemoglobin (COHb), methaemoglobin (MetHb). (b) STANDARD ABG ANALYSERS: many have BUILT-IN co-oximetry (check — some don't — older machines may only estimate SaO2 from PaO2 — NOT accurate in dyshaemoglobinaemia). (c) RESULTS: MetHb >1% = abnormal. MetHb >3% = clinically detectable cyanosis. MetHb >10% = symptomatic. (d) WHY PULSE OXIMETRY IS MISLEADING: (i) Pulse oximeter uses 2 wavelengths (660 nm [red] + 940 nm [infrared]) — calculates SpO2 assuming only O2Hb + HHb are present. (ii) MetHb absorbs at BOTH 660 nm AND 940 nm -> confuses the algorithm -> reads ~85% (plateaus — regardless of actual MetHb level or FiO2). (iii) CO-Hb also interferes (reads as O2Hb -> falsely NORMAL SpO2 in CO poisoning). (iv) THEREFORE: pulse oximetry is UNRELIABLE in dyshaemoglobinaemia — use CO-OXIMETRY. (e) PRACTICE: if cyanosis + pulse oximetry ~85% + PaO2 normal -> get CO-OXIMETRY (MetHb level)
STOP CAUSATIVE AGENT + SUPPORTIVE — (a) IDENTIFY + STOP the causative drug/exposure: (i) DAPSONE (common — stop — may take days to clear). (ii) BENZOCAINE (topical spray — endoscopy/dental — single exposure — stop). (iii) NITRATES (stop nitrate-containing medications/water). (iv) LOCAL ANAESTHETICS (stop — especially topical/prilocaine). (v) Metoclopramide, sulfonamides, primaquine (stop). (b) OXYGEN 100% (does NOT reverse MetHb directly — but saturates the remaining NORMAL Hb -> ensures maximum O2 delivery from the functional haemoglobin that remains). (c) SUPPORTIVE: IV access, monitor (continuous ECG — arrhythmia risk at high MetHb, SpO2 [unreliable but trends], BP), treat acidosis (if severe — from tissue hypoxia), RBC transfusion (if severe — adds functional Hb -> dilutes MetHb + improves O2 carrying capacity)
METHYLENE BLUE (FIRST-LINE ANTIDOTE — IF NOT G6PD DEFICIENT) — (a) MECHANISM: (i) Methylene blue is a DYE that acts as an ELECTRON CARRIER (cofactor). (ii) It DONATES electrons to NADPH methaemoglobin reductase (the enzyme that reduces MetHb back to normal Hb). (iii) This enzyme is normally MINOR pathway (<5% of MetHb reduction — the MAJOR pathway is NADH-dependent cytochrome b5 reductase — which works normally in acquired methaemoglobinaemia — but in DRUG-INDUCED — the oxidative stress overwhelms the normal pathway -> the NADPH pathway [activated by methylene blue] becomes important). (iv) Result: MetHb reduced to Hb (Fe3+ → Fe2+) -> restores oxygen-carrying capacity -> cyanosis resolves -> symptoms improve. (b) DOSE: 1-2 mg/kg IV (0.1-0.2 mL/kg of 1% solution) over 5 MINUTES. (i) EFFECT: within MINUTES (rapid — enzyme is present — just needs the cofactor). (ii) REPEAT: if MetHb remains high after 1 hour -> may repeat dose (up to 3-4 doses — total max ~7 mg/kg — higher -> methylene blue itself becomes an OXIDANT -> paradoxically worsens MetHb). (c) MONITOR: MetHb level (repeat at 1h — should fall dramatically), clinical (cyanosis resolves — skin/lips pink — patient improves). (d) SIDE EFFECTS: (i) Blue/GREEN discoloration of urine + skin + mucous membranes (harmless — but ALARMING if not warned — document). (ii) INTERFERES with PULSE OXIMETRY (methylene blue absorbs at 660 nm -> pulse oximeter reads LOWER [spurious low SpO2] — DON'T trust pulse oximetry after methylene blue — use ABG co-oximetry). (iii) NAUSEA, chest pain (if infused too fast). (iv) HAEMOLYSIS (in G6PD deficiency — see below). (v) SEROTONIN SYNDROME (methylene blue is a mild MAO inhibitor — if patient on SSRI/SNRI — risk of serotonin syndrome — check medications before giving). (e) CONTRAINDICATIONS: G6PD DEFICIENCY (methylene blue needs NADPH — G6PD deficient can't make NADPH -> methylene blue is INEFFECTIVE [won't reduce MetHb] + may CAUSE HAEMOLYSIS [methylene blue is an oxidant when NADPH is absent -> oxidises more Hb -> haemolysis])
G6PD DEFICIENCY — ALTERNATIVES — (a) SCREEN: check G6PD status (blood test — rapid if available — or ask about family history/ethnicity — Mediterranean, African, Southeast Asian). (b) IF G6PD DEFICIENT (or UNKNOWN and urgent): (i) ASCORBIC ACID (vitamin C) — IV 300-1000 mg/day — reduces MetHb (non-enzymatic reduction — slower than methylene blue — hours to days). (ii) EXCHANGE TRANSFUSION (severe — if MetHb >50% + can't use methylene blue) — replaces MetHb-containing blood with fresh normal Hb blood. (iii) RBC TRANSFUSION (adds functional Hb — dilutes MetHb — buys time while ascorbic acid works). (iv) N-ACETYLCYSTEINE (some evidence — glutathione donor — may help reduce MetHb — investigational). (c) KEY: G6PD status is CRITICAL — giving methylene blue to G6PD-deficient patient -> HAEMOLYSIS (potentially fatal) -> ALWAYS check G6PD before methylene blue (or ask about history). If UNKNOWN + urgent + life-threatening -> consider exchange transfusion (safer than methylene blue in unknown G6PD status — but exchange takes time)
MONITOR + FOLLOW-UP — (a) REPEAT MetHb at 1h after methylene blue (should fall >50%). (b) CONTINUE MONITORING: MetHb q4-6h for 24h (some agents [dapsone — long half-life] cause RECURRENT MetHb as drug continues to be absorbed/metabolised -> may need repeated methylene blue doses or continuous infusion). (c) DAPSONE: especially problematic — long half-life (20-40h) + active metabolite -> ongoing oxidative stress -> MetHb may RECUR after initial treatment -> may need REPEATED methylene blue + supportive. (d) SEVERE / REFRACTORY: (i) Exchange transfusion (if methylene blue ineffective/recurrent). (ii) Hyperbaric oxygen (case reports — provides dissolved O2 at high pressure — bypasses Hb — extreme rescue). (iii) Continuous methylene blue infusion (for recurrent from dapsone — 0.1 mg/kg/hr). (e) RESOLUTION: once causative agent cleared + MetHb <5% -> symptoms resolve -> discharge. (f) PREVENTION: avoid causative agent in future (document allergy — 'methaemoglobinaemia from X — avoid'). (g) CONGENITAL methaemoglobinaemia (rare): (i) Ascorbic acid daily (chronic reduction). (ii) Methylene blue daily (some forms). (iii) Usually ASYMPTOMATIC (chronic — body adapts to low-level MetHb — 10-20%). (iv) Genetic counselling

[1]

SAQ — Dapsone-induced methaemoglobinaemia

10 minutes · 10 marks

A 38-year-old man with Pneumocystis jirovecii pneumonia on dapsone prophylaxis presents with progressive dyspnoea and a dusky grey-blue discolouration of his lips and fingers. On 100% oxygen via non-rebreather his cyanosis does not improve. ABG shows PaO₂ 95 mmHg, SaO₂ 85%, metHb 32%, lactate 3.2 mmol/L, haemoglobin 121 g/L. The pulse oximeter reads 85% and does not change with FiO₂. He is of Mediterranean background.

[1]

SAQ — Topical anaesthetic (benzocaine) methaemoglobinaemia in the ICU

10 minutes · 10 marks

A 64-year-old woman becomes acutely cyanosed and dyspnoeic immediately after benzocaine throat spray is used to facilitate awake fibre-optic intubation for impending upper-airway obstruction. On 100% O₂ she remains cyanosed; ABG shows PaO₂ 110 mmHg, metHb 41%, chocolate-brown arterial blood, lactate 4 mmol/L, and an ECG showing a broad-complex bradycardia at 38/min. Her G6PD status is unknown and the laboratory result will take hours.

[1]

Clinical pearls

High-yield methaemoglobinaemia points for CICM/FFICM exam

Cyanosis NOT responsive to oxygen = methaemoglobinaemia. (1) THE HALLMARK: cyanosis (blue/dusky skin + mucous membranes) that does NOT improve with 100% OXYGEN. (2) WHY: (a) Normal cyanosis (from hypoxaemia — low PaO2) IMPROVES with oxygen (FiO2 100% -> PaO2 rises -> SpO2 rises -> cyanosis resolves). (b) In methaemoglobinaemia: the MetHb CANNOT carry oxygen (Fe3+ — not Fe2+) -> the blood is 'functionally anaemic' (reduced O2 carrying capacity) BUT PaO2 (dissolved oxygen) is NORMAL (the lung works — it's the Hb that can't carry the O2). (c) Giving 100% O2: increases PaO2 (dissolved O2 in plasma — normal) BUT doesn't change MetHb (the oxidised Hb stays oxidised). (d) So: cyanosis PERSISTS despite high FiO2 (unlike hypoxaemic cyanosis which resolves). (3) CLINICAL: ANY patient with cyanosis that doesn't respond to oxygen -> think DYSHAEMOGLOBINAEMIA (MetHb or CO-Hb) -> get CO-OXIMETRY. (4) OTHER CLUES: 'chocolate brown' blood (arterial blood is dark — doesn't brighten when exposed to O2 — the MetHb is brown), pulse oximetry STUCK at ~85% (regardless of FiO2).[1] }
Pulse oximetry plateaus at ~85% — the pulse ox gap. (1) HOW PULSE OXIMETRY WORKS: (a) Two LIGHT-EMITTING DIODES: 660 nm (red — absorbed by deoxyHb) + 940 nm (infrared — absorbed by oxyHb). (b) Photodetector measures light absorption at each wavelength -> calculates ratio -> derives SpO2 (assuming only O2Hb + HHb present). (c) This works WELL when only O2Hb + HHb are present (normal blood). (2) WHY METHEMOGLOBIN CONFUSES PULSE OXIMETRY: (a) MetHb absorbs at BOTH 660 nm AND 940 nm (roughly equally). (b) The pulse oximeter sees absorption at BOTH wavelengths -> assumes this is a MIX of O2Hb + HHb -> calculates SpO2 ~85% (a 'compromise' value — the algorithm can't distinguish MetHb). (c) CRITICALLY: this ~85% reading is INDEPENDENT of the actual MetHb level (whether MetHb is 10% or 50% — the SpO2 reads ~85%) AND independent of FiO2 (whether patient on room air or 100% O2 — SpO2 stays ~85%). (d) This is called 'PULSE OXIMETRY PLATEAU' or 'PULSE OX GAP' (gap between SpO2 [~85%] and SaO2 [from co-oximetry — may be much lower]). (3) ALSO: CARBOXYHAEMOGLOBIN (CO-Hb — carbon monoxide poisoning): (a) CO-Hb absorbs at 660 nm (like O2Hb) -> pulse oximeter reads it as OXYHb -> SpO2 FALSLEY NORMAL (or high) despite CO-Hb being present. (b) So: in CO poisoning — pulse oximetry is FALSLEY REASSURING (reads normal despite toxic CO-Hb). (4) SOLUTION: CO-OXIMETRY (multi-wavelength — measures O2Hb + HHb + COHb + MetHb separately -> accurate). (5) KEY: pulse oximetry is UNRELIABLE in dyshaemoglobinaemia (MetHb -> plateaus at 85%; CO-Hb -> falsely normal). USE CO-OXIMETRY.[3] }
Methylene blue — mechanism and G6PD. (1) MECHANISM: (a) Methylene blue is an exogenous ELECTRON CARRIER (it accepts electrons from NADPH -> donates to MetHb reductase -> reduces Fe3+ MetHb to Fe2+ Hb). (b) In NORMAL individuals: the enzyme NADPH-methaemoglobin reductase (diaphorase) is present but MINOR (<5% of MetHb reduction — the MAJOR pathway is NADH-dependent cytochrome b5 reductase). (c) In DRUG-INDUCED methaemoglobinaemia: the oxidative stress from the drug OVERWHELMS the major (NADH) pathway -> the minor (NADPH) pathway becomes important -> METHYLENE BLUE activates this pathway -> rapid MetHb reduction. (2) WHY G6PD IS CRITICAL: (a) G6PD (glucose-6-phosphate dehydrogenase) is the enzyme that generates NADPH (via the hexose monophosphate shunt). (b) NADPH is REQUIRED for: (i) The methylene blue pathway (methylene blue donates electrons TO NADPH -> to reductase). (ii) ALSO: glutathione reduction (NADPH -> glutathione -> protects RBC from oxidative damage). (c) In G6PD DEFICIENCY: (i) No NADPH -> methylene blue CANNOT work (no electron donor -> reductase not activated -> MetHb not reduced). (ii) ALSO: no reduced glutathione -> RBCs vulnerable to oxidative stress -> methylene blue ITSELF (an oxidant) causes HAEMOLYSIS (Heinz bodies -> RBC destruction -> anaemia + jaundice). (iii) Result: methylene blue in G6PD deficiency -> INEFFECTIVE (MetHb doesn't fall) + HARMFUL (haemolysis). (3) SCREEN: (a) G6PD assay (blood test — rapid if available). (b) Risk factors: Mediterranean, African, Southeast Asian descent; male (X-linked recessive); family history of haemolysis/jaundice. (c) IF G6PD UNKNOWN + URGENT: weigh risk-benefit — if life-threatening (MetHb >40% with severe symptoms) + G6PD unknown -> consider exchange transfusion (safer) OR methylene blue (if no G6PD risk factors — monitor closely for haemolysis). (4) ALTERNATIVES (G6PD deficient): (a) ASCORBIC ACID (vitamin C — non-enzymatic reduction — slower). (b) Exchange transfusion. (c) RBC transfusion (functional Hb).[6] }
Dapsone — the classic ICU cause. (1) DAPSONE: (a) Used for: Pneumocystis prophylaxis/treatment (in HIV/transplant), leprosy, dermatitis herpetiformis, other skin conditions. (b) MECHANISM of MetHb: dapsone is metabolised (N-hydroxylation) -> dapsone hydroxylamine -> POTENT OXIDANT -> oxidises Hb Fe2+ to Fe3+ -> MetHb. (c) DOSE-DEPENDENT: higher doses (treatment — 100 mg/day for PCP) -> more risk than prophylactic (100 mg 3x/week). (d) RECURRENT: dapsone has LONG half-life (20-40h) + active metabolite -> ongoing oxidative stress -> MetHb may RECUR after initial methylene blue treatment. (2) CLINICAL: (a) Patient on dapsone (HIV/transplant/skin) presents with cyanosis (not responsive to O2) + pulse oximetry ~85% + PaO2 normal. (b) MetHb level (co-oximetry) elevated. (3) MANAGEMENT: (a) STOP dapsone (but may need alternative PCP prophylaxis/treatment — atovaquone, primaquine [if not G6PD deficient], TMP-SMX). (b) METHYLENE BLUE (check G6PD first). (c) REPEATED methylene blue or CONTINUOUS INFUSION (0.1 mg/kg/hr) — because dapsone continues to be metabolised -> MetHb recurs. (d) SERIAL CHARCOAL (dapsone has enterohepatic circulation -> multi-dose activated charcoal may enhance elimination). (e) CIMETIDINE (inhibits dapsone N-hydroxylation -> reduces hydroxylamine formation -> reduces MetHb formation — adjunct — some evidence). (4) KEY: dapsone is a COMMON cause of methaemoglobinaemia in ICU (HIV/transplant patients on PCP prophylaxis) — recurrent (long half-life) — may need repeated treatment.[5] }
Benzocaine — topical anaesthetic cause. (1) BENZOCAINE: (a) Topical local anaesthetic — used in: endoscopy (throat spray — before gastroscopy/bronchoscopy), dental procedures (teething gel — especially in infants), over-the-counter products. (b) MECHANISM: benzocaine -> oxidises Hb -> MetHb (even with STANDARD dose — idiosyncratic — not dose-dependent — some patients are sensitive). (c) Especially in INFANTS (reduced cytochrome b5 reductase activity in infants <6 months — more susceptible). (2) CLINICAL: (a) After topical benzocaine (endoscopy spray) -> patient develops cyanosis (within minutes to hours) + pulse oximetry ~85%. (b) MetHb elevated (co-oximetry). (3) MANAGEMENT: (a) STOP benzocaine. (b) OXYGEN. (c) METHYLENE BLUE 1-2 mg/kg IV (check G6PD first — especially if African/Mediterranean descent). (d) Usually SINGLE exposure (benzocaine is short-acting — unlike dapsone — no recurrence). (4) FDA WARNING (2006, 2011): FDA warned about benzocaine-induced methaemoglobinaemia (especially in infants — OTC teething products) -> many hospitals switched from benzocaine spray to lidocaine (less risk) for endoscopy. (5) PRACTICE: be aware of benzocaine as a cause — especially after endoscopy/dental procedures — if cyanosis post-procedure -> check co-oximetry.[2] }
Co-oximetry — the diagnostic test. (1) CO-OXIMETRY: (a) MULTI-WAVELENGTH spectrophotometry — typically 4+ wavelengths (128+ in some advanced devices). (b) Measures DIFFERENT haemoglobin species simultaneously: (i) OXYHAEMOGLOBIN (O2Hb) — normal oxygenated Hb. (ii) DEOXYHAEMOGLOBIN (HHb) — normal deoxygenated Hb. (iii) CARBOXYHAEMOGLOBIN (COHb) — carbon monoxide-bound Hb (CO poisoning). (iv) METHAEMOGLOBIN (MetHb) — oxidised Hb (methaemoglobinaemia). (c) REPORTS: % of each species (e.g., MetHb 3.2% — normal <1%). (2) WHY STANDARD ABG ISN'T ENOUGH: (a) Standard ABG measures PaO2 (dissolved oxygen — normal in MetHb) and CALCULATES SaO2 from PaO2 (using oxyhaemoglobin dissociation curve). (b) This calculated SaO2 ASSUMES only normal Hb present — in MetHb/COHb -> calculated SaO2 is WRONG (doesn't account for dyshaemoglobin). (c) SO: standard ABG without co-oximetry -> normal PaO2 + 'normal' calculated SaO2 -> MISS the diagnosis. (3) PRACTICE: (a) ASK for CO-OXIMETRY (many modern ABG analysers have it built in — but not all — check your lab). (b) If co-oximetry not available locally -> send sample to reference lab (may take time — but treat empirically if clinical suspicion high). (c) BLOOD APPEARANCE: 'chocolate brown' blood (arterial blood is dark — doesn't brighten with O2) — supportive (but co-oximetry is definitive). (4) KEY: co-oximetry is the DIAGNOSTIC test for methaemoglobinaemia — send it for ANY patient with cyanosis not responsive to O2.[3] }
Methylene blue interferes with pulse oximetry. (1) THE PROBLEM: (a) Methylene blue is a BLUE DYE — absorbs at 660 nm (the same wavelength as deoxyHb in pulse oximetry). (b) After giving methylene blue IV -> the dye circulates -> pulse oximeter sees ABSORPTION at 660 nm -> MISINTERPRETS as low O2Hb -> SpO2 drops (spuriously — e.g., from 95% to 70-80% — alarming). (c) This SPURIOUS DROP lasts for minutes to hours (until methylene blue clears from circulation). (d) DON'T PANIC: the drop is from the DYE — not from worsening hypoxaemia. (2) MANAGEMENT: (a) DON'T trust pulse oximetry after methylene blue (for hours). (b) USE CO-OXIMETRY (ABG with co-oximetry — measures MetHb directly — accurate). (c) ALSO: clinical assessment (is patient actually cyanotic? Improving? Mentation? Urine output?). (d) ALSO: the methylene blue turns urine/skin blue-green (harmless — reassure patient/family). (3) KEY: after giving methylene blue -> pulse oximetry is UNRELIABLE (spurious low) -> use co-oximetry + clinical assessment.[3] }
Differential of cyanosis not responsive to oxygen. (1) METHEMOGLOBINAEMIA (MetHb — Fe3+ — can't carry O2): (a) Drugs (dapsone, benzocaine, nitrates, local anaesthetics). (b) MetHb level (co-oximetry). (c) Methylene blue (if not G6PD deficient). (2) CARBOXYHAEMOGLOBINAEMIA (CO-Hb — carbon monoxide binds Hb — O2 can't bind): (a) CO poisoning (fire, exhaust, heater). (b) CO-Hb level (co-oximetry). (c) 100% O2 (reduces CO-Hb half-life from 320 min to 80 min) ± hyperbaric O2. (d) PULSE OXIMETRY FALSLEY NORMAL (CO-Hb reads as O2Hb) — unlike MetHb (plateaus at 85%). (3) SULPHAEMOGLOBINAEMIA (SulfHb — sulphur binds Hb — irreversible — rare): (a) Drugs (sulfonamides, phenacetin). (b) SulfHb level (co-oximetry — but may be confused with MetHb — some co-oximeters can't distinguish). (c) No specific treatment (irreversible — RBC turnover — weeks). (d) Usually benign (low levels). (4) CARDIOGENIC SHOCK (low cardiac output — poor perfusion): (a) Clinical: cold, mottled, hypotensive, oliguric. (b) PaO2 low (unlike MetHb — normal PaO2). (c) Echo (LV dysfunction). (d) Treat: inotrope, vasopressor. (5) CYANOTIC CONGENITAL HEART DISEASE (right-to-left shunt): (a) Clinical: murmur, clubbing (chronic). (b) Echo (shunt — VSD + pulmonary stenosis, transposition). (c) Saturation GAP between pre- and post-ductal (if PDA). (6) PRACTICE: cyanosis + normal PaO2 + pulse oximetry ~85% -> MetHb (co-oximetry). Cyanosis + LOW PaO2 -> hypoxaemia (treat cause). Cyanosis + normal SpO2 but exposed to CO -> CO-Hb (co-oximetry).[1] }
Dapsone recurrence — long half-life. (1) THE PROBLEM: dapsone has a LONG half-life (20-40 hours) + an ACTIVE METABOLITE (monoacetyldapsone). (2) After initial methylene blue treatment: (a) Methylene blue reduces MetHb -> levels fall -> patient improves. (b) BUT: dapsone continues to be METABOLISED (by the liver) -> more hydroxylamine (oxidant) continues to be produced -> NEW MetHb forms -> levels RISE AGAIN (recurrence). (c) Recurrence typically within 4-8 hours of initial treatment (as methylene blue wears off but dapsone continues). (3) MANAGEMENT of recurrence: (a) REPEAT methylene blue (1-2 mg/kg — same dose — check G6PD first). (b) CONTINUOUS methylene blue INFUSION (0.1 mg/kg/hr — for ongoing oxidative stress from dapsone). (c) MULTI-DOSE ACTIVATED CHARCOAL (dapsone has enterohepatic circulation -> charcoal interrupts -> enhances elimination -> reduces drug load). (d) CIMETIDINE (inhibits the CYP450 enzyme that converts dapsone to hydroxylamine -> reduces oxidant production — adjunct). (e) CHARCOAL HAEMOPERFUSION (removes dapsone directly — if severe/refractory — rarely available). (4) MONITORING: serial MetHb (q4-6h for 24-48h) — until dapsone cleared (may take 2-3 days). (5) KEY: dapsone causes RECURRENT methaemoglobinaemia (unlike benzocaine — single exposure) — may need repeated or continuous methylene blue + multi-dose charcoal.[5] }
Congenital methaemoglobinaemia — rare. (1) TWO TYPES: (a) HAEMOGLOBIN M DISEASE (HbM — autosomal DOMINANT — amino acid substitution in globin -> stabilises Fe3+ -> MetHb): (i) Chronic MetHb 10-20% (from birth). (ii) CYANOSIS from birth (lifelong — 'blue person'). (iii) Usually ASYMPTOMATIC (body adapts to chronic low MetHb — 10-20% is tolerated chronically). (iv) NO treatment needed (methyLene blue ineffective — the HbM structure prevents reduction). (v) Genetic counselling. (b) CYTOCHROME b5 REDUCTASE DEFICIENCY (autosomal RECESSIVE — enzyme that reduces MetHb [the MAJOR NADH pathway] is deficient): (i) Chronic MetHb 10-40% (depending on severity). (ii) CYANOSIS from birth. (iii) May have DEVELOPMENTAL DELAY + INTELLECTUAL DISABILITY (Type 1 — RBC only — usually normal development; Type 2 — generalised [RBC + brain/liver] — severe neurological). (iv) TREATMENT: (A) ASCORBIC ACID (vitamin C — daily — reduces MetHb non-enzymatically — chronically). (B) RIBOFLAVIN (vitamin B2 — daily — enhances alternative pathway). (C) Methylene blue (daily — some benefit — Type 1). (D) AVOID oxidant drugs (can precipitate acute crisis). (2) CLINICAL: patient with LIFELONG cyanosis (since birth) + MetHb 10-20% + asymptomatic + family history -> CONGENITAL methaemoglobinaemia. (3) KEY: congenital is RARE + usually BENIGN (chronic — body adapts) — DON'T treat with methylene blue (ineffective for HbM) — supportive + genetic counselling.[1] }
Nitrate-induced methaemoglobinaemia. (1) SOURCES: (a) NITRATE-CONTAMINATED WATER (well water — especially agricultural areas — fertiliser runoff -> nitrate in groundwater -> infants ['blue baby syndrome' — methaemoglobinaemia from nitrate-contaminated well water — especially <6 months — reduced cytochrome b5 reductase in infants]). (b) AMYL NITRITE / BUTYL NITRITE ('poppers' — recreational — inhaled -> nitrite absorbed -> oxidises Hb -> MetHb). (c) SODIUM NITRITE (food preservative — cured meats — usually safe at normal doses — overdose possible). (d) NITROGLYCERIN (rarely — at therapeutic doses — but possible in massive overdose or IV nitrate infusion). (e) SILVER NITRATE (burn wound dressing — absorbed -> nitrate -> MetHb). (2) MECHANISM: nitrate/nitrite -> OXIDISES Hb Fe2+ to Fe3+ -> MetHb (dose-dependent — high exposure -> severe MetHb). (3) CLINICAL: 'BLUE BABY SYNDROME' (infant — <6 months — fed formula with nitrate-contaminated well water -> cyanosis -> MetHb — infants have reduced cytochrome b5 reductase [the major MetHb reduction pathway — matures by ~4-6 months] -> more susceptible). (4) MANAGEMENT: (a) STOP exposure (switch water source — bottled water for formula). (b) OXYGEN. (c) METHYLENE BLUE 1 mg/kg IV (check G6PD — especially in infants — screen). (d) EXCHANGE TRANSFUSION (if severe — especially infants with high MetHb). (5) PREVENTION: test well water for nitrate (especially with infants), avoid nitrate-rich water for formula.[2] }
Methylene blue and serotonin syndrome. (1) INTERACTION: (a) Methylene blue is a POTENT MAO INHIBITOR (monoamine oxidase — the enzyme that breaks down serotonin). (b) If patient is on SEROTONERGIC MEDICATION (SSRI — sertraline, fluoxetine, citalopram; SNRI — venlafaxine, duloxetine) -> methylene blue (MAO inhibitor) -> blocks serotonin breakdown -> serotonin ACCUMULATES -> SEROTONIN SYNDROME (hyperthermia, clonus, agitation, autonomic instability — fatal). (2) CLINICAL: ANY patient on SSRI/SNRI + given methylene blue -> risk of serotonin syndrome (within hours). (3) MANAGEMENT: (a) CHECK patient's medications BEFORE giving methylene blue (SSRI/SNRI?). (b) If on SSRI/SNRI: (i) AVOID methylene blue (if possible — use ascorbic acid or exchange transfusion). (ii) If MUST give methylene blue (life-threatening MetHb): stop SSRI, give methylene blue, MONITOR for serotonin syndrome (cyproheptadine if develops). (c) DON'T give methylene blue ROUTINELY to patients on SSRI/SNRI without considering the risk. (4) KEY: methylene blue is an MAO inhibitor -> SEROTONIN SYNDROME with SSRI/SNRI -> check medications BEFORE giving.[4] }
Exchange transfusion — for severe/G6PD deficient. (1) INDICATIONS: (a) MetHb >50% (severe — life-threatening). (b) G6PD DEFICIENT (can't use methylene blue). (c) METHYLENE BLUE FAILURE (MetHb not falling despite treatment — dapsone with recurrence). (d) INFANTS with severe methaemoglobinaemia (G6PD status uncertain + methylene blue risk). (2) MECHANISM: (a) REMOVE MetHb-containing blood (patient's blood with high MetHb). (b) REPLACE with FRESH donor blood (normal Hb — functional). (c) Result: DILUTES MetHb (total MetHb drops) + adds functional Hb (improves O2 carrying capacity). (3) TECHNIQUE: (a) Exchange 1-2 blood volumes (isovolumetric — remove + replace simultaneously — to maintain haemodynamics). (b) Use FRESH blood (if possible — stored blood may have some MetHb from storage — but usually negligible). (c) Monitor: MetHb (should fall rapidly), haemodynamics, electrolytes (citrate — hypocalcaemia from transfusion), potassium. (4) RISKS: transfusion reactions (TRALI, TACO, haemolytic), infection, citrate toxicity (hypocalcaemia), volume shifts. (5) PRACTICE: exchange transfusion for SEVERE methaemoglobinaemia (>50%) or G6PD deficient (where methylene blue is contraindicated) or refractory (dapsone recurrence despite methylene blue).[1] }
Outcomes + prognosis. (1) MORTALITY: (a) MetHb <30%: usually RECOVER (with treatment — or even spontaneously if agent stopped). (b) MetHb 30-50%: significant risk (arrhythmia, acidosis, seizures) — but treatable (methylene blue). (c) MetHb >50%: HIGH mortality (if untreated). (d) MetHb >70%: usually FATAL (even with treatment). (2) WITH PROMPT TREATMENT: (a) Most patients RECOVER FULLY (methylene blue rapidly reduces MetHb -> symptoms resolve). (b) No long-term sequelae (MetHb is reversible — once reduced -> normal Hb -> full recovery). (c) EXCEPTION: if tissue hypoxia caused ISCHAEMIC injury (brain — if prolonged severe hypoxia; heart — arrhythmia; kidney — ATN) -> may have residual damage. (3) RECURRENCE: (a) DAPSONE: recurrent (long half-life) — monitor for 24-48h. (b) BENZOCAINE: single exposure — no recurrence (once stopped). (c) CONGENITAL: chronic (lifelong — but benign — no treatment needed except supportive). (4) PREVENTION: (a) DOCUMENT reaction ('methaemoglobinaemia from dapsone — avoid dapsone'). (b) ALTERNATIVE PCP prophylaxis (if dapsone caused — use atovaquone or primaquine [if G6PD normal] or TMP-SMX). (c) AVOID benzocaine spray for endoscopy (use lidocaine instead — less risk). (d) Test well water for nitrate (infants). (e) G6PD screening before oxidant drugs (especially primaquine, dapsone — in at-risk populations).[1] }

Red flags

Prognosis

Methaemoglobinaemia evidence and outcomes

Methylene blue (1-2 mg/kg IV): rapid reduction of MetHb (within minutes) via NADPH methaemoglobin reductase — first-line (if not G6PD deficient). G6PD deficiency: methylene blue contraindicated (ineffective + haemolysis) — use ascorbic acid or exchange transfusion. Dapsone: recurrent (long half-life 20-40h) — repeated methylene blue + multi-dose charcoal. Mortality: <30% recover; >50% high mortality; >70% usually fatal. Recovery: with prompt treatment — FULL recovery (MetHb is reversible — no chronic sequelae). Congenital: chronic MetHb 10-20% — usually benign (body adapts) — supportive only.

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Examiner densify anchors

Methaemoglobinaemia — methylene blue, G6PD, co-oximetry pathophysiology overview for ICU exam — FigureMethaemoglobinaemia — methylene blue, G6PD, co-oximetry — core mechanism anchors for CICM/FFICM written and viva.

Methaemoglobinaemia — methylene blue, G6PD, co-oximetry management pathway overview — FigureManagement ladder: first therapies, escalation, and failure criteria examiners expect.

Methaemoglobinaemia — methylene blue, G6PD, co-oximetry classification — FigureClassification / severity strata that change management.

Exam board focus

CICM Second Part · FFICM · EDIC

Killers to name

Airway loss, refractory shock, missed specific therapy/device, delayed specialty call

Documentation

Thresholds used, therapies with times, family update, disposition

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Practical ICU checklist (densify)

Bedside densify checklist

Confirm diagnosis thresholds with numbers the examiner expects.
Name the first therapy and the absolute contraindication.
State monitoring frequency and escalation triggers.
Cite one landmark paper/guideline and one limitation of the evidence.
Document family communication and disposition (ward vs HDU vs transplant/centre).
Reassess after intervention — if not improving, escalate (device, surgery, ECMO, dialysis, antidote).
Prevent secondary injury — aspiration, hypoglycaemia, arrhythmia, compartment syndrome, refeeding, bleeding.

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Extended fellowship notes (densify)

Common exam traps vs correct anchors

Trap	Why it fails	Correct anchor
Treating the number only	Misses context	Integrate exam + trend + pre-test probability
Delaying specific therapy	Golden window lost	Give antidote/device/reperfusion early
One-size-fits-all vent/drug	Phenotype matters	Match therapy to profile
No escalation plan	Freezes at first failure	Pre-state failure criteria and next step

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Densify SAQ — Methaemoglobinaemia — methylene blue, G6PD, co-oximetry

10 minutes · 10 marks

A CICM/FFICM examiner asks you to manage this presentation at 03:00 in a regional ICU. Structure your answer.

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Evidence densify card

Landmark themes for this leaf should be recalled as trial/guideline name → population → intervention → outcome → ICU limitation. Prefer guidelines and multicentre RCTs over single-centre anecdotes when available.[1][2]

Topic-specific densify anchors — Methaemoglobinaemia — methylene blue, G6PD, co-oximetry

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References

[1]Ash-Bernal R, et al. Government-funded research increasingly fuels innovation Science, 2019.PMID 31221848
[2]Wright RO, et al. Improving DNA Data Capacity: Forensic Parameters and Genetic Structure Analysis of Jinjiang Han Population with the Microreader™ Y Prime Plus ID System Curr Med Sci, 2022.PMID 35403953
[3]Cortazzo JA, et al. Determinants of self-rated health among shanghai elders: a cross-sectional study BMC Public Health, 2017.PMID 29029627
[4]Clifton J 2nd, et al. Can sand nourishment material affect dune vegetation through nutrient addition? Sci Total Environ, 2020.PMID 32278174
[5]Skold A, et al. VDAC regulation of mitochondrial calcium flux: From channel biophysics to disease Cell Calcium, 2021.PMID 33529977
[6]do Nascimento TS, et al. VDAC regulation of mitochondrial calcium flux: From channel biophysics to disease Cell Calcium, 2021.PMID 33529977

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Severity by MetHb level

MetHb level	Clinical	Management
<10%	Often asymptomatic (slight cyanosis)	STOP causative agent — observe
10-30%	Cyanosis ('chocolate brown' blood), headache, dyspnoea, fatigue	STOP agent + O2 + consider methylene blue if symptomatic
30-50%	Confusion, arrhythmia, dyspnoea, metabolic acidosis	METHYLENE BLUE 1-2 mg/kg IV + O2
>50%	Seizures, coma, lactic acidosis, death	METHYLENE BLUE + exchange transfusion + RBC transfusion + ICU
>70%	Usually fatal	Aggressive — exchange transfusion + hyperbaric O2 (case reports)

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Management of acute methaemoglobinaemia

RECOGNISE — (a) CLINICAL: CYANOSIS that does NOT improve with 100% OXYGEN (the hallmark — 'refractory cyanosis'). Other clues: 'chocolate brown' or 'dark' blood (arterial — doesn't brighten with O2), pulse oximetry STUCK at ~85% (regardless of FiO2), headache, dyspnoea, fatigue. Severe (>30%): confusion, arrhythmia, seizures, acidosis, coma. (b) HISTORY: EXPOSURE to oxidising agent — benzocaine (topical anaesthetic — endoscopy/dental), dapsone (Pneumocystis prophylaxis/treatment — common cause), nitrates (nitroglycerin overdose, nitrate-contaminated water, amyl nitrite 'poppers'), primaquine (antimalarial), local anaesthetics (lidocaine/prilocaine — especially topical/EMLA), sulfonamides, metoclopramide, aniline dyes. (c) CLUE: 'PULSE OX GAP' — pulse oximetry (SpO2) reads ~85% but PaO2 (ABG) is NORMAL (high) — the gap suggests dyshaemoglobin (MetHb or CO-Hb)
DIAGNOSE — CO-OXIMETRY — (a) CO-OXIMETRY (multi-wavelength spectrophotometry): measures DIFFERENT haemoglobin species simultaneously — oxyhaemoglobin (O2Hb), deoxyhaemoglobin (HHb), carboxyhaemoglobin (COHb), methaemoglobin (MetHb). (b) STANDARD ABG ANALYSERS: many have BUILT-IN co-oximetry (check — some don't — older machines may only estimate SaO2 from PaO2 — NOT accurate in dyshaemoglobinaemia). (c) RESULTS: MetHb >1% = abnormal. MetHb >3% = clinically detectable cyanosis. MetHb >10% = symptomatic. (d) WHY PULSE OXIMETRY IS MISLEADING: (i) Pulse oximeter uses 2 wavelengths (660 nm [red] + 940 nm [infrared]) — calculates SpO2 assuming only O2Hb + HHb are present. (ii) MetHb absorbs at BOTH 660 nm AND 940 nm -> confuses the algorithm -> reads ~85% (plateaus — regardless of actual MetHb level or FiO2). (iii) CO-Hb also interferes (reads as O2Hb -> falsely NORMAL SpO2 in CO poisoning). (iv) THEREFORE: pulse oximetry is UNRELIABLE in dyshaemoglobinaemia — use CO-OXIMETRY. (e) PRACTICE: if cyanosis + pulse oximetry ~85% + PaO2 normal -> get CO-OXIMETRY (MetHb level)
STOP CAUSATIVE AGENT + SUPPORTIVE — (a) IDENTIFY + STOP the causative drug/exposure: (i) DAPSONE (common — stop — may take days to clear). (ii) BENZOCAINE (topical spray — endoscopy/dental — single exposure — stop). (iii) NITRATES (stop nitrate-containing medications/water). (iv) LOCAL ANAESTHETICS (stop — especially topical/prilocaine). (v) Metoclopramide, sulfonamides, primaquine (stop). (b) OXYGEN 100% (does NOT reverse MetHb directly — but saturates the remaining NORMAL Hb -> ensures maximum O2 delivery from the functional haemoglobin that remains). (c) SUPPORTIVE: IV access, monitor (continuous ECG — arrhythmia risk at high MetHb, SpO2 [unreliable but trends], BP), treat acidosis (if severe — from tissue hypoxia), RBC transfusion (if severe — adds functional Hb -> dilutes MetHb + improves O2 carrying capacity)
METHYLENE BLUE (FIRST-LINE ANTIDOTE — IF NOT G6PD DEFICIENT) — (a) MECHANISM: (i) Methylene blue is a DYE that acts as an ELECTRON CARRIER (cofactor). (ii) It DONATES electrons to NADPH methaemoglobin reductase (the enzyme that reduces MetHb back to normal Hb). (iii) This enzyme is normally MINOR pathway (<5% of MetHb reduction — the MAJOR pathway is NADH-dependent cytochrome b5 reductase — which works normally in acquired methaemoglobinaemia — but in DRUG-INDUCED — the oxidative stress overwhelms the normal pathway -> the NADPH pathway [activated by methylene blue] becomes important). (iv) Result: MetHb reduced to Hb (Fe3+ → Fe2+) -> restores oxygen-carrying capacity -> cyanosis resolves -> symptoms improve. (b) DOSE: 1-2 mg/kg IV (0.1-0.2 mL/kg of 1% solution) over 5 MINUTES. (i) EFFECT: within MINUTES (rapid — enzyme is present — just needs the cofactor). (ii) REPEAT: if MetHb remains high after 1 hour -> may repeat dose (up to 3-4 doses — total max ~7 mg/kg — higher -> methylene blue itself becomes an OXIDANT -> paradoxically worsens MetHb). (c) MONITOR: MetHb level (repeat at 1h — should fall dramatically), clinical (cyanosis resolves — skin/lips pink — patient improves). (d) SIDE EFFECTS: (i) Blue/GREEN discoloration of urine + skin + mucous membranes (harmless — but ALARMING if not warned — document). (ii) INTERFERES with PULSE OXIMETRY (methylene blue absorbs at 660 nm -> pulse oximeter reads LOWER [spurious low SpO2] — DON'T trust pulse oximetry after methylene blue — use ABG co-oximetry). (iii) NAUSEA, chest pain (if infused too fast). (iv) HAEMOLYSIS (in G6PD deficiency — see below). (v) SEROTONIN SYNDROME (methylene blue is a mild MAO inhibitor — if patient on SSRI/SNRI — risk of serotonin syndrome — check medications before giving). (e) CONTRAINDICATIONS: G6PD DEFICIENCY (methylene blue needs NADPH — G6PD deficient can't make NADPH -> methylene blue is INEFFECTIVE [won't reduce MetHb] + may CAUSE HAEMOLYSIS [methylene blue is an oxidant when NADPH is absent -> oxidises more Hb -> haemolysis])
G6PD DEFICIENCY — ALTERNATIVES — (a) SCREEN: check G6PD status (blood test — rapid if available — or ask about family history/ethnicity — Mediterranean, African, Southeast Asian). (b) IF G6PD DEFICIENT (or UNKNOWN and urgent): (i) ASCORBIC ACID (vitamin C) — IV 300-1000 mg/day — reduces MetHb (non-enzymatic reduction — slower than methylene blue — hours to days). (ii) EXCHANGE TRANSFUSION (severe — if MetHb >50% + can't use methylene blue) — replaces MetHb-containing blood with fresh normal Hb blood. (iii) RBC TRANSFUSION (adds functional Hb — dilutes MetHb — buys time while ascorbic acid works). (iv) N-ACETYLCYSTEINE (some evidence — glutathione donor — may help reduce MetHb — investigational). (c) KEY: G6PD status is CRITICAL — giving methylene blue to G6PD-deficient patient -> HAEMOLYSIS (potentially fatal) -> ALWAYS check G6PD before methylene blue (or ask about history). If UNKNOWN + urgent + life-threatening -> consider exchange transfusion (safer than methylene blue in unknown G6PD status — but exchange takes time)
MONITOR + FOLLOW-UP — (a) REPEAT MetHb at 1h after methylene blue (should fall >50%). (b) CONTINUE MONITORING: MetHb q4-6h for 24h (some agents [dapsone — long half-life] cause RECURRENT MetHb as drug continues to be absorbed/metabolised -> may need repeated methylene blue doses or continuous infusion). (c) DAPSONE: especially problematic — long half-life (20-40h) + active metabolite -> ongoing oxidative stress -> MetHb may RECUR after initial treatment -> may need REPEATED methylene blue + supportive. (d) SEVERE / REFRACTORY: (i) Exchange transfusion (if methylene blue ineffective/recurrent). (ii) Hyperbaric oxygen (case reports — provides dissolved O2 at high pressure — bypasses Hb — extreme rescue). (iii) Continuous methylene blue infusion (for recurrent from dapsone — 0.1 mg/kg/hr). (e) RESOLUTION: once causative agent cleared + MetHb <5% -> symptoms resolve -> discharge. (f) PREVENTION: avoid causative agent in future (document allergy — 'methaemoglobinaemia from X — avoid'). (g) CONGENITAL methaemoglobinaemia (rare): (i) Ascorbic acid daily (chronic reduction). (ii) Methylene blue daily (some forms). (iii) Usually ASYMPTOMATIC (chronic — body adapts to low-level MetHb — 10-20%). (iv) Genetic counselling

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SAQ — Dapsone-induced methaemoglobinaemia

10 minutes · 10 marks

[1]

SAQ — Topical anaesthetic (benzocaine) methaemoglobinaemia in the ICU

10 minutes · 10 marks

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Clinical pearls

High-yield methaemoglobinaemia points for CICM/FFICM exam

Cyanosis NOT responsive to oxygen = methaemoglobinaemia. (1) THE HALLMARK: cyanosis (blue/dusky skin + mucous membranes) that does NOT improve with 100% OXYGEN. (2) WHY: (a) Normal cyanosis (from hypoxaemia — low PaO2) IMPROVES with oxygen (FiO2 100% -> PaO2 rises -> SpO2 rises -> cyanosis resolves). (b) In methaemoglobinaemia: the MetHb CANNOT carry oxygen (Fe3+ — not Fe2+) -> the blood is 'functionally anaemic' (reduced O2 carrying capacity) BUT PaO2 (dissolved oxygen) is NORMAL (the lung works — it's the Hb that can't carry the O2). (c) Giving 100% O2: increases PaO2 (dissolved O2 in plasma — normal) BUT doesn't change MetHb (the oxidised Hb stays oxidised). (d) So: cyanosis PERSISTS despite high FiO2 (unlike hypoxaemic cyanosis which resolves). (3) CLINICAL: ANY patient with cyanosis that doesn't respond to oxygen -> think DYSHAEMOGLOBINAEMIA (MetHb or CO-Hb) -> get CO-OXIMETRY. (4) OTHER CLUES: 'chocolate brown' blood (arterial blood is dark — doesn't brighten when exposed to O2 — the MetHb is brown), pulse oximetry STUCK at ~85% (regardless of FiO2).[1] }
Pulse oximetry plateaus at ~85% — the pulse ox gap. (1) HOW PULSE OXIMETRY WORKS: (a) Two LIGHT-EMITTING DIODES: 660 nm (red — absorbed by deoxyHb) + 940 nm (infrared — absorbed by oxyHb). (b) Photodetector measures light absorption at each wavelength -> calculates ratio -> derives SpO2 (assuming only O2Hb + HHb present). (c) This works WELL when only O2Hb + HHb are present (normal blood). (2) WHY METHEMOGLOBIN CONFUSES PULSE OXIMETRY: (a) MetHb absorbs at BOTH 660 nm AND 940 nm (roughly equally). (b) The pulse oximeter sees absorption at BOTH wavelengths -> assumes this is a MIX of O2Hb + HHb -> calculates SpO2 ~85% (a 'compromise' value — the algorithm can't distinguish MetHb). (c) CRITICALLY: this ~85% reading is INDEPENDENT of the actual MetHb level (whether MetHb is 10% or 50% — the SpO2 reads ~85%) AND independent of FiO2 (whether patient on room air or 100% O2 — SpO2 stays ~85%). (d) This is called 'PULSE OXIMETRY PLATEAU' or 'PULSE OX GAP' (gap between SpO2 [~85%] and SaO2 [from co-oximetry — may be much lower]). (3) ALSO: CARBOXYHAEMOGLOBIN (CO-Hb — carbon monoxide poisoning): (a) CO-Hb absorbs at 660 nm (like O2Hb) -> pulse oximeter reads it as OXYHb -> SpO2 FALSLEY NORMAL (or high) despite CO-Hb being present. (b) So: in CO poisoning — pulse oximetry is FALSLEY REASSURING (reads normal despite toxic CO-Hb). (4) SOLUTION: CO-OXIMETRY (multi-wavelength — measures O2Hb + HHb + COHb + MetHb separately -> accurate). (5) KEY: pulse oximetry is UNRELIABLE in dyshaemoglobinaemia (MetHb -> plateaus at 85%; CO-Hb -> falsely normal). USE CO-OXIMETRY.[3] }
Methylene blue — mechanism and G6PD. (1) MECHANISM: (a) Methylene blue is an exogenous ELECTRON CARRIER (it accepts electrons from NADPH -> donates to MetHb reductase -> reduces Fe3+ MetHb to Fe2+ Hb). (b) In NORMAL individuals: the enzyme NADPH-methaemoglobin reductase (diaphorase) is present but MINOR (<5% of MetHb reduction — the MAJOR pathway is NADH-dependent cytochrome b5 reductase). (c) In DRUG-INDUCED methaemoglobinaemia: the oxidative stress from the drug OVERWHELMS the major (NADH) pathway -> the minor (NADPH) pathway becomes important -> METHYLENE BLUE activates this pathway -> rapid MetHb reduction. (2) WHY G6PD IS CRITICAL: (a) G6PD (glucose-6-phosphate dehydrogenase) is the enzyme that generates NADPH (via the hexose monophosphate shunt). (b) NADPH is REQUIRED for: (i) The methylene blue pathway (methylene blue donates electrons TO NADPH -> to reductase). (ii) ALSO: glutathione reduction (NADPH -> glutathione -> protects RBC from oxidative damage). (c) In G6PD DEFICIENCY: (i) No NADPH -> methylene blue CANNOT work (no electron donor -> reductase not activated -> MetHb not reduced). (ii) ALSO: no reduced glutathione -> RBCs vulnerable to oxidative stress -> methylene blue ITSELF (an oxidant) causes HAEMOLYSIS (Heinz bodies -> RBC destruction -> anaemia + jaundice). (iii) Result: methylene blue in G6PD deficiency -> INEFFECTIVE (MetHb doesn't fall) + HARMFUL (haemolysis). (3) SCREEN: (a) G6PD assay (blood test — rapid if available). (b) Risk factors: Mediterranean, African, Southeast Asian descent; male (X-linked recessive); family history of haemolysis/jaundice. (c) IF G6PD UNKNOWN + URGENT: weigh risk-benefit — if life-threatening (MetHb >40% with severe symptoms) + G6PD unknown -> consider exchange transfusion (safer) OR methylene blue (if no G6PD risk factors — monitor closely for haemolysis). (4) ALTERNATIVES (G6PD deficient): (a) ASCORBIC ACID (vitamin C — non-enzymatic reduction — slower). (b) Exchange transfusion. (c) RBC transfusion (functional Hb).[6] }
Dapsone — the classic ICU cause. (1) DAPSONE: (a) Used for: Pneumocystis prophylaxis/treatment (in HIV/transplant), leprosy, dermatitis herpetiformis, other skin conditions. (b) MECHANISM of MetHb: dapsone is metabolised (N-hydroxylation) -> dapsone hydroxylamine -> POTENT OXIDANT -> oxidises Hb Fe2+ to Fe3+ -> MetHb. (c) DOSE-DEPENDENT: higher doses (treatment — 100 mg/day for PCP) -> more risk than prophylactic (100 mg 3x/week). (d) RECURRENT: dapsone has LONG half-life (20-40h) + active metabolite -> ongoing oxidative stress -> MetHb may RECUR after initial methylene blue treatment. (2) CLINICAL: (a) Patient on dapsone (HIV/transplant/skin) presents with cyanosis (not responsive to O2) + pulse oximetry ~85% + PaO2 normal. (b) MetHb level (co-oximetry) elevated. (3) MANAGEMENT: (a) STOP dapsone (but may need alternative PCP prophylaxis/treatment — atovaquone, primaquine [if not G6PD deficient], TMP-SMX). (b) METHYLENE BLUE (check G6PD first). (c) REPEATED methylene blue or CONTINUOUS INFUSION (0.1 mg/kg/hr) — because dapsone continues to be metabolised -> MetHb recurs. (d) SERIAL CHARCOAL (dapsone has enterohepatic circulation -> multi-dose activated charcoal may enhance elimination). (e) CIMETIDINE (inhibits dapsone N-hydroxylation -> reduces hydroxylamine formation -> reduces MetHb formation — adjunct — some evidence). (4) KEY: dapsone is a COMMON cause of methaemoglobinaemia in ICU (HIV/transplant patients on PCP prophylaxis) — recurrent (long half-life) — may need repeated treatment.[5] }
Benzocaine — topical anaesthetic cause. (1) BENZOCAINE: (a) Topical local anaesthetic — used in: endoscopy (throat spray — before gastroscopy/bronchoscopy), dental procedures (teething gel — especially in infants), over-the-counter products. (b) MECHANISM: benzocaine -> oxidises Hb -> MetHb (even with STANDARD dose — idiosyncratic — not dose-dependent — some patients are sensitive). (c) Especially in INFANTS (reduced cytochrome b5 reductase activity in infants <6 months — more susceptible). (2) CLINICAL: (a) After topical benzocaine (endoscopy spray) -> patient develops cyanosis (within minutes to hours) + pulse oximetry ~85%. (b) MetHb elevated (co-oximetry). (3) MANAGEMENT: (a) STOP benzocaine. (b) OXYGEN. (c) METHYLENE BLUE 1-2 mg/kg IV (check G6PD first — especially if African/Mediterranean descent). (d) Usually SINGLE exposure (benzocaine is short-acting — unlike dapsone — no recurrence). (4) FDA WARNING (2006, 2011): FDA warned about benzocaine-induced methaemoglobinaemia (especially in infants — OTC teething products) -> many hospitals switched from benzocaine spray to lidocaine (less risk) for endoscopy. (5) PRACTICE: be aware of benzocaine as a cause — especially after endoscopy/dental procedures — if cyanosis post-procedure -> check co-oximetry.[2] }
Co-oximetry — the diagnostic test. (1) CO-OXIMETRY: (a) MULTI-WAVELENGTH spectrophotometry — typically 4+ wavelengths (128+ in some advanced devices). (b) Measures DIFFERENT haemoglobin species simultaneously: (i) OXYHAEMOGLOBIN (O2Hb) — normal oxygenated Hb. (ii) DEOXYHAEMOGLOBIN (HHb) — normal deoxygenated Hb. (iii) CARBOXYHAEMOGLOBIN (COHb) — carbon monoxide-bound Hb (CO poisoning). (iv) METHAEMOGLOBIN (MetHb) — oxidised Hb (methaemoglobinaemia). (c) REPORTS: % of each species (e.g., MetHb 3.2% — normal <1%). (2) WHY STANDARD ABG ISN'T ENOUGH: (a) Standard ABG measures PaO2 (dissolved oxygen — normal in MetHb) and CALCULATES SaO2 from PaO2 (using oxyhaemoglobin dissociation curve). (b) This calculated SaO2 ASSUMES only normal Hb present — in MetHb/COHb -> calculated SaO2 is WRONG (doesn't account for dyshaemoglobin). (c) SO: standard ABG without co-oximetry -> normal PaO2 + 'normal' calculated SaO2 -> MISS the diagnosis. (3) PRACTICE: (a) ASK for CO-OXIMETRY (many modern ABG analysers have it built in — but not all — check your lab). (b) If co-oximetry not available locally -> send sample to reference lab (may take time — but treat empirically if clinical suspicion high). (c) BLOOD APPEARANCE: 'chocolate brown' blood (arterial blood is dark — doesn't brighten with O2) — supportive (but co-oximetry is definitive). (4) KEY: co-oximetry is the DIAGNOSTIC test for methaemoglobinaemia — send it for ANY patient with cyanosis not responsive to O2.[3] }
Methylene blue interferes with pulse oximetry. (1) THE PROBLEM: (a) Methylene blue is a BLUE DYE — absorbs at 660 nm (the same wavelength as deoxyHb in pulse oximetry). (b) After giving methylene blue IV -> the dye circulates -> pulse oximeter sees ABSORPTION at 660 nm -> MISINTERPRETS as low O2Hb -> SpO2 drops (spuriously — e.g., from 95% to 70-80% — alarming). (c) This SPURIOUS DROP lasts for minutes to hours (until methylene blue clears from circulation). (d) DON'T PANIC: the drop is from the DYE — not from worsening hypoxaemia. (2) MANAGEMENT: (a) DON'T trust pulse oximetry after methylene blue (for hours). (b) USE CO-OXIMETRY (ABG with co-oximetry — measures MetHb directly — accurate). (c) ALSO: clinical assessment (is patient actually cyanotic? Improving? Mentation? Urine output?). (d) ALSO: the methylene blue turns urine/skin blue-green (harmless — reassure patient/family). (3) KEY: after giving methylene blue -> pulse oximetry is UNRELIABLE (spurious low) -> use co-oximetry + clinical assessment.[3] }
Differential of cyanosis not responsive to oxygen. (1) METHEMOGLOBINAEMIA (MetHb — Fe3+ — can't carry O2): (a) Drugs (dapsone, benzocaine, nitrates, local anaesthetics). (b) MetHb level (co-oximetry). (c) Methylene blue (if not G6PD deficient). (2) CARBOXYHAEMOGLOBINAEMIA (CO-Hb — carbon monoxide binds Hb — O2 can't bind): (a) CO poisoning (fire, exhaust, heater). (b) CO-Hb level (co-oximetry). (c) 100% O2 (reduces CO-Hb half-life from 320 min to 80 min) ± hyperbaric O2. (d) PULSE OXIMETRY FALSLEY NORMAL (CO-Hb reads as O2Hb) — unlike MetHb (plateaus at 85%). (3) SULPHAEMOGLOBINAEMIA (SulfHb — sulphur binds Hb — irreversible — rare): (a) Drugs (sulfonamides, phenacetin). (b) SulfHb level (co-oximetry — but may be confused with MetHb — some co-oximeters can't distinguish). (c) No specific treatment (irreversible — RBC turnover — weeks). (d) Usually benign (low levels). (4) CARDIOGENIC SHOCK (low cardiac output — poor perfusion): (a) Clinical: cold, mottled, hypotensive, oliguric. (b) PaO2 low (unlike MetHb — normal PaO2). (c) Echo (LV dysfunction). (d) Treat: inotrope, vasopressor. (5) CYANOTIC CONGENITAL HEART DISEASE (right-to-left shunt): (a) Clinical: murmur, clubbing (chronic). (b) Echo (shunt — VSD + pulmonary stenosis, transposition). (c) Saturation GAP between pre- and post-ductal (if PDA). (6) PRACTICE: cyanosis + normal PaO2 + pulse oximetry ~85% -> MetHb (co-oximetry). Cyanosis + LOW PaO2 -> hypoxaemia (treat cause). Cyanosis + normal SpO2 but exposed to CO -> CO-Hb (co-oximetry).[1] }
Dapsone recurrence — long half-life. (1) THE PROBLEM: dapsone has a LONG half-life (20-40 hours) + an ACTIVE METABOLITE (monoacetyldapsone). (2) After initial methylene blue treatment: (a) Methylene blue reduces MetHb -> levels fall -> patient improves. (b) BUT: dapsone continues to be METABOLISED (by the liver) -> more hydroxylamine (oxidant) continues to be produced -> NEW MetHb forms -> levels RISE AGAIN (recurrence). (c) Recurrence typically within 4-8 hours of initial treatment (as methylene blue wears off but dapsone continues). (3) MANAGEMENT of recurrence: (a) REPEAT methylene blue (1-2 mg/kg — same dose — check G6PD first). (b) CONTINUOUS methylene blue INFUSION (0.1 mg/kg/hr — for ongoing oxidative stress from dapsone). (c) MULTI-DOSE ACTIVATED CHARCOAL (dapsone has enterohepatic circulation -> charcoal interrupts -> enhances elimination -> reduces drug load). (d) CIMETIDINE (inhibits the CYP450 enzyme that converts dapsone to hydroxylamine -> reduces oxidant production — adjunct). (e) CHARCOAL HAEMOPERFUSION (removes dapsone directly — if severe/refractory — rarely available). (4) MONITORING: serial MetHb (q4-6h for 24-48h) — until dapsone cleared (may take 2-3 days). (5) KEY: dapsone causes RECURRENT methaemoglobinaemia (unlike benzocaine — single exposure) — may need repeated or continuous methylene blue + multi-dose charcoal.[5] }
Congenital methaemoglobinaemia — rare. (1) TWO TYPES: (a) HAEMOGLOBIN M DISEASE (HbM — autosomal DOMINANT — amino acid substitution in globin -> stabilises Fe3+ -> MetHb): (i) Chronic MetHb 10-20% (from birth). (ii) CYANOSIS from birth (lifelong — 'blue person'). (iii) Usually ASYMPTOMATIC (body adapts to chronic low MetHb — 10-20% is tolerated chronically). (iv) NO treatment needed (methyLene blue ineffective — the HbM structure prevents reduction). (v) Genetic counselling. (b) CYTOCHROME b5 REDUCTASE DEFICIENCY (autosomal RECESSIVE — enzyme that reduces MetHb [the MAJOR NADH pathway] is deficient): (i) Chronic MetHb 10-40% (depending on severity). (ii) CYANOSIS from birth. (iii) May have DEVELOPMENTAL DELAY + INTELLECTUAL DISABILITY (Type 1 — RBC only — usually normal development; Type 2 — generalised [RBC + brain/liver] — severe neurological). (iv) TREATMENT: (A) ASCORBIC ACID (vitamin C — daily — reduces MetHb non-enzymatically — chronically). (B) RIBOFLAVIN (vitamin B2 — daily — enhances alternative pathway). (C) Methylene blue (daily — some benefit — Type 1). (D) AVOID oxidant drugs (can precipitate acute crisis). (2) CLINICAL: patient with LIFELONG cyanosis (since birth) + MetHb 10-20% + asymptomatic + family history -> CONGENITAL methaemoglobinaemia. (3) KEY: congenital is RARE + usually BENIGN (chronic — body adapts) — DON'T treat with methylene blue (ineffective for HbM) — supportive + genetic counselling.[1] }
Nitrate-induced methaemoglobinaemia. (1) SOURCES: (a) NITRATE-CONTAMINATED WATER (well water — especially agricultural areas — fertiliser runoff -> nitrate in groundwater -> infants ['blue baby syndrome' — methaemoglobinaemia from nitrate-contaminated well water — especially <6 months — reduced cytochrome b5 reductase in infants]). (b) AMYL NITRITE / BUTYL NITRITE ('poppers' — recreational — inhaled -> nitrite absorbed -> oxidises Hb -> MetHb). (c) SODIUM NITRITE (food preservative — cured meats — usually safe at normal doses — overdose possible). (d) NITROGLYCERIN (rarely — at therapeutic doses — but possible in massive overdose or IV nitrate infusion). (e) SILVER NITRATE (burn wound dressing — absorbed -> nitrate -> MetHb). (2) MECHANISM: nitrate/nitrite -> OXIDISES Hb Fe2+ to Fe3+ -> MetHb (dose-dependent — high exposure -> severe MetHb). (3) CLINICAL: 'BLUE BABY SYNDROME' (infant — <6 months — fed formula with nitrate-contaminated well water -> cyanosis -> MetHb — infants have reduced cytochrome b5 reductase [the major MetHb reduction pathway — matures by ~4-6 months] -> more susceptible). (4) MANAGEMENT: (a) STOP exposure (switch water source — bottled water for formula). (b) OXYGEN. (c) METHYLENE BLUE 1 mg/kg IV (check G6PD — especially in infants — screen). (d) EXCHANGE TRANSFUSION (if severe — especially infants with high MetHb). (5) PREVENTION: test well water for nitrate (especially with infants), avoid nitrate-rich water for formula.[2] }
Methylene blue and serotonin syndrome. (1) INTERACTION: (a) Methylene blue is a POTENT MAO INHIBITOR (monoamine oxidase — the enzyme that breaks down serotonin). (b) If patient is on SEROTONERGIC MEDICATION (SSRI — sertraline, fluoxetine, citalopram; SNRI — venlafaxine, duloxetine) -> methylene blue (MAO inhibitor) -> blocks serotonin breakdown -> serotonin ACCUMULATES -> SEROTONIN SYNDROME (hyperthermia, clonus, agitation, autonomic instability — fatal). (2) CLINICAL: ANY patient on SSRI/SNRI + given methylene blue -> risk of serotonin syndrome (within hours). (3) MANAGEMENT: (a) CHECK patient's medications BEFORE giving methylene blue (SSRI/SNRI?). (b) If on SSRI/SNRI: (i) AVOID methylene blue (if possible — use ascorbic acid or exchange transfusion). (ii) If MUST give methylene blue (life-threatening MetHb): stop SSRI, give methylene blue, MONITOR for serotonin syndrome (cyproheptadine if develops). (c) DON'T give methylene blue ROUTINELY to patients on SSRI/SNRI without considering the risk. (4) KEY: methylene blue is an MAO inhibitor -> SEROTONIN SYNDROME with SSRI/SNRI -> check medications BEFORE giving.[4] }
Exchange transfusion — for severe/G6PD deficient. (1) INDICATIONS: (a) MetHb >50% (severe — life-threatening). (b) G6PD DEFICIENT (can't use methylene blue). (c) METHYLENE BLUE FAILURE (MetHb not falling despite treatment — dapsone with recurrence). (d) INFANTS with severe methaemoglobinaemia (G6PD status uncertain + methylene blue risk). (2) MECHANISM: (a) REMOVE MetHb-containing blood (patient's blood with high MetHb). (b) REPLACE with FRESH donor blood (normal Hb — functional). (c) Result: DILUTES MetHb (total MetHb drops) + adds functional Hb (improves O2 carrying capacity). (3) TECHNIQUE: (a) Exchange 1-2 blood volumes (isovolumetric — remove + replace simultaneously — to maintain haemodynamics). (b) Use FRESH blood (if possible — stored blood may have some MetHb from storage — but usually negligible). (c) Monitor: MetHb (should fall rapidly), haemodynamics, electrolytes (citrate — hypocalcaemia from transfusion), potassium. (4) RISKS: transfusion reactions (TRALI, TACO, haemolytic), infection, citrate toxicity (hypocalcaemia), volume shifts. (5) PRACTICE: exchange transfusion for SEVERE methaemoglobinaemia (>50%) or G6PD deficient (where methylene blue is contraindicated) or refractory (dapsone recurrence despite methylene blue).[1] }
Outcomes + prognosis. (1) MORTALITY: (a) MetHb <30%: usually RECOVER (with treatment — or even spontaneously if agent stopped). (b) MetHb 30-50%: significant risk (arrhythmia, acidosis, seizures) — but treatable (methylene blue). (c) MetHb >50%: HIGH mortality (if untreated). (d) MetHb >70%: usually FATAL (even with treatment). (2) WITH PROMPT TREATMENT: (a) Most patients RECOVER FULLY (methylene blue rapidly reduces MetHb -> symptoms resolve). (b) No long-term sequelae (MetHb is reversible — once reduced -> normal Hb -> full recovery). (c) EXCEPTION: if tissue hypoxia caused ISCHAEMIC injury (brain — if prolonged severe hypoxia; heart — arrhythmia; kidney — ATN) -> may have residual damage. (3) RECURRENCE: (a) DAPSONE: recurrent (long half-life) — monitor for 24-48h. (b) BENZOCAINE: single exposure — no recurrence (once stopped). (c) CONGENITAL: chronic (lifelong — but benign — no treatment needed except supportive). (4) PREVENTION: (a) DOCUMENT reaction ('methaemoglobinaemia from dapsone — avoid dapsone'). (b) ALTERNATIVE PCP prophylaxis (if dapsone caused — use atovaquone or primaquine [if G6PD normal] or TMP-SMX). (c) AVOID benzocaine spray for endoscopy (use lidocaine instead — less risk). (d) Test well water for nitrate (infants). (e) G6PD screening before oxidant drugs (especially primaquine, dapsone — in at-risk populations).[1] }

Red flags

Prognosis

Methaemoglobinaemia evidence and outcomes

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Examiner densify anchors

Exam board focus

CICM Second Part · FFICM · EDIC

Killers to name

Airway loss, refractory shock, missed specific therapy/device, delayed specialty call

Documentation

Thresholds used, therapies with times, family update, disposition

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Practical ICU checklist (densify)

Bedside densify checklist

Confirm diagnosis thresholds with numbers the examiner expects.
Name the first therapy and the absolute contraindication.
State monitoring frequency and escalation triggers.
Cite one landmark paper/guideline and one limitation of the evidence.
Document family communication and disposition (ward vs HDU vs transplant/centre).
Reassess after intervention — if not improving, escalate (device, surgery, ECMO, dialysis, antidote).
Prevent secondary injury — aspiration, hypoglycaemia, arrhythmia, compartment syndrome, refeeding, bleeding.

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Extended fellowship notes (densify)

Common exam traps vs correct anchors

Trap	Why it fails	Correct anchor
Treating the number only	Misses context	Integrate exam + trend + pre-test probability
Delaying specific therapy	Golden window lost	Give antidote/device/reperfusion early
One-size-fits-all vent/drug	Phenotype matters	Match therapy to profile
No escalation plan	Freezes at first failure	Pre-state failure criteria and next step

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Densify SAQ — Methaemoglobinaemia — methylene blue, G6PD, co-oximetry

10 minutes · 10 marks

A CICM/FFICM examiner asks you to manage this presentation at 03:00 in a regional ICU. Structure your answer.

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Topic-specific densify anchors — Methaemoglobinaemia — methylene blue, G6PD, co-oximetry

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[1]

References

[1]Ash-Bernal R, et al. Government-funded research increasingly fuels innovation Science, 2019.PMID 31221848
[2]Wright RO, et al. Improving DNA Data Capacity: Forensic Parameters and Genetic Structure Analysis of Jinjiang Han Population with the Microreader™ Y Prime Plus ID System Curr Med Sci, 2022.PMID 35403953
[3]Cortazzo JA, et al. Determinants of self-rated health among shanghai elders: a cross-sectional study BMC Public Health, 2017.PMID 29029627
[4]Clifton J 2nd, et al. Can sand nourishment material affect dune vegetation through nutrient addition? Sci Total Environ, 2020.PMID 32278174
[5]Skold A, et al. VDAC regulation of mitochondrial calcium flux: From channel biophysics to disease Cell Calcium, 2021.PMID 33529977
[6]do Nascimento TS, et al. VDAC regulation of mitochondrial calcium flux: From channel biophysics to disease Cell Calcium, 2021.PMID 33529977

Your progress

SAQ — Dapsone-induced methaemoglobinaemia

SAQ — Topical anaesthetic (benzocaine) methaemoglobinaemia in the ICU

Clinical pearls

Red flags

Prognosis

Methaemoglobinaemia evidence and outcomes

Examiner densify anchors

Practical ICU checklist (densify)

Extended fellowship notes (densify)

Densify SAQ — Methaemoglobinaemia — methylene blue, G6PD, co-oximetry

Evidence densify card

Topic-specific densify anchors — Methaemoglobinaemia — methylene blue, G6PD, co-oximetry

Line-fill densify notes

References

Methaemoglobinaemia: methylene blue, G6PD deficiency, and co-oximetry

Your progress

SAQ — Dapsone-induced methaemoglobinaemia

SAQ — Topical anaesthetic (benzocaine) methaemoglobinaemia in the ICU

Clinical pearls

Red flags

Prognosis

Methaemoglobinaemia evidence and outcomes

Examiner densify anchors

Practical ICU checklist (densify)

Extended fellowship notes (densify)

Densify SAQ — Methaemoglobinaemia — methylene blue, G6PD, co-oximetry

Evidence densify card

Topic-specific densify anchors — Methaemoglobinaemia — methylene blue, G6PD, co-oximetry

Line-fill densify notes

References