Methaemoglobinaemia
ā¢Download as PPTX, PDFā¢
24 likesā¢10,663 views
Methaemoglobinaemia
Recommended
More Related Content
What's hot
What's hot (20)
Viewers also liked
Similar to Methaemoglobinaemia
Similar to Methaemoglobinaemia (20)
More from SCGH ED CME
More from SCGH ED CME (20)
Recently uploaded
Recently uploaded (20)
Methaemoglobinaemia
- 2. OVERVIEW ā¢ What is methaemoglobinaemia? ā¢ Clinical presentation ā¢ Toxicology ā¢ Antidote ā Methylene Blue ā¢ Case ā¢ Quiz
- 3. WHAT IS METHAEMOGLOBIN? ā¢ Hb that is unable to carry O2 due to reduction of ferrous iron to ferric form ā¢ Fe2+ ļ Fe3+ ā¢ O2 that is bound to Fe2+ has increased affinity, shifts O2-dissociation curve to the left ā¢ Occurs physiologically (auto-oxidation), but physiological reduction by cytochrome b5 reductase (NADH-dependent) maintains low levels 1-2% ā¢ Congenital vs acquired causes
- 4. TOXICOLOGY ā¢ What drugs / substances can cause MetHb? ā¢ Many: nitrites, dapsone, local anaesthetics, aniline & derivates ā¢ Therapeutic MetHb ā¢ Historically used in cyanide poisoining (amyl nitrite & Na nitrite induce MetHb, the Fe3+ binds cyanide ā cyanomethaemoglobin). MetHb may then need to be treated if reaches significant levels.
- 5. CLINICAL PRESENTATION ā¢ Patients may be asymptomatic! Pre-existing conditions e.g. cardiac, respiratory may cause symptoms at lower levels MetHb ā¢ Symptomatic patients ā¢ Clinical HYPOXAEMIA ā cyanosis, chest pain, end organ ischaemia, conduction abnormalities / dysrhythmia, CNS changes ā seizure, confusion, coma.
- 6. CLINICAL PRESENTATION ā¢ Cyanosis does not respond well to supplemental oxygen ā¢ āChocolate-brownā blood ā¢ Pulse oximetry cannot properly differentiate between HbO2 & MetHb ā SpO2 must be interpreted with caution ā trends towards 80-85%. ā¢ SaO2 on ABG will be falsely normal as it is a calculated value based on dissolved O2 in blood
- 7. ANTIDOTE ā METHYLENE BLUE ā¢ How does it work? ā¢ Dose 1-2mg/kg ā onset within 20 minutes, give over 5/60. Can repeat doses. ā¢ Should not be used in G6PD deficiency ā¢ Consider use of methylene blue when: ā¢ MetHb level >20% in asymptomatic patients ā level found on blood gas ā¢ Any symptomatic acquired MetHb case If not responding, consider blood transfusion, exchange transfusion. Role for ascorbic acid infusions ?? Slower onset
- 8. CASE ā¢ Call from Burnie, North West Tasmania ā¢ 52 yr old, intentional polypharmacy overdose, of note dapsone 5g (100 x 50mg tablets) 2.5 hrs ago ā¢ GCS 15, SpO2 74% on 15L NRBM, RR 18, HR 97 sinus, SBP 105 ā¢ Met Hb level 16.4% ā¢ Mx ā 1mg/kg methylene blue with excellent effect; MDAC; high flow O2, HDU for ongoing serial MetHb levels / close observation / repeat MB doses if needed ā¢ Following morning, patient clinically well SpO2 92% 3L NP MetHb 4-6%, repeat MB if level >10%, If for ascorbic acid infusions. MDAC working well.
- 9. SUMMARY ā¢ MetHb forms by oxidation of ferrous to ferric ion in haem, unable to bind O2 ā¢ Acquired MetHb can be caused by a variety of drugs / chemicals ā¢ Symptoms include cyanosis, SOB, cardiovascular & CNS features ā¢ Not responsive to supplemental O2, sats monitoring not reliable ā¢ Can check MetHb levels of ABG/VBG ā¢ Methylene blue antidote in symptomatic patients 1-2mg/kg
- 10. QUIZ (U READY FOR THIS?)
- 11. QUESTION 1 ā¢ What is the name of the villain who hates these lovable creatures? Bonus point for the name of the cat.
- 12. QUESTION 2 ā¢ Who is this group and what are they famous for?
- 13. QUESTION 3 ā¢ In what year did Nestle introduce the blue smartie?
- 14. QUESTION 4 ā¢ Your patient tells you he takes a little blue pill in the evening for his āatrial fibillationā. What pill might he be taking and at what strength? Note ā there is more than one correct answer.
- 15. QUESTION 5 ā¢ Leanne Rimes released the single āBlueā in 1996. A packet of blue M&Ms to the first person to stand up and croon a few barsā¦
- 16. QUESTION 6 ā¢ Apart from blueberries, name 2 naturally occurring foods that are āblueā?
- 17. QUESTION 7 ā¢ Which Australian actor played this blue character in the 2009 wank fest Avatar?
- 18. QUESTION 8 ā¢ What is this? And where is it?
- 19. QUESTION 9 ā 2 POINTS! ā¢ Methylene blue comes in 10mg/ml solution in a 10ml vial. Your patient is symptomatic with acquired methaemoglobinaemia after your Beirās blocked them with prilocaine. He is 70kg. What is the maximum dose in millilitres your patient can have before they are at risk of haemolysis or paradoxical MetHb? How many vials will you need?
- 20. QUESTION 10 ā¢ In 60 seconds, name as many songs (with artist) as you can with the word āBlueā in the title. 1 point per song AND artist.
Editor's Notes
- A form of Hb unable to carry O2. The iron in the haem is normally in the Fe2+ form and can carry O2. Due to oxidative stress, the Fe2+ is oxidated to Fe3+ which cannot carry oxygen. The remaining Fe2+ in haem then binds oxygen with higher affinity, making it increasingly difficult to offload oxygen to tissues. This is a left ward shift of the oxygen dissociation curve (lower PaO2 for a given SpO2). Results in functional anaemia i.e. there is sufficient haemoglobin, but it is non functional, resulting in hypoxaemia. MetHb formation occurs naturally due to oxidative stress on red cells, however, NADH dependent cytochrome b5 reductase reduces MetHb(Fe3+) back to HbO2, so that physiological levels of MetHb are usually only 1-2% of total Hb. Congenital causes ā rare but well described. Tend to not respond to usual antidote, may be associated with other neurological abnormalities. Acquired causes are usually due to drugs / oversdoses or occupational / accidental exposures and are antidote responsive. This is where the toxicologists start to get interested. A non-physiological pathway to reduce MetHb back to HbO2 can be utilised with methylene blue, but this will be discussed under the antidote section.
- Local anaesthetics especially benzocaine (not available in Australia outside of topical skin preparation of 2.5%), prilocaine (think Beirās block), lignocaine. The molecular mechanism for MetHb with these drugs is unknown. Nitrites ā may be present in sufficiency quantities in contaminated water or foods where high doses of nitrites are used as preservatives. In 2006 ā public health investigations into clusters of MetHb in Sydney found food additives / flavour enhancers āGoldfish brane Nutre Powderā and āBorax powderā imported into Australia where actually 100% sodium nitrite. Packages were not labelled to Australian standards and were recalled. NITRITES ARE OXIDISING AGENTS Patients presented with rapid onset symptoms ā headache, N&V, SOB, cyanosis and were all treated successfully with MB and discharged. Aniline and its derivates ā used in chemical production as precursor to polyurethane. Can have inhalational and dermal systemic exposures. Also in dyes. Dapsone ā antibacterial agent ā thought to block folate synthesis. May act as an immunomodulatory in treatment of skin diseases. Used in leprose, dermatitis herpetiformis, treatment & prevention of PJP and prevention toxoplasmosis. Known to cause low levels of MetHb at therapeutic levels. Presented as 25mg and 100mg tablets. THERAPEUTIC METHAEMOGLOBINAEMIA ā traditionally used to treat cyanide poisoining ā use of inhaled anyl nitrite & sodium nitrite to induce formation Fe3+ which binds cyanide and cyanomethaemglobinaemia. Cyanide dissociates from cytochrome oxidase.
- Methylene blue is reduced by NAPDH-methaemoglobin reductase to leucomethylene blue. The leucomethylene blue then reduces the Fe3+ back to Fe2+. Because this pathway relies on NAPDH as a co-enzyme, patients with G6PD deficiency who have low levels of NAPDH cannot use MB to reduce Fe3+ and can lead to haemolysis. MB dosing can be repeated where levels of MetHb remain high, where there are ongoing effects of the drug (e.g. dapsone), or where there is an inadequate response to the initial dose.
- Resuscitation ā airway patent, mentating well, on maximal O2 therapy Risk assessment ā massive dose of dapsone with clinical and biochemical features of significant methaemoglobinaemia needing urgent treatment Supporitve Care ā oxygen therapy, resuscitation area Investigations ā MetHb level, APAP ets Decontamination ā stat AC 50g Enhanced Elimination ā MDAC ā undergoes enterhepatic circulation. 25g AC 4-6 hourly, 3-4 doses Antidote ā urgent 1-2mg/kg methylene blue. May require repeat doses up to 7mg/kg. May need to consider blood or exchange transfusion. Disposition ā HDU area, high flow O2, possible airway for 100% O2, serial levels, MDAC management, serial bloods.