Tasty Morsels of EM 084 – #FRCEM Local Anaesthetic Doses and Toxicity

27 Jul

(featured image, JL Johnson on FLickr, CC license)

I’m entering a few months prep for the UK and Ireland exit exam in Emergency Medicine: the FRCEM. I’ll be adding lots of little notes on pearls I’ve learned along the way. A lot of my revision is based around the Handbook of EM as a curriculum guide and review of contemporary, mainly UK guidelines. I also focus on the areas that I’m a bit sketchy on. With that in mind I hope they’re useful.

You can find more things on the FRCEM on this site here.

Classes

  • Amide
    • Lignocaine
    • Bupivacaine
    • Priolcaine
  • Esters
    • Cocaine
    • Benzocaine
    • Tetracaine

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How do they work?

  • All work by
    • “inhibiting sodium influx through sodium-specific ion channels in the neuronal cell membrane, in particular the so-called voltage-gated sodium channels.” (Wiki)
    • Different pain fibres are affected in  different ways with sympathetic fibres being most sensitive followed by pain fibres. Hence the sensory block without motor block that we often see.

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Max Doses

  • Bupivacaine 2mg/kg
  • Lignocaine 3mg/kg
  • Lignocaine with Adrenaline (1:200000) 7mg/kg
  • Prilocaine 6mg/kg (of note remember the MetHb risk with this – I’ve seen it and been responsible for it!)
  • NUH have a lovely table with patient weights and different concentrations all worked out.
  • Also of note if you’ve given your 2mg/kg of bupivacaine you can’t just have a go with lignocaine

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Discuss Local Anaesthetic Systemic Toxicity

(From the AAGBI 2010)

  • Signs
    • altered mental staus
    • CV collapse, brady, conduction blocks, asystole and VF/VT
    • apparently you can also get a rash (if you need a third body system for an exam question…)
    • of note can be delayed
  • Management
    • Stop injecting the stuff dummy
    • HELP!!!
    • ABCs
    • Stop the seizure with your agent of choice (benzo, prop, thio…)
  • If cardiac arrest
    • CPR
    • Lipid Emulsion
    • May take >1 hr for recovery so keep going!
    • From the realms of the obvious
      • propofol not a suitable substitute for intralipid despite being made from it
      • don’t give lignocaine for arrhythmias
      • (sigh… eye roll… repeat)
  • If no cardiac arrest
    • supportive yadda yadda
    • consider lipid emulsion

How to give the intralipid

From the AAGBI 2010 Click image for full PDF

  • Bolus
    • 20% intralipid 1.5ml/kg over 1 min (eg about 100mls in a 70kg patient)
    • Can be repeated twice (once every 5 mins)
  • Infusion
    • as soon as the 1st bolus given
    • infusion of 15ml/kg/hr (1000ml/hr of 20% solution in a 70kg patient)
    • if repeating the bolus then double the infusion rate
  • 12ml/kg of the 20% is the maximum dose

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