Primer: Local Anaesthetics

edited January 14 in Procedures

DISCLAIMER: The following is for educational purposes only and is not meant as an instructional tool. Any surgical procedures should be undertaken under the supervision and by a medical professional.

PRIMER ON LOCAL ANAESTHETICS

Local anaesthetics block pain by inhibiting certain electrolyte channels at the cellular membrane of neurones, inhibiting nociception and numbing the affected area. As with all medical procedures, local anaesthesia carries a risk of side effects and care must be taken with its use.

COMMON TYPES OF LOCAL ANAESTHETIC

Lidocaine

  • Likely the most common type of local you will encounter
  • Onset: 3-5 minutes
  • Duration: 30 minutes to an hour

Bupivocaine & Ropivocaine

  • Long acting local
  • Onset: 15 minutes
  • Duration: 2-8 hours

For the purposes of simple surgeries Lidocaine should be more than sufficient.

THE USE OF VASOCONSTRICTIVE AGENTS WITH LOCAL ANAESTHETIC

Vasoconstrictive = reduces the calibre of blood vessels, reducing blood flow and subsequent bleeding from the surgical wound. In almost all cases this will be adrenaline (AKA epinephrine). Local anaesthetic comes prepared either with adrenaline or without.

Adrenaline is an excellent agent to reduce bleeding and increase the visibility of the surgical field. As long as the agent is injected SC (subcutaneously) there will be no systemic effects (i.e. it will only be an issue if you hit a blood vessel).

WARNING - Adrenaline is NOT to be used on structures with relatively small blood supply. This includes fingers and toes, nose and ears and the penis (among other structures). These have relatively small vascular structures, and the use of adrenaline can cut off blood supply, inducing ischaemia and necrosis if used improperly. Unless you are a medical specialist DO NOT ATTEMPT THIS. Fingers get local anaesthetic ONLY.

LOCAL ANAESTHETIC TOXICITY

Local anaesthetic is meant to be used locally. That is, subcutaneously. When infiltrating the needle into tissue, retract the plunger BEFORE pushing it down - if blood is drawn into your syringe you have hit a blood vessel. If local anaesthetic is injected into the systemic circulation the patient is in for a bad time - certain agents are actually cardiotoxic.

Early symptoms

  • Tinnitus
  • Blurring of vision
  • Dizziness
  • Anxiety and confusion
  • Tingling around the lips and tongue
  • Metallic taste in the mouth

Late symptoms

  • Seizures, coma
  • Bradycardia, hypotension, cardiac dysrhythmias
  • Respiratory depression

If ANY of these symptoms occurs following surgery call an ambulance - these patients will need full tertiarty work up.

Comments

  • Nice. This is great. A few things I'd like your input on.. how do you feel about nerve blocks? There are a few simple ones like digital.. mouth.. tragus. I know people in the community are using these. There are also crazy regional blocks etc.n that definitely shouldnt be being used.

    Also.. duration. The combo of trauma and lido sometimes leaves people worried. They expect the lido to wear off.. but it occurs slowly and they end up terrified they've eliminated their ability to heal.

    So that do you consider to be an adequate dose say for a infiltration of the finger tip?

  • Digital blocks are very useful - I'll be doing a separate primer on them later on. The issue with digital blocks is that the arteries and nerves that supply fingers are very close to where you want to inject. I'd formally advise against trying it unless you've been trained to do so.

    Lidocaine should wear off slowly - there is a difference between effective duration (as above) and the time that full sensation should occur. If the numbness has not fully resolved by 24 hours I would go see a doc. Another primer I'm interested in doing is on aftercare including how to recognise when things have gone wrong.

    Dosage depends on the strength of the lidocaine. A dose of 200mg should be more than adequate for most procedures

    • 40mL if the concentration is 0.5%
    • 20mL if the concentration is 1%

    Dosages of lidoocaine above 3mg/kg are enough to cause serious adverse effects and death.

  • Wow. Those are large volumes. I've seen 1ml of 1 percent work just on a small person. 40ml.. that's 2 full multidose bottles.
    Yeah, I've cut most procedural stuff out of my blog but there are people working on the wiki. Do you mind if someone takes this post and folds it into the wiki?
  • These would be the maximum recommended doses for any area, there is no real hard and fast rule for dosing, I usually just inject a nerve block and then inject some more if the area is not fully anaesthetised. My usual dose for a digital block would be around 5 mls.

    Just as an aside, there is no reason you should need to inject the tip of the finger if you perform a good digital block, and in fact considering the amount of delicate structures packed together in the tip of the finger I would actually recommend against it.

    Happy for this to be put on the wiki as long as there is a big disclaimer above it reiterating that any procedures should be carried out by or under the supervision of a medical professional.

  • Yeah. I wonder if we can add a floating disclaimer to the wiki.. no matter where you scroll it tells you not to be stupid. Lol.
  • What are your opinions on dorsal blocks for finger magnets? I injected 1ml dorsally at the second joint of the finger just under the skin. I found it worked much much better than previous digital blocks I've had.
  • Would recommend avoiding, I've never seen or heard of them used in surgery. Nerves that supply the digits run either side of the digit, not on the dorsal and palmar surfaces. There is no reason a digital block should fail if its done properly - if the finger is not properly anaesthetised just add more anaesthetic.

    The issue with the technique you describe is that there is relatively little expandable space over the DIP (Distal InterPhalangeal) joint (the 2nd finger joint) for the anaesthetic bolus to spread. Injection in this area if not done carefully could damage the ligaments of the dorsal finger. Furthermore if its done over a joint this could magnify any surgical adverse events - anything is made much more complicated by the addition of a joint into the mix.

  • I've heard transthecal works well. If someone is going to use a finger tourniquet.. any changes to how someone would perform anything?
  • My bad, not dorsal. I meant on the sides of the finger. I do get what you're saying though.

  • Tranthecal injections have worked well in studies, I just don’t have any experience in them. Take a look at this paper

    https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1285922/pdf/archemed00037-0042.pdf

    Seems pretty effective.
  • Awesome. For the wiki, I think it would be good to add needle and syringe selection etc. Honestly, if person was just going directly into the area an insulin needle could be used. For things like digital blocks though, people usually use a longer needle. At minimum, a 5/8 inch 25 gauge needle. For intrathecal in larger people, you'll need a 1 inch 23 gauge.
  • > @Rezlep said:
    > Tranthecal injections have worked well in studies, I just don’t have any experience in them. Take a look at this paper
    >
    > https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1285922/pdf/archemed00037-0042.pdf
    >
    > Seems pretty effective.

    That's wild. I think I'll ask my preceptor about that and report back.
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