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Local anaesthetics and anaesthesia

 

Introduction to local anaesthetics

  • Local anaesthetics (LA) are drugs which cause reversible local inhibition of nociception and therefore pain perception
  • Can be used pragmatically to cause numbness of an area of skin through local infiltration or used to target specific nerves to give predictable and targeted anaesthesia
  • Commonly used in many medical and surgical specialties for patient comfort and ease of procedure
  • Many different types and techniques available

 

Mechanism of action of local anaesthetics

  • Act by inhibition of action potential generation within nerve cells
  • Bind reversibly to Na+ channels in the neuron cell membrane
    • Pass across lipid membrane in an non-ionised state and become ionised inside the axon (intracellular environment more acidic)
    • Ionised form able to bind to the Na+ channel intracellular surface preventing Na+ ion entry and action potential generation
  • Nerve fibres are blocked in a predictable sequence
    • Preferentially block small diameter, myelinated and high frequency nerve fibres
    • Sequence of fibres blocked: B fibres (autonomic), C & A∂ fibres (pain and temperature sensation), Aß fibres (Light touch and pressure sensation), A⍺ & Aγ fibres (Motor and proprioception)

 

Click here for the detailed pharmacology of local anaesthetic action 

 

Contraindications for local anaesthetics

  • Previous allergic reaction to local anaesthetic
  • Inflamed or Infected tissues- unlikely to have effect if used locally due to low pH of tissue
  • Caution in patients with family history of malignant hyperthermia

 

Doses of local anaesthetic

  • Dose variable depending on LA
  • Concentration of LA solution expressed as % eg Lidocaine 1%
    • Can be converted into mg/ml by remembering 1% solution = 10mg/ml, 0.5% solution =5mg/ml
  • Each LA has a maximum dose
    • See Table 1 below
    • Calculating maximum dose requires patients weight and concentration of LA solutions
  • Typical solutions (with or without adrenaline)
    • Lidocaine 1% (10mg/ml), 2% (20mg/ml)
    • Bupivacaine 0.25% (2.5mg/ml), 0.5% (5mg/ml)

 

Adrenaline use with local anaesthetics

  • Adrenaline (vasoconstrictor) can be combined with LA
  • Improves local haemostasis by vasoconstriction
  • Prolongs LA effect through reduced absorption from tissues
  • Almost instantaneous effect and half life of 2 minutes (vasoconstriction effect lasts significantly longer)
  • High doses risk ischaemia, tachycardia, hypertension and arrhythmias
  • Contraindications to Adrenaline use:
    • End-arteries eg digits, penis
    • Use with caution in patients with poor peripheral circulation eg Raynaud’s, diabetes, peripheral vascular disease
    • Avoid using on skin flaps
  • Typical doses expressed as grams of solvent in millitres of solvent eg 1:200,000 is 1g adrenaline in 200,000ml solution
    • Commonly used solutions of LA contain 1:200,000 or 1:80,000 adrenaline

 

Pre-Procedure

  • Consent the patient explaining the risks and benefits of LA (see Complications, below)
  • Check for contraindications (see above)
  • Explain to the patient that the purpose is to numb the skin and prevent pain, but they may still feel movement/pressure
  • Warn the patient that the LA injection will be painful during administration, but this quickly settles
    • Topical LA is painless but causes erythema & swelling of the skin if left for too long

 

Equipment for local anaesthetic administration

  • Sterile Gloves
  • Skin preparation- povidone-iodine, chlorhexidine or alcohol wipe
  • Sterile drapes/sheets
  • Sharps Bin
  • Gauze
  • Syringe
  • Needle- ideally small gauge e.g. 25G or less
  • Local Anaesthetic solution- with or without adrenaline (eg 1% Lidocaine with 1:200000 adrenaline)
  • Good Lighting
  • Equipment for procedure about to be performed e.g. suture kit

 

General principles of local anaesthetic administration

  • Draw up LA into syringe removing any air bubbles
  • Clean skin using skin preparation or alcohol wipe
  • Insert needle confidently through skin and dermis
    • Inadvertent intradermal injection is very painful
    • LA should be injected into subcutaneous space
  • Before injecting LA, aspirate to ensure you are not injecting into a blood vessel
    • If you do aspirate blood, withdraw or advance the needle slightly more, aspirate again and if no further blood it is safe to to infiltrate LA
    • Keeping the needle in constant motion whilst injecting prevents any significant volume of LA being injected intravascularly
  • Inject LA slowly
  • Advance the needle to cover larger area, manouevering within subcutaneous space.
    • Try to avoid withdrawing the needle from the skin to avoid having to re-insert

 

Techniques for local anaesthesia

  • Local infiltration: LA injected directly into/around site of interest to create an area of numbness
  • Ring Block: Injection of LA around the digital neurovascular bundles of the fingers/thumb to cause numbness distal to the site of ring block and allow operative intervention on the fingers
  • Nerve Block: LA injection around a specific nerve designed to cause numbness in the territory supplied by that nerve
  • Topical LA: creams applied to the skin under an adhesive dressing eg EMLA™ (5% mixture of lidocaine & prilocaine)- useful LA in children for blood taking/cannulas but allow 15-30minutes for LA to take effect
  • Bier’s Block: intravenous regional anaesthesia (IVRA) using Prilocaine
  • Eye Drops: LA eye drops used in ophthalmology for emergency treatment and surgery

 

Post-Procedure

  • Monitor the patient for signs of LA toxicity
  • Inform the patient of the likely duration of action and advice to protect the anaesthetised area until sensation returns e.g. no hot drinks or chewing if LA used on the lips

 

Top Tips for using local anaesthetic:

  • Use nerve blocks where possible as they provide more reliable anaesthesia and are often injected well away from the site of trauma/infection
  • Using a mixture of a rapid onset, short duration LA and a slow onset, long duration LA gives the ideal combination of rapid onset and prolonged duration of anaesthesia
    • A classical example is a lidocaine and bupivacaine mixture e.g. 1% lidocaine & 0.5% bupivacaine
  • Introduce the needle through a previously anaesthetised area or wound edges to reduce pain (or consider using topical LA before first needle insertion)
  • Heating the needle and LA before injection reduces the initial pain
  • Use the smallest volume possible and the smallest needle!
  • Stimulating surrounding skin by pressure or a pinch (activates A-fibres) can inhibit C-fibre activity and thus reduce the pain of injection
  • Buffering the solution with bicarbonate increases the pH of the solution (making it closer to physiological pH) and therefore reduces pain but in practice this is inconvenient to do

 

Complications of local anaesthetia

  • Pain: injection of LA is painful!
  • Allergic Reaction:
    • More commonly associated with reaction to preservative rather than LA itself
    • Symptoms common to other allergic reactions
    • Treat as per allergic reaction/anaphylaxis (see Anaphylaxis) and withdraw offending agent
    • NOTE: can be confused with patient anxiety
  • Local Anaesthetic Toxicity
    • Central nervous system (CNS) toxicity:
      • Increased levels in CNS result in decreased inhibitory neuron activity
      • Symptoms include: perioral paraesthesiae; tinnitus; twitching; visual disturbances; light-headedness/dizziness; tongue numbness/metallic taste; anxiety; confusion; ‘feeling of impending doom’; seizures
      • Further increased toxicity inhibits cardiorespiratory centres in the medulla leading to: respiratory failure; arrhythmias; cardiac arrest; hypotension; coma
      • NOTE: CNS toxicity tends to precede cardiovascular toxicity
      • Management:
        • Remove toxic agent
        • Call for senior/anaesthetic support
        • ABC management including high flow O2 and ventilation if required
        • Consider benzodiazepines for seizures
        • Consider lipid emulsion eg Intralipid™ 20% solution (LA ‘antidote’) bolus and infusion
          • g. 1.5mL/Kg IV over 1 minute
          • Infusion of 0.25ml/Kg/min
        • Cardiovascular toxicity:
          • Decreased myocardial activity & contractility
          • Symptoms: Bradycardia; hypotension; arrhythmias; cardiac arrest
          • Some degree of direct peripheral vasodilation
          • NOTE: Bupivacaine is particularly cardiotoxic (Ropivicaine & levobupivacaine have much better safety profiles)
          • Management:
            • Remove toxic agent
            • Call for senior/anaesthetic support
            • ABC management including high flow O2, ventilation if required and IV fluid resuscitation
            • ALS management if arrhythmia develops (see Advanced Life Support)
            • Consider lipid emulsion eg Intralipid™ 20% solution (LA ‘antidote’) bolus and infusion as above
          • For more on intralipid click here
        • Other: neuromuscular toxicity- direct injection into muscle causes decreased muscle excitability and possible myotonic effects
      • Methaemoglobinaemia
        • Rare complication associated with metabolite of prilocaine (0-toluidine)
        • Symptoms: cyanosis; tachypnoea; confusion; decreased level of consciousness; dizziness; death
        • Management:
          • ABC management with high flow O2
          • IV methylene blue 1% solution e.g. 1 to 2ml/kg over 5 minutes

 

Useful links

 

Click here for medical student OSCE and PACES questions about Local Anaesthetic and anaesthesia

Common Local Anaesthetic and anaesthesia exam questions for medical students, finals, OSCEs and MRCP PACES