top of page

LOCAL ANAESTHESIA

History of local anaesthesia

  • In 1860, cocaine was extracted from the leaves of the Erythroxylon coca bush. 

  • In 1884 Koller used it as a topical ophthalmic anaesthetic. 

  • In 1904, procaine was synthesised. Amethocaine was released in 1930. 

    • These agents were esters and, despite a risk of allergic reactions, were widely used. 

  • In 1943 Lofgren developed lidocaine

    • This was an amide and had a low risk of allergic reactions

    • This was followed by mepivacaine (1957), prilocaine (1960), bupivacaine (1963), ropivacaine(1997) and levobupivacaine (2000).

History of local anaesthesia

How does local anaesthetic work?

  • Two types of local anaesthetics

    • Amides​: lidocaine, prilocaine, mepivacaine, bupivacaine, ropivacaine and levobupivacaine

      • Commonly used in the UK​

    • Esters: cocaine, procaine, amethocaine and chloroprocaine

  • Mechanism: Reversible conduction block in nerves by blocking the sodium channels in the nerve membrane

    • Interferes with the nerve action potential and thus prevent propagation of the action potential along the axon

    • These agents cause vasodilation, which increases their tissue absorption

    • By adding a vasoconstrictor, the duration of block increases and the local anaesthetic toxicity risk decreases

  • Local anaesthetics with lower pK have a more rapid onset of action

    • More unionised (uncharged) form allows rapid diffusion into cytoplasmic side of sodium channel​

How does LA work?

Types of local anaesthetic

  • Lidocaine (Lignocaine)

    • Short acting

    • Can be carbonated 

    • Maximum dose​

      • Without adrenaline - 3mg/kg​

      • With adrenaline - 7mg/kg

  • Prilocaine

    • ​Similar to Lidocaine

    • It is more rapidly metabolised and hence less toxic

      • LA of choice in Bier's blocks​

    • It can cause methaemoglobinaemia when used in high dosage (> 600 mg)

      • Methaemoglobinaemia causes a blue skin discolouration and results in false pulse oximeter readings

      • The condition is usually benign and resolves within a couple of hours

    • Maximum dose​

      • Without adrenaline - 6mg/kg​

      • With adrenaline - 9mg/kg

  • Bupivacaine​​

    • ​Four times more potent than lidocaine

    • Long acting

    • Maximum dose

      • Without adrenaline - 2mg/kg​

      • With adrenaline - 2mg/kg

  • Levobupivacaine 

    • Similar to Bupivacaine but contains only the S enantiomer of the racemic mixture of Bupivacaine​

    • Said to have greater vasoconstrictive action and less motor block

    • Maximum dose

      • Without adrenaline - 2.5-3mg/kg​

  • Ropivacaine

    • ​Less toxic than bupivacaine

    • Maximum dose 

      • With or without adrenaline - 3-4mg/kg​

Types of LA

Common additives

  • Adrenaline/ epinephrine

    • Vasocontrictor added to reduce systemic resorption, therefore prolonging onset of anaesthetic effect and reducing toxicity

    • Also provides haemostasis in surgical field in field blocks

    • Commercially available in 1:1000 and 1:10000 ampoules

    • Available pre-mixed with Lidocaine and Bupivacaine

    • If mixing separately, remember that 1:1000 containes 1 mg of adrenaline per ml

    • Maximum safe dose - 4 micrograms/kg

  • Sodium bicarbonate​

    • Used to reduce the pKa of local anaesthetic, allowing more of it to be in its uncharged form, permeate the neuron, and act of the cytoplasmic sodium channels

    • Increases speed of onset and prolongs intensity and duration of block

    • Reduces pain of injection

Common additives

Toxicity

  • Cardiotoxic and neurotoxic in high doses

  • In the event of suspected toxicity:

    • Stop administration

    • Cardiovascular status must be assessed

    • In the event of local anaesthetic-induced cardiac arrest, standard cardiopulmonary resuscitation should be initiated immediately

      • Lidocaine must not be used as anti-arrhythmic therapy

    • ​If the patient does not respond rapidly to standard procedures, 20% lipid emulsion such as Intralipid® should be given intravenously at an initial bolus dose of 1.5 mL/kg over 1 minute, followed by an infusion of 15 mL/kg/hour

      • ​After 5 minutes, if cardiovascular stability has not been restored or circulation deteriorates, give a maximum of two further bolus doses of 1.5 mL/kg over 1 minute, 5 minutes apart, and increase the infusion rate to 30 mL/kg/hour

      • Continue infusion until cardiovascular stability and adequate circulation are restored or maximum cumulative dose of 12 mL/kg is given

      • Standard cardiopulmonary resuscitation must be maintained throughout lipid emulsion treatment

      • Propofol is not a suitable alternative to lipid emulsion

      • Further advice on ongoing treatment should be obtained from the National Poisons Information Service

Toxicity

Safe administration of local anaesthetic

  • Before administration:

    • Decide on the type of block​

      • Field block - Local anaesthesia administered subcutaneously around a surgical field

        • Useful for small areas​

        • May require large amounts of local anaesthetic if the surgical field is large

      • Regional block - Local anaesthesia administered directly to a nerve sheath to achieve sensory block over the nerve territory

        • Risk of nerve injury​

        • Adrenaline does not provide haemostatic effect in surgical field

    • Decide on suitable type of local anaesthetic, taking into account:​

      • Surgical area

      • Duration of surgery​

      • Use of additives

    • Decide on administration device​

      • Size of syringe

        • Poorer control in larger syringes

        • Smaller syringes need to be refilled often

      • Size and type of needle​

        • Finer needles tend to be shorter​

        • Finer needles are less painful

        • Flexible needles are available and useful for staying within a plane

    • Prepare local anaesthetic​​

      • Warm to room temperature if refrigerated​

      • Mix with required additives

      • Check expiry dates

    • Aseptic preparation of administration area

      • Clean injection area with chlorhexidine wipes and allow to dry

  • When infiltrating:

    • ​Insert the needle

    • Aspirate to ensure no flashback of blood

    • Inject local anaesthesia locally and as you retract

    • Aim to penetrate the skin as little as possible

      • When the tip of the needle is close to the skin, rotate the angle of the needle and insert needle to an uninfiltrated area to allow maximal infiltration through one puncture site

      • If required, additional punctures should be through an area already infiltrated

  • After administration:

    • Test all areas of the surgical field for effect​

      • Bear in mind some local anaesthetics take longer to work

      • When in effect, only the pain sensation is anaesthetised - patient will still feel pressure, touch and propioception

    • Haemostasis - pressure with gauze to haemostase area

Safe administration of LA
bottom of page