Esters & Amides
Local anaesthetics fall into one of these two categories. A simple method for differentiating them, is by following this general rule (be warned - it may not always work!):
- Esters: have only one 'i' in the name - e.g. procaine, cocaine. Tend to be faster acting.
- Amides: have over two 'is' in the name - e.g. lidocaine or bupivicaine, prilocaine
Lidocaine & Epinephrine
- Lidocaine (formerly known as lignocaine) solution is available in 3 strengths (0.5%, 1% and 2%). Each preparation is available with and without epinephrine (adrenaline).
- Generally, 1% is adequate for most purposes.
- 1% = 1 gram / 100 mls.
The use of combined local anaesthetic & epinephrine preparations can have several advantages:
- The procedure can be made easier by reducing local bleeding (especially in very vascular areas such as the scalp).
- Epinephrine reduces systemic absorption (by causing local vasoconstriction) and allows larger volumes of anaesthetic to be used (reduces the systemic distribution of the anaesthetic).
- The vasoconstrictory 'blanching' of local skin can help demarcate the extent of the anaesthetised field.
- Dose limit for epinephrine: total dose in one administration = <500 micrograms.
IMPORTANT NOTE: Never inject epinephrine containing preparations into end-artery areas (e.g. the digits/penis).
Lidocaine maximum safe doses
The dose in a 'single application' should not exceed:
- Lidocaine (without epinephrine) = 3mg/kg body weight (e.g. max 200mg for 70kg person)
- Lidocaine (with epinephrine, 1:200,000) = 7mg/kg body weight (e.g. max 500mg for a 70kg person)
Bear in mind, that the effects of local anaesthetics depend on a number of factors including the size of the patient, the relative 'vascularity' of the infiltration site, cardiac output, drug distribution & metabolism, as well as local factors such as whether infection is present at the site of injection (this will reduce the effects of the anaesthetic). For this reason, doses should always be tailored for the individual patient.
In reality, only a few mls are usually required for most minor procedures. Do not forget to consider the cumulative effects of injecting multiple small lesions in a single application, and never even draw-up more than the maximum safe dose for a patient, as this may lead to accidental over administration.
Local anaesthetic administration
- Wide-bore needles (green) should be used to draw up the LA, and narrow-bore (blue or smaller) should be used to administer the drug itself (depends upon the site and nature of the administration).
- Slowly inject the anaesthetic around the target lesion. Use a more 'fan-shaped' approach for smaller lesions, while with larger lesions aim to 'encircling' the site.
- Make sure you remember to start superficially, and infiltrate the deeper areas as the superficial anaesthetic takes effect.
- Always pull-back on the syringe to check that there is no blood (that you aren't in a vessel) before pushing down on the plunger.
- Drug name, with/without epinephrine, best-before (expiry) date of the vial, calculation for maximum safe dose.
- You should always get a second person to verify these details prior to administration.
- Always document details of the anaesthetic in the notes.
- These are used for anaesthetising areas such as the digits (for example prior to suturing a finger wound).
- A 'ring' of LA is injected at the proximal end (base) of a digit, leading to a distal anaesthetic effect.
- A ring block takes longer to work than standard subcutaneous infiltration, and remember never to use epinephrine.
- Ring blocks are less effective and should therefore not be injected through infected skin (can also cause spreading of infection through tissue plains).
- 1-3ml is usually enough to use.
- Avoid piercing the skin on the palmar/plantar aspect. Beware of accidental needle-stick injuries.
Ring block technique:
- Raise a 'bleb' of anaesthetic over the dorsal aspect of the digit, just distal to the metatarsophalangeal (MTP - feet) or metacarpophalangeal (MCP - hands), to anaesthetise the superficial aspects of the digital nerves. Always pull back prior to injecting in order to check that you haven't gone into a vessel.
- Angle the needle towards the palm or the sole and inject about 0.5ml of LA along one side of the digit. Withdraw the needle partially, and inject a further small quantity just deep to the skin.
- Withdraw completely and repeat on the other side of the digit.
Causes & management
- Lidocaine toxicity is caused by high circulating plasma levels of the drug.
- This may be because of a high total administered dose, or due to the distribution in an individual.
- If toxicity is suspected, do not administer any more anaesthetic, and be prepared to stabilise the patient ('ABC') as required.
Lidocaine toxicity - symptoms & signs
- Early signs: mainly CNS related, eg. restlessness, tingling of limbs/around mouth (peri-oral), confusion
- Late signs: CNS related (depression, convulsions, unconsciousness) & cardio-respiratory (respiratory depression, arrhythmias, arrest)
NB: cardiac toxicity can be resistant to treatment (this is particularly true when bupivicaine is used).