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anaestheticslocal

local anaesthetic agents

see also:

  • remember 1% solution = 10mg/ml
  • thus for lignocaine, max. s/c dose without adrenaline of 4.5mg/kg for a 20kg child = 9mls (4.5 x 20 / 10)
  • toxicity may be managed by use of lipid emulsion infusion
    • iv lipid emulsion therapy - 1.5ml/kg bolus of 20% lipid emulsion, then 0.25-0.5ml/kg/min infusion1)

introduction

  • most local anaesthetics (LA) are classified as either amides or esters.
  • amides
    • include lignocaine, bupivicaine, prilocaine, dibucaine, ropivicaine, mepivicaine, etidocaine
    • tend to have cross-sensitivity in patients with hypersensitivity reactions
      • 0.5-1% diphendydramine, an antihistamine, may be used as a LA in these patients with similar LA onset and duration as for lignocaine
        • using a 50mg/ml iv solution, add 1ml of the solution to 4ml normal saline to achieve 5ml of 1% solution.
        • avoid higher concentrations as risk of tissue necrosis and dermal sloughing, as well as increased pain.
    • most are metabolised by the liver and excreted in the urine, so dose should be reduced in patients with impaired function of either.
    • mechanism of action:
      • uncharged entities diffuse through interstitial fluid and across neuronal membranes by lipid diffusion
        • dispersion of the local anesthetic through the tissue occurs more rapidly as the percentage of uncharged entity increases
        • the pH of the solution determines the proportion of charged and uncharged anesthetic.
        • increased acidity of the solution reduces the proportion of anesthetic in the uncharged form as they are bases, hence part of the reason why LA's do not work as well in the acidic milieu of infected regions.
        • most local anesthetic solutions are marketed in acidic forms with pH of 5.0 to 7.0 as they are more stable at this pH and thus have a longer shelf life of 3-4 years
        • increasing pH to 7.2-7.4, enhances its LA effect, speeds the onset of action, and reduces pain from infiltration but shelf life is reduced to 1-3 weeks at room temperature
      • once inside the neuron, the charged ion form (from acid-base equilibration and thus pH dependent) block neuronal membrane sodium channels and thus prevents neuronal depolarisation and thus blocks nerve impulse conduction.
    • adverse effects of the amide group:
      • pain from infiltration
      • hypersensitivity reaction
      • if excessive dosage or inadvertent intravascular dose:
        • CNS toxicity including metallic taste, tinnitus, tingling of lips, agitation, and seizures (esp. if PH epilepsy]]
        • CVS toxicity including bradycardia, vasodilatation, AV block, ventricular arrhythmia, death (more likely with bupivicaine)
      • pregnancy may be a relative C/I to the more cardiotoxic amides such as mepivicaine and bupivicaine due to poor fetal metabolism due to hepatic immaturity
      • prilocaine in particular may cause methaemoglobinaemia if excessive absorption/dose, particularly in susceptible patients such as G6PD deficiency or with other agents that increase the risk such as nitrous oxide anaesthesia.
  • esters:
    • include procaine, tetracaine, cocaine, benzocaine
    • because of their toxicity and allergic reactions, clinical use is restricted to mainly topical applications
    • mucosal application of benzocaine has caused methaemoglobinaemia in susceptible patients
    • metabolized in plasma by pseudocholinesterase
    • metabolism may be inhibited by anticholinesterases
    • may competitively enhance the neuromuscular blocking action of suxamethonium

the amide local anaesthetics

  • lignocaine (Xylocaine/lidocaine)
  • bupivicaine (Marcaine)
    • slower onset but more prolonged action of LA compared with lignocaine
      • better than lignocaine when prolonged action required - eg. ongoing pain (regional blocks for fracture pain), post-op pain.
    • more cardiotoxic than lignocaine → NOT for IV use and avoid in pregnancy, acidosis or hypoxia
    • max. dose s/c: 1-2mg/kg (3mg/kg if combined with adrenaline)
  • prilocaine (Citanest)
    • similar to lignocaine but:
      • onset of action & duration of action longer than lignocaine
      • its metabolites may cause methaemoglobinaemia2) - avoid in infants
    • usage for IV regional anaesthesia:
  • articaine (Septanest, Deltazine, Bucanest)
    • introduced in Australia in 2005 for dental use as 4% with adrenaline
    • amide type but with additional ester
    • half-life 1.8hrs
    • onset within 6 minutes; duration action usually 1hr

the ester local anaesthetics

  • procaine (Novocaine)
    • the prototype LA, now largely replaced by lignocaine.
    • it is hydrolysed in vivo to para-aminobenzoic acid (inhibits action of sulfonamides)
  • tetracaine (Pontocaine, Amethocaine)
    • deriv. of paraaminobenzoic acid.
    • 10x more toxic & potent as procaine after IV injection ⇒ NOT for IV use
    • similar toxicity to bupivicaine but shorter acting
    • useful in topical applications to skin but no longer used for topical ophthalmic use
    • generally too toxic and expensive for clinical use

esters suitable only for ophthalmic use

  • oxybuprocaine HCl (Benoxinate)
    • a benzoic acid ester related to procaine.
    • i-ii drops of 0.4% → LA within 1 min.
  • proparacaine HCl (Alcaine, Ophthaine)
    • a benzoate ester but distinct from procaine, benoxinate & tetracaine
    • produces little or no irritation;
    • no cross-sensitisation between it & other LA's.

LA of low solubility thus too slowly absorbed from wounds/denuded skin to be toxic

  • benzocaine (ethyl aminobenzoate)

infiltrative LA

  • in general only need to use 1% not 2% lignocaine:
    • infiltrative use of lignocaine (s/c):
      • max. dose s/c: 4.5mg/kg (7mg/kg if combined with adrenaline)
      • onset of action 2-5 minutes
      • duration of action 30-120 minutes
  • when longer duration LA is required, and no C/I such as pregnancy, hypoxia, acidosis, consider bupivicaine instead:
    • max. dose s/c: 1-2mg/kg (3mg/kg if combined with adrenaline)
  • always aspirate first to exclude needle being intravascular before infiltrating
  • wear protective eye wear when infiltrating as eye splashes are not uncommon and resultant bodily fluid infection risk
  • be aware of maximal infiltrative doses otherwise risk of systemic toxicity such as seizures
  • use 1:100,000 adrenaline / epinephrine as well if:
    • not an end-artery region (ie. penis, tip nose, ear, digits, intracutaneous)
    • need relatively bloodless field
    • prolonged anaesthesia or need for larger volume LA
      • adrenaline is said to double the duration of anaesthesia
      • adrenaline reduces LA toxicity by slowing the dissemination into the systemic circulation through local vasoconstriction
    • low risk of wound infection (adrenaline increases infection rate)
    • not involving skin flaps with doubtful blood supply
    • NB. adding adrenaline increases the pain of infiltration;
    • NB. consider keeping vials of LA with adrenaline at a separate location to avoid accidental use of adrenaline vials.
  • minimise pain from infiltration:
    • small needles - 27G or 30G
    • inject slowly, preferably into edge of wound where there are less nerve endings
    • inject under the skin not intradermally
    • warm the local anaesthetic to body temperature
    • consider pre-application of topical anaesthetic
    • consider additional analgesia for children eg. Entonox
    • consider adding sodium bicarbonate to reduce pain of infiltration by raising pH, although randomised trials in children have not been able to demonstrate clear benefit

topical cutaneous application

intact skin

  • application of local anaesthetics to intact skin as a premed for IV cannulation in children
  • EMLA cream:
    • an acronym for Eutectic Mixture of Local Anesthetics
    • consists of micron-sized droplets
    • contains 2.5% lignocaine and 2.5% prilocaine
    • eutectic mixture has a lower melting point than its individual components; consequently, it is in liquid form at room temperature. These properties allow increased penetration of the active ingredients through the skin.
    • risk of methaemoglobinaemia if widespread or prolonged use, particularly in infants under 3 months age, or those with G6PD deficiency or use of nitrous oxide, due mainly to the prilocaine component although lignocaine has also been reported to cause it.
    • requires ~1 hour application for best effect providing 3mm depth of LA, but this further improves over the next 30-60min after removal of the cream
    • duration of action is 1-4 hours after removal of the cream
    • dose: 1-2g per 10 sq cm of skin, cover with occlusive dressing (eg. Tegaderm or OpSite) for 45-60min then remove
    • avoid prolonged contact > 2hrs as may cause blistering
  • LMX cream:
    • topical preparation of lignocaine using liposomes to enhance delivery
    • does not require occlusive dressing as does EMLA, application duration 30min instead of 60min, and no risk of methaemoglobinaemia, and appears to be as effective as EMLA
    • systemic lignocaine toxicity may occur if applied to extensive areas for prolonged periods or on skin rashes or broken skin.
    • may be toxic if accidental oral ingestion
    • dose: 1-2g per 10 sq cm of skin
    • max. skin area coverage3):
      • < 10kg: 100 sq cm
      • 10-20kg: 600 sq cm
      • > 20kg: 2000 sq cm
  • needleless lignocaine delivery:
    • use compressed gas to deliver a liquid or powdered form of lignocaine
    • the degree of pain relief varies, and their clinical utility is uncertain for non-emergent procedures.
    • their ability to provide analgesia within one to three minutes of use may make them very useful for patients undergoing emergent skin breaking procedures.

open wounds

  • application of local anaesthetics to open wounds as primary anaesthesia for wound repair or premed for infiltrative LA
  • those containing vasoconstrictors such as adrenaline (epinephrine) as in LET or TAC should not be used where adrenaline is C/I (see under infiltrative LA above)
  • excessive dosing or usage on mucosal membranes may result in systemic toxicity
  • LET:
    • solution or gel made up of 4% lignocaine, 0.1% epinephrine (adrenaline / epinephrine), and 0.5% tetracaine
    • remove clots from wound then apply 1-3ml of solution (via cotton ball) or gel (via cotton-tipped applicator) and leave for 20-30min
    • allows adequate LA for 70-90% of repair of superficial scalp and facial wounds in children (comparable to TAC), but, like TAC, is less effective on truncal and extremity wounds 4)
    • duration of action following removal of LA is 45-60 minutes.
  • TAC solution or gel:
    • solution made up of 0.25-0.5% tetracaine, 0.025-0.05% adrenaline / epinephrine and 4-11.8% cocaine
    • suitable for scalp and facial lacerations in children
    • serious systemic toxicity may occur if excessive dose or use on mucosa including:
      • hyperexcitability, seizures, stroke, cerebral haemorrhage, tachyarrhythmias, malignant hypertension and death.
    • the issues with cocaine and cost have meant that its use has been replaced by LET.

topical ophthalmic use

tropicamide is NOT a local anaesthetic even though it has amide in the name, it is used to dilate the eyes!
  • topical LA drops are often required to facilitate slit lamp examination, eye irrigation, and usually needed for corneal FB removal
  • ophthalmic LA agents typically degrade more quickly if left at room temperature for more than one week, thus they are usually stored at 2-8deg C in a refrigerator.
  • discard agents that turn from clear to light brown.
  • use unit dose vials to reduce the risk of infection.
  • tend to cause initial burning sensation that usually lasts less than 1 minute
  • duration of action may be longer in the elderly
  • they are NOT recommended for long-term pain control as they may delay wound healing by decreasing the tear film stability and tear break-up times through disruption of the surface microvilli and tend to by toxic to the corneal epithelium with prolonged usage.
  • patients should be warned to wear an eye pad whilst the eye has been anaesthetised to protect against dust and bacterial contamination.
  • proparacaine:
    • causes less pain, provides longer LA and appears to be less cytotoxic to the cornea than tetracaine
  • oxybuprocaine 0.4% (Benoxinate)
    • still commonly used
    • surface anaesthesia occurs in approximately one minute with 0.4% intraocular solution and peak response is between 1 and 15 minutes.
    • anaesthesia persists for about 20 to 30 minutes, with full corneal sensitivity taking 40 minutes or more to return.
  • tetracaine (amethocaine) 1%:
    • now superceded by proparacaine

other resources

1)
Clin. Toxicol. (Phila.) 2010; 48(1):1-27
2)
methaemoglobinemia
anaestheticslocal.txt · Last modified: 2013/08/01 14:12 (external edit)