Local Anaesthetics Flashcards
Two chemical types of local anaesthetics:
- Esters: Procaine, Chloroprocaine, Oxybuprocain, Tetracaine, Benzocaine (cocaine)
- Amides: Lidocaine (lignocaine), Prilocaine, Mepivacaine, Bupivacaine, Ropivacaine
Local Anaesthetics mechanism of action:
- They act by blocking conduction in nerve fibers
- They block action potential generation by blocking Na+ channels
- They act in their cationic form (they have to be weak bases), but must reach their site of action by penetrating the nerve sheat and axonal membrane as unionised species.
Modulated receptor hypothesis:
- status of Ion-channel
- Affinity of the LA
- Effect on the channel
Pharmacodynamic actions:
LA blocks conduction in the following order:
- Axons, Non-myelinated axons, Small myelinated, Large myelinated axons
Nociceptice and sympathetic transmission is thus blocked first.
Disappearance of nervous function:
- Pain -> Warmth -> Touch -> Deep pressure -> Motor function
Return of nerve function are in the reverse order
Types of peripheral nerve-endings:
Kinetics of Local Anaethetics:
Factors determine the absorption
- site of application
- dose
- vasoconstrictive drug (Epinephrine 0.001%)
- features of LAs (eg. lipid solubility)
Effects of LAs until the concentration falls below the critical level due to their absorption from the site of application
Metabolism of LAs:
- Ester-type LAs are hydrolysed primarily by plasma (pseudocholin) esterase and degraded by hepatic metabolism
–> the safer LAs are sters (BUT tetracaine)
- Amides are largerly degraded by hepatic inactivation (+ some plasma hydrolysis)
- metabolites are excreted by the kidneys
- order of clearance of amides most to less rapid:
Prilocaine > Lidocaine > Mepi-/Ropivacaine > Bupivacaine
Comparison of Local anaesthetics:
Side effects of LAs:
They are relatively free from harmful side effects, Toxicity in increased order:
Mepivacaine < Procaine < Lidocaine < Tetracaine, Bupivacaine
Side effects:
- Allergy (mostly common with ester-type LAs)
- Asthmatic seizure
- Local tissue irritation
- Systemic effects (due to elevated plasma conc.)
–> CNS effects: first mild sensory inhibition, agitation, confusion, tremors progressing to convulsions and respiratory depression
–> Cardiovascular effects: namely myocardial depression and vasodilatation, leading to fall in blood pressure
–> the absorption may cause toxic effect if the rate of absorption is faster than the rate of elimination. The rate of absorption can be reduced with vasoconstrictors
- Effect of adjuvant (adrenaline)
Uses of Local anaesthtics:
- Topical anaesthesia
- Local infiltration
- Perineural injection
- Intra-articular use
- IV regional anaesthesia (IVRA)
- Epidural and spinal (subarachnoid) block
Topical Anaesthesia:
- The drug is applied to the cornea, mucous membranes or skin
- To cause loss of sensation by paralyzing sensory nerve endings (Normal skin is too thich, although creams of Lidocaine or Prilocain cause anaesthesia in about 60 min)
Indications:
- ulcers, opthalmology, oral, rectal, pharyngeal painful diseases before examinations (introduction of catheters), burnings, sun-irritation, inflammation of outer or middle ear.
Local Infiltration:
- Common methd, consists of numerous SC inj. of small volumes.
Indications:
- minor surgery, dentistry (may be ineffective!), Castration of males
Subcutaneous regional anaesthesia (“field block)
- Local anaestitic is injected along a line, blocking conduction in the nerves that pass through the tissue. All regions supplied by the distal section of these nerves will be anaesthetised
Perineural injections:
- Peripheral nerve block (conduction block) injection of the anaesthetic in the immediate vicinity of peripheral nerves
Indications:
- dentistry, minor surgery (head, legs, hoofs)
Further types:
- paravertebral, intercosta, brachial plexus blocks.
Intra-articular use:
Relatively rare
- Diagnostic goal: to facilitate the diagnosis of lameness
- Therapeutic indication: operation of the affected joint
Intravenous regional anaesthesia (IVRA):
- Local anaesthtics is injected IV distal to a tourniquet
- extreme isolation from the rest of the circulation, only less toxic agents are allowed, eg. Prilocain.
- frequently used for operations of the digit in cattle
