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CNS lectures (Bal) > Module 2.1.1 (Local Anaesthetics) > Flashcards

Module 2.1.1 (Local Anaesthetics) Flashcards

1
Q

What are ester type LA

A

Amethocaine
Cocaine
Procaine

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2
Q

What are the amide type LA?

A

Lignocaine
Bupivacaine
Levobupivacaine
Ropivacaine
Prilocaine

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3
Q

What does LA preferrentially block?

A

LA preferentially blocks small nerve fibres

  • Small nerve fibres are preferentially blocked because of their high surface-area to volume ratio.
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4
Q

What does LA prolong?

A

LA prolongs the sodium channel inactivation state

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5
Q

What does potency and DOA directly related to for LA?

A

Potency, onset and duration of
action are dependent on lipid
solubility

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6
Q

Where does LA act by binding?

A

LA acts by binding of the
hydrophilic centre
(ionic form) of the
anaesthetic to the binding
site on the inside of
the sodium channel

  • The local anaesthetic
    binds to amino acid
    residues located on
    domain 4.
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7
Q

What is the importance of pH and ionization in LA potency

A

LAs are weak bases (pKa of 8-9)

Only uncharged form can
penetrate lipid membranes

Activity increased at alkaline pH

Quaternary ammonium
compounds must be injected
directly into the nerve axon

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8
Q

Explain where the nonionised form/ionized of LA acts on specific site of sodium channel?

A

Nonionised form of the local
anaesthetic (R-NH2) penetrates
the axonal membrane and is
then converted to the ionized
form (R-NH3+).

Ionized form binds to the
sodium channel in the open
state, and this prolongs the
sodium channel inactivation
state.

Sodium entry is blocked during
the inactivation state.

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9
Q

What is the half life of LA with ester bond?

A

LA with ESTER Bond
Is inactivated by
plasma cholinesterase
Very short half life
<3min-1h

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10
Q

What is the half life of LA with amide bond?

A

Is degraded by N-dealkylation in liver
Short half life 1-3h

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11
Q

What is the MOA of lignocaine?

A
  • Lignocaine is classified as an amide type local anaesthetic
  • Cause reversible inhibition of nerve conduction
  • by binding to specific site in sodium channel to cause closure of inactivation gate
  • Keeps channel in the inactivation stage hence preventing propagation of action potential.
  • NET result is an inhibition of depolarisation
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12
Q

Why are vasocontrsictors used in LA? Which ones are used?

A

Vasoconstrictors are added to some formulations of
LAs to increase the duration of their contact with
nerves

Adrenaline (epinephrine)

  • Most commonly used vasoconstrictor

Felypressin

  • used in some dental formulations of LAs
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13
Q

What are the adverse reactions to vasoconstrictors?

A
  • Tachycardia, hypertension, sweating & arrhythmias
  • contraindicated in IV regional anaesthesia (IVRA)
  • should never be injected near terminal arterioles,
    such as those in fingers, toes or ears
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14
Q

Why did EM’s stinging pain arrive before the dull
aching pain, and why did it resolve more quickly
after lignocaine administration?

A

Pain is conducted by two main types of nociceptive nerve fibers the A-delta and C fibres. A-delta fibres are fast fibres which are involved in the transmission of the “first pain” characterised by the immediate intense stinging and sharp pain. This is followed by the “second pain” characterised the dull chronic/aching pain and is conducted via the C fibre

  • LAs unlike opioids are non –specific and will inhibit ALL neurons. They however preferentially block the fast, small and myelinated fibres (eg A-delta pain fibres) followed by small non-myelinated fibres (eg C pain fibres) and finally by larger motor neurons. Hence the first pain will be preferentially blocked first before the second pain
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15
Q

Why is epinephrine sometimes administered with lignocaine

A

Adrenaline (epinephrine) is the most commonly used vasoconstrictor

  • Added to some formulations of LAs to increase the duration of their contact with nerves
  • It is a powerful vasoconstrictor and should never be injected near terminal arterioles, such as those in fingers, toes or ears, as it can cause peripheral ischaemia and which can result is peripheral necrosis
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16
Q

Explain the adverse effects of LAs:

A) CNS effects

B) Cardiovascular effects

C) Hypersensitivity

A

A)

CNS stimulation: restlessness, nervousness, agitation, confusion, tingling around the mouth, tinnitus, dizziness and blurred vision, that may be followed by convulsions.

CNS depression: excitation followed by drowsiness, loss of consciousness, respiratory arrest and death.

B)

Vasovagal attack may occur suddenly with little warning

  • bradycardia, arteriolar vasodilatation, hypotension, syncope, sweating, pallor, nausea, vomiting, cardiovascular collapse, cardiac arrest

Arrhythmia

  • ​Lignocaine - Class Ib antiarrhythmic agent

Methaemoglobinaemia

  • prilocaine - oxyhaemoglobin transformed to a product useless for respiration, due to oxidation of Fe2+ to Fe3+

C)

  • Allergic dermatitis or asthma
  • Usually to the ester group of local anaesthetic
  • Procaine metabolised to PABA (paminobenzoic acid) - allergen
  • Allergy to amides is rare, no crossreactivity
17
Q

What are adverse drug interaction with LAs

A

Limited significant drug interactions

  • CNS depressant drugs as some LAs are antiarrhythmic – additive CNS depression
  • Anticholinesterase drugs – inhibit metabolism of ester-type LAs (procaine, amethocaine)
  • Anaesthetics containing vasoconstrictors – use with caution in patients taking antihypertensives, MAOIs, TCAs
18
Q

What are the techniques of administration?

A

Topical anaesthesia

Subcutaneous infiltration

Intra-articular /intra-tendonous injection

Peripheral nerve or nerve plexus block

Central neural blockade (epidural/caudal/intrathecal)

IV regional anaesthesia (IVRA) (Bier’s Block)

Sympathetic plexus block

19
Q

What LA are used for topical anaesthesia? What are the uses?

A

amethocaine, cocaine, lignocaine, prilocaine

> Many formulations – eye and ear drops, cream, ointment, solution, gel, spray and powder

> Topical anaesthetics do not effectively penetrate unbroken skin except EMLA = EMLA (eutectic mixture for local anaesthesia) combination cream of lignocaine and prilocaine

Uses

  • Intact skin/mucous membranes 
  • Relieve pain or itching 
  • Used before venipuncture or split skin grafting 
  • Facilitates instrumentation (e.g. gastroscopy, laryngoscopy, cystoscopy) 
  • For procedures involving surfaces of the eye and dental practice
20
Q

Which LA are use for subcutaneous infiltration? What is it used for?

A

Procaine, prilocaine, lignocaine

  • For localised anaesthesia where motor nerve block is not required 
  • Minor surgery, skin lesions 
  • Drainage of cyst 
  • Major procedures in dental practice 
  • Adrenaline and felypressin often added as vasoconstrictors to prolong duration of action 
  • NOT with fingers and toes!!
21
Q

What is LA used with for intra-articular/intra-tendonous injection?

A

Used for relief of pain caused by inflammation

  • LA with corticosteroid
22
Q

Which LA is used for Peripheral nerve or nerve plexus block? what is it used for?

A

Procaine, prilocaine, lignocaine, bupivacaine

  • Used for surgery to structures within specific nerve or plexus distribution

> LA alone or with sedation or general anaesthesia

  • For postoperative pain relief
23
Q

Which LA is used for central neural blockade? Include for epidural and spinal. What is it used for?

A

Epidural

  • lignocaine, bupivacaine, Levobupivacaine, ropivacaine

Spinal

  • lignocaine, bupivacaine, Levobupivacaine

> Major surgery to lower limbs, pelvic organs, abdomen and thorax. LA alone or with sedation or general anaesthesia

> Acute pain relief (postoperative and during labour and delivery)

> Chronic pain relief. LA with corticosteroid

> Cancer and neuropathic pain. LA with drugs such as opioids, midazolam and clonidine

24
Q

Which LA is used for IV regional anaesthesia (IVRA) (Bier’s Block)? How is it used? What is it CI with?

A

Prilocaine and lignocaine

  • LA injected IV distal to a pressure cuff to arrest blood flow
  • Remains effective until circulation is restored
  • Used for peripheral limb surgery and for sympathetic limb blockade

CI with bupivacaine, Levobupivacaine, ropivacaine

25
Q

What are specific considerations for cocaine and coexisting conditions?

A

Cardiac disease, hypertension - relative risk of cardiotoxicity

Damaged mucosa - risk of systemic toxicity from enhanced absorption

Treatment with sympathomimetics - increases risk of cardiac arrhythmias

Given after 14 days of stopping MAOI drug therapy

CNS lectures (Bal) (29 decks)

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