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Anesthetics > Local Anesthesia > Flashcards

Local Anesthesia Flashcards

1
Q

Classification - Chemical Properties

A

Esters

  • Procaine
  • Chlorprocaine
  • Cocaine
  • Benzocaine
  • Tetracaine

Amides

  • Lignocaine
  • Bupivacaine
  • Ropivacaine
  • Prilocaine
  • Mepivacaine
  • Etidocaine
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2
Q

Classification - Duration of Action and Potency

A

Short DOA, low potency

  • Procaine
  • Chlorprocaine

Intermediate DOA and potency

  • Lignocaine
  • Mepivicaine
  • Prilocaine
  • Cocaine

Long DOA, high potency

  • Bupivacaine
  • Tetracaine
  • Etidocaine
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3
Q

Factors affecting onset of action

A
  • pKa
  • pH
  • Concentration
  • Lipid solubility
  • CT surrounding nerve
  • Temperature
  • Vasoconstrictors
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4
Q

Shortening onset of action

A
  • Warm local anaesthetic
  • Add sodium bicarbonate 1ml/ 10ml LA
  • Add adrenaline separately
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5
Q

Factors affecting duration of action

A
  • Lipid solubility
  • Protein binding
  • Dose
  • Site of Injection
  • Additives
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6
Q

Additives prolonging duration of action

A
  • Alpha-adrenergic agonists (e.g. adrenaline, clonidine or dexmetetomidine)
  • Opioids (neuraxial blocks)
  • NaHCO3 (epidural blocks)
  • Hyluronidase (ophthalmic blocks)
  • Ketamine (caudal and epidural blocks)
  • Magnesium (peripheral nerve - controversial)
  • Dextrose (increase baricity for intrathecal injection)
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7
Q

Factors affecting absorption

A
  • Site of injection (IVI > tracheal > intercostal > caudal > paracervical > epidural > brachial plexus > subcutaneous)
  • Vasoconstrictors
  • Protein bindings
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8
Q

Mechanism of action

A

Primarily: block conduction via Na+ channels by binding to the α subunit

Secondarily: block K+ and Ca2+ channels, and NMDA glutamate receptors

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9
Q

Metabolism and Excretion

A

Esters: plasma pseudocholinesterase (butyrylcholinesterase) breaks LA down to PABA (may trigger allergic reaction) - excreted unchanged in urine

Cocaine: partially metabolised by the liver, partially excreted unchanged by the kidney

Amides: microsomal P450 system (lignocaine > ropivacaine > bupivacaine)

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10
Q

Potency

A

Bupivacaine > levobupivacaine > ropivacaine > lignocaine > procaine

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11
Q

Toxicity - CNS

A

Initial Phase

  • Circumoral paraesthesia
  • Tinnitus
  • Confusion

Excitatory Phase

  • Convulsions

Depressive Phase

  • Loss of consciousness
  • Coma
  • Respiratory depression

Management

  • Stop injection
  • Apply oxygen
  • Secure airway
  • Avoid hypoventilation and acidosis
  • Treat seizures
    • Midazolam 0.05-0.1mg/kg
    • Propofol 0.5-1mg/kg
    • Thiopentone 1-2mg/kg
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12
Q

Toxicity - CVS

A

Initial phase

  • Hypertension
  • Tachycardia

Intermediary phase

  • Myocardial depression
  • Decreased CO
  • Hypotension

Terminal phase

  • Peripheral vasodilation
  • Severe hypotension
  • Sinus bradycardia
  • Conduction defects
  • Dysrhythmias

Management

  • Resuscitate according to ACLS
  • Intravenous fluids, vasopressors
  • Amiodarone and vasopressors instead of lignocaine and adrenaline
  • Intralipid infusion
  • CPR
  • Insulin, 50% glucose, and potassium infusion (2IU/kg/hr, 2ml/kg/hr, 2mmol/kg/hr respectively)
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13
Q

Intralipid (Dosage)

A
  • 20% solution
  • 1.5ml/kg bolus IVI over 1 minute
  • Followed by 0.25ml/kg/min infusion
  • Bolus can be repeated twice at 5 minute intervals
  • Infusion can be increased to 0.5ml/kg/min
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14
Q

Side Effects

A
  • Direct neuronal toxicity
    • Cauda equina syndrome (persistent neurological injury)
    • Transient neurological syndrome (marked dysaesthesia)
  • Allergy
    • PABA from ester metabolism
    • Methylparaben preservative in amide formulations
  • Myotoxicity (hyper contraction, vacuole formation, edema, and necrosis)
  • Hematological
    • Reduced coagulation, enhanced fibrinolysis (lignocaine)
    • Prilocaine (metabolised to orthotoline which oxidises Hb to metHb - treated with methylene blue 1-2mg/kg IVI over 5 minutes)
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15
Q

Contraindications to adrenaline

A
  • CVS
    • Unstable angine
    • Arrhythmia
    • Uncontrolled hypertension
  • Uteroplacental insufficiency
  • Drugs
    • MAOI
    • TCA
  • Poor collateral flow (digits, penis)
  • IV regional anesthesia
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16
Q

Maximum Dosages

A
  • Bupivacaine, ropivacaine, tetracaine, and cocaine - 3mg/kg
  • Procaine - 12mg/kg
  • Lignocaine - 4.5/7mg/kg

1% means 1ml = 10mg

17
Q

Duration of block

A
  • Procaine and cocaine - 0.5-1 hour
  • Lignocaine - 0.75-2 hours
  • Tetracaine - 1.5-6 hours
  • Bupivacaine and ropivacaine - 1.5-8 hours
18
Q

Cocaine

A
  • MOA: inhibit adrenaline and NA reuptake
  • Use: topical anesthesia in ENT surgery and bronchoscopy
  • Onset: 2-5 min
  • Duration: 30-40 min
  • Dose: 3mg/kg
19
Q

Lignocaine - Uses

A
  • Regional anesthesia (spinal -controversial, epidural, plexus and peripheral nerve blocks)
  • Local infiltration
  • Topical (4%)
  • Intravenous regional
  • Blunt hemodynamic response to intubation
  • Antiarrhymic
  • Acute postoperative pain
  • Chronic neuropathic pain
  • Neuro- and cardioprotective
20
Q

Lignocaine - Dosing

A

Maximum dose

  • Neonates - 3mg/kg (5mg/kg with adrenaline)
  • Adults - 4.5mg/kg (7mg/lg with adrenaline)

Onset of action: rapid (5-10 min)

Duration of action: 30-60 min (120 min with adrenaline; half life 1.5-2 hours)

21
Q

Bupivacaine - Uses

A
  • Infiltration
  • Topical mucous membrane anesthesia
  • Spinal
  • Epidural and caudal
22
Q

Bupivacaine - Dosing

A

Maximum dose: 2-3mg/kg

Onset of action: 10-15 min

Duration of action: 180-300 min (120-480 with adrenaline) (half life 2.7 hours (8 in neonates))

23
Q

EMLA

A
  • Eutectic mixture of LA
  • 2.5% lignocaine + 2.5% prilocaine in oil-water emulsion
  • 3-5mm penetrating
  • Applied 1-2 hours prior procedure; duration 1-2 hours
  • Use: IV line insertion, suturing minor wounds, split thickness skin grafts, circumcision
  • Dose:
    • Paeds: 1g/10cm skin
    • Adult: 1-2g/10cm skin
  • Side effects: blanching, erythema, edema, metHb
  • Alternative: Amethocaine (tetracaine 4%)
24
Q

EMLA - Maximum Dosage

25
Q

Precautions to prevent toxicity

A
  • Check for past history of adverse events from local anaesthetic administration (e.g. abnormal pseudocholinesterase)
  • Calculate the maximum dosage, and remain within the lower limit of safety
  • If large volumes are to be used, use drugs with low toxicity
  • Good technique (e.g. draw back to prevent accidental intra-vascular injection, slow injection, and incremental dosing)
  • Prevent accidental subarachnoid administration of epidural dosage by administering a test dose with adrenaline
  • Use of adrenaline to slow systemic absorption
  • Understand that toxicity of different agents are not independent of each other
  • Consider renal and liver disease, which may decrease the toxic dose
  • High index of suspicion for toxicity
26
Q

Why are amides used more commonly

A
  • Lower incidence of allergic reaction
  • Can be stored for longer
  • More heat stable (can be autoclaved)

Anesthetics (10 decks)

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