Local Anaesthetics Flashcards
Describe the process of the generation of a neuronal action potential
- Resting sodium channels open and Na+ enters the cells
- Na+ channels close, K+ channels open and K+ leaves the cell
- Na+ channels are restored to resting state but K+ channels still open therefore cell refractory
- Na+ and K+ channels restored to resting state therefore cell will respond normally to further depolarising stimulus
Describe the interaction of local anaesthetics with sodium channels
- Local anaesthetic (weak base) injected close to the connective tissue sheath
- Recaches equilibrium between unionised and ionised forms
- Unionised form crosses the connective tissue sheath and then the axonal membrane
Cationic form (NH+) binds inside the voltage sensitive sodium channel and hinders sodium influx
What are the effects of local anaesthetics
Prevent generation and conduction of APs
May also influence channel gating
Selectively blocks small diameter and non-myelinated fibres (C, A-delta)
What are the effects of local anaesthetics on resting membrane potential
No influence on resting membrane potential
What is the pKa of local anaesthetics
8-9
Weak bases
What are the 6 main routes of administration of local anaesthetic
Surface Infiltration Intravenous regional Nerve block Spinal Epidural
Describe surface anaesthesia (location of admin, admin, dosage and side effect of this)
Mucosal surface (mouth, bronchial tree)
Spray (or powder)
High concentrations → systemic toxicity
Describe infiltration anaesthesia (location of admin, uses, co-admin)
Directly into tissues → sensory nerve terminals
Minor surgery
Adrenaline co-injection (NOT extremities)
Describe intravenous regional anaesthesia (location of admin, uses, toxicity)
i.v. distal to pressure cuff
Limb surgery
Systemic toxicity of premature cuff release
Describe nerve block anaesthesia (were it acts, what it’s used for, onset and admin)
Close to nerve trunks e.g. dental nerves
Widely used – low doses – slow onset
Vasoconstrictor co-injection
Describe spinal anaesthesia (location of admin, uses, dosage, effects)
Sub-arachnoid space – spinal roots Abdominal, pelvic, lower limb surgery Low doses ↓ b.p.; prolonged headache Glucose (↑ specific gravity)
Describe epidural anaesthesia (location of admin, uses, onset, effect)
Fatty tissue of epidural space – spinal roots
Uses same as spinal anaesthesia and painless childbirth
Slower onset – higher doses
More restricted action – less effect on b.p.
Describe the pharmacokinetics of lidocaine (absorption, PPB, metabolism, half life)
Good absorption
70% bound to plasma binding proteins
Hepatic metabolism - N-dealkylation
half life of 2 hours
Describe the pharmacokinetics of cocaine (absorption, PPB, metabolism, half life)
Good absorption
90% PPB
Liver and plasma non-specific esterases
1 hour half life
What is bupivacaine used for and what is its duration of a action
Epidural anaesthesia
6 hours
