Principles of local anaesthesia Flashcards
6 routes of local anaesthetic administration
1) Surface anaesthesia
2) Infiltration anaesthesia
3) Intravenous regional anaesthesia
4) Nerve block anaesthesia
5) Spinal anaesthesia
6) Epidural anaesthesia
Epidural anaesthesia (local)
- administered into fatty tissue of epidural space to act on spinal roots (does not penetrate dura or arachnoid membrane)
- similar use as spinal anaesthesia but also used in painless childbirth
- slower onset as anaesthesia needs to diffuse across fatty tissue for spinal root access (higher doses needed)->leads to increased percentage of systemic side effects
- more restricted action so no headache and less effect on blood pressure
- reduced infection risk as anaesthesia does not penetrate the meninges
Intravenous regional anaesthesia (local)
- IV injection distal to pressure cuff (pressure cuff cuts off blood supply)
- used in limb surgery
- systemic toxicity of premature cuff release (keep cuff on for 20 mins to allow local anaesthesia to diffuse into tissue)
Spinal anaesthesia (local)
- administered into subarachnoid space (spinal roots L3/L4 to minimise damage to spinal cord)
- goes through tough outer dural membrane, arachnoid membrane and into space below subarachnoid membrane (CSF)
- used in abdominal, pelvic and lower limb surgery as well as hip replacement
- low doses
- results in very sharp blood pressure reduction (preganglionic SNS neurones are smaller in diameter->sensitive to anaesthesia->vasodilation) and prolonged headache (anaesthesia mixes with CSF)
- add glucose to local anaesthesia (increases specific gravity->anaesthesia stays in same injected place instead of diffusing freely in CSF to brain)
Surface anaesthesia (local)
- spray or powder form
- on mucosal surface (mouth, bronchial tree etc)
- need high concentrations but results in systemic toxicity
The 3 common features of local anaesthetic structures
- aromatic region (lipid soluble)
- basic amide side chain (hydrophilic)
- ester or amide bond (bridging group)->cocaine has ester bond and lidocaine has amide bond
Pharmacokinetic properties of Lidocaine
- amide
- good absorption
- 70% plasma protein binding
- hepatic metabolism (N-dealkylation)
- 2 hour plasma half life (more metabolism resistance)
Unwanted effects of Lidocaine
CNS
-stimulation, restlessness, confusion and tremor -paradoxical effect where inhibitory system is more sensitive to local anaesthesia than excitatory system is=initial stimulation then dampening of CNS activity
CVS
-myocardial depression, vasodilation, hypotension
-results from sodium channel blockade on heart and smooth muscle
Pharmacokinetic properties of Cocaine
- ester
- good absorption (used as surface anaesthetic)
- 90% plasma protein binding (high drug interactions)
- liver and plasma metabolism (non-specific esterases)
- 1 hour plasma half life
Unwanted effects of Cocaine
CNS
-euphoria and excitation
CVS
-increased cardiac output, vasoconstriction and hypertension
Both result from sympathetic actions rather than sodium channel blockade (Cocaine inhibits noradrenaline re-uptake)
Generation of a neuronal action potential
1) resting sodium channels open and sodium ions enter cells
2) sodium channels close (inactivation), potassium channels open and potassium ions leave cells
3) sodium channels restored to resting state but potassium channels still open so cell is refractory
4) sodium and potassium channels restored to resting state so cell responds normally to further depolarising stimulus
Interaction of local anaesthetics with sodium channels
/
Local anaesthetic effect
- prevent generation and conduction of action potentials
- do not influence resting membrane potential
- may also influence channel gating
- selectively block small diameter fibres and non-myelinated fibres
Local anaesthetic properties
- weak bases (pKa 8-9) so are mostly ionised
- infected tissues are slightly acidic=local anaesthetics less effective
Infiltration anaesthesia
- subcutaneous injection directly into tissues (sensory nerve terminals)
- used in minor surgeries
- adrenaline co-injected
