Analgesics:
Mechanism of local anaesthetic:
Bind to voltage gated sodium channels and keep them inactive, preventing depolarisation of cells and subsequent nerve transmission
May also be incorporated into cell membrane
Small nerve fibres more sensitive cf. large nerve fibres, and myelinated fibres blocked before non myelinated.
Acidic environments (inflammation, sepsis, ischaemia) reduce effectiveness of LA – the agent remains ionised and unable to enter the neuron.
Infected tissue may also have increased blood supply which washes away local quicker.
Biphasic response on blood vessels – low concentration causes vasoconstriction, but at clinical doses cause vasodilation (esp lignocaine) – therefore add adrenaline to prevent quick absorption of agent.
Tachyphylaxis can occur with local leading to reduced effectiveness over time
Toxicity directly related to blood concentration of anaesthetic.
Early CNS – oral numbness, numb fingertips, tinnitus, visual disturbance, anxiety, tremor, dizziness
CVS – hypotension, myocardial depression, arrest, arrhythmias (with bupivacaine/marcain)
Resp – tachypnoea
Late CNS – seizures, coma, respiratory arrest, death
With bupivacaine (marcain) – cardiovascular collapse likely to occur same time as CNS toxicity
When combining, such as lignocaine and marcain (bupivacaine) – toxic effects are additive, so can’t use max dose of both
Management of local anaesthetic toxicity
- Prevention – use correct dose, do not inject directly into vessel, early recognition of symptoms
- Stop injection
- Oxygen
- Call for help
- Diazepam if seizures
- Injection of 100 mL of 20 % lipid emulsion
Regional anaesthesia and spinal anaesthetic
Contraindications:
- Patient refusal
- Cardiac outflow obstruction – significant aortic stenosis
- Anticoagulation / bleeding diathesis
- Skin infections over the back
- Significant hypotension or instability or shock
- Local anaesthetic allergy or hypersensitivity
Sympathetic block causes vasodilation and can cause sudden hypotension
Spinal – into subarach space below L1 vertebral level, bathing spinal cord and nerve rami inside their sheaths – early onset, dense block, smaller volume
Epidural – outside the dura, bathing nerve roots outside their sheath – single shot or continuous through catheter
Penile block
Innervation of penis – dorsal nerve mostly, perineal branches of pudendal nerve ventrally
Dorsal block – 10 and 2 o’clock at base of penis, under Buck’s fascia, aiming for caudal to pubic bone
Circumferential block – aspirating to avoid venous injection