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Summary Anaesthetic notes
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Local Anaesthetic AgentsRestricts loss of sensation to area supplied by limited number of nerves. Can be sole
anaesthesia for procedure, but most commonly used to reduce intraoperative and
postoperative pain.
Work by reversibly blocking neural transmission in autonomic, sensory, and motor nerves.
Binds to Na+ channels in axon membrane from the inside and prevent Na+ entry during
depolarisation
Methods of delivery
1. Topical e.g. creams/gels for skin and mucous membranes
2. Infiltrative e.g. field-blocks
3. IV e.g. Bier’s block for fractured wrist
4. Epidural or subarachnoid: regional anaesthesia blocking spinal nerves
Commonly used drugs
- Lidocaine
o Rapid onset, duration 1-2 hrs
o Can be used topically, infiltration, nerve blocks, epidural and IV
o Max dose 3mg/kg w/o adrenaline, 7 mg/kg w/ adrenaline
- Bupivacaine
o Slower onset, longer duration (3-4 hrs spinal anaesthesia, 12 hrs nerve block)
o IV contraindicated due to CNS/CVS toxicity
o Max dose 2mg/kg
- Levobupivacaine – less cardiotoxic isomer of bupivacaine
o Max dose 2mg/kg
Factors influencing LA action
1. Lipid solubility
a. Individual nerve fibres are lipid membranes and the unionized form of the
drug must cross the membrane to reach the inside. The more lipid soluble
the agent the more potent it is
b. Bupivocaine > Lidocaine
2. pKa and pH
a. Unionised form must cross axonal lipid membrane but ionized form of drug is
effective in blocking the channel. The more unionized the more rapid the
passage across the membrane and the faster onset of the block
b. Factors affecting proportion of drug in ionized form: unionized form
i. Is the drug a weak acid or weak base? LAs are weak bases – this
means they are ionised below their pKa
ii. pKa of the drug – both lidocaine and bupivacaine have pKa above
plasma pH
iii. pH of the environment – at pH 7.4 lidocaine is approx. 75% ionised
and bupivacaine is approx. 85% ionised
c. Lidocaine has faster onset of block than bupivacaine
3. Increasing the pH of the solution to increase the proportion of drug in ionised form
by adding sodium bicarbonate
a. Do not take too high as can be damaging to tissues
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