Local Anaesthetics Flashcards
When should you use local anaesthetics?
When loss of consciousness is neither necessary or desirable.
Should local anaesthetics be used in large operations?
For major surgery with sedation and as post-operative analgesia.
What are the two general structures of local anaesthetics?
Procaine and lidocaine (lignocaine).
Which type of anaesthetic is more common?
Amides are more commonly used, while esters are rarely used.
Which type of local anaesthetic is associated with allergic reactions?
Esters are more unstable and can be metabolised into compounds associated with allergic reactions.
What are examples of amide containing local anaesthetics?
Lidocaine, prilocaine, bupivacaine and articaine.
What are the properties of lidocaine?
Medium acting with rapid onset.
What are the actions of prilocaine?
Medium acting with no vasodilation.
What are the properties of bupivacaine?
Long acting with slow onset.
What are the actions of articaine?
Short acting with rapid onset.
What are examples of ester-containing local anaesthetics?
tetracaine, chloroprocaine and benzocaine.
What are the actions of tetracaine?
Long acting with very slow onset.
What are the properties of chloroprocaine?
Medium acting.
What are the actions of benzocaine?
Atypical mechanism of action.
How do local anaesthetics work in general
By reversibly blocking voltage-gated sodium ion channels.
What do local anaesthetics do to membranes?
Stabilises excitable membranes and prevents membranes from being depolarised.
What does a peripheral nerve consist of?
Fibres of different function, diameter and insulation.
Can the constituents of the peripheral nerve be blocked by LAs?
All can be blocked, but at different rates.
What is the order of loss in the peripheral loss due to LAs?
Pain, temperature, proprioception and finally skeletal muscle tone.
What are the factors that affect the probability that a LA will block an impulse?
Diameter of the fibre and the myelination status.
How does the diameter of the fibre affect the probability that a LA will block an impulse?
Smaller nerve fibres are blocked more easily.
How does the myelination status affect the probability that a LA will block an impulse?
Myelinated fibres are blocked more easily than unmyelinated ones.
What other non-fibre factors affect the probability that a LA will block an impulse?
Length of nerve exposed to drug, length of time exposed to drug and conc of drug.
What is the chemical status of LAs?
They are weak bases.
When do LAs block voltage-gated sodium channels?
They block when they are ionised via an intracellular binding site.
What is the side effect of ionised LAs?
They cannot pass through the membrane when ionised.
What is the no use-dependence pathway?
The hydrophobic pathway.
What is the use-dependent pathway?
The hydrophilic pathway.
What is the pKa of most LAs?
Between 8 and 9.
What is the Henderson-Hasselbalch equation for a weak base?
pKa-pH=log10((BH+)x(B)).
How do use-dependence LAs work?
They bind to open or inactive channels.
Why are they called use-dependence LAs?
Related to frequency of neuronal firing. There is faster onset in faster firing neurones.
What is the duration of the use-dependent LAs dictated by?
The rate of removal of the LA.
What is the rate of removal of an LA dictated by?
Blood flow, action of plasma esterases and the hydrophobicity of the drug.
What do most LAs cause?
Vasodilation.
Which LAs do not cause vasodilation?
Cocaine and prilocaine
What is ischaemia?
An inadequate flow of blood to a part of the body, caused by a constriction or blockage of vessels supplying it.
Why do some LAs aim to cause vasoconstriction?
Decreased blood flow to the area means decreased rate of removal of the drug, therefore longer lasting.
What are the different routes of administration of an LA?
Surface, infiltration, nerve block, intravenous regional, extradural and subarachnoid.
What are examples of surface routes of administration of LA?
Nose, mouth, bronchial tree, cornea and urinary tract.
What are examples of infiltration routes of administration of LA?
Injection into tissues to reach nerve branches/terminals.
What are examples of nerve block routes of administration of LA?
Small or large regional block by injection around the nerve.
What are examples of intravenous regional routes of administration of LA?
Double-cuff method to contain LA to a limb.
What are examples of extradural routes of administration of LA?
Used in thoracic, lumbar and sacral regions.
What are examples of subarachnoid routes of administration of LA?
Drug injected into the subarachnoid space (CSF).
What is in EMLA?
Mixture of lignocaine and prilocaine.
What does EMLA stand for?
Eutectic mixture of local anaesthetics.
What are the chemical properties of EMLA?
properties of EMLA?
The melting point of the mixture is lower than that of the individual chemicals.
Why do the properties of EMLA allow it to work?
Because of the properties, a higher conc of both chemicals in the mix can be used.
What causes side effects of LAs?
Unwanted side effects due to entry into systemic circulation.
What side effects could occur in the cardiovascular system?
Dysrhythmias and a sudden fall in blood pressure.
What is dysrhythmias?
An abnormal heart beat.
What are the side effects in the CNS caused by LAs?
Restlessness, tremors, convulsions, respiratory centre depression and death.
