Local anaesthetics/dysrhthmics Flashcards
What does use dependency mean?
The local anaesthetic is more effective when action potentials are conducted with greater frequency. The degree of block is proportional to the rate of nerve stimulation. This is be cause more protonated drugs can enter open sodium channels and cause inactivation.
Does the activated or inactivated form of Na channel have higher affinity for local anaesthetic?
Inactivated form has higher affinity for LA
Which types of fibres are blocked more readily?
Smaller fibres are blocked before large fibres.
Small myelinated fibres are blocked before small unmyelinated fibres.
pain sensation tends to disappear first.
Why are vasoconstrictors sometimes used with LAs?
Adrenaline/NA/vasopressin are added to LA solutions to delay absorption from site of injection, prolonging duration of the LA at the tissues and reducing risk of toxicity.
felypressin is commonly used.
What are the toxic effects of LAs?
CNS effects: hyperactivity, manic behaviour, convulsions-> respiratory coma.
CV effects: vasodilation, cardiac slowing, hypotension
Spinal anaesthesia -> local tissue damage
Dangerous allergic reactions are possible
Which segment of the Na channel plays a dominant role in binding to LA?
IVS6 segment
Inner cavity pore of Na channel - alpha subunit (aa residues from at least three of the four domains
contribute to the receptor site)
How are amino amides metabolised?
They are slowly metabolised by hepatic amidases
eg lidocaine, prilocaine
What is an ester breakdown product which can cause allergic reaction?
para-amino benzoate (PABA)
Which drugs may be topically applied?
Lidocaine, benzocaine
Which drugs may be used for infiltration anaesthesia?
Lidocaine, prilocaine
with vasopressin/adrenaline/felypressin
Which drugs may be used for spinal anaesthesia?
Procaine, tetracaine, cinchocaine
What is delayed after-depolarisation?
A cause of dysrhythmia.
[Ca]i increased above normal, due to transient inward current possibly carried by Na/Ca exchanger.
What is reentry or disordered conduction?
A cause of dysrhythmia.
Damaged myocardium developed unidirectional conduction block, conduction circulates indefinitely in area of heart. the activation wavefront of the anterograde signal interacts with the repolarisation phase of a preceding excitation wave.
What occurs with heart block?
Failure of impulse generation in SA node or failure of propagation through AV node.
Ventricular beating is maintained by abnormal pacemaker (e.g. Purkinje system) which is usually slow and unreliable.
What do K+ channel blockers alter?
the AP duration, and thus the refractory period