Local Anaesthetics and Cardiac Dysrhythmias Flashcards
Why are neurons that fire at a higher frequency more susceptible to Local Anaesthetics?
- the neurons are depolarized more frequently so the inside of the cell is more positive. This drives the entry of uncharged LA
- the inactivated Na channel has a higher affinity for LA’s than the resting channel
What are the main effects of Local Anaesthetics on the CNS?
Stimulatory effects. They result in hyperactivity and manic behaviour, convulsions followed by coma and respiratory depression
What are the main effects of Local Anaesthetics on the CVS?
Vasodilatory effects, depression on the myocardium and decrease of heart rate. These lead to hypotension (apart from cocaine)
State the 6 methods for the administration of Local Anaesthetics
1) Surface
2) Subarachnoid - between 2nd and 5th lumbar vertebrae
3) Epidural
4) Infiltration - directly into tissue to get nerve endings
5) Nerve Block - close to nerve trunk
6) Intravenous Regional
In terms of the movement and permeability of ions, what happens in the myocardium during Phases 0, 1, and 2 of the action potential?
Phase 0: fast inward Na+ current
Phase 1: inactivation of Na+ channels, entry of Cl- and efflux of K+
Phase 2: L-type Ca channels activate. Efflux of K+ and influx of Ca+ balances
In terms of the movement and permeability of ions, what happens in the myocardium during Phases 3 and 4 of the action potential?
Phase 3: L-type Ca channels deactivate while K+ efflux is sustained
Phase 4: K+ channels close and membrane returns to resting
What is the cause of Delayed After-depolarization?
Larger than usual Calcium influx
State the two ways in which damaged myocardium can be the cause of a dysrhythmia
1) Disordered Conduction Pattern (Re-entry) - damaged myocardiam can develop unidirectional block
2) Abnormal Pacemaker activity
What is the mechanism of action of Class I anti-dysrhythmia drugs
They block the voltage sensitive Na channels thus reduce the excitability and slow conduction velocity
= Lidocaine
What is the mechanism of action of Class II anti-dysrhythmia drugs
They are Beta Blockers. They block the excitatory effects of SNS stimulation by blocking Beta 1 receptors.
= Alprenolol
What is the mechanism of action of Class III anti-dysrhythmia drugs
They block K channels, prolong the length of the action potential and increase the refractory period
= Amiodarone
What is the mechanism of action of Class IV anti-dysrhythmia drugs
They block Ca channels, this results in slow AV conduction.
= Verapamil
