Antidysrhythmic drugs Flashcards
List the 4 stages of myocardial depolarisation.
0- Fast depolarisation, Na+ channels open, allowing an inward current
1- Na+ channels begin to close
2- Opening of Ca2+ channels, allows sustained contraction
3- Outward K+ movement
4- Slow depolarisation, due to Ca+ and Na+.
Which aspect of the ECG demonstrates ventricular conduction?
QT
What is used to classify dysrhythmias?
Site and rate
Compare types of atrial/supraventricular tachydysrhythmias.
Atrial fibrillation - persistent, irregular
Supraventricular tachycardia - rapid, regular (AV node or higher)
Paroxymal supraventricular tachycardia - intermittent
List drugs used to control ventricular rate.
Digoxin, Ca2+ channel blocker, beta-blocker
What is used to control sinus rhythm.
Direct current cardioversion
Compare types of ventricular dysrhythmias
Ventricular ectopics - premature contractions
Ventricular tachycardia - rapid, regular
Ventricular fibrillation - irregular
What are the two major mechanisms of dysrhythmias?
Abnormal pulse generation and abnormal impulse conduction/repolarisation.
List mechanisms of abnormal impulse generation.
Altered normal automaticity
Ectopic aytomaticity
List mechanisms of abnormal impulse conduction/depolarisation.
Conduction block
Re-entry circuits
After-depolarisations
What are the different types of conduction block?
1st AV block - prolonged PR interval
2nd AV block - impulses not conducted, no QRS
3rd AV block - no conduction
What are re-entry circuits?
Retrograde conduction re-exiting tissue that has already passed through the refractory period
What is the caused of QT prolongation in after-depolarisation?
Impaired K+ efflux.
When are antidysrhythmic drugs used?
Bradydysrhythmics: acutely
Tachydysrhythmics: only for life-threatening dysrhythmias or for moderate/severe symptoms
What drugs are used for bradycardia?
Acute - atropine, adrenaline, isoprenaline
Chronic - Stop bradycardic medication, pacemaker
List the classes of antidysrhythmics.
Class I - Na+ channel blockers, block conduction, phases 0, 4
Class II - beta blockers
Class III - block cardiac K+ channels and other actions, phase 3
Class IV - Ca2_ channel blockers, particularly in nodal tissue
Describe class I antiarrhythmics.
Local anaesthetic properties
Block fast Na+ current at phase 0, slow cardiac conduction, depress automaticity.
When are class I antuarrythmics used?
Used for ventricular and supraventricular dysrhythmias, often I.V.
Can be pro-dysrhythmic
What gives class I antiarhythmics some selectivity?
Use dependent block of Na+ channels
Lis the subclasses of class I antiarrhythmics
Class I A - prolongs AP
Class I B - shortens AP
Class I C - does not change AP duration
Describe class II antidysrhythmics.
Reduce effects of Adr, NA and SNS (i.e. catecholamine toxicity).
Decrease sinus node automaticity and affect atrio-ventricular communication.
When are class II antidysrhythmics used?
AF (exercise, mental stress), post-MI
Describe class III antidysrhythmics.
Prolong AP by blocking K+ efflux, prolong refractory period (QT interval)
Can be pro-dysrhythmic as increase ventricular conduction (QT interval)
Give examples of class III antidysrhythmics.
Sotalol - beta blocker with class III activity Amiodarone - serious ADRS - thyroid, lungs, liver, deposits in eyes and skin)
