Antiarrhytmic Drugs

Question 1

Classification

Answer 1

Class 1 - Sodium channel blockers
1A - moderate phase 0 depolarisation depression: quinidine, procainamide, disopyramide
1B - minimal effect on phase 0 depolarisation: lignocaine, mexiletine.
1C - markedly depress phase 0 : felcainide, propafenone.

Class 2 - beta adrenergic blockers :
Class 3 - drugs that prolong duration of AP :
Amiodarone, sotalol, dofetilide, bretylium.
Class 4 - CCBs : verapamil, dilitazem.

Question 2

Quinidine

Answer 2

Pharmac actions -
1. Heart : blocks Na channels in open state.
Blocks potassium channels.
Prolongs refractory period.
Suppresses ectopic foci and blocks re entry of impulses.
2. AV Node : vagolytic and dorecy depressant action on AV node.
3. ECG : prolongs QRS and QT
4. BP : fall in BP

Question 3

Lignocaine

Uses

Answer 3

1. For emergency treatment of ventricular arrthymias associated with MI, digitalis toxicity, cardiac surgery.
2. Preference in ventricular arrthymias:
   - rapid onset of action, short duration
   - has no action on AV nodal conduction velocity, hence does not intensify AV block in digitalis toxicity.
3. Not useful in atrial arrthymias

Question 4

Beta blockers

Anti arrthymic properties

Answer 4

1. Depress phase 4 depolarisation
2. Prolong refractory period and decrease the conduction velocity in AV node :
   - useful in re-entrant arrthymias
   - to control ventricular rate in atrial flutter and fibrillation.

Question 5

Amidarone

Answer 5

1. Blocks potassium channels > increases AP duration > prolongs Refractory period
2. Blocks sodium channels in inactive state
3. Prolongs refractory period
4. Weak beta adrenergic blocking and CCB actions.

Question 6

Amidarone

Drug interactions

Answer 6

Drug interactions:

1. Amidarone + verapamil - additive depressant on SA and AV nodes.
2. Amidarone inhibits renal clearance of digoxin.
3. Increases concentration of quinidine and procainamide.
4. Potentiates anticoagulant effect of warfarin.