Anti-arrhythmic drugs: how does each class work on the heart?
Four Vaughan Williams classes, each changing one phase of the heartbeat — plus digoxin and adenosine, which work on the AV node.
Select any card for the reasoning, what to watch for and an exam tip.
Anti-arrhythmic drugs
Where each class acts
Heart cells fire in two ways. Pacemaker cells in the SA and AV node drift up to firing point by themselves; muscle cells wait to be triggered, then hold a long plateau. Each class changes one phase.
Pacemaker cellsSA node and AV node
Heart muscleAtria and ventricles
- Digoxin and adenosine sit outside the four classes: both act on the AV node.
- Sotalol is a beta-blocker (Class II) with Class III activity too.
Key points
- Rate control slows the AV node (beta-blocker, diltiazem or verapamil, digoxin); rhythm control restores sinus rhythm (flecainide, amiodarone or electrical cardioversion).
- Their weakening effects on the heart add up: take special care combining two, especially if the heart is already failing.
- Most anti-arrhythmics can also provoke arrhythmias, and low potassium makes that more likely.
- The BNF notes the Vaughan Williams classes matter less in practice than which arrhythmia a drug treats.
Key takeaway
Class I blocks sodium, II blocks beta-receptors, III blocks potassium and IV blocks calcium. Digoxin and adenosine slow the AV node in their own ways.