Sodium blockers, Beta blockers, Potassium blockers, Calcium blockers
Mnemonic
“Some Block Potassium Channels”, for the four classes:
- I - Sodium channel blockers
- Ia quinidine, procainamide, disopyramide: prolong the QT
- Ib lidocaine, phenytoin: shorten repolarisation, act on ischaemic ventricle
- Ic flecainide, propafenone: marked conduction slowing, avoided in structural heart disease
- II - Beta blockers
- III - Potassium channel blockers: amiodarone, sotalol. Prolong the QT
- IV - Calcium channel blockers: verapamil, diltiazem
Digoxin and adenosine sit outside the classification, which is a common examination point.
Amiodarone has properties of all four classes, which is why it is so broadly effective and so toxic: thyroid disease (both ways), pulmonary fibrosis, hepatitis, corneal microdeposits, slate grey skin and photosensitivity.
“Ic is for a normal heart, III for a damaged one” captures the main practical division.
Expansion
Some Block Potassium Channels:
- Class I - Sodium channel blockers, acting on phase 0
- Ia (quinidine, procainamide, disopyramide): moderate block, prolongs the action potential
- Ib (lidocaine): weak block, shortens the action potential; ventricular arrhythmias
- Ic (flecainide, propafenone): strong block, no change in duration; avoided in structural heart disease
- Class II - Beta blockers, acting on phase 4 in nodal tissue
- Class III - Potassium channel blockers (amiodarone, sotalol), prolonging phase 3 and the refractory period
- Class IV - Calcium channel blockers (verapamil, diltiazem), acting on phase 0 of nodal tissue
Adenosine, digoxin and magnesium fall outside the classification.
The scheme maps onto the action potential, which makes it easy to reconstruct: phase 0 is sodium in muscle and calcium in nodes, phase 3 is potassium, and phase 4 is the pacemaker current modulated by sympathetic tone.