Class IA sodium-channel blockers vs Class IB and IC sodium-channel blockers
How to tell Class IA sodium-channel blockers and Class IB and IC sodium-channel blockers apart when a board question makes them look alike.
IA prolongs action potential/QT; IB shortens action potential; IC strongly slows conduction and widens QRS.
Side by side
Quinidine, procainamide, or disopyramide pattern with sodium blockade plus action-potential prolongation and torsades risk.
IB means lidocaine/mexiletine with shortened action potential in ischemic ventricular tissue; IC means flecainide/propafenone with strong conduction slowing.
Intermediate sodium-channel blockade plus potassium-channel effects prolong repolarization.
IB drugs bind rapidly and shorten repolarization; IC drugs dissociate slowly and markedly slow phase 0 conduction.
QT prolongation, torsades risk, and procainamide lupus-like syndrome support class IA.
Lidocaine points IB; QRS widening with flecainide and no structural heart disease points IC.
The trap
All are class I sodium blockers, so the action-potential and QRS clues do the separating.
The repair move
For class I drugs, sort IA, IB, and IC by action-potential duration and conduction effect before memorizing names.
Try a free question
CoreA patient receives procainamide for arrhythmia and develops QT prolongation with torsades risk plus a lupus-like syndrome. Which sodium-channel blocker subclass best fits?
Show answer and explanation
Answer: A. Class IA. Class IA drugs slow sodium channels and prolong repolarization, so QT prolongation and torsades risk are key clues.
Why the other choices are tempting
- B. Class IB drugs shorten action potential duration and are typified by lidocaine or mexiletine.
- C. Class IC drugs strongly slow conduction with little action-potential duration effect.
- D. Class II drugs are beta blockers.
- E. Class IV drugs are non-dihydropyridine calcium channel blockers.
Flashcards
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What is the fastest separator for Class IA sodium-channel blockers vs Class IB and IC sodium-channel blockers?
IA prolongs action potential/QT; IB shortens action potential; IC strongly slows conduction and widens QRS.
What clue should make you choose Class IA sodium-channel blockers?
Quinidine, procainamide, or disopyramide pattern with sodium blockade plus action-potential prolongation and torsades risk.
Quick answers
What is the fastest way to tell Class IA sodium-channel blockers from Class IB and IC sodium-channel blockers?
IA prolongs action potential/QT; IB shortens action potential; IC strongly slows conduction and widens QRS.
What is the key clue for Class IA sodium-channel blockers?
Quinidine, procainamide, or disopyramide pattern with sodium blockade plus action-potential prolongation and torsades risk.
What is the key clue for Class IB and IC sodium-channel blockers?
IB means lidocaine/mexiletine with shortened action potential in ischemic ventricular tissue; IC means flecainide/propafenone with strong conduction slowing.
What is the common trap with Class IA sodium-channel blockers vs Class IB and IC sodium-channel blockers?
All are class I sodium blockers, so the action-potential and QRS clues do the separating.