ECG characteristics:

Class IA:

Quinidine, disopyramide, procainamide

Mechanism of action:

• Blockade of the voltage-dependent sodium channels raising the threshold potential (phase 0), delay of the rise of the slope (phase I, slope flattening), delay of phase IV of the action potential, and anticholinergic effect on the AV node

ECG characteristics:

• Knotting of the P wave
  
• Shortening of the PT interval
  
• Widening of the QRS complex
  
• Depression of the ST segment
  
• Prolongation of the QT interval

Class IB:

Lidocaine, mexiletine, tocainide

Mechanism of action:

• Blockade of the fast sodium channels
  
• Shortening of the duration of the action potential
  
• Reduction in the speed of the rise of the action potential (class I property)

ECG characteristics:

• No typical ECG changes

Class I C:

Flecanide, propafenone, ajmaline

Mechanismus of action:

• Blockade of the fast sodium channels causing reduction in the speed of the rise of the action potential (class I property); no changes in the duration of the action potential
  
• Prolongation of impulse conduction

ECG characteristics:

• Prolongation of the PQ interval
  
• Widening of the QRS complex
  
• Only minimal prolongation of the QT interval