• Phase 3 of the Action Potential
• Action Potential and Refractory Period
• Block in Phase 3
• ECG and Block in Phase 3
• Atrial Extrasystole and Block in Phase 3
• Tachycardia-Dependent Blockade

Phase 3 of the Action Potential

• Action potential (impulse) is a rapid change in the electrical voltage of cardiomyocytes
• It occurs due to the change in the concentration of intracellular and extracellular ions
• The action potential propagates
• Rapidly through the conduction system
• Slowly through the working myocardium

Phase 3 of the Action Potential

• On the ECG, it appears as the T wave (ventricular repolarization)
• It is the relative refractory period (RRP)
• If an impulse arrives at a cardiomyocyte (in the conduction system or myocardium) during the RRP
• This impulse will be blocked

Block in Phase 3

• A block occurs when a supraventricular impulse arrives at some part of the ventricular conduction system
• in the 3rd phase of the action potential (Relative Refractory Period)
• This supraventricular impulse is blocked in the affected part of the conduction system
  
  
• It is the most common cause of aberrant conduction
• It is a temporary disorder of the ventricular conduction system
• It is an abnormal conduction of the impulse (action potential) through the ventricular conduction system
• It is a physiological response of the conduction system to overload
  
  
• On an ECG, it may appear as
• Right Bundle Branch Block (RBBB)
• This is the most common block pattern in Phase 3
• Because the right bundle branch has a longer refractory period than the left
• Left Bundle Branch Block (LBBB)
• Complete AV Block
• Fascicular Block (anterior or posterior)
  
  
• Phase 3 block creates a tachycardia-dependent block during supraventricular tachycardias
  
  
• Ashman Phenomenon
• The Ashman Phenomenon is a Phase 3 block that occurs after a prolonged RR interval
• It is most commonly seen in atrial fibrillation

Atrial Extrasystole and Block in Phase 3

Sinus Rhythm

• Sinus rhythm without atrial extrasystole
• Every P wave is conducted through the bundle branches

Atrial Extrasystole (with RBBB Pattern)

• Extrasystole conducts to the ventricles during the RRP (phase 3) of the right bundle branch
• It is blocked in the right bundle branch
• On the ECG, a pattern of right bundle branch block (RBBB) is observed
• This is a case of aberrant conduction of the atrial extrasystole

Tachycardia-Dependent Blockade

• Block in phase 3 is most commonly seen in supraventricular tachycardias (SVT)
• During SVT, impulses begin to pass through the bundle branches during the RRP (in phase 3)
• This is a functional block of the conduction system
• The block is most often in the right bundle branch (due to the longest refractory period)
• The block is present only during tachycardia
• When tachycardia subsides, the functional block disappears

Functional Bundle Branch Block and Supraventricular Tachycardia

• Initially, there is sinus rhythm
• Then a paroxysm of SVT follows
• Supraventricular impulses pass through the ventricular system during the RRP of the right bundle branch
• Thus, with tachycardia, a functional right bundle branch block (RBBB) occurs
• This is a case of aberrant conduction
• When tachycardia subsides, the functional block disappears

Functional Right Bundle Branch Block (block in phase 3)

• With sinus rhythm at a rate of 80/min.
• The patient did not have a bundle branch block
• Later experienced palpitations, and the recorded EKG showed the mentioned block
• It is paroxysmal SVT with a rate of 180/min.
• Not ventricular tachycardia (V1 shows rsR' - larger right rabbit ear)
• With an increase in rate to 180/min. a right bundle branch block appeared
• Because supraventricular impulses reach the right bundle in phase 3 of the action potential
• This is a tachycardia-dependent block
• Present only during tachycardia

Functional Block of the Left Bundle Branch (Phase 3 Block)

• This is a sinus rhythm with a frequency of 70/min.
• With an increase in frequency above 70/min.
• An intermittent block of the left bundle branch began to occur
• This is a tachycardia-dependent block (even though a frequency above 70/min. is not considered tachycardia)
• If a functional bundle branch block occurs at a frequency below 100/min.
• This indicates damage to the conduction system, such as in cardiomyopathy
• The left bundle branch is more damaged in this case than the right