• Action Potential
• ECG and Action Potential
• Block in Phase 4 (Bradycardia-dependent Block)
• Action Potential and Left Bundle Branch

Action Potential

• Action potential is a rapid change in the electrical voltage of a cardiomyocyte
• The action potential spreads
• Rapidly through the conduction system
• Slowly through the working myocardium
• Impulses arise spontaneously in the SA node and the conduction system
• Because the 4th phase of the action potential spontaneously and slowly depolarizes
• When it reaches the threshold of -50mV, the depolarization triggers fully (phase 0)
• Cardiomyocytes of the working myocardium and Purkinje fibers
• Have a flat 4th phase, they do not spontaneously depolarize
• They begin to depolarize only if stimulated by an external impulse (from the conduction system)

Block in Phase 4 (Bradycardia-dependent Block)

• It is an aberrant conduction of a supraventricular impulse through the ventricular conduction system
• This is a temporary conduction system disorder during bradycardia (with prolonged RR interval)
• It most commonly occurs in structurally damaged myocardium (e.g., cardiomyopathy, ischemia)
• In the presence of damaged Purkinje fibers
• In a damaged Purkinje system, cardiomyocytes begin to spontaneously depolarize during phase 4
• The longer phase 4 lasts (the TQ interval), the greater the spontaneous depolarization in phase 4 becomes
• Until it reaches the threshold, thereby blocking an external (supraventricular) impulse
• This is rare and occurs with a significant RR pause (frequency < 40/min.)
• Block in phase 4 most often manifests as a left bundle branch block
• Because the left ventricle (Purkinje fibers) is more sensitive to ischemic damage
  
  
• Occurs in a damaged Purkinje system with prolonged RR interval, such as:
• Bradycardia
• Sinus pause
• Second-degree AV block
• It is often referred to as a Bradycardia-dependent block

Undamaged Left Bundle Branch (Action Potential)

• This is an action potential in the undamaged Purkinje fibers of the left bundle branch
• During a sinus pause, there is no spontaneous depolarization in phase 4
• The supraventricular impulse is conducted through the left bundle branch

Block in Phase 4 (Bradycardia-dependent Block)

• After a sinus pause, a sinus QRS with the appearance of LBBB (arrows) follows
• The sinus impulse is conducted to the ventricles only through the right bundle branch
• In the left bundle branch, it is blocked in phase 4 of the action potential
• A phase 4 block occurs only in a damaged Purkinje system (ischemia, cardiomyopathy)