• Pacemaker
• Subsidiary Pacemakers
• 3 Cardiac Pacemakers
• Overdrive Suppression

Pacemaker

• The heart beats according to the site with the highest frequency
• In a healthy heart, this is the sinoatrial (SA) node
• Pacemaker means "pace setter"
• In a healthy heart, the pacemaker is the SA node
• A pacemaker is also referred to as a cardiac stimulator
• which generates impulses instead of the SA node
  
  
• Pacemaker potential (the ability to spontaneously generate impulses)
• Decreases in the conduction system from the SA node towards the ventricles
• The lowest pacemaker activity is in the working myocardium
  
  

3 Cardiac Pacemakers

• The heart has 3 main pacemakers (SA node, AV node, Ventricular Purkinje fibers)
• These are activated and ensure the heart rhythm
• Other sites act as pacemakers rarely

3 Main Pacemakers

• Primary Pacemaker (SA node)
• Sinus rhythm (f: 50-100/min.)
• Secondary Pacemaker (AV node)
• Junctional rhythm (f: 40-50/min.)
• Tertiary Pacemaker (Purkinje fibers in the ventricles)
• Ventricular rhythm (f: 20-40/min.)
  
  
• If the primary pacemaker is damaged
• The secondary pacemaker is activated
• If the secondary (and primary) pacemaker is damaged
• The tertiary pacemaker is activated

Overdrive Suppression (Suppression of the SA Node)

• Initially, there is a sinus rhythm with a frequency of 75/min.
• Impulses are generated by the SA node
• Then a paroxysm of AV nodal re-entry tachycardia (AVNRT) occurs
• AVNRT was triggered by an atrial extrasystole (4th QRS complex)
• During AVNRT, the AV node generates impulses with a frequency of 150/min.
• The AV node thus surpassed and deactivated the SA node, resulting in overdrive suppression of the SA node
• At the end of the ECG, the AVNRT ended (the AV node stopped generating impulses)
• And the SA node was reactivated (sinus rhythm)

Overdrive Suppression (Suppression of the AV Node)

• The rhythm is a sinus rhythm with a frequency of 60/min.
• The SA node generates impulses with the highest frequency in the heart
• And deactivates all foci with a lower frequency (AV junction, and others)

Sick Sinus Syndrome and Overdrive Suppression

• Initially, there is atrial fibrillation
• In atrial fibrillation, the atria generate impulses with a frequency of 350-600/min.
• SA node suppression occurs
• After spontaneous termination of atrial fibrillation, a post-tachycardic pause follows
• During the pause, there is no suppression of the SA node, and the SA node activates - sinus rhythm
• The length of the pause corresponds to the time required for SA node recovery (sinus node recovery time)
• Sinus node recovery time is < 1.5 seconds (with an intact SA node)
• This EKG shows 4.2s, indicating a SA node dysfunction associated with sick sinus syndrome
• Following the first sinus beat, atrial fibrillation recurs