• Heart Rhythm
• Ectopic Ventricular Focus
• Unidirectional Electrical Block
• Parasystole
• Transplanted Heart and Atrial Parasystole
• Ventricular Extrasystole (VES)
• ECG and Parasystole

Heart Rhythm

• Heart rhythm is always determined by the focus (foci)
• which generates impulses with the highest frequency (overdrive suppression)
• There are 3 basic heart rhythms
• Sinus rhythm (Impulses are generated by the SA node)
• Junctional rhythm (Impulses are generated by the AV junction)
• Ventricular rhythm (Impulses are generated by the Purkinje fibers in the ventricles)
• Sinus rhythm is the physiological rhythm of the heart
• Because the SA node generates impulses with the highest frequency
• The SA node is referred to as the primary pacemaker

Unidirectional Electrical Block

• In a structurally altered heart, a unidirectional electrical block can form around the ventricular focus
  
  
• Entrance block
• No external impulse (from the SA node) passes into the focus
• Exit impulses from the focus are not blocked
• A functional exit block can occur
• If an impulse exits the focus during the refractory period of the ventricles
• The ventricles are currently activated by the SA node
• thus this impulse will be functionally blocked (overdrive suppression)

Transplanted Heart and Atrial Parasystole

• Transplanted Heart
• Is sewn onto a part of the recipient's atrial myocardium
• The recipient's atria are then electrically isolated
• From the atria of the transplanted heart
• Because they are separated by a suture
  
  
• In the recipient's atrial myocardium
• An ectopic atrial rhythm is activated
• Generating a P wave
• The depolarization wave however stops at the suture
• In the transplanted heart
• The SA node generates a sinus rhythm
• The SA node is isolated from the ectopic focus by the suture
  
  
• Atrial parasystole appears on the ECG (2 independent rhythms)
• P waves from the ectopic focus (recipient's atrial myocardium)
• P waves from the SA node (SA node of the transplanted heart)

ECG and Parasystole

• Parasystole
• An ectopic focus generates impulses constantly at a frequency of 20-40/min. (ventricular rhythm)
• Only some are blocked in the ventricles due to the refractory period (functional exit block)
• On ECG, there are ventricular extrasystoles
• Broad QRS complex (> 0.12s)
• Discordance
• Full compensatory pause
  
  
• Characteristic triad:
• 1. The coupling interval changes
• Distance between VES and the preceding QRS complex
• 2. Fusion beats
• The ventricles are activated simultaneously from the SA node and the ventricular ectopic focus
• 3. Inter-ectopic interval
• VES are equidistant from each other (because the ventricular focus generates impulses regularly)
• If a VES is blocked, it results in a multiple of the inter-ectopic interval

Ventricular Parasystole

• Sinus rhythm
• Sinus rhythm is interrupted by ventricular extrasystoles, between which there is a temporal relationship
• The coupling interval changes (760, 960, 670...)
• F: Fusion beat
• Inter-ectopic interval is always an X multiple of 2400ms
• Ventricular rhythm has a frequency of 25/min. (2400ms)

Atrial Parasystole and Heart Transplantation

• Heart Transplant
• Is sewn onto a part of the atrial myocardium of the recipient
• The recipient's atria are then electrically isolated
• From the atria of the transplanted heart
• Because they are separated by a suture
• Parasystole
• The heart has 2 independent foci that generate impulses
• 1. Focus is activated in the recipient's atrium
• 2. Focus is the SA node in the transplanted heart
• Ectopic atrial rhythm (1st focus)
• P waves (marked in blue)
• Each P wave is blocked at the suture
• QRS complexes are independent of P waves
• Sinus rhythm (2nd focus)
• SA node generates P waves (marked in green)
• Each P wave passes to the ventricles and generates a QRS
• PQ interval is the same