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AV Dissociation

Atrioventricular (AV) dissociation

Ventricular Rhythm

• Impulses are generated by an ectopic ventricular focus
• The impulse spreads retrogradely through the AV junction from the ventricles to the atria
• Depolarizes the ventricles (QRS complex) and then the atria (retrograde P wave)
• The retrograde P wave is often not seen in ventricular rhythm
• According to frequency, ventricular rhythm is classified into:
• Ventricular rhythm
• Accelerated ventricular rhythm
• Ventricular tachycardia
• Overdrive suppression
• The site with the highest frequency deactivates sites with lower frequency
• In ventricular rhythm, the pacemaker is the ventricular focus

Accelerated Ventricular Rhythm

• Frequency 75/min.
• The activity of the atria and ventricles is synchronized
• First, the ventricles depolarize (QRS complex) and then the atria (retrograde P waves)
• The retrograde P wave is often not seen in ventricular rhythm

Third-Degree AV Block (Complete AV Block)

• The only electrical connection between the atria and the ventricles is the AV junction
• Unless it is a WPW syndrome
• In third-degree AV block, the AV junction is "cut off"
• The atria and ventricles are electrically isolated from each other
• Third-degree AV block is the most common cause of AV dissociation
  
  
• The heart has 2 independent pacemakers in third-degree AV block:
• 1st pacemaker (SA node, Sinus rhythm)
• SA node generates impulses regularly (P waves)
• Impulses from the SA node do not pass to the ventricles through the cut-off AV junction
• 2nd pacemaker (Ventricular focus, Ventricular rhythm)
• A focus activates spontaneously in the ventricles
• This is a protective electrophysiological mechanism against asystole
• The ventricular focus generates wide QRS complexes
• QRS complexes occur independently of P waves (AV dissociation)

AV Dissociation - Classification

• Complete AV Dissociation
• Occurs in Third-Degree AV Block (the atria and ventricles are electrically isolated)
• Frequency of P waves > QRS complexes
  
  
• Incomplete AV Dissociation
• The mechanism is functional block in the AV junction
• 2 impulses (atrial and ventricular) meet in the AV junction during the refractory period and cancel each other out
• Occurs in
• SA node slowing down, then activates a subsidiary pacemaker (Junctional rhythm, Ventricular rhythm)
• Acceleration of the ventricular focus (Ventricular tachycardia)
• Frequency of P waves and QRS complexes is almost the same
• On the ECG contractions are recorded (When the P wave intermittently passes to the ventricles and creates a QRS complex)
  
  
• Isoelectric AV Dissociation
• The mechanism is functional block in the AV junction
• 2 impulses (atrial and ventricular) meet in the AV junction during the refractory period and cancel each other out
• The AV junction is not interrupted
• Frequency of P waves and QRS complexes is the same on the recorded ECG paper
• But on a longer ECG record, a captured contraction will definitely be recorded
• Essentially, this is incomplete AV dissociation

Complete AV Dissociation

• P waves and QRS complexes are independent of each other
• There are 2 pacemakers in the heart that generate impulses independently
• 1st pacemaker (SA node) produces P waves
• 2nd pacemaker (ventricular focus) produces QRS complexes
• The cause of AV dissociation is Third-Degree AV Block (Complete AV Block)
• There is a "cut" AV junction

Incomplete AV Dissociation

• The heart has 2 pacemakers simultaneously
• Sinus Rhythm: P waves (red arrows)
• Accelerated Ventricular Rhythm: QRS complexes
• Both pacemakers have the same frequency initially
• In each cycle, 2 impulses meet in the AV junction during the refractory period
• The impulses from the SA node and the ventricles thus reset in the AV junction
• At the end of the ECG, the SA node overtakes the ventricular focus
• The impulse from the SA node passes to the ventricles and depolarizes them (blue arrow)
• The AV junction is not in the refractory period from the ventricular impulse
• After the P wave, a narrow QRS complex follows - blue arrow (sinus rhythm)
• The patient does not have a "cut" AV junction
• Does not have Third-Degree AV Block
• At the end of the ECG, the impulse (P wave) passed to the ventricles (QRS) through the AV junction
• The cause of AV dissociation is
• Accelerated Ventricular Rhythm during sinus rhythm

Isorhythmic AV Dissociation

• Laddergram (Rebríkový graf) illustrates impulse propagation through the conduction system
• A - Atria, AV - AV junction, V - Ventricles
• The heart has 2 pacemakers simultaneously
• Sinus Rhythm: P waves
• Junctional Rhythm: Narrow QRS complexes
• Both pacemakers have the same frequency
• In each cycle, 2 impulses meet in the AV junction during the refractory period
• The impulse from the SA node and the AV junction thus resets in the AV junction
• No impulse from the SA node passes to the ventricles
• In a longer ECG recording, a captured beat would definitely appear
• In which the impulse from the SA node would pass to the ventricles
• The patient does not have a "cut" AV junction
• Does not have Third-Degree AV Block