• Fusion Beat Mechanism
• Fusion Beat
• Causes of Fusion Beats
• Fusion Beat and WPW Syndrome

Fusion Beat Mechanism

Fusion Beat Mechanism

• Sinus Beat
• The ventricles are activated by an impulse from the SA node through the conduction system of the ventricles
• The impulse spreads quickly through the conduction system and depolarizes the ventricles within 0.12s
• Therefore, on the ECG, the QRS complex is narrow < 0.12s
• Ventricular Beat
• The impulse originates in the ventricle and depolarizes the ventricles through the myocardium (not through the conduction system)
• Through the myocardium, the impulse spreads slowly, with depolarization lasting more than 0.12s
• Therefore, there will be a wide QRS complex > 0.12s
• Ventricular beats occur with:
• Ventricular Extrasystole
• Ventricular Tachycardia
• Fusion Beat
• The ventricles are depolarized by 2 impulses
• One from the atria, the other from the ventricles
• Results in a distorted QRS complex
• Which is wider than the sinus beat and narrower than the ventricular beat

Fusion Beats

• Sinus Rhythm
• Ventricular Extrasystole (VES)
• The ventricles are depolarized only by the ventricular impulse
• Sinus impulse is blocked at the AV junction
• Because it encounters the refractory period (the ventricles are already fully depolarized from the ventricular extrasystole)
• Fusion Beat
• The ventricles started depolarizing from the ventricular extrasystole
• The sinus impulse also reached the ventricles, as the ventricles were not yet fully depolarized
• Thus, the ventricles are depolarized by two impulses
• Fusion beats have a different shape
• Because the impulses met at different locations in the ventricles (resulting in a different vector)

Ventricular Tachycardia

• Ventricular Tachycardia
• Wide QRS complexes with a heart rate of 125/min.
• AV Dissociation (is a hallmark of ventricular tachycardia)
• Wide QRS complexes and P waves (blue dots) are independent of each other
• Fusion Beat (is a hallmark of ventricular tachycardia)
• A sinus impulse (P wave) occurred at a time when it could be conducted to the ventricles (outside of the refractory period)
• The ventricles are activated by two impulses:
• One from the SA node
• Another from a ventricular ectopic focus
• Captured Beat
• A sinus impulse (P wave) occurred at a time when it was conducted to the ventricles and reset the ventricular ectopic focus
• The ventricles are thus activated only by the impulse from the SA node
• This is a sinus rhythm
• Then ventricular tachycardia continues

Ventricular Parasystole

• Ventricular Parasystole occurs when the heart has 2 independent rhythms (ventricular and sinus)
• Sinus Rhythm
• Frequency of 90/min.
• The sinus rhythm is interrupted by ventricular beats
• Variable conduction interval
• Fusion Beat
• Created by the merging of sinus and ventricular impulses
• Inter-ectopic interval is always a multiple of 1200ms
• Ventricular rhythm has a frequency of 50/min. (1200ms)
• The ventricles are activated every third ectopic impulse (3x1200ms) or by every fifth impulse (5x1200ms)
• Because the ventricular impulse depolarizes the ventricles only if they are not depolarized by the sinus impulse

Fusion Beat and WPW Syndrome

• In WPW syndrome, there is an accessory pathway (Kent bundle)
• Kent bundle activates one ventricle earlier than the impulse from the AV junction
• The impulse from the Kent bundle creates a delta wave on the ECG
• Delta Wave
• Widening of the QRS complex
• Shortening of the PQ interval
  
  
• Fusion Beat
• The 1st impulse activates the ventricles via the AV junction (QRS complex)
• The 2nd impulse activates the ventricles via the Kent bundle (Delta wave)