Definition

  Depolarization of the ventricle partly by a supraventricular stimulus and partly by a VPC is called ventricular fusion beat.

  Ventricular fusion beat occurs due to
fortuitous meeting of a supraventricular impulse and ventricular impulse in the ventricle.

  Since ventricular rhythms are independent of supraventricular stimuli, fusion beats may be seen in any type of ventricular rhythms:
- end-diastolic VPC
- accelerated idioventricular rhythm
- ventricular parasystole
- ventricular tachycardia
- ventricular pacing (VVI)




How to recognize a ventricular fusion beat?

  The fusion beat has a QRS morphology different than that of a supraventricular beat and a ventricular beat.

  Its morphology depends on the proportion of the ventricular myocardium activated by each impulse.

  Since it is partly formed by a supraventricular impulse, it usually has a P wave preceding it. In this case, the PR interval is generally shorter than expected.




Ventricular fusion beat during artificial cardiac pacing

  The fusion beat has a QRS morphology different than that of an intrinsic QRS or a paced QRS.

  The shape of each
fusion beat depends on the contribution of intrinsic and paced stimuli. Therefore, fusion beats with varying QRS morphologies may be seen.



Clinical significance of ventricular fusion beat

  Appearance of a fusion beat during wide-QRS complex tachycardia confirms that it is of ventricular origin. However, a fusion beat is rarely seen during ventricular tachycardia.

  Appearance of a fusion beat during frequent VPCs suggest the diagnosis of ventricular parasystole
.



How to differentiate intermittent preecitation from ventricular fusion beat?

  The fusion beat arises due to fortuitous meeting of a supraventricular impulse and ventricular impulse in the ventricle.

  This fortuitous meeting of impulses result in
QRS complexes with different shapes.

  On the other hand,
intermittent preexcitation results in similar QRS complexes with delta waves.



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Reference

  Circulation 1962;26:880.





ECG 1. The 3-channel Holter tracings above belong to a middle-aged woman. After
the first three normally conducted
supraventricular impulses
, the first beat of the ventricular tachycardia (VT) attack fuses with the 4th supraventricular
impulse to form
the fusion beat (the lower 3-channel Holter tracing is the continuation of the upper tracing).
This Holter tracing also shows that short bursts of VT may be grossly irregular.

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ECG 2. The ECG above belongs to an old-man with atrial fibrillation and intermittent pacemaker rhythm (VVI).
Pacemaker-induced beats are seen among the normally conducted supraventricular impulses .
However,
the fifth beat from the left does not look like to either the supraventricular or pacemaker-induced beats.
Although
it has a pacemaker spike in front of it, its width is less than other pacemaker-induced beats .
This is a fusion beat
.

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ECG 3a. The above ECG was recorded during a ventricular tachycardia (VT) attack.
In the precordial lead section,
the 4th beat from the left is different than the other VT beats.
This is a fusion beat
.

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ECG 3b. The ECG above belongs to the same patient and was recorded 3 days after the termination of VT attack.
The fusion beat seen in ECG 3a is not similar to any of the beats during sinus rhythm.

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ECG 4. The ECG above belongs to another man with permanent cardiac pacemaker rhythm.
Pacemaker-induced beats are seen among the normally conducted supraventricular impulses .
However,
the eighth beat from the left does not look like to either the supraventricular or pacemaker-induced beats.
Although
it has a pacemaker spike in front of it, its width is less than other pacemaker-induced beats .
This is a fusion beat
.

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ECG 5.
Two fusion beats are seen among pacemaker-induced beats and normally conducted supraventricular impulses .

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ECG 6. The ECG above belongs to a man with frequent VPCs.
On the left hand,
the 2nd and 3rd beats are VPCs .
The 7th beat is a fusion beat
due to end-diastolic VPC. Its configuration is different than both VPCs and sinus beats .
By looking at leads C2 to C6, it is difficult to recognize
the fusion beat at first glance. Limb leads show it better.

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ECG 7. In the above ECG,
the first beat is a fusion . Although it starts with a pacemaker spike , its width is not as wide as the
pacemaker-induced beat . The fused components of the simultaneous intrinsic and the pacemaker stimuli had the chance to
depolarize the ventricle in a shorter time. The
7th beat from the left is another fusion beat with a different QRS morphology .
The second beat from the right
is not a fusion, but a pseudofusion .
The ventricle is depolarized intrinsically and the pacemaker spike has not contributed to it.
If the pacemaker spike is removed, the remaining QRS will look similar to the preceding intrinsic QRS complexes.
(
pseudofusion is a matter of timing ).

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ECG 8. The ECG above is from a 67 years-old woman with previous coronary artery bypass graft surgery.
The rhythm is sinus tachycardia.
The sixth beat from the left is a fusion beat and is not causally related to the preceding P wave.
The fusion beat in lead I looks very similar to the sinus beats.
In case this patient was being monitorized from lead I, it would be almost impossible to realize this fusion beat.

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ECG 9a. The ECG above is from a 74 years-old man who had undergone previous coronary artery byppass graft surgery.
Fusion beat shows that this wide QRS complex tachycardia is of ventricular origin.
The fusion beat in lead aVR looks very similar to the VT beats at the same lead.
The computer of the ECG also failed to diagnose the VT in this patient.

Cardiologist Dr. Cegergun Polat has donated the above ECG to our website.

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ECG 9b. Above is his ECG which was recorded after termination of the VT attack by medical therapy.

Cardiologist Dr. Cegergun Polat has donated the above ECG to our website.

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ECG 10. The above ECG is from a 60 yars-old man who had inferior myocardial infarcton in the past.
Frequent VPCs are seen.
The first 2 VPCs on the left are fusion beats. The 3rd and 4th VPCs are not.
The PR interval is slightly prolonged (210 msec). Tre is also left atrial abnormality.

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ECG 11. The ECG above is from a 82 years-old woman with a DDDR pacemaker, left heart failure and pulmonary hypertension.
Pacemaker spikes originating from the atrial electrode and pacemaker spikes originating from the ventricular electrode
are clearly seen.
Two
ventricular premature contractions (VPCs) are seen
A
fusion beat is also seen.

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ECG 12. The ECG above belongs to a 68 years-old woman.
Only some of the beats show short PR interval and
delta wave (preexcitation) (denoting intermittent WPW).
At first glance, these two beats may seem like fusion beats.
Especially, if there was only one of them, it would be more easy to misdiagnose it as a fusion beat.
However, the fusion beat arises due to fortuitous meeting of a supraventricular impulse and ventricular impulse in the ventricle.
Therefore it is almost impossible to see fusion beats with similar morphology.
On the other hand, the ECG above shows two beats with
similar delta waves, suggesting intermittent preexcitation.

Dr. Sinan Altan Kocaman has donated the above ECG to our website.

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ECG 13. The ECG above belongs to a 65 years-old hypertensive woman.
A VVI pacemaker was implanted in the past. She is not pacemaker dependent.
Her basic rhythm is
sinus rhythm with right bundle branch block and 1st degree AV block.
During recording of the above ECG, her heart rate was increased by us.
Since
her intrinsic heart rate was 103/minute (left side of the ECG), we increased the lower rate of her pacemaker
to 110/minute (right side of the ECG) and the pacemaker started to
pace (give ventricular outputs).
Since her intrinsic heart rate and pacemaker rhythm were almost similar, we observed
frequent fusion beats with different shapes.

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ECG 14. Above is a 6-channell ECG from a 90 years-old woman.
The basic rhythm is atrial fibrillation: P waves are not seen, RR intervals are iregular.
Fibrillatory waves are best seen in leads V1 and V2.
She had undergone VVI pacemaker implantation in the past.
Some QRS complexes have preceding
pacemaker spikes.
The lower rate limit of her pacemaker is about 80/minute.
When a QRS complex is not formed in a pre-defined period (about 18 small squares)
the pacemaker gives an impulse (spike)
resulting in
pacemaker-induced QRS complexes.
Some QRS complexes are formed by fusion of supraventricular and pacemaker-induced depolarization (fusion beat).
These fusion beats resemble neither the naturally occuring QRS complexes nor the paced QRS complexes.

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ECG 15a. The ECG above belongs to an 85 years-old hypertensive woman.
A VVI-R pacemaker was implanted 4 years ago.
Her basic rhythm is atrial fibrillation.
Pacemaker-induced beats, Fusion beats and a Native beat are seen.
Lower-rate limit for this pacemaker is 60/minute.

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ECG 15b. Above is a rhythm tracing from the same patient.
When
RR interval prolongs to a pre-set interval (one second / 5 large squares) in other words when her intrinsic heart rate
drops to 60/minute
and still no QRS appears, then the pacemaker gives an impulse.
When this pacemaker impulse coincides with the native beat, a
Fusion beat occurs.
The following 3 beats are
pacemaker-induced beats.

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ECG 16. The ECG above belongs to a 70 years-old woman who had undergone temporary pacemaker implantation
(in VVI mode) for complete AV block.
At first glance, lead V3 may suggest that this pacemaker fails to sense properly.
However, this pacemaker is functioning properly at this time.
The first pacemaker spike in lead V3 results in wide QRS complex.
The small positive deflection preceding this pacemaker spike is a P wave, not a spontaneously occuring QRS complex
(this is not a fusion beat).
Why not?
The timing of
this deflection is in accordance with regularly appearing P waves (interval between P waves).

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