Definition

  Depolarization of the ventricle partly by a supraventricular stimulus and partly by a VPC

      is called
fusion beat.

  The fusion beat arises due to fortuitous meeting of a supraventricular impulse and ventricular impulse

     
in the ventricle.

  Since ventricular rhythms are independent of the 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 the 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.




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 the fusion beat

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

      However, it is very rare to observe a fusion beat during ventricular tachycardia.

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



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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Click here to go to PSEUDOFUSION in cardiac pacemakers