The atria are depolarized partially by the atrial ectopic beat and partially by the sinus beat.

  Atrial fusion beat arises due to fortuitous meeting of an atrial ectopic beat and a sinus beat

     
in the atria.

  The morphology of an atrial fusion beat
is different from that of a sinus beat.

  The width of an atrial fusion beat is generally
more than the width of a sinus beat.

 
Caution: If an atrial premature beat comes too early and merges with its preceding U wave,

      it may falsely give the impression of atrial fusion
.

 
Caution: Don't forget to remember interatrial block in the differential diagnosis of widened P wave.



Reference

  Chou's Electrocardiography in Clinical Practice. Adult and Pediatric.

      6th ed. Philadelphia. WB Saunders 2008:345-346
.





ECG 1. The ECG above belongs to a middle-aged woman.
The fifth P wave from the left is an
atrial fusion beat. This P wave is wider than a sinus P wave.
An
atrial premature beat has merged with a sinus beat resulting in an atrial fusion beat.

Click here for a more detailed ECG





ECG 2a. The ECG above belongs to a 66 years-old man who had undergone coronary artery bypass grafting operation.
ECHOcardiography (ECHO) showed inferior and posterior hypokinesia (mild left ventricular systolic dysfunction).
Limb leads were recorded at a calibration of
20 mm/mV while chest leads were recorded at a standard calibration of 10 mm/mV.
Therefore, at first glance it may seem like the left ventricle is hypertrophic. However, ECHO didn't show hypertrophy.
At first glance,
the second P wave from the right side seems like a wide P wave, suggesting the diagnosis of atrial fusion.
However, this P wave widening is due to
fusion of the atrial premature beat with its preceding U wave.
Similar QU intervals from other leads support U-P wave fusion.

Click here for a more detailed ECG




ECG 2b. The above ECG belongs to the same man.
This time, it is recorded at a standard calibration of
10 mm/mV.
At first glance,
the fourth P wave from the left seems like a wide P wave, suggesting the diagnosis of atrial fusion.
However, this P wave widening is due to
fusion of the atrial premature beat with its preceding U wave.
Similar QU intervals from other leads support U-P wave fusion.

Click here for a more detailed ECG





ECG 3. The ECG above belongs to a 54 years-old man.
The second P wave in lead I may seem like a widened P wave suggesting
atrial fusion at first glance.
However,
checking other leads will show that it is due to baseline drift.
The amplitude of
baseline drift is unacceptably high for a partial P wave depolarization.

Click here for a more detailed ECG