How to diagnose pseudofusion?

  Shape of the QRS complex of a pseudofusion is similar to that of the intrinsic (unpaced) QRS complex.

      However, there is a superimposed
pacemaker spike on the intrinsic QRS complex.

  Although a pacemaker spike is superimposed on the intrinsic QRS complex,

     
the pacemaker has NOT contributed to its generation.

  The T wave following a pseudofusion is similar to the T wave following an intrinsic (unpaced) QRS complex.

  If pacemaker spike occurs
before or after the intrinsic QRS complex, it is NOT a pseudofusion.

  If pacemaker spike occurs
after the intrinsic QRS complex (in the ST segment),

      it denotes
improper SENSE.

  If pacemaker spike occurs just
before the intrinsic QRS complex (in the PR interval),

      it denotes
improper CAPTURE.



What are the differences between Fusion and Pseudofusion beats?

  Fusion beat looks like neither the intrinsic QRS complex nor the paced beat.

  On the contrary, the pseudofusion beat has a QRS shape similar to the intrinsic beat

      but with a superimposed pacemaker spike.

  Shape of the T wave following a pseudofusion is similar to the shape of the T wave following an intrinsic beat,

      while that following a fusion is NOT.



In patients with cardiac pacemakers, what do Pseudofusion and Fusion beats have in common?

  Both are related to the bad timing of the pacemaker output (spike). They occur due to

     
the competition between the lower rate of the pacemaker and the patient's intrinsic heart rate.



Clinical significance of pseudofusion

  Pseudofusion is a matter of bad-timing.

      It occurs when intrinsic heart rate is very close to the lower rate of the pacemaker.

  Pseudofusion is
not dangerous to the patient.

  Since the pacemaker does not contribute to ventricular depolarization during pseudofusion,

     
evaluation of that cardiac cycle for the signs of ischemic changes is still possible.

  Since the pacemaker does not contribute to ventricular depolarization during pseudofusion,

      it discharges in vain, resulting in further
consumption of the battery.

      Therefore, adjustment of pacemaker settings will be helpful when pseudofusion starts to occur frequently.

     
In suitable patients,

            -
activation of the hysteresis function                                     OR

           
- further lowering of the lower rate of the pacemaker (unless the patient's intrinsic heart rate is very low)

            will ususally suffice to eliminate the pseudofusion.

  Pseudofusion
neither confirms nor rebuts CAPTURE.



Reference

  The nuts and bolts of paced ECG interpretation.

      Tom Kenny. 2009. St. Jude Medical. ISBN: 978-1-405-18404-5.





ECG 1a. The 2-channel rhyhtm strip above is from the Holter recording of a patient with permanent cardiac pacemaker.
The basic rhythm is first degree AV block with bundle branch block.
The first 6 beats from the left are
Paced beats which are followed by a Fusion beat .
Then
intrinsic Normal beats resume.
The sweep speed is 25 mm/second.

Click here for a more detailed ECG




ECG 1b. Another 2-channel rhyhtm strip from the same patient's Holter recording is seen above.
The first 4 beats from the left are
intrinsic Normal beats with first degree AV block. They are followed by 2 PseudoFusion beats .
If the pacemaker spikes on these two beats are removed, the remaining QRS complexes will look like to normal intrinsic beats.
They are followed by 3 fusion beats (look like neither normal beats, nor paced beats).
Next come wide QRS complexes starting with the pacemaker spikes (
paced beats ).
The sweep speed is 25 mm/second.

Click here for a more detailed ECG




ECG 1c. A section of the ECG 1b is depicted at a sweep speed of 50 mm/second in the above tracing.
The first beat from the left is
intrinsic normal beat and is followed by 2 PseudoFusion beats .
If you remove the pacemaker spikes from
these two beats they will resemble the normal intrinsic beats.
They are followed by 3 fusion beats (look like neither normal beats nor paced beats).
Then comes a
paced beat (wide QRS complex starting with a pacemaker spike).

Click here for a more detailed ECG




ECG 1d. Another 2-channel rhyhtm strip from the same patient's Holter recording is seen above. In a patient with cardiac
pacemaker, fusion and pseudofusion appear when
the intrinsic heart rate and the pacemaker's lower rate become similar.
In the rhythm strip above, when heart rate decreases from 83/minute to 80/minute, the pacemaker is triggered
and
PseudoFusion beats appear.
Later, when intrinsic heart rate increases up to 86/minute, pseudofusion disappears and
normal intrinsic beat reappears.
The sweep speed is 25 mm/second.

Click here for a more detailed ECG




ECG 1e. Another 2-channel rhyhtm strip from the same patient's Holter recording is seen above.
The shape of
fusion beats vary due to the varying contributions of the pacemaker stimuli to ventricular depolarization.
The 6th beat from the left is a
PseudoFusion .
If you remove the superimposed pacemaker spike from the
PseudoFusion , normal intrinsic beat will be left.
The pacemaker spike in
Pseudofusion does not contribute to ventricular depolarization.
The sweep speed is 50 mm/second.

Click here for a more detailed ECG




ECG 1f. Another 2-channel rhyhtm strip from the same patient's Holter recording is seen above.
Pacemaker does not give impulse when the intrinsic heart rate is above 70/minute.
When it decreases below 70/minute (68/minute),
PseudoFusion appears.
The sweep speed is 25 mm/second.

Click here for a more detailed ECG




ECG 1g. Another 2-channel rhyhtm strip from the same patient's Holter recording is seen above.
Paced beats , Normal intrinsic beats , Fusion and PseudoFusion are seen.
The sweep speed is 25 mm/second.

Click here for a more detailed ECG




ECG 1h. Another 2-channel rhyhtm strip from the same patient's Holter recording is seen above.
The sweep speed is 50 mm/second.
Paced beats , Normal intrinsic beats , Fusion and PseudoFusion are seen.

Click here for a more detailed ECG





ECG 1i. Another 2-channel rhyhtm strip from the same patient's Holter recording is seen above.
The sweep speed is 25 mm/second.
Paced beats , Normal intrinsic beats , Fusion and PseudoFusion are seen.

Click here for a more detailed ECG






ECG 2. The ECG above is from a 61 years-old woman with permanent cardiac pacemaker.
Paced beats and unpaced intrinsic beats are seen.
Among them, the one on the left side is a pseudofusion : if you remove the pacemaker spike only the intrinsic QRS will be left.

Click here for a more detailed ECG