Definition

  Ventricular parasystole is a protected, independent and regular ectopic ventricular rhythm.

 
Protection is due to the failure of other cardiac pacemaker sites to disrupt the rhythmicity

      of this ectopic focus.

  The R-R intervals of the widely seperated parasystolic VPCs are some

     
multiple of the shortest interectopic intervals.

  Since parasystolic focus is independent of the basic cardiac rhythm, the parasystolic VPCs may sometimes

      find the surrounding myocardium refractory, resulting in a failure to depolarize the ventricle. This is the

      reason why interectopic interval is not constant but instead it is some multiple of the shortest interectopic

      intervals.

 
Independence of the parasystolic focus from the basic cardiac rhythm is the reason for the occurence of

      1.
fusion beats.

      2.
variable coupling intervals.

  Ventricular parasytolic rhythm is believed to be
automatic.



How to diagnose ventricular parasystole on the ECG

  There is no fixed coupling interval: variable coupling interval

      (the interval between the parasystolic VPC and its preceding QRS complex is not fixed).

 
Fusion beats may be seen.

  The R-R intervals of the widely seperated parasystolic VPCs are some

     
multiple of the shortest interectopic intervals.



References

  Learning Electrocardiography. A Complete Course.

      Jules Constant. Third Edition. 1987. Little, Brown and Company.





ECG 1. Above is an ECG from a 78 years-old man with a permanent cardiac pacemaker and
frequent VPCs.
Normally conducted beats have preceding P waves .
Parasystolic VPCs have neither preceding P waves nor pacemaker spikes ,
Paced beats come after the pacemaker spikes .
A fusion beat is also seen (looks like neither paced nor normally conducted beats) .
The interval between the 2nd and 3rd VPCs is a multiple (2x) of the interval between the 1st and the 2nd VPCs .
Varying coupling intervals of the
VPCs (620, 660, 655 mseconds) are additional evidence for the presence of parasystole.

Click here for a more detailed ECG