Ventricular parasystole

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Ventricular Parasystole

1. An ectopic ventricular pacemaker activates the ventricle concurrently with, but independent of, the impulse of the basic rhythm.
2. It is protected from the impulse of the basic rhythm by entrance block.
3. Its own impulse is able to depolarize the ventricle.
4. Probably due to increased automaticity.
5. EKG findings:
   • varying coupling intervals

     1. With PVCs there is usually fixed coupling

   • RR intervals of the ectopic beats being mathematically related to each other

     1. The interectopic forum is variable, are multiples of the shorter interval
     2. Although the ectopic focus fires regularly, only those that find the ventricles responsive are manifested electrocardiographically.
     3. The ratio between the intervals is seldom exact whole numbers (3:1) and more often 3:2 etc.
     4. Parasystolic rates vary from 20 to 400 BPM
     5. In most patients the rate varies between 30 to 56 BPM
     6. Parasystolic rates greater than 70 BPM is called parasystolic VT.

   • the presence of fusion beats

     1. May discharge at the same time that an impulse of the basic rhythm arrives at the ventricles.
     2. The ventricles are then depolarized by the two wavefronts.
     3. Can occur with regular PVCs, but with parasystole more frequently.

6. It is relatively uncommon
   • Seen in 1 to 1.5/1000 EKGS
   • Long rhythm strip required to demonstrate variability in coupling interval