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| Parasystole | |
|---|---|
| Specialty | Cardiology |
Parasystole is a kind ofarrhythmia caused by the presence and function of a secondarypacemaker in the heart, which works in parallel with theSA node. Parasystolic pacemakers are protected fromdepolarization by the SA node by some kind ofentrance block. This block can be complete or incomplete.
Parasystolic pacemakers can exist in both theatrium or theventricle. Atrial parasystolia are characterized by narrowQRS complexes
Two forms of ventricular parasystole have been described in the literature, fixed parasystole and modulated parasystole. Fixed ventricular parasystole occurs when an ectopic pacemaker is protected by entrance block, and thus its activity is completely independent from the sinus pacemaker activity. Hence, theectopic pacemaker is expected to fire at a fixed rate.Therefore, onECG, the coupling intervals of the manifestectopic beats will wander through the basic cycle of thesinus rhythm. Accordingly, the traditional electrocardiographic criteria used to recognize the fixed form of parasystole are:
According to the modulated parasystole hypothesis, rigid constancy of a pacemaker might be expected if the entrance block were complete, but if there is an escape route available for the emergence of ectopic activity, then clearly there must be an effective ionic communication, not complete insulation, between the two tissues. If there is an electricalcommunication between the two, then the depolarization of the surrounding ventricle may influence the ectopic pacemaker. That influence will be electrotonic; depolarization of the surrounding field will induce a partial depolarizationof the pacemaker cells. Therefore, appropriate diagnosis of modulated parasystole relies upon the construction of a “phase response curve” as theoretical evidence of modulation of the ectopic pacemaker cycle length by the electrotonic activity generated by the sinus discharges across the area of protection. In this case, the timing of the arrival of the electronic stimulus will serve to delay or advance the subsequent pacemaker activation. In this case, the coupling intervals between the manifest ectopic and sinus discharges will be either fixed or variable, depending on the cycle length relations between the two pacemakers.