Parasystole
Encyclopedia
Parasystole is a kind of arrhythmia caused by the presence and function of a secondary pacemaker
in the heart, which works in parallel with the SA node. Parasystolic pacemakers are protected from depolarization
by the SA node by some kind of entrance block. This block can be complete or incomplete.
Parasystolic pacemakers can exist in both the atrium or the ventricle
. Atrial parasystolia are characterized by narrow QRS 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, the ectopic pacemaker
is expected to fire at a fixed rate.
Therefore, on ECG, the coupling intervals of the manifest ectopic beats will wander through the basic cycle of the sinus 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 electrical
communication 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 depolarization
of 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.
Cardiac pacemaker
right|thumb|350px|Image showing the cardiac pacemaker which is the SA nodeThe contraction of heart muscle in all animals with hearts is initiated by chemical impulses. The rate at which these impulses fire controls the heart rate...
in the heart, which works in parallel with the SA node. Parasystolic pacemakers are protected from depolarization
Depolarization
In biology, depolarization is a change in a cell's membrane potential, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential...
by the SA node by some kind of entrance block. This block can be complete or incomplete.
Parasystolic pacemakers can exist in both the atrium or the ventricle
Ventricle (heart)
In the heart, a ventricle is one of two large chambers that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The Atria primes the Pump...
. Atrial parasystolia are characterized by narrow QRS 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, the ectopic pacemaker
Ectopic pacemaker
An ectopic pacemaker or ectopic focus is an excitable group of cells that causes a premature heart beat outside the normally functioning SA node of the human heart. Acute occurrence is usually non-life threatening, but chronic occurrence can progress into tachycardia, bradycardia or ventricular...
is expected to fire at a fixed rate.
Therefore, on ECG, the coupling intervals of the manifest ectopic beats will wander through the basic cycle of the sinus rhythm
Sinus rhythm
In medicine, sinus rhythm is the normal beating of the heart, as measured by an electrocardiogram . It has certain generic features that serve as hallmarks for comparison with normal ECGs.- ECG structure :...
. Accordingly, the traditional electrocardiographic criteria used to recognize the fixed form of parasystole are:
- the presence of variable coupling intervals of the manifest ectopic beats;
- inter-ectopic intervals that are simple multiples of a common denominator;
- fusion beatFusion beatFusion beat and Capture beat.In some cases of VT with AV dissociation the SA node may transiently "capture" the ventricles, producing a capture beat, which has a normal QRS duration, or a fusion beat, in which the sinus and ventricular beats coincide to produce a hybrid complex.- References...
s.
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 electrical
communication 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 depolarization
of the pacemaker cells. Therefore, appropriate diagnosis of modulated parasystole relies upon the construction of a “phase response curve
Phase response curve
A phase response curve illustrates the transient change in the cycle period of an oscillation induced by a perturbation as a function of the phase at which it is received...
” 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.