[SCD-FORUM] 117E Parasystole and syncope. Dr. Perez Riera.

[SCD Symposium]

Dear Dr. Ekaterina Pervova from Russia Moscow. Eric Andrés Ricardo Pérez 
Riera from Sao Paulo Brasil answer.

Parasystole is usually an extrasystolic rhythm which can occur at SA node 
(1), atrium, junction and particularly in the ventricles. The dysrhythmia, 
resulting from the co-existence of two pacemakers, the sinus node and an 
ectopic focus, presents distinctive regular patterns, with transitions from 
one pattern to another occurring abruptly. Parasystole is the expression of 
a pacemaker that is protected from, and thus independent of, the dominant 
rhythm. Parasystole is an arrhythmia characterized by a second automatic 
rhythm existing simultaneously with normal SA rhythm.

 It is admitted that the parasystolic focus is protected from the environing 
myocardium by an entry block but can manifest itself. Parasystolic 
pacemakers are protected from depolarization by the SA node by some kind of 
entrance block. This block can be complete or incomplete. Actually, a pure 
unidirectional block does not exist and the environing myocardium affects 
the parasystolic rhythm by an electrotonic current which modulates the 
output.



The classic criteria for the diagnosis of parasystole are:

1)       Variable coupling intervals: Fixed coupling refers to a fixed 
interval between the sinus QRS complex and the VPC; this indicates reentry 
or a triggered focus as the possible cause. Variable coupling could be due 
to parasystole or multifocal ectopy;

2)       Constant shortest interectopic intervals. Vagal stimulation causing 
temporary sinus arrest is the optimal method for differentiation between 
parasystole and extrasystoles in cases without spontaneous pure ectopic 
cycles (2).

3)       Mathematically related interectopic intervals;

4)       Fusion beats: Simultaneous activation of the ventricle by 2 sources 
can lead to a beat with characteristics between the conducted sinus beat and 
the ectopic beat.



Three quantitative indices as necessary conditions have been used as 
diagnostic criteria for parasystole with high sensitivity and high 
specificity (3):



1) Take the earliest recorded eight interectopic intervals in which at least 
four intervals containing sinus beats or other beats having activated to the 
area within the ectopic focus. When in case of deficiency, it will fill up a 
vacancy in order. The ratios of the shortest coupling interval to the 
shortest ectopic cycle length are all less than 80%;



2) The coefficients of variation of the eight ectopic cycle length are all 
less than 6%;



3) The maximal differences of coupling intervals are  equal to or more than 
110ms.



A non-parasystolic complex which occurs prematurely in the parasystolic 
cycle delays it. Conversely it accelerates the cycle when it occurs late. By 
this fact, a parasystole pacing is possible and can lead to a fixed 
coupling.

This arrhythmia is frequently unknown and can be experimentally, reproduced 
by a sucrose gap preparation. A large amount of experimental and clinical 
data has pointed out several atypical phenomena that make the recognition of 
parasystole difficult. This especially occurs in the presence of influence 
exerted from sinus impulses upon the parasystolic rhythm. Several different 
expressions of parasystolic rhythm may be present within the same tracing. 
Supernormal modulation is responsible for the occurrence of couplets.

In the case of a heart-transplant patient (4) with a small heart-rate 
variability as a result of heart denervation, the model predicts the RR 
intervals with an error of less than 6% for an 80-beat sequence. From a 
physiological point of view, the results imply that the interaction between 
the two pacemakers in the heart is fairly weak, and hence the parasystole 
observed in the heart-transplant patient can be modelled as pure 
parasystole. A minimal or absent modulation results in the classical picture 
of parasystole; when a mild modulating influence is present, the typical 
pattern of modulated parasystole ensues, whereas a strong modulation leads 
to disappearance of the typical features of parasystole and manifestation of 
concealed bigeminy.(5)

   In two cases, studied by Itoh et al (6).  ventricular parasystole was 
associated with VT  and in one patient, catheter ablation was successful. In 
patient 1, with dilated cardiomyopathy, ventricular parasystole led to VT, 
which converted to VF.  Both ventricular parasystole and VT disappeared 
immediately, and no recurrence has been observed during a follow-up of 8 
months. Catheter ablation to the parasystolic focus was effective and a 
relationship between VP and VT was strongly suggested.

The ventricular parasystole revealed a new depressive effect of adenosine on 
ventricular parasystolic activity. Disappearance of "true" parasystole with 
adenosine was related (7).

A case of VF during Holter monitoring was described by Szpotz from Poland. 
Tape analysis doesn't shows any significant changes of QT intervals, 
increasing in HR and ventricular ectopic activity. It was attempt 
possibility that modulated parasystole may lead to VF (8).



Referentes



1)       Satullo G, Oreto G, Luzza F, Sinus parasystole. Am Heart J. 1991; 
121:1507-1512.

2)       Kinoshita S, Okada F, Konishi G et al. Differentiation between 
parasystole and extrasystoles. Influence of vagal stimulation on 
parasystolic impulse formation. J Electrocardiol. 1994;27:169-174.

3)       Ren Z, Zhou J, Xu G. et al., The diagnostic criteria for classic 
parasystole. Chin Med J (Engl). 1999; 112:992-4.

4)       Costa M, Pimentel IR, Santiago T. et al.Modelling a parasystolic 
rhythm in a heart-transplant patient. Med Biol Eng Comput. 1999;37:492-496.

5)       Satullo G, Oreto G, Cavallaro L. The many faces of parasystolic 
rhythm G Ital Cardiol. 1993;23:699-712.

6)       Itoh E, Aizawa Y, Washizuka T et al. Two cases of ventricular 
parasystole associated with ventricular tachycardia. Pacing Clin 
Electrophysiol. 1996;19:370-373

7)       Tomcsanvi J, Tenczer J, Horvath L. Effect of adenosine on 
ventricular parasystole. J Electrocardiol. 1996;29:61-63.

8)       Szpotz M. Case of ventricular fibrillation recorded during Holter 
monitoring and initiated by ventricular parasystole. Przegl Lek. 1994; 
51:319-321.

All the best

Andrés Ricardo Pérez Riera

Chief of Electro-Vectocardiology Sector of the Discipline of  Cardiology, 
ABC Faculty of Medicine (FMABC), Foundation of ABC (FUABC) - Santo André - 
Sao Paulo - Brazil. Rua Sebastiao Afonso  885 - Zip Code: 044417-100- Jardim 
Miriam   S.P  Brazil- Phone: 5504-6243  Fax: 5506-0398.