Differentiation between parasystole and extrasystoles

Journal of Electrocardiology Vol. 27 No. 2 1994

Differentiation B e t w e e n Parasystole and Extrasystoles Influence of Vagal Stimulation on Parasystolic Impulse Formation

Shinji Kinoshita, MD, Fumihiko Okada, MD, Go Konishi, PharmB, Makoto Kinoshita, BAgr, and Yoshito Ito, MD

Abstract: Recently, it has been shown that when a sinus impulse falls late in the parasystolic cycle, it usually hastens the next ectopic discharge. Thus, in many cases, the classic criteria for the diagnosis of parasystole (ie, varying coupling intervals and constant shortest interectopic intervals) cannot be used. To differentiate between parasystole and extrasystoles in such cases, the influence of vagal stimulation on parasystolic impulse formation was investigated in seven cases of "true" parasystole in which one or more "pure" ectopic cycles without any intervening nonectopic QRS complexes were found spontaneously. In all cases pure ectopic cycles were found during sinus arrest caused by vagal stimulation; namely, none of the cases showed extreme prolongation of the parasystolic cycle. These results strongly suggest that instead of the classic criteria, vagal stimulation causing temporary sinus arrest is the optimal method for differentiation between parasystole and extrasystoles in cases without spontaneous pure ectopic cycles. Key words: parasystole, extrasystoles, vagal stimulation.

In concealed bigeminy the n u m b e r of sinus QRS complexes intervening between two successive extrasystoles is ordinarily uneven. This p h e n o m e n o n was originally reported by us 1 in 1960 in cases of ventricular extrasystoles. Recently, several investigators 2-4 have attempted to explain the mechanism of concealed bigeminy according to the theory of modulated parasystole. 5 However, it seems to us that in most cases of concealed bigeminy, the mechanism is governed not by parasystole but by reentry. In our

previous reports 6-8 it was shown that to differentiate between parasystole and ordinary extrasystoles in cases of concealed bigeminy, vagal stimulation causing temporary sinus arrest was useful because vagal stimulation hardly suppresses the parasystolic impulse formation, s-16 Namely, it was shown that in concealed bigeminy due to parasystole s one o r more " p u r e " parasystolic cycles without any intervening nonparasystolic QRS complexes were found during temporary sinus arrest, whereas in concealed bigeminy due to reentry 6-8 pure parasystolic cycles were not found during sinus arrest. In a few cases it was reported that vagal stimulation extremely prolonged the parasystolic cycle bey o n d 150% because of suppression of the parasys-

From the Health Administration Center and Graduate School of Environmental Science, Hokkaido University, Sapporo, and Health Administration Center, Kitami Institute of' Technology, Kitami, Japan. Reprint requests: Shinji Kinoshita, MD, Health Administration Center, Hokkaido University, Sapporo 060, Japan.

169

170

Journal of Electrocardiology Vol. 27 No. 2 April 1994

tolic impulse formation 17 or the occurrence of exit block. 18 However, it seems to us that such previously reported cases were not parasystole but intermittent or concealed bigeminy due to reentry~'8; namely, that such cases were not "true" parasystole but "seeming" parasystole. Thus, in order that vagal stimulation causing temporary sinus arrest can be used for differentiating between parasystole and ordinary extrasystoles, it seems important to investigate the influence of vagal stimulation in cases of true parasystole. In this study the influence of vagal stimulation on parasystolic impulse formation was investigated in seven cases of true parasystole in which one or more pure parasystolic cycles without any intervening nonparasystolic QRS complexes were found spontaneously (ie, without vagal stimulation) on the same day. Before this study there were few reported cases of such true parasystole in which vagal stimulation was performed. 1x,~,~9

Materials and Methods Seven patients with ectopic QRS complexes of the same configuration were selected for this study because the following findings were seen on their ordi~nary electrocardiogram (ECG), which was taken without vagal stimulation: ( 1 ) one or more pure ectopic cycles without any intervening nonectopic QRS complexes, XX and (2) ectopic cycles with an intervening sinus QRS complex, XSX or (X)SX. X and S represent ectopic and sinus QRS complexes, respectively, and (X) represents a concealed ectopic depolarization. These ECG findings indicate the coexistence of ectopic rhythm with (second- or thirddegree) entrance block, namely, the presence of true parasystole. In seven cases (including 1 previously reported case 2°) of such true parasystole vagal stimulation due to pressure on the eyeball was performed on the same day, and the influence of vagal stimulation on

parasystolic impulse formation was investigated. None of the patients had organic heart disease or was receiving antiarrhythmic therapy. In all cases except one (case 7) the parasystolic pacemaker was located in the ventricles; in case 7 the pacemaker was located in the atrioventricular junction. In all cases, w h e n a sinus impulse fell late in the parasystolic cycle it hastened the occurrence of the next parasystolic impulse, and the parasystolic cycle with a late intervening sinus QRS complex (short XSX) was considerably shorter than the cycle with an early intervening sinus QRS complex (long XSX), which indicates the presence of "irregular" parasystole. ~5,21 Differences between the long XSX and the short XSX intervals are shown in Table 1.

Results Table 1 shows pure XX intervals that were found spontaneously (without vagal stimulation) and those that were found during vagal stimulation in the seven cases of true parasystole. In all cases spontaneous pure XX cycles and pure XX cycles during vagal stimulation were found in the long ECGs taken on the same day. As shown in Table 1, none of these cases showed complete suppression of parasystolic impulse formation by vagal stimulation; namely, even w h e n the parasystolic rate was slowed by vagal stimulation, prolongation of the pure XX interval was not beyond 8% of the spontaneous pure XX interval. Figure 1 shows parts of a long continuous recording in case 1. The upper three strips show the coexistence of spontaneous pure XX cycles (1.73 and 1.75 seconds) with (X)SX cycles, which indicate the presence of true parasystole. The long interectopic intervals of 4.89 seconds (the top strip) and 3.18 seconds (the third strip) are considerably shorter than three times and twice the spontaneous pure XX interval, respectively; namely, the parasystolic rhythm in this case is irregular. The lower four strips, which are con-

Table 1. Influence of Vagal Stimulation on Parasystolic Cycle Length Case No.

Age/Sex (years)

Difference Between Long XSX a n d Short XSX (seconds)

S p o n t a n e o u s Pure XX (seconds)

D u r i n g Vagal Stimulation Pure XX (seconds)

1 2 3 4 5 6 7

44/M I9/M 36/M 21/M 35/M 27/M 40/F

0.15 0.03 0.15 0.12 0,11 0.39 0.21

1.73-1.75 1.50-1.51 1.56 1,32 1.43 1.75 1.42

1.72-I.86 1.38-1.56 1.55-1.56 1.33-1.34 1.40-1.52 1.51-1.65 1.43-1.48

The findings in case 3 were obtained from our previously reported case. ~6 XX and XSX = ectopic cycles with no and one intervening sinus QRS complex, respectively,

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Fig. 1. Case 1 : vagal stimulation in true parasystole. The upper three strips show the coexistence of spontaneous pure XX cycles with (X) SX cycles, indicating the presence of true parasystole. The second and third strips are continuous. The lower four strips, which are continuous, show that marked prolongation of the pure ectopic cycle does n o t occur during temporary sinus arrest caused by vagal stimulation. Time intervals in this and subsequent figures are expressed in hundredths of a second. Numerals marked with a n asterisk (*) indicate pure parasystolic cycle lengths. S a n d X = sinus and ectopic QRS complexes, respectively; (X) = concealed ectopic depolarization; X + S or S + X = fusion QRS complex.

tinuous, show that marked sinus bradycardia and t h e n t e m p o r a r y s i n u s a r r e s t are c a u s e d b y v a g a l s t i m u l a t i o n , T h e first p u r e XX i n t e r v a l d u r i n g v a g a l s t i m u l a t i o n (1.86 s e c o n d s ) is s o m e w h a t l o n g e r t h a n t h e s p o n t a n e o u s p u r e XX i n t e r v a l . This s u g g e s t s t h a t t h e p a r a s y s t o l i c rate is s l o w e d d o w n b y v a g a l s t i m u l a t i o n , b u t t h a t p r o l o n g a t i o n o f t h e p u r e XX i n t e r v a l is n o t b e y o n d 8 % of t h e s p o n t a n e o u s p u r e XX i n t e r val. T h e b o t t o m strip s h o w s t h a t w h e n v a g a l s t i m u l a t i o n is s t r e n g t h e n e d f u r t h e r , p u r e XX cycles o c c u r r e p e a t e d l y . H o w e v e r , t h e XX i n t e r v a l s s h o r t e n to alm o s t t h e s a m e l e n g t h as t h e s p o n t a n e o u s p u r e XX interval. F i g u r e 2 s h o w s parts of a l o n g c o n t i n u o u s r e c o r d i n g i n case 2. T h e t o p strip s h o w s t h e c o e x i s t e n c e o f s p o n t a n e o u s p u r e XX cycles w i t h (X)SX cycles, w h i c h i n d i c a t e t h e p r e s e n c e of t r u e p a r a s y s t o l e . T h e s e c o n d a n d t h i r d strips s h o w t h e p o r t i o n t h a t w a s recorded immediately before vagal stimulation, in w h i c h t h e p a r a s y s t o l i c cycle w i t h a n e a r l i e r i n t e r v e n i n g s i n u s QRS c o m p l e x ( l o n g XSX i n t e r v a l o f 1.61

s e c o n d s ) is s o m e w h a t l o n g e r t h a n t h e cycle w i t h a l a t e r QRS c o m p l e x ( s h o r t XSX i n t e r v a l o f 1.57 o r 1.58 s e c o n d s ) . T h e l o w e r f o u r strips, w h i c h are c o n tinuous, show that sinus bradycardia and then sinus a r r e s t are c a u s e d b y v a g a l s t i m u l a t i o n . T h e first p u r e XX i n t e r v a l ( 1 . 5 6 s e c o n d s ) is s l i g h t l y l o n g e r t h a n t h e s p o n t a n e o u s p u r e XX i n t e r v a l , b u t n o t l o n g e r t h a n t h e XSX i n t e r v a l s i m m e d i a t e l y b e f o r e v a g a l s t i m u l a t i o n . T h e b o t t o m strip s h o w s t h a t w h e n v a g a l s t i m u l a t i o n is s t r e n g t h e n e d f u r t h e r , p u r e XX i n t e r v a l s b e come considerably shorter than the spontaneous p u r e XX i n t e r v a l . I n t h e o t h e r five cases (cases 3 - 7 ) m a r k e d p r o l o n g a t i o n of t h e p u r e XX i n t e r v a l b e y o n d 8 % d i d n o t occur during vagal stimulation.

Discussion It h a d b e e n b e l i e v e d for a l o n g t i m e t h a t p a r a s y s tolic r h y t h m is r e g u l a r a n d i n d e p e n d e n t of t h e b a s i c (usually sinus) rhythm; namely, that the following

172

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criteria for the diagnosis of parasystole can be used: (1) varying coupling intervals with frequent appearances of fusion beats and (2) constant shortest interectopic intervals. 22-24 In 1974, however, Kinoshita 2° reported{ a case of ventricular parasystole in w h i c h w h e n a sinus impulse fell late in the parasystolic cycle it hastened the occurrence of the next parasystolic impulse. Since then, m a n y cases of such irregular parasystole have been reported. ~4"~,~,x~ and it has been s h o w n that quite "regular" parasystole is a rather u n c o m m o n occurrence.~ ~, ~ It has also been rePorted[ that in some cases of parasystole, fixed coupling intervals are occasionally seen. 2~ On the other hand, ordinary premature beats (extrasystoles) occasionally have varying coupling intervals 27"28 or almost constant shortest interectopic intervals, particularly in cases of concealed bigeminy. ~'~'x~ These features in parasystole and extrasysto]es show that in not a few cases the classic :

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criteria for the diagnosis of parasystole (ie, varying coupling intervals and constant shortest interectopic intervals) canrrot be used. The observations in this study demonstrated that in cases of true parasystole extreme prolongation of the parasystolic cycle was not caused by vagal stimulation. Thus, it seems that instead of the classic criteria, vagal stimulation is very useful as a diagnostic tool of parasystole. Figures 3 and 4 show two exemplary cases in which vagal stimulation differentiates between parasystole and extrasystoles: In Figure 3 the upper strip seems to,show ordinary premature beats (extrasystoles) with almost fLxed coupling intervals. The lower strip shows that during temporary sinus arrest caused by vagal stimulation, pure parasystolic cycles without any intervening nonparasystolic QRS complexes, XX, are found. This indicates that ectopic QRS complexes in this case are not extrasystoles but parasystolic complexes.

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Fig. 4. Seemingly parasystolic QRS complexes with markedly variable coupling intervals and almost constant shortest interectopic intervals (the first two strips). Disappearance of the ectopic QRS complexes during temporary sinus arrest caused by vagal stimulation (the bottom strip) suggests that the ectopic QRS complexes in this case are not parasystolic complexes but extrasystoles (ordinary premature beats). This case can be explained by the concept of concealed conduction and two-level block !in the reentrant pathway of extrasystoles. 6 The diagram below the second strip shows the conduction of impulses in the reentrant pathway. All sinus impulses cannot enter the reentrant pathway through the exit (the upper side of the pathway in the diagram) because of unidirectional block. Intraventricular conduction of impulses leading to the entrance of the pathway (the lower side) is indicated by dashed lines. Shaded bars represent abnormally long effective refractory periods of the proximal and distal portions of the pathway. The diagram shows that sinus impulse S~ proceeds very slowly through the proximal portion of the pathway and then is blocked before the distal portion. The next sinus impulse, S2, is blocked without an appreciable invasion of the pathway. Thus, impulse $3 passes through the pathway without sufficient conduction delay and becomes a concealed reentrant extrasystole, (X). Impulse $4 passes through both the proximal and distal portions of the pathway with marked delay and becomes a reentrant extrasystole, X, with a markedly long coupling interval. V = ventricles; RP = reentrant pathway. Other abbreviations are the same as in Figure l.

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Journal of Electrocardiology Vol. 27 No. 2 April 1994

Figure 4 s h o w s a n o t h e r e x e m p l a r y case. I n the first two strips ectopic QRS complexes s h o w varying coupling intervals a n d almost c o n s t a n t shortest interectopic intervals, w h i c h s e e m to s h o w parasystole. The b o t t o m strip s h o w s that during t e m p o r a r y sinus arrest caused b y vagal stimulation, p u r e parasystolic cycles, XX, are n o t found. This indicates that ectopic QRS complexes in this case are n o t parasystolic c o m plexes b u t extrasystoles. Prior to this study, to o u r k n o w l e d g e , in o n l y five other cases ~1,15,19 of such true parasystole h a s the influence of vagal stimulation b e e n reported. In all of these present a n d previous cases extreme p r o l o n gation of the parasystolic cycle b e y o n d 35% of the s p o n t a n e o u s pure cycle w a s n o t seen during vagal stimulation. In all cases of parasystole that w e h a v e analyzed so far the longest pure parasystolic cycle is n o t b e y o n d 3.00 seconds. Thus, it is strongly suggested that in cases w i t h o u t s p o n t a n e o u s pure ectopic cycles, vagal stimulation causing t e m p o r a r y sinus arrest b e y o n d 3.5 seconds is the optimal m e t h o d for differentiation b e t w e e n parasystole a n d extrasystoles.

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