Variation in parasystolic cycle length

At the time of the first visit to our clinic, an electrocardiographic examination of a 73-year-old female patient revealed ventricular premature contractions (VPCs) with variable coupling intervals that were diagnosed as parasystole. Characteristically many of the parasystoles had no sinus contractions between two consecutive VPCs, which we referred to as pure parasystole. We first repeatedly examined variations in the length of the parasystolic cycles between January 6, 1997 and March 2, 2003 using electrocardiography. The time courses recorded over this period showed that the length of the parasystolic cycle did not remain constant, but varied irregularly within a relatively narrow range. We also recorded the length of the parasystolic cycles over 3 hours using Holter monitoring. The interectopic intervals plotted against mean sinus cycle length showed that the cycle length of pure parasystoles remained almost constant at about 1,300 ms over the 3 hours. We also examined the cycle length during exercise and found that it was slightly prolonged thereafter, while the sinus cycle length was clearly shortened after exercise. The average of six deep breathing tests showed that parasystolic cycle length did not significantly differ between deep inspiration and deep expiration, whereas the sinus cycle length during expiration was significantly longer than that during inspiration. These results indicate that the responses to both exercise and deep breathing obviously differed between the parasystolic and sinus cycle lengths.

Extrapotentials and allorhythmias as an expression of experimental parasystole

The aim of the study was to investigate the dynamics of experimental parasystole taking into consideration the peculiarities of recurrent arrhythmias recorded in clinical settings.

MATERIAL AND METHODS

The experiments were conducted on isolated right atria of seven chinchilla rabbits. Parasystolic arrhythmias using periodical one-site electrostimulation were provoked in one atrium, where the sinus node was not affected, and in two atria with the spontaneous low value activity of pacemakers. The parasystolic arrhythmias by the dual-site periodical pacing were provoked in four atria, in which the spontaneous activity had disappeared, while the membrane potential of cardiomyocytes remained at the level of 70 to 80 mV.

RESULTS

The parasystolic arrhythmias of the shape of single extrapotentials were obtained in atria when the periods of excitation impulses were within the limits of 0.9-1.2 s, and the differences between these periods being relatively small (0.04-0.2 s). The increase of these differences resulted the various allorhythmias. In cases of single extrapotentials, the recurrence periods of arrhythmias reached 5.6-29 s; while in cases of allorhythmias they shortened to 2.4-4.8 s.

CONCLUSION

The parasystoles in isolated atria of rabbits can be induced by two competitive excitation sources. They may manifest themselves through single extrapotentials or allorhythmias, whose form depends on the duration of the periods of excitation impulses, the difference between these durations, as well as on effective refractory periods of atrial cardiomyocytes. The determination and evaluation of the recurrence period of these arrhythmias can serve in any given clinical situation as a supplementary criterion.

His-bundle parasystole masquerading as exercise-induced 2:1 atrioventricular block

We describe a case of symptomatic pseudo-AV block due to His-bundle parasystole masquerading as exercise-induced 2:1 AV block. Electrophysiologic study revealed the presence of His-bundle parasystole, and the fluctuation of parasystolic cycle length could be explained by the concept of modulated parasystole. Modulated parasystole is a possible explanation for maintenance of stable 2:1 AV conduction at an atrial rate of specific range during exercise.

Clinical evaluation of excitability measures in sensory nerve

A recently described method for recording multiple excitability parameters of human motor nerves has been adapted to the study of sensory nerves. The protocol measures stimulus-response behavior using two stimulus durations (from which the distribution of strength-duration time constants is estimated), threshold electrotonus to 100 ms polarizing currents, a current-threshold relationship (indicating inward and outward rectification), and the recovery of excitability following supramaximal activation. The method was tested on 50 healthy volunteers, stimulating the median nerve at the wrist and recording the antidromic compound sensory nerve action potential (SNAP) from digit 2. The excitability measurements were similar, where comparisons were possible, with published sensory nerve data, and confirmed differences from motor nerves, particularly in strength-duration behavior and recovery cycle, likely to reflect functional differences between sensory and motor nerves. Although slower than for motor nerves, the sensory nerve recordings were sufficiently quick (16 to 18 min) to allow them to be included in routine clinical studies. We propose that this method, which provides quite different and complementary information about nerve function to conventional conduction studies, provides a useful new approach for exploring the pathophysiology of sensory neuropathies.

Ventricular parasystolic couplets originating in the pathway between the ventricle and the parasystolic pacemaker: mechanism of "irregular" parasystole

This article explains the mechanism of "irregular" parasystole. Two theories have been suggested: "electrotonic modulation" and "type I second degree entrance block." This study attempts to clarify the mechanism of irregular parasystole in cases of true ventricular parasystole associated with ventricular parasystolic couplets. Cases associated with ventricular parasystolic couplets were selected from 37 clinical cases of true ventricular parasystole in which one or more pure parasystolic cycles with no intervening nonectopic QRS complexes were found. Of the 37 cases of true ventricular parasystole, ventricular parasystolic couplets were found in 4 cases. In none of the other 33 cases, ventricular parasystolic couplets were found. In all the cases coexisting with ventricular parasystolic couplets, the latter ectopic QRS complex of the couplet failed to reset the parasystolic rhythm. The above findings suggest that the latter ectopic QRS complex of the parasystolic couplet originated not in the parasystolic pacemaker but in the pathway between the ventricle and the parasystolic pacemaker. It seems that when a sinus impulse fell late in the parasystolic cycle, it passed through the site of second degree entrance block and that the parasystolic couplets originated from the reentrant pathway between the ventricle and the pacemaker. This strengthens our previous suggestion that the mechanism of irregular parasystole is governed by "type I second degree entrance block" and not by "electrotonic modulation."

The diagnostic criteria for classic parasystole

OBJECTIVE

To establish a diagnostic criteria of parasystole with high sensitivity and high specificity.

METHODS

After excluded from nonparasystole and each variant parasystoles, based on the electrocardiographic data obtained from 61 patients with classic parasystole, we selected the quantitative indices which could reflect the features of ectopic focus with complete entrance block as the diagnostic criteria for parasystole.

RESULTS

The common features of the electrocardiograms of this group were: 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 (ECL) were all less than 80%; 2) The coefficients of variation of the eight ECLs were all less than 6%; 3) The maximal differences of coupling intervals were all equal to or more than 0.11 second.

CONCLUSION

Three quantitative indices as necessary conditions have been used as diagnostic criteria for parasystole with high sensitivity and high specificity.

Intermittent sinus bigeminy as an expression of sinus parasystole: a case report

A case of sinus parasystole is reported. The diagnosis of sinus parasystole is relatively difficult because there is no difference between the basic sinus P wave and the parasystolic wave. Sinus parasystole is diagnosed according to the following electrocardiographic criteria: (1) premature P waves having contour identical to P waves of basic beats; (2) intervals between premature P waves mathematically related. In the case reported, the coupling intervals during long phases of intermittent sinus bigeminy were nearly fixed, because there was little variability in the returning cycles, making the diagnosis of sinus parasystole difficult.

Effects of exercise and standing on atrial parasystole: prolongation and shortening of the parasystolic cycle length

In recently reported cases of ventricular parasystole, it was shown that after exercise the parasystolic cycle length is prolonged, in contrast to a shortening of the sinus cycle length, whereas during standing the parasystolic cycle length and the sinus cycle length both shortened. In this report, to explore whether the same features as occur in ventricular parasystole are seen in atrial parasystole, effects of exercise and standing on the parasystolic cycle length were investigated in two men with atrial parasystole. The atrial parasystolic cycle length was prolonged after exercise, whereas it shortened during standing, similar to what occurs in ventricular parasystole. This is the first report to show such changes of cycle length in atrial parasystole. These findings suggest that in atrial parasystole, as in ventricular parasystole, influences on the parasystolic cycle length do not always act in the same direction as those on sinus cycle length.

Mechanism of atrial escape-capture bigeminy: second-degree sinoatrial exit and entrance block

Two women with atrial escape-capture bigeminy were reported in whom sinus P waves and escape P waves alternated with each other. Their electrocardiographic findings showed that, during the bigeminal rhythm, 2:1 exit block of sinus impulses associated with entrance block of escape impulses occurred in the sinoatrial junction. Three phenomena are suggested to explain the observations. (1) Seeming bradycardia-dependent sinoatrial exit block may have occurred, probably as a result of phasic changes in vagal tone due to respiration, with decreased vagal tone depressing conductivity in the sinoatrial junction to a greater degree than automaticity in the sinus node. (2) In case 1, type II second-degree entrance block in the sinoatrial junction may have occurred, reflecting the presence of sinus parasystole with incomplete entrance block. (3) Sinus escape also may have occurred, especially in case 2. These phenomena have never been reported before.

Asymptomatic temporary atrioventricular dissociation complicating meningococcal meningitis

We present a case of a young man with meningococcal meningitis and various asymptomatic temporary ECG abnormalities, including sinus bradycardia, atrioventricular dissociation and non specific ST-T changes.

[The use of adenosine in the diagnosis and treatment of cardiac arrhythmias]

The authors investigated the effect of adenosine or ATP on narrow QRS tachycardia in 56 pts, 3 pts with wide QRS tachycardia 9 pts with suspected latent preexcitation and 10 pts with PVC suspected to be ventricular parasystole. After the bolus iv. administration of adenosine or ATP every SVT was stopped related to AV node (44 pts), but in the rest twelve related to atrial origin of SVT only one automatic atrial tachycardia could be stopped. From the 9 patient suspected to have concealed WPW 2 pts had delta wave during the effect of adenosine, and in four pts parasystole was demonstrated among the pts had varying coupling interval PVC. None of the pts who had wide QRS tachycardia was the tachycardia stopped, but in two cases the supraventricular origin--atrial flutter and tachycardia--was discovered. The authors emphasize the favourable effect of adenosine in narrow complex tachycardia and suggest that it can given safely in wide QRS tachycardia of unknown origin either. The diagnostic effect of adenosine can be used in sinus rhythm too if latent preexcitation or ventricular parasystole is suspected.

Intermittent parasystole with exit block

A 53-year-old patient had a complex ventricular arrhythmia, which most likely was an intermittent pure (nonmodulated) parasystole, due to protection limited to the first part of the cycle coexisting with exit block. However, other interpretations of the observations were possible. Foremost among these was modulated parasystole with partial protection or with an attenuated or nondetectable early delaying phase, with exit block. Interestingly, the parasystole appeared to have fulfilled the dynamic rules regarding the number of sinus beats interposed between manifest parasystolic beats, as has been reported for pure or weakly modulated parasystole. This case corroborates unusual manifestations of an arrhythmia, which because of its newly found complexities and various possible interpretations seems to be discussed with decreasing frequency in most textbooks on general cardiology.

Atrial parasystole

A case of the relatively rare atrial parasystole is reported, and this interesting cardiac arrhythmia is reviewed. Atypical deviations from classical parasystole are reviewed, and modern concepts of parasystole as they differ from traditional, classical parasystole are addressed.

Reverse effects of exercise on the sinus and parasystolic cycle lengths

Ventricular arrhythmias are the primary concern in the exercise laboratory. It has recently been suggested that in not a few cases, ventricular premature complexes are governed by modulated parasystole and not by ordinary extrasystolic rhythm. In many cases, however, it is difficult to differentiate between parasystole and ordinary extrasystoles. Few reports are available on the effect of exercise in cases of "true" parasystole. This study investigated the effect of exercise on the parasystolic cycle length 11 cases of true ventricular parasystole, in which one or more "pure" parasystolic cycles containing no intervening nonectopic QRS complexes were found. In all cases, in contrast to an acceleration of the sinus rate, a definite decrease in the parasystolic rate was found. In no case did complete suppression occur. These findings suggest that the effects of exercise on ordinary ventricular extrasystolic rhythm and on ventricular parasystole may be considerably different from each other.

A different approach to the analysis of pure ventricular parasystole

Until recently, it had not been recognized that predictions regarding the number of sinus beats interposed between two consecutive parasystolic beats could be made. In a case of perfect, pure parasystole resulting from unintentional fixed rate ventricular pacing, the following was observed: there were consistently three different values (0,2,3) for the number of interposed sinus beats; only one of these values was odd, and the sum of the two smaller values was one less than the larger value. Our findings, which are in keeping with those obtained in an mathematical model, may be of additional help in the diagnosis of this elusive arrhythmia.

[An accelerated idioventricular rhythm and sports activity. Comments on a clinical case and a characterization of the arrhythmia]

In the evaluation of an accelerated idioventricular rhythm (AIVR) case presented by an athlete, even though considered qualified for agonistic sport practice in compliance with the COCIS protocol, the authors made some considerations relative to such type of arrhythmia. AIVR are characterized by a wide oscillation of frequency (from 40 to 120 b/min) and are distinguished as active AIVR when the ventricular center exceeds the discharge frequency in a non depressed sinusal activity; and it's passive AIVR when an automatic ventricular center substitutes the physiological pacemaker in the presence of sinusal bradycardia. This leads to think that it could be two different phenomenons. The first is characterized by a low frequency and is determined by the activation of some automatic cells located under the His bundle, and the second arises with the extrasystolic modality. Moreover, AIVR manifest a parasystolic type of behaviour that complicates the diagnostic differentiation. Therefore it can be considered that the arrhythmia of ventricular genesis (AIVR, ventricular tachycardia, parasystole) represents the varied expression of the same electrogenic substratum with different discharge potentiality. Thus was formulated the proposal of unifying the ventricular rhythms in one group which includes: rhythm of ventricular escapement, AIVR, parasystole and ventricular tachycardia.

[The many faces of parasystolic rhythm]

BACKGROUND

Parasystole is the expression of a pacemaker that is protected from, and thus independent of, the dominant rhythm. The arrhythmia is not always associated with the 3 classic signs: variable coupling, mathematically related interectopic intervals, and fusion beats. A large amount of experimental and clinical data have 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.

METHODS

A pattern of ventricular parasystole was evident throughout a 24-hour Holter recording obtained from a 55-year-old female. The following data were analyzed: a) distribution of ectopic complexes; b) parasystolic cycle duration; c) regularity of parasystole, as assessed by means of the variation index. In some sections of the tracing in which an electrotonic influence (modulation) exerted by the sinus rhythm on the parasystolic rhythm appeared as very likely, phase-response curves were constructed in order to express the time-dependent modulation effected by sinus impulses.

RESULTS

Parasystole occurred in two separate periods, lasting 4 hours and 90 minutes, respectively. On both occasions, the arrhythmia was apparently precipitated by a brief episode of atrial tachycardia. Phases of regular parasystole, as well as periods of irregular modulated parasystole, were observed. The distribution of ectopic complexes was at times typical for concealed bigeminy (intervening beats always in odd numbers), but on other occasions reflected the even variant of concealed bigeminy where the intervening beats conform to the formula 1 + n (n being zero or an odd number). Some couplets of identical ectopic complexes were also observed.

CONCLUSIONS

The study shows that several different expressions of parasystolic rhythm may be present within the same tracing. 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. Finally, supernormal modulation is responsible for the occurrence of couplets.