Differentiation between parasystole and extrasystoles. Influence of vagal stimulation on parasystolic impulse formation

The heart side of brain neuromodulation

Neuromodulation refers to invasive, minimally invasive or non-invasive techniques to stimulate discrete cortical or subcortical brain regions with therapeutic purposes in otherwise intractable patients: for example, thousands of advanced Parkinsonian patients, as well as patients with tremor or dystonia, benefited by deep brain stimulation (DBS) procedures (neural targets: basal ganglia nuclei). A new era for DBS is currently opening for patients with drug-resistant depression, obsessive-compulsive disorders, severe epilepsy, migraine and chronic pain (neural targets: basal ganglia and other subcortical nuclei or associative fibres). Vagal nerve stimulation (VNS) has shown clinical benefits in patients with pharmacoresistant epilepsy and depression. Non-invasive brain stimulation neuromodulatory techniques such as repetitive transcranial magnetic stimulation (rTMS) and transcranial direct current stimulation (tDCS) are also being increasingly investigated for their therapeutic potential in several neurological and psychiatric disorders. In this review, we first address the most common neural targets of each of the mentioned brain stimulation techniques, and the known mechanisms of their neuromodulatory action on stimulated brain networks. Then, we discuss how DBS, VNS, rTMS and tDCS could impact on the function of brainstem centres controlling vital functions, critically reviewing their acute and long-term effects on brain sympathetic outflow controlling heart function and blood pressure. Finally, as there is clear experimental evidence in animals that brain stimulation can affect autonomic and heart functions, we will try to give a critical perspective on how it may enhance our understanding of the cortical/subcortical mechanisms of autonomic cardiovascular regulation, and also if it might find a place among therapeutic opportunities in patients with otherwise intractable autonomic dysfunctions.

Apparent disappearance of ventricular parasystole due to a marked difference between the long form and the short form of the ectopic cycles

Electrocardiograms were taken from a 44-year-old man with irregular ventricular parasystole in whom pure parasystolic cycles without any intervening nonectopic QRS complexes were found. When a sinus impulse fell late in the parasystolic cycle, it hastened occurrence of the next parasystolic discharge. This suggested that type I second degree entrance block occurred in the re-entrant pathway containing the parasystolic focus. When a sinus impulse fell early in the parasystolic cycle, it delayed occurrence of the next parasystolic discharge. This suggested that electrotonic modulation occurred in the parasystolic focus. As a result, the difference in length between the short form and the long form of the parasystolic cycle became markedly great. When the length of two adjacent sinus cycles ranged between the short and the long parasystolic cycle, manifest parasystolic QRS complexes disappeared for a long time. In true ventricular parasystole with pure ectopic cycles, such long disappearance has never been reported before.

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."

Direct discrimination and full-day disclosure of ventricular parasystole on new heart rate tachograms

INTRODUCTION

To discriminate ventricular parasystole from fixed coupling interval ventricular premature complexes (VPCs), we developed a new diagnostic method using a dot distribution pattern corresponding to VPCs recorded on a heart rate tachogram using ambulatory ECG monitoring data. We tested our hypothesis that widely scattered VPC dots on instantaneous heart rate tachograms indicate a constant VPC-VPC interval compatible with parasystole.

METHODS AND RESULTS

Patients with frequent VPCs > 5,000/day) were divided into two groups depending on the tachogram dot distribution patterns: group S (n = 10, aged 61 +/- 16 years) showed widely scattered VPC dot distribution, whereas group F (n = 10, 60 +/- 17 years) showed fixed VPC dot distribution limited to a narrow zone. Using digitized R-R interval data, full-day heart rate tachograms and VPC-VPC intervals were depicted simultaneously. Group S demonstrated constant basic VPC-VPC intervals (1,285 to 2,052 msec, mean 1,738 +/- 219), with a coefficient of variation (CV) of 0.061 +/- 0.018. Their VPC coupling intervals were markedly variable (651 +/- 113 msec; CV = 0.193 +/- 0.034). Each patient's basic VPC-VPC intervals showed small diurnal alterations (minimum -13% +/- 3% to maximum +15% +/- 6%). VPC-VPC intervals in group F were not constant and showed marked variation. Group F VPC coupling intervals were shorter and constant (480 +/- 30 msec, P = 0.0002; with CV = 0.076 +/- 0.013, P < 0.0001).

CONCLUSION

Ventricular parasystole with constant VPC-VPC intervals consistently became evident based on VPC dot patterns recorded on heart rate tachograms.

Concealed conduction in the reentrant pathway as a mechanism of stable ventricular quadrigeminy

This is the first report on the stable occurrence of ventricular quadrigeminy as a manifestation of concealed bigeminy in a case of fixed and late coupled ventricular extrasystoles. A 46-year-old man is reported in whom the period of ventricular bigeminy alternated with the period of ventricular quadrigeminy. Coupling intervals of the extrasystoles were fixed and much longer than sinus QT intervals. When the heart rate is decreased, the period of bigeminy changed to the period of quadrigeminy without gradual decrease in coupling of the preceding extrasystoles. Once such a change occurred, stable quadrigeminy is maintained for a period. These findings suggest the possibility that concealed electrotonic conduction of blocked impulses and interference of conducted impulses may occur in the reentrant extrasystolic pathway as a mechanism of stable ventricular quadrigeminy.

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.

Effect of standing on ventricular parasystole: shortening of the parasystolic cycle length

OBJECTIVE

To investigate the effect of standing on the parasystolic cycle length in cases of "true" ventricular parasystole.

METHODS

Parasystolic cycle length and sinus cycle length were measured during lying and standing in eight men with true ventricular parasystole. These cycle lengths were also measured after exercise in the lying position.

RESULTS

In all cases, parasystolic cycle length and sinus cycle length both shortened on standing, by a mean of 6.4% and 17.8%, respectively, compared to lying. In all cases, the rate of shortening of the parasystolic cycle length was less than that of the sinus cycle length. Parasystolic cycle length was prolonged after exercise, in contrast to a shortening of the sinus cycle length.

CONCLUSIONS

Influences on the parasystolic cycle length are not always in the same direction as on the sinus cycle length. This suggests that the effect of autonomic changes on parasystolic rhythm is not always parallel to that on sinus rhythm.

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.