1/5. 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."- - - - - - - - - - ranking = 1keywords = block (Clic here for more details about this article) |
2/5. 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.- - - - - - - - - - ranking = 2keywords = block (Clic here for more details about this article) |
3/5. Double atrial parasystole showing intermittent trigeminy.An extremely rare instance of atrial parasystole arising from two different ectopic atrial foci, i.e. double atrial parasystole, is presented, in which both parasystolic intrinsic cycles were found to be around 0.80-0.89 sec. Atrial parasystolic trigeminy with a positive P wave in leads II and III and interectopic intervals (IEIs) of 2.40-2.68 sec continued, then intermittent atrial parasystole with a negative P wave in leads II and III and IEIs of 2.45-2.69 sec took over. A wandering pacemaker between the sinus node and the upper atrio-ventricular (A-V) junction due to autonomic imbalance, i.e. increased vagal tone, in an elderly patient with bronchial asthma was supposed to be the underlying mechanism. Resetting of the parasystolic pacemaker by the third preceding atrial impulse or mutual entrainment and the subsequent 3:1 exit block may play a role in the development of intermittent atrial parasystole with a negative P wave in this case on account of the intrinsic ectopic cycle of 0.84 sec. Alternative explanation of IEIs 2.40-2.69 sec was a 2:1 exit block with the altered basic cycle of mainly 1.24-1.29 sec in consideration of the IEI of 1.24 sec. An explanation of manifest parasystolic cycles on the basis of the theory or cardiac resonant oscillation was put forward for the first time in this report. The IEIs take certain discrete values, which may represent a quantitized property of atrial parasystolic pacemakers as oscillating systems.- - - - - - - - - - ranking = 0.66666666666667keywords = block (Clic here for more details about this article) |
4/5. 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.- - - - - - - - - - ranking = 2keywords = block (Clic here for more details about this article) |
5/5. 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.- - - - - - - - - - ranking = 3keywords = block (Clic here for more details about this article) |